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EngineeringDrawing,Communication,and Design

PETER COOLEY

C.Eng., M.I.Mech.E.Department of Mechanical EngineeringUniversity of Aston in Birmingham

Pitman Publishing

First published 1972

SIR ISAAC PITMAN AND SONS LTD.Pitman House, Parker Street, Kingsway, London, WC2B 5PBP.O.Box 6038, Portal Street, Nairobi, Kenya

SIR ISAAC PITMAN (AUST.) PTY . LTD.Pitman House, Bouverie Street, Carlton, Victoria 305 3

PITMAN PUBLISHING COMPANY S.A. LTD.P.O.Box 9898, Johannesburg, S.Africa

PITMAN PUBLISHING CORPORATION6 East 43rd Street, New York, N.Y. 10017, U.S.A.

SIR ISAAC PITMAN (CANADA) LTD.Pitman House, 381-383 Church Street, Toronto, 3, Canada

THE COPP CLARK PUBLISHING COMPANY517 Wellington Street, Toronto, 2B, Canada

P. Cooley 1972

Cased Edition ISBN 273 43940 5

Paperback Edition ISBN 273 31798 9

G2-(T199/523:76)

There is an old practical joke in which a groupof people who are "in the know" ask a newcomer todescribe a spiral staircase. Very often the new-comer does as expected and traces out the locus ofthe stairs with a finger whilst saying "It's astaircase that goes round like this". The rest ofthe group are prepared for this and can laugh athis predictable behaviour. The trick works becausesome objects are very difficult to describe in wordsand most people resort to pictures or models whenthey have to deal with them in detail.

The engineering designer has to cope with thisproblem throughout most of his professional life.In order to communicate ideas and instructions heuses sketches and drawings as well as written andspoken words. There are many books on engineeringdrawing and on the various aspects of engineeringdesign but little has been written which shows therelationship between the two activities. Surely thisis a very strange state of affairs? Engineeringdrawings and sketches are the engineering designer'sspecial method of communicating his ideas to otherpeople but why should the study of this communica-tion be isolated from the process which uses it? Inthis book I have attempted to treat the basic topicof drawing and sketching and then to show how theymay be used in the synthesis of a new product.

The earlier chapters cover most of the essentialengineering drawing background required by moststudents of engineering. They can be used either asa text or as a self-instruction manual. To facilit-ate the latter, solutions are provided for many ofthe exercises. Whenever possible the exercises aredesign-orientated: students choose the most suitableviews of a given object rather than being told ex-actly what to draw. The units used are SI through-out.

The last eight chapters illustrate the designprocess. It is extremely difficult to present arealistic project in a book; only in an industrialenvironment are there real constraints on the des-igner which could be easily appreciated. However,I believe that the unusual and, frankly, experimentalscripted case-history approach will prove stimulatingto most readers . My own experience with this tech-nique has been most encouraging and I hope that otherstudents of engineering design will find it inter-esting and informative.

I would like to thank Mr. A. Dunsmore forseveral helpful suggestions, my wife for her helpand encouragement, and my children for not makingtoo much noise.

Peter Cooley

Streetly 3 Staffs

September 1971.

Contents

Preface

1 ORTHOGRAPHIC PROJECTION

First and Third Angle Projection • 7

Lines and Dimensions • 7

Pictorial and Orthographic Drawings • 7

Clarity in Engineering Drawings • 9

2 VISUALIZATION

Engineering Practice and Visual Noise • 15Intersection of Planes and Curved Surfaces • 19

3 SECTIONS

Other Types of Sectional Views • 25Ribs • 25Exceptions and Conventions • 27Special Section Lining • 27

4 THE USE OF DRAWING INSTRUMENTS AND EQUIPMENT

Materials • 29Supporting the Drawing Material • 29Marking the Drawing Material • SIRemoving Marks • S3Other Drawing Instruments • S3At the Drawing Board • 33The Use of Springbows and Compasses • 35Arc Blending • 35

5 DIMENSIONING

General Principles • 39Arrangement of Dimensions • 41Circles^ Radii and Holes • 41Dimensioning for Manufacture and Function • 43

6 AUXILIARY PROJECTIONS

Circles • 55Double Auxiliary Projection • 57Surfaces with All Sides Foreshortened • 59Circles in Double Auxiliary Projection • 63Projective Geometry • 63Isometric Projection • 65

7 DEVELOPMENTS 69

Modified Cylinder • 71

Right Cone • 71

Oblique Cone • 75Triangulation • 77

8 PICTORIAL DRAWINGS 79

Isometric Projection • 79Isometric Drawing • 79Sloping Surfaces • 81Circles • 83Blended Arcs • 85Small Circles • 8 5

Dimetric Projection • 85Dimetric Drawing • 85Trimetric Projection • 88Trimetric Drawing • 88Methods of Producing Dimetric and Trimetric Drawings • 88Oblique Drawing • 88

9 MATING PARTS 89

Tolerances 3 Deviations and Limits • 89Selection of Fits • 95British Standard 4500:1969 • 95Fits for General Engineering Products • 96Typical Applications of Hole Basis Fits • 96Other Manufacturing Tolerances • 96

10 ENGINEERING DRAWINGS IN INDUSTRY 99

General Arrangement Drawings • 99Assembly and Sub-assembly Drawings • 99Detail Drawings • 99Conventions in Assembly Drawings • 101Fasteners • 101Surface Texture • 103Modifications to Detail Drawings • 103

11 SKETCHING 109

Sketching Circles • 109Pictorial Drawings - Oblique Sketches • 113Pictorial Drawings - Isometric Sketches • 113The Sketching of Ideas • 115

12 INTRODUCTION TO SYNTHESIS 118

13 PROBLEM DEFINITION 119

Phase 1 - Discovering the true nature ofthe problem posed • 119

14 PROBLEM ACCEPTABILITY 123

Phase 2 - Determining whether the task is onethat the organization will be ableto carry out • 123

15 DESIGN FACTORS 128

Check List of Design Factors • 128Phase 3 - Examining the factors that will

influence design • 129Specification for Automatic Toothbrush • 133

16 PROBLEM SOLVING 135

Phase 4 - Finding possible solutions andselecting the most suitable one • 135

17 SOLUTION DIVISION 142

Phase 5 - Preparing an overall design anddividing up the work that it willentail • 143

18 SOLUTION DEVELOPMENT 147

Phase 6 - Making decisions on items of increasingdetail 3 in consultation with the otherpeople affected • 149

19 SOLUTION EXECUTION 160

Phase 7 - Directing the manufacture of therequired quantity • 161

23

-

THIRD ANGLEPROJECTION

ALL DIMENSIONSARE IN MMSCALE 1 /1

THIRD ANGLE PROJECTION

4 8

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FIG. 1.1FIG. 1.3

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FIG. 1.4

FIG. 1.2

CMAraa u

Orthographic Projection

Figure 1.1 is an engineering draw-ing of a simple, solid object but itcontains many of the features of allworking drawings . The first thing tonotice is that three separate views ofthe object have been drawn. Study theviews carefully and try to picture thesolid object. Make a sketch of the ob-ject and then insert arrows to indicatethe directions in which the views ofFig. 1.1 could be seen.

You will have made some assumptionsabout the arrangement of the views inFig. 1.1; can you justify these assump-tions by relating Fig. 1.1 to your sketch?When you have checked on this point,compare your sketch with Figure 1.2.Your sketch may be drawn as seen from aviewing point different from that inFig. 1.2, but the important thing is tohave the same concept of the object asthe figure illustrates.

In an engineering drawing, the ex-act shapes of the surfaces of the ob-ject are constructed with reference totwo or more perpendicular planes . Inthe above example, there are no curvedsurfaces to complicate matters andevery surface is rectangular. The ob-ject is referred to imaginary planesthat are parallel to the surfaces ofthe object and, therefore, every surfaceappears in its true shape. The distan-ces need not be full size, although itis often convenient to make them so.

Fig. 1.9 is to a scale of 1/2, i.e. 1 mmon the paper represents 2 mm on theobject.

Figs. 1.1 and 1.2 illustrate how asolid object is represented in Ortho-graphic Projection. By accurately draw-ing out the surfaces that appear whenone looks in the direction of arrow X,the lower left-hand view (of Fig. 1.1)would be produced. Similarly, arrow Yindicates the source of the lower right-hand view and arrow Z indicates thesource of the upper view.

It must be emphasized that theviews contain the true shapes of thesurfaces. It is neither necessary nordesirable to employ the rules of per-spective in order to draw what a person(having two eyes, about 60 mm apart)would actually see from a particularviewing point.

Figure 1.3 shows three views of amore complicated object than the oneabove. Try to visualize the solid ob-ject and then make a sketch of it. In-sert arrows to indicate the directionsin which the views of Fig. 1.3 could beseen. If you have difficulty, try ref-erring back to Figs. 1.1 and 1.2. Al-though the views in Figs. 1.1 and 1.3are arranged differently, the relation-ship between the views is the same; itis known as THIRD ANGLE PROJECTION andit will be explained below.

FIG. 1.5

c

AB

The three ways ofarranging the samethree views inThird Angle Projection

FIG. 1.6

<

\ ^y~THIRD ANGLE PROJECTION

Poororientationsof objectwi th a

def ini tebase

FIG. 1.7

When you have finished your sketch,compare it with Figure 1.4 . Your sketchmay be drawn as seen from a viewingpoint different from that in Fig. 1.4,but it should be, essentially, the sameobject. By accurately drawing out thesurfaces that appear when one looks inthe direction of arrow X, the upperright-hand view (of Fig. 1.3) would beproduced. Notice that the long (44 mm)edge has been placed vertically. ArrowY indicates the source of the lowerview and arrow Z indicates the sourceof the upper left-hand view.

Why have the views been positionedas they have in Figs. 1.1 and 1.3? Inother words , can you work out the rulefor positioning the views called THIRDANGLE PROJECTION? Write it down andthen compare your answer with the ex-planation given below.

The rule is very simple. Pick anyof the views in Figs. 1.1 and 1.3 (thisview will be referred to as the "selec-ted" view) . What would be seen of thesolid object, from the left of the sel-ected view, is drav/n, horizontally, tothe left . What would be seen from theright of the selected view is drawn,horizontally, to the right . What wouldbe seen from above the selected view isdrawn, vertically, above . What wouldbe seen from below the selected view isdrawn, vertically, below .

Corresponding surfaces line upalong vertical and horizontal lines:this is of considerable assistance tothe person making the drawing and tothe person reading the drawing.

Examine all the views of Figs. 1.1and 1.3 and you will find that the ruleholds good. Notice that there is oneview which links the other two together,because it has views projected both hor-izontally and vertically from it. Thisview is dubbed the Link View and itwill be shown that, in any drawing withthree views from mutually perpendiculardirections, any of the views may act asthe link view in third angle projection.

Figure 1.5 is a pictorial view ofa simple block. Views are required as

seen in the directions of arrows A, Band C; they are to be arranged inthird angle projection. Any one of theviews can be the link view. Use asheet of graph paper and estimate theproportions of the object, then sketchout the three views with the view inthe direction of arrow A as the linkview.

Now sketch out the same threeviews, but make the view in the direc-tion of arrow B the link view. It isstill possible to follow the rule forthird angle projection.

Thirdly, sketch out the same threeviews but with the view in the direc-tion of arrow C acting as the link view.When you are satisfied that the sets ofviews are all in accordance with therule for third angle projection, compareyour sketches with those in Figure 1.6.

You may have a combination thatlooks different from those shown in thefigure. Try turning your paper through90° or 180° and you should see that itis the same as one of those in Fig. 1.6.

It is not always easy to decideupon which arrangement to use for anEngineering Drawing; much depends onthe object in the drawing and on theshape of the paper on which the viewsare to be drawn. Some objects have sur-faces and edges that would look verystrange if they were not orientated inthe normal way. For example, a housewould look most peculiar if its verticalwalls were shown horizontally in a draw-ing. A very long object looks strangeif its length is not along the horizon-tal, and there may be some difficulty infitting it on the paper. Other objects,like the one shown in Figure 1.7, havean obvious base which should be drawn inthe normal position, and not with theobject appearing to stand on its heador its side. However, an object, likethe one in Fig. 1.5, could be drawn outin any of the possible arrangements con-sistent with the type of projectionused; a draughtsman would use the onethat he found most convenient.

ALL DIMENSIONSARE IN MM

FIG. 1.8


SCALE 1/2

n ! n

i~n

FIG. 1.9

EXERCISE 1 .1

Figure 1.8 is a pictorial view of abracket. Views, as seen in the direc-tions of arrows X, Y and Z, are to beused for an engineering drawing. Why-are these views better than those inthe directions of arrows X', Y' and Z'?

Draw the views out, FULL SIZE (ascale of 1/1) , in third angle projection.Choose a link view so that the objectdoes not appear with an unnatural orien-tation. Make the drawing on graph paper;do not attempt to use instruments atthis stage. Do not put any dimensionson your drawing; dimensioning is a sub-ject in its own right and is dealt within Chapter 5 .

When you have finished, compareyour drawing with Figure 1.9.

First and thirdangle projection

So far, the examples in this Chap-ter have dealt exclusively with THIRDANGLE PROJECTION. This is the systemof orthographic projection used through-out the United States of America, inseveral Commonwealth countries, and inmany British industries. The other sys-tem used in Britain (and in most ofEurope) is known as FIRST ANGLE PROJEC-TION. It is the exact opposite of thirdangle projection, i.e. what is seen fromabove a selected view is drawn vertic-ally below that view; what is seen fromthe left of the selected view is drawnhorizontally to the right of that view,etc.

The British Standards Institutionissues its recommendations for Engin-eering Drawing Practice in British Stan-dard 308 (B.S.308). Both First AngleProjection and Third Angle Projectionare acceptable as British Standards.Both systems have their supporters butit cannot be proved that, for a trainedperson, one method has any advantagesover the other. However, there is someevidence 1 that a person who has neverstudied an engineering drawing beforewill tend to assume that the views arearranged in third angle projection.

For this reason, third angle projectionwill be used for explaining each newprinciple but, whenever space permits,both types of projection will be intro-duced for the exercises.

Because there are two methods ofprojection in use in Britain, it is im-portant that drawings are clearly label-led to indicate whether FIRST or THIRDANGLE PROJECTION has been used.

Lines and dimensionsFig. 1.3 illustrates several of tne

other recommendations contained in B.S.308. Notice that, for the lines usedin the drawing, there are two distinctthicknesses. Thick lines are used forvisible outlines and thin lines are usedfor (amongst other purposes) the dimen-sion lines (those with arrowheads) andthe projection lines (those touched bythe arrowheads) . Thick lines should befrom two to three times the thicknessof thin lines. All the lines in Fig.1.3 are continuous but other types willbe encountered later on.

The dimensions are expressed inmillimetres and there is a note on thedrawing to make this clear. Centimetresare not recommended units for engineer-ing drawings; millimetres are satis-factory for most detail drawings, with-out using a larger or a smaller unit.

Pictorial and ortho-graphic drawings

Engineering drawings are normallymade in first or third angle ortho-graphic projection, which means thatthe views are drawn as seen in two ormore perpendicular directions. Thereare always two views at least and thesehave to be considered together in orderto visualize the object. An engineeringdrawing must also provide informationfor the manufacture of the object:namely, as dimensions and notes. It istoo risky to allow the dimensions to bescaled from the drawing. However, itwould be possible to put the dimensionsaround a pictorial view of the objectbut such a method is rarely used.

1 SPENCER, J., Experiments on Engineer-ing Drawing Comprehension, Ergonomics

,

vol.8, pp. 93-110, January 1965

FILLETS AND OTHERRADII 3

2 HOLES<j) 15


Notes

1 The holes are not blind, i.e. theygo right through tne metal.The bracket is symmetri cal about tneverti cal centre line.Thin chain lines are used inengineering drawings to indicate a

centre line or an axis of symmetry.The symbol means 'diameter 1

.

FIG. 1.10

THIRD ANGLE P RCLIECTION

_ r -A\- ~^ ! H^T T

SCALE 1/2

FIG. 1.11

Figure 1.10 illustrates some ofthe disadvantages of dimensioned pictor-ial views. The first point to noticeis that , because the bracket is made bya casting process, it has very fewsharp edges. The rounded corners, tog-ether with the circular holes and thegeneral geometry of the object, make itquite awkward to draw. The shading hasdeliberately not been done very skill-fully, but some shading is essential forindicating the true shape of the object.

Therefore, even a simple bracketlike this one would be difficult andexpensive to illustrate in this way.Once the object has been drawn, the dim-ensions have to be added. Notice that,although the bracket's geometry is quitestraightforward, the dimensions take upa lot of space, are not always easy tofollow, and confuse the pictorial qual-ity of the drawing. A more complicatedobject would be extremely difficult todraw and to dimension and the cost ofdoing so could be prohibitive.

The advantages of orthographic pro-jection are that drawings may be pre-pared quickly and cheaply by a personwith no particular artistic skill andthe views may be dimensioned with littledifficulty and confusion. The disad-vantage of orthographic projection isthat one has to acquire some skill invisualizing the object in order to readthe drawing. This is not a very seriousdisadvantage as most people can, withpractice, develop the ability to under-stand orthographic projection.

EXERCISE 1 .2

Which views will best illustratethe bracket shown in Figure 1.107 Arethree views needed, or will two be suf-ficient? If you cannot decide on thispoint, look quickly at Fig. 1.11, butuse it only as a guide for which viewsto draw.

Decide on a suitable scale and planthe layout of the views before you beginto draw. Do not be caught with toolittle space left for one of the views

.

Use millimetre graph paper, a ruler anda coin to make a reasonable engineeringdrawing in third angle projection. When

you have finished, compare your solu-tion with Fig. 1.11

Clarity in engineeringdrawings

Figure 1.11 shows two views, inthird angle projection, of the bracketillustrated in Fig. 1.10. Only twoviews have been drawn and these showthe circular holes and the rectangularhole quite clearly. A third view wouldadd nothing to the clarity of the draw-ing and, for this fairly simple object,would not help to distribute the dimen-sions more evenly.

Notice that every thick line onthe drawing represents either the bound-ary between a metal surface and the sur-rounding air, or the intersection oftwo metal surfaces. At first sightthere may appear to be some lines mis-sing, but a comparison with Fig. 1.10will reveal that the line is missingbecause there is a radius that blendstwo surfaces smoothly. There are veryfew sharp edges and it is necessary tosketch all the circular arcs or to usesome form of template

.

As both views are symmetricalabout the vertical centre line, it isconvenient to work from this line andto transfer equal distances from oneside to the other.

In addition to the requirements ofExercise 1.2, a new type of line hasbeen used in Fig. 1.11 to indicate theboundaries of the holes . It is conven-ient to show hidden details of thiskind with thin short dashes and manyengineering drawings include such lines

.

The centre lines of the circularholes are shown in both views: anotheraid to clarity. Remember that centrelines, like dimension and hidden detaillines, should be thin lines; visible,thick lines are, at least, twice asthick as the thin lines. Drawings aremuch more difficult to read if this dis-tinction is not maintained.

There are two labels in Fig. 1.11which should appear on every" engineeringdrawing: the type of projection andthe scale

.

60

20 .,10

40

30

oCM

15

m

FIRST ANGLEPROJECTION

SCALE 1/2

10

r

'20 20



SCALE 1/2

FIG. 1.12

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FIG. 1.13

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FIG. 1.14

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EXERCISE 1.3 Conversion from Thirdto First Angle Projection

Figure 1.12 shows three views of aVee block. The views are badly chosenand there is a great deal of hidden det-ail. The object is to be redrawn infirst angle projection, and it shouldbe possible to find some more suitableviews. Study Fig. 1.12 carefully andget to know the object. Make a simplesketch if you find that a pictorialview helps you. Decide on the numberof views and the viewing directions fora clearer drawing.

Make the drawing on a piece ofgraph paper and use first angle projec-tion. As explained above, first andthird angle projection are exact oppos-ites and you may find it helpful tomake a sketch of the views that you pro-pose to draw, in order to check thatthey are correctly positioned. Whenyou have completed the views , add thenecessary labels and then compare yourdrawing with Figure 1.13.

Summary The Vee block of Exercise 1.3is a simple object and two views aresufficient to show all its features.The two left-hand views in Fig. 1.12 arepoorly chosen because much of the detailis hidden behind the visible surfaces.The opposite viewing directions resultin no hidden details and these viewshave been chosen for Fig. 1.13. Thelower view is what would be seen fromthe top of the upper view, so the draw-ing is correctly arranged for FIRSTangle projection. If a view from theright-hand side were to be drawn, itshould be placed on the left.

EXERCISE 1 .4

Figure 1.14 shows a pictorial viewand orthographic drawings of variousobjects. State whether first or thirdangle projection has been used in theorthographic drawing. Answers are atthe end of this Chapter.

EXERCISE 1 .5

Fig. 1.1 5 shows orthographic draw-ings of various objects. State whetherfirst or third angle projection hasbeen used. N.B. Some of the arrange-ments are ambiguous. Answers are atthe end of Chapter.

EXERCISE 1 .6

Figure 1.16 shows pictorial viewsof various objects . Select views thatwould most clearly show the object andshow how these views should be arrangedin both first and third angle projec-tion.

Answers to Exercise 1.4

1. 1st;5. 1st;9. 1st;

2.

6.

10.

1st;3rd;3rd.

3rd;1st;

4. 3rd;8. 1st;


1. 3rd; 2. 3rd; 3. 1st;5. 1st; 6. 3rd; 7. Both;9. Both; 10. 1st.

1st;3rd;

FIG. 1.16

11

FIG. 2.1

VIEW ALONG Z

VIEW ALONG Y


VIEW ALONG X

FIG. 2.2

THIRD ANGLE PROJECTION THIRD ANGLE PROJECTION

A BOUNDARY LINE

BINTER-SECTIONLINE

A BOUNDARY LINE

FIG. 2.3

V

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FIG. 2.4

Visualization

In the previous Chapter, it wasstated that the disadvantage of ortho-graphic projection is that one has toacquire some skill in order to visual-ize the object. The purpose of thisChapter is to provide some practice atthis activity so that future drawingsmay be read and understood as rapidlyas possible.

Figures 2.1 and 2.2 show a pictor-ial drawing and an orthographic drawingof the same object. Fig. 2. 2 showsviews as seen in the directions ofarrows X, Y and Z of Fig. 2.1.

It is not too difficult to visual-ize the separate views from a pictorialdrawing of the object. However, untilone has a certain degree of fluency inorthographic projection, it is not soeasy to picture the solid object fromthe separate views. You can see thisfor yourself by turning to Fig. 2. 2 fora time and trying to picture the solidobject, without reference to Fig. 2.1.

It is important to remember that,in any orthographic drawing, a contin-uous thick line and a hidden detail(thin) line can indicate either:

(1) the boundary between the materialof which the object is made andthe surrounding medium (whichwill be referred to as "air") , or

(2) the intersection of two surfaces.

The distinction is not alwaysclear and in Fig. 2. 2 nearly every lineis both a boundary line and an inter-section line. There is an example of a

boundary line which is not also an in-tersection line in Figure 2.3. Thelines labelled A do not represent theintersection of two surfaces: theironly function is to show the boundarybetween the material and the air

.

The line labelled B representsonly the intersection of two surfaces.The test for distinguishing between thetwo types is that, if it were possibleto grind and smooth the material at theline, the intersection line would beremoved, whereas the boundary linewould merely be shifted.

Sometimes, lines will be superim-posed because of the viewing direction}what appears as a single line in oneview may be two or more boundary or in-tersection lines overlaid. The otherviews of the drawing have to be con-sulted before the situation becomesclear. If a visible line is coincidentwith a hidden detail line then, natur-ally, the visible line takes precedence.This situation may be avoided by a care-ful choice of view on the part of thedraughtsman, but it is something forwhich the reader of a drawing should bewatching.

13

THIRD ANGLEPROJECTION U


3

FIRST ANGLEPROJECTION

n_n

LTUFIRST ANGLE PROJECTION


a VW b

EiiTHIRD ANGLE PROJECTION

FIG. 2.5

The following steps are essentialwhen reading any orthographic drawingfor the first time:(1) Identify the type of projection.

The drawing will often be labelledFIRST ANGLE PROJECTION or THIRDANGLE PROJECTION, or one of thesymbols shown in B.S.308 may beused. In the absence of a defin-ite specification, it may be poss-ible to discover the type of pro-jection by inspection, just as inthe exercises at the end of thelast Chapter. If the projectionis ambiguous or if you are uncer-tain about the actual system, itis much better to ask than to makethe wrong assumption.

(2) Identify the various views andmake sure that you have understoodtheir relationship to one another.

(3) Look at the views in more detailand identify the same feature ineach view in which it appears . Itmay sometimes be necessary to usea straight-edge in order to seehow the features line up betweenviews . There are various unusualcombinations of lines that occurwith sufficient frequency to beworth special attention: theywill be dealt with in the exer-cises below. Bear in mind that arectangular shape can be manythings, as well as a horizontal orvertical surface: it may be aninclined surface or it may be acurved surface

.

EXERCISE 2.1

Figure 2.4 shows two views of a

block. Go through the steps describedabove and try to get a clear idea ofthe solid object. Quite a lot of theinformation about the block is presen-ted as hidden detail lines. Make asketch of the object and check that itagrees with the two views in the figure.

When you have finished, compareyour sketch with Figure 2.6. Noticehow the various features produce thefull and hidden detail lines of theorthographic drawing.

EXERCISE 2.2

Figure 2.5 shows orthographic draw-ings of various objects, each with aselection of pictorial 'views. Studythe drawings and try' to picture thesolid object without looking at thepictorial views. When you have visual-ized the object, select the pictorialview which is correctly matched withthe orthographic drawing.

When you have finished, turn tothe answers at the end of this Chapter.You should pay special attention to anywrong answers to Fig. 2. 5. Most peoplefind that planes normal to the viewingdirections give very little trouble;planes that are inclined, but appear asa line in one of the views , give moretrouble and planes that appear neitherin their true shape, nor as -a straightline, give most trouble at first.

Engineering practiceand visual noise

Engineering components are rarelymachined on all surfaces; only a fewhave no rounded edges at all. The major-ity of the objects shown in this Chaptershould not be regarded as typical ofengineering practice; they are only in-tended as exercises in visualization.

Dimensions can make a drawing moredifficult to understand, although theyare an essential part of the communic-ation medium. For this reason, some ofthe exercises at the end of this Chapterhave dimensions and notes added. Theseexercises provide practice at concentra-tion, as it is important to ignore the"visual noise" and observe only theshape of the object.

15

FIG. 2.6

FIG. 2.9

FIG. 2.7

FIRST ANGLE PROJECTION

:TO4*?-

FIG. 2.8


EEEEEno±r:

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FIG. 2.10


N

FIG. 2.11

EXERCISE 2.3

Figure 2.7 is a pictorial view ofa casting. Study it carefully and getto know the object. Figure 2.8 showstwo views (not to the same scale) ofthe same object; check that they agreewith Fig. 2. 7.

The bracket is to be modified sothat it fits on to a different shape ofbar and its general appearance is imp-roved. These modifications are shownin Figure 2.9. How should the views ofFig. 2. 8 be modified? Make a sketch ofan orthographic drawing of the modifiedbracket.

When you have finished, compareyour drawing with Figure 2.10.

Fig. 2. 10 shows two views of thebracket illustrated in Fig. 2. 9. Noticethe differences between Figs. 2.8 and2 . 10 . Because of the viewing direc-

tions, not all the curves introduced inFig. 2. 9 can be seen. There is a lot ofhidden detail which would be very diff-icult to interpret without the pictor-ial view. The views shown do not fullydefine the bracket for it would be poss-ible to interpret them as indicatingsquare corners on the forked end. Thisambiguity can usually be avoided by acareful choice of views.

EXERCISE 2.4

Figure 2.11 contains a pictorialview of a component and two views , inorthographic projection, of the samesort of object but not an identical one.Study the orthographic drawing and id-entify the differences between the twoobjects. By partially tracing the ex-isting pictorial view, make a sketch ofthe modified component.

17

FIG. 2.12

FIRST ANGLEPROJECTION FIRST ANGLE

PROJECTION

FIG. 2.14.1 FIG. 2.14.2

4>

FIG. 2.14.3 FIG. 2.14.4.

*&

8n±

FIG. 2.13

I I

I 1

1

I ! !

FIG. 2.14.5

J

FIG. 2.14.6.

10

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FIG. 2.14.7

///

//

/

//

FIG. 2.14.8

Intersection of planesand curved surfaces

This type of intersection occursvery frequently in engineering compon-ents, so many of which are turned on alathe. Figure 2.12 is an example ofhow to sketch a pictorial view of abasically cylindrical component. Theorthographic drawing at the top may befound, by inspection, to be in firstangle projection. The first stage inthe preparation of a pictorial view isa sketch of the basic cylinder. Next,two parallel lines are drawn on the topsurface and down the curved surface.In the third stage, the other visiblelines are sketched in. Finally, thesuperfluous lines are removed. Thistype of construction can be used forall the objects in the next exercise.

EXERCISE 2.5

Figure 2.13 shows ten objects;each of them could be manufactured froma cylindrical piece of material, 20 mmdiameter, 30 mm long. For each drawing,identify and state the type of projec-tion used and make a sketch of a pictor-ial view of the object. When you havefinished, check that your sketches

agree with the original drawings. Ans-wers, for the type of projection, areat the end of this Chapter.

EXERCISE 2.6

The final set of exercises in thischapter is graded and contains some ofthe visual noise mentioned above. Workthrough the sixteen objects in Figures2.14.1 to 2.14.16 by the usual methodof first identifying the projection,then examining the relationship betweenthe views, and then familiarizing your-self with the various features. Make apictorial sketch of each of the sixteenobjects, in order.

When you have finished, compareyour solutions with Figures 2.15.1 to2. 15.16.


1. a 2. b 3. b and d


1. 1st; 2. 3rd; 3. 1st; 4. 1st;5. 3rd; 6. 1st; 7. 3rd; 8. 3rd;9. 3rd; 10. Both.

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FIG. 3.3


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FIG. 3.4

FIG. 3.5


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SECTION X-X

FIG. 3.6

FIG. 3.7

aaAPfna

In the previous chapters, the onlydrawings that have been examined arethose with views that show the externalappearance of the object. Many engin-eering components are more complicatedinternally than they are externally.Figure 3.1 is a sketch of a block madeof some transparent material and allthe edges are visible. Two views of-this block are shown in third angleprojection in Figure 3.2; here thematerial is opaque, like most practicalmaterials are. The transparent mater-ial indicated in Fig. 3.1 is a convenienttechnique for showing the internal det-ails of the object. Notice how ortho-graphic projection, with its use of vis-ible and hidden detail lines, tends tomake the object more difficult to vis-ualize. Without the benefit of Fig. 3.1,it would not be quite so easy to under-stand what all the hidden detail linesmean.

The internal details of the objectmay be shown by drawing a sectionalview, normally labelled as a Section.In other words, that part of the objectwhich conceals the details of interestis removed and only what remains isdrawn. Normally, sections are drawnfor planes cutting the object. Figure3.3 contains a section of the blockthat has been produced by removingeverything to the right of the planeX-X and, in this way, one gets a clearerimpression of the holes.

Notice how the section plane X-Xis indicated. It is in accordance withthe recommendations in British Standard308 : 1964. The plane is shown as athick chain line:

complete with arrowheads and letters

.

The section is labelled accordingly.Where material has been cut by the sec-tion plane, this is indicated by means

of continuous thin lines at, in thiscase, 45°; this evenly-spaced set oflines is called section lining orhatching

.

Figure 3.4 shows three views ofthe same block. The sectional view isthe link view (see Chapter 1) . Noticethat the upper view shows the completeblock. It would be most confusing ifthe upper view were made a pedanticview on to the top of the section andshowed only part of the object. Allviews that are not specifically chosenas sections should represent the com-plete object.

Many objects cannot be convenientlysectioned by taking only one sectionplane . It is necessary in such casesto consider a section on two or moreplanes. Figure 3.5 shows such an ob-ject and Figure 3.6 shows the same ob-ject cut on two parallel planes, withtwo discontinuities in the sectionline. Notice how, in the left-handside of the section, the discontinuitiesin the section line are not shown;there is no way of avoiding showingthem on the right-hand side.

EXERCISE 3.1

Figure 3.7 is a pictorial drawingof a block. The internal details havebeen indicated by thin lines, as thoughthe object were made of a transparentmaterial. In the squared grid is partof the view that would be seen in thedirection of arrow Y. Complete thisview using thin, hidden detail linesfor the internal features, as for anopaque material. _ Choose a section thatwill show all the holes and projectthis section, from the existing view,to the right side of the grid. Completethe drawing by section-lining the sec-tional view and inserting the appro-priate thick chain lines and labels.

23

^ Ef«SECTION X-X

FIG. 3.8

FIG. 3.9

FIG. 3.13.1

p^j jsssasssstj

PARTSECTION

FIG. 3.10

THIRD ANGLE PROJECTION—•-A

SECTION A-A —"-A

FIG. 3.13.2

REVOLVEDSECTIONS

FIG. 3.11 THIRD ANGLE PROJECTIONr—A

SECTION A-A L_»a

FIG. 3.13.3

FIG. 3.12

Other types ofsectional views

Figure 3.8 shows, in the right-hand view, a circular object with fouraxes of symmetry. There are two basictypes of hole, excepting the centralone, and, in order to section throughboth types, the section planes aredrawn as shown. As the section planesare not parallel, it would be imposs-ible to project from both planes in thesame direction . The sectibnal view onthe left is not projected along eitherhorizontal or vertical lines but is con-structed independently of the otherview.

Notice that the sectional view rep-resents the two parts of the section inalignment, i.e. it would not actuallybe possible to produce such a view bycutting the solid object only along thesection planes, unlike the section inFig. 3. 6. Nevertheless, this sectioningdevice is so useful that it has beenaccepted as a recommendation in B.S.308.Part of the sectional view may be pro-jected from the adjacent view, if sodesired.

Figure 3.9 shows a half section.This is another useful method and isoften used for objects with an axis ofsymmetry. There is no need to have thehidden detail lines on the left-handside but they have been inserted inorder to emphasize the symmetry in thisexample.

Figure 3.10 is a part section.There would be no point in sectioningthe whole of such an object and a partsection is taken to show the hole. Theboundary of the section is shown by anirregular thick boundary line; this isthe sixth type of line for engineeringdrawings which has been introduced inthe book. (What are the other five?)

Figure 3.11 has two revolved sec-tions. This is a convenient method ofshowing the shape of varying cross-sections. The section planes are indic-ated with thin chain lines and the sec-tional views are revolved about theselines . The outline of the sectionshould be thin , to distinguish it fromthe remainder of the view.

Figure 3.12 has two removed sec-tions. These are similar to revolvedsections but are used when it is diffic-ult to superimpose the cross-section onthe existing view. The section planesare indicated with extended thin chainlines and the sectional views are rev-olved about these lines, outside theexisting view. The outlines of the sec-tions are thick .

RibsA rib is any thin, flat part of a

casting or moulding incorporated inorder to give the object greaterstrength and rigidity. A triangularrib is shown in Figure 3.13.1; the ob-ject is not typical of engineering prac-tice and serves only to illustrate thesectioning of a rib. Two views of theobject are shown in orthographic projec-tion in Figure 3.13.2; a section planeis indicated which passes LONGITUDINALLYthrough the rib. Notice the resultwhen the sectional view is pedanticallydrawn. The impression of solidness andbulk is quite unmistakable but complete-ly misleading.

It is better to ignore the factthat the section plane passes throughthe rib and to draw the rib in outsideview, as in Figure 3.13.3. This is theB.S.308 recommendation for ribs; otherexamples of this important exception tothe principles of sectioning will befound below.

25

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z_J_sL -,7 "^ 75 7 s t

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FIG. 3.14

FIG. 3.16.1


^^SW^ws

!^^^SECTION A-A A

FIG. 3.16.2

FIG. 3.17.1

THIRD ANGLE PROJECTION"B

SECTION B-B

FIG. 3.17.2

EXERCISE 3.2

Study the pictorial view shown inFigure 3.14. Part of one view of theobject is drawn on the grid below; com-plete this view and consider how bestto section the object in order to showboth sizes of non-central hole. Drawthe sectional view in the grid provided;add all necessary section lines, sec-tion-lining and labels. When you havefinished, compare your solution withFigure 3.1S.

Fig. 3. 15 shows one reasonable meth-od of sectioning the object. It issimilar to the method shown in Fig. 3. 8,but here the top part of the sectionhas been projected, horizontally, fromthe adjacent view. The lower part hasbeen drawn by transferring radial dis-tances, measured along the section line,to the appropriate vertical position inthe sectional view. Notice how the ribhas not been sectioned in the upperhalf and how it is visible beyond thesection plane in the lower half.

An alternative method of section-ing this object would be to use twoparallel planes; one passing vertic-ally through the larger hole and theother passing through the smaller hole.The discontinuity could be at any con-venient place.

Exceptions andconventions

In the same way that a rib is notshown in section when the section planepasses longitudinally through it, soother common parts are treated in thesame way. It would be pointless andmisleading to section shafts, bolts,nuts, rods, rivets, keys, pins, shimsand washers. Such components will nor-mally appear with a section plane long-itudinally through them in GeneralArrangement (Assembly) drawings only:they are illustrated in Chapter 10.However, two threaded features are soimportant that they will be treatedhere.

The tapped hole, or internal screwthread, is shown in Figure 3.16.1. Thepart section shows a blind tapped hole

with the end of the hole made by thetapping size drill and the helices cutby the tap, in quite accurate detail.The cost of producing such a drawingwould be prohibitive, so the conventionshown in Figure 3.16.2 is recommended.The crests and roots of the thread arerepresented by parallel lines and thesection lining is taken as far as thetapping size hole. The end view of thehole is represented as shown, with theouter circle broken, to indicate thethread.

The external screw thread of abolt, stud or screw is shown realistic-ally in Figure 3.17.1. Again, the costof the drawing would be prohibitive andFigure 3.17.2 shows the recommended con-vention. The end view of this fasteneris represented by two concentric circlesbut, in this case, the inner one isbroken

.

One way of remembering the conven-tion is that the circle which wouldappear First in the process of manufac-ture (the tapping hole of the plain bar)is shown as the Full circle.

In Fig. 3. 17. 2, the section planepasses through the fastener but it isnot sectioned; plane B-B would normallybe drawn so as to reveal the internaldetails of an assembly of parts.

Special section liningWhen a large area of a section has

to be shown, the recommendation of B.S.308 is that the edges alone should besection-lined. Such an area is shownin Figure 3.18.

Normally, section lining is at 45°to the edges of the paper and spaced inrelation to the area to be covered.However, there are cases, as in Figure3.19, when 45° lining is inappropriateand another angle is used. The senseand spacing of section-lining can bevaried in order to distinguish betweenadjacent parts of an assembly.

EXERCISE 3.3

Describe the errors or omissionsin the drawings shown in Figures 3.20to 3,23.

27


^

i

SECTION X-X

FIG. 3.15

THIRD ANGLE PROJECTIONA

SECTION A-A

FIG. 3.18

S

THIRD ANGLE PROJECTIONB,

-j--H-ift

SECTION B-B B

FIG. 3.19

—j*

SECTION X-X

FIG. 3.20 FIG. 3.21

:d-6- y^i

^»SECTION Z-Z

FIG. 3.22

M

HT^

oSECTION M-M M

FIG. 3.23

CMAPfiia 41

The Use of DrawingInstruments and Equipment

This Chapter is not intended to bea description of Drawing Office Pract-ice in the use of draughting machinesand other equipment; it is a summaryof those instruments a student is mostlikely to use. Drawing equipment willbe considered under five headings: thematerial on which the drawings are made,the methods of supporting that material,methods of marking the material, meth-ods of removing marks from the material,and miscellaneous aids.

MaterialsMost engineering drawings are made

in pencil on translucent materials tofacilitate the rapid preparation ofprints. Translucent paper can tear alltoo readily in inexperienced hands and,especially if prints are not required,it is convenient to use opaque drawingpaper for some exercises. The papershould be of sufficiently high qualityto withstand some rough handling andthe erasure of the inevitable mistakes

.

Translucent cloth and plasticdrawing film are other very useful mat-erials from which prints can be made.Many companies have standard sizes of

drawing sheets with the company ' s nameand other titles and headings printedon the sheet so that they are reproducedduring the copying process.

The International Standards Organ-ization's A-series of drawing sheetsizes is based on a rectangle of 1square metre area, the sides of whichare in the ratio l:/2. The sizes areobtained by dividing the next largersize into two equal parts, the divisionbeing parallel to the shorter side. TheA-series has the following dimensions:

Desiqnation Millimetres

AO 841 x 1 189Al 594 x 841A2 420 x 594A3 297 x 420A4 210 x 297

Supporting thedrawing material

It is important that the paper orother material is supported on a flat,dimensionally stable surface which isnot so hard that there is no "give"when pnassure is applied.

29

FIG. 4.1

FIG. 4.2

MAOC IN EHSLANO

FIG. 4.4

rFIG. 4.5

FIG. 4.3

Nearly all drawing boards containwood and their cost reflects the qual-ity of the timber. The cheapest aremade of plywood but these are only suit-able for low-quality work and they havea relatively short useful life. Betterquality boards are constructed fromblockboard with a wood veneer or plasticsurface. The best boards are construc-ted from well-seasoned timber and havebattens on the underside. If a tee-square is to be used, a hard insert islet into the working edge . A typicalgood quality board for student use isshown in Figure 4.1.

The paper, on which the drawing isto be made, may be laid directly on tothe plastic-surfaced type of board, oron to a paper backing-sheet if theboard has a wooden surface. There mustalways be some resilience under thedrawing material. The paper is held inplace with draughting tape or withspring clips, like the one shown inFigure 4.2. Drawing pins can ruin anyboard

.

Drawing boards are, of course,slightly larger than the correspondingA-series paper sizes:

Designation

AOAlA2A3

Board SizeMillimetres

950 x i 270650 x 920470 x 650336 x 470

The board may be used on a suit-able table, propped up at a convenientangle . The larger boards are oftenmounted on a positioning mechanism andprovided with a parallel motion straightedge . Such draughting machines arevery expensive and their use can usual-ly be justified only by the productiv-ity of the draughtsman who would beemploying one continually.. A typical,medium size draughting machine is shownin Figure 4. 3.

For the board shown in Fig. 4.1,horizontal lines are drawn with the aidof a tee-square. The stock of the tee-

square must always be in line contactwith the edge of the board. Verticallines are drawn by employing a set-square, resting on the tee-square. Forcertain angles, the sloping edge of theset-square is used. To cover all poss-ible angles, the three instruments ill-ustrated in Figure 4.4 are required:they are, from left to right, a 60° set-square, a 450 set-square, and a protrac-tor.

An adjustable set- square is oftenuseful. With one of these it is poss-ible to dispense with the 45° set-squareand the protractor, but not entirelywith the 60° set-square. Figure 4.5illustrates a robust adjustable set-square.

Marking thedrawing material

Most drawings are made with penciland the leading manufacturers produceranges of about twenty degrees of hard-ness. For drawings on translucent mat-erials, it is generally found thatdraughtsmen use mainly 2H grade forline work and F grade for lettering.For students' drawings on opaque paper,the most suitable grades are HB forlettering and H and 2H for line work.Sometimes, it may be convenient to usea 3H grade for certain constructionlines.

The mechanical type of "lead"holder shown in Figure 4.6 is very use-ful. The "lead" is firmly held in themetal jaws but it can be quickly rel-eased for changing or resharpening

.

"Leads" are sharpened to giveeither a conical point, as in Figure4.7 or a chisel point, as in Figure 4.8.After removing sufficient wood from thepencil with a sharp blade, the point isprepared by rubbing the "lead" on apiece of fine glass paper. Blocks ofabout ten sheets are available for thispurpose. The conical point is used forlettering and dimensioning and thechisel point, which wears less rapidly,is ideal for line work.

31

771 S STAEDTLER MARS owmanv

FIG. 4.6

i - = r~

FIG. 4.7

FIG. 4.8

FIG. 4.11

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FIG. 4.9

FIG. 4.12

FIG. 4.13

Front

1:1 10mm 20 30 40 50 60 70 SO

UIUI009 09S 09S 0»S OZS 005 09t> 09t> 0«»

1:1

1:2

TBack

1 1 1

1

1 1 1 1

1

1 1 1 1

1

1 1 1 1 1 |ll 1

1

1 1 1 1

1

1 1 1 1

1

1 1 1 1

1

1 1 1 1

1

1

1200 1300 1400 1500mm

1:1000* - U1U1Q0Z

iiiiliiiiliiiiliiiiimiliiiiliiiiliimiiiiliiiiliiiiliinli

FIG. 4.10

Occasionally, it is necessary touse drawing ink for illustrations,charts, graphs and some special engin-eering drawings. The need to use inkis far less than it was before printmaking machines reached their presentstandard of performance . Ink drawingshould be attempted only when a certainskill with pencil drawings has beenacquired. Special pens are requiredfor ink lines but some pencil compassesmay be fitted with a pen head.

Removing marksPencil marks may be removed from

most types of drawing materials with agood quality eraser which is effectivewithout damaging the paper. There arevarious types on the market; some areencased in wood and can be sharpened,like a pencil, so that their action isrestricted to a small area of the paper.Alternatively, a thin sheet-metal eras-ing shield, as in Figure 4.9, may beused to prevent the erasure of correctlines adjacent to mistakes.

Tracing erasers will remove bothpencil marks and dry drawing ink but itis all too easy to damage the paper.If the printing process to be used makesuse of reflected light, it is possibleto "paint out" mistakes with the com-pound "Snopake" and to draw on top ofthe dry painted area.

A soft "art gum" or "art cleaner"block is very useful for cleaning fin-ished drawings. Eraser dirt should beremoved by flicking with a clean stock-inette duster, which can also be usedfor wiping the tee-square and set-squares.

Other drawinginstruments

Measurements are made using ascale like the one shown in Figure 4.10.It should have an oval cross-sectionand sets of millimetre graduations.Modern plastic scales are far more rob-ust than the traditional box-wood type.

Circles and circular arcs, of upto about 25 mm radius, are drawn usingthe type of spring bows illustrated inFigure 4.11. Notice that they have aremovable shouldered point (A) , a repla-ceable "lead" (B) and a knurled wheel(C) for setting the radius. The "lead"should be sharpened on one side only.

Radii should never be set bysticking the point into the scale. In-stead, mark the required radius on an

odd piece of paper, stick the point inone mark, and adjust the knurled wheeluntil the "lead" draws through the othermark.

For larger radii, of up to about120 mm, spring_ compasses of the typeshown in Figure 4.12 are ideal forstudents. When fitted with an extensionbar they may be used for radii of up to240 mm. The traditional design ofdraughtsman's compasses, shown in Figure4.13, will also draw these larger radiusarcs but there is no positive settingof the radius . They tpo may be fittedwith an extension bar.

Very small radius arcs (1-12 mmradius) may be drawn very quickly, with-out having to mark the position of thecentre, with the aid of a template. Twotypes are illustrated in Figure 4.14.

For curves other than circulararcs, a set of french curves or, atleast, one similar to that shown inFigure 4. IS is very useful. The flex-ible curve shown in Figure 4.16 is afairly good substitute

.

At the drawing boardIt is well worth developing the

ability to produce neat, attractivedrawings, even if this is not the meansby which one earns one's livelihood.The following suggestions are made withthat aim in view:

(1) Always plan the arrangement of thewhole drawing on the paper beforemaking any marks whatsoever

.

(2) Build up the drawing from centrelines and base lines which arecommon to two views

.

(3) Do not try to complete one viewbefore proceeding to another. Itis much better to draw in parts ofeach view which are aligned alonghorizontal or vertical lines andwork on each of the views in somesort of rotation

.

(4) Keep your pencils sharp so thatyour lines are always crisp. Usethe chisel point as much as poss-ible.

(5) Practice, from the start, themaintenance of a clear distinctionbetween the two thicknesses oflines. Thick lines, whetherstraight or curved, should bebetween two and three times thickerthan thin lines

.

(6) Keep all your instruments clean.Remove all loose matter from yourdrawing with a duster, not withyour hand.

33

FIG. 4.14

FIG. 4.15FIG. 4.16

ALL DIMENSIONS ARE IN MMFIG. 4.17

ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789&

FIG. 4.18

EXERCISE 4.1

Figure 4.17 is a pictorial drawingof a block. Draw, scale 1/1, in thirdangle projection, views as seen in thedirections of arrows X, Y and Z. Planthe layout and spacing of the drawingbefore you begin it. Insert centrelines and hidden detail lines but donot add dimensions

.

When the views are completed, prac-tise freehand printing of the styleshown in Figure 4.18 by adding thelabels THIRD ANGLE PROJECTION and SCALE1/1. Also print a title and your name.Use very thin horizontal lines as guidelines to make your printing about 3 mmhigh.

The use of springbowsand

Circular arcs should always bedrawn in before the straight lines thatare tangential to them. It is verymuch easier to blend the two togetherif they are drawn in this order . Whendrawing the arc, the centre should belocated and marked first, then theradius should be set by making two markson an unwanted piece of paper , stickingthe point in one mark and adjusting theinstrument so that the "lead" draws

through the other mark

.

Construction lines required forthe location of the centre of the arcshould be so thin that they do not haveto be erased.

EXERCISE 4.2

Figure 4.19 shows two views of acomponent in first angle projection.Draw, scale 1/1 and in third angle pro-jection, the following views:

(a) the existing left-hand view and(b) a sectional view on A-A.

Add all the necessary section-lining,centre lines and section line, labelsand titles. Do not add dimensions.Check that you have a consistent differ-ence between the thick lines and thethin lines.

Arc blendingIt is easy enough to blend an arc

with a straight line but much more diff-icult to blend two arcs having a commontangent. Figure 4.20 illustrates thistype of problem. (The tiny objectlooks rather unusual when drawn to sucha scale. Cultivate the habit of immed-iately looking at the scale when yousee a drawing for the first time.)

35


2 SLOTS 8 WIDE

"~:i*— R 3

,

11

«

—

3.1 .__

ALL DIMENSIONS ARE IN MMSCALE 1/2

FIG. 4.19

R1

"iZ

1^

V

T^.\L

eft

5-5 It-

*3

*%-

.R 3

PENCIL SHARPENER BODY


FIG. 4.20

\ f N^J

1

B-© )A /

\ (

ALL DIMENSIONSARE IN MMSCALE 2/1

FIG. 4.21

Figure 4.21 illustrates the stagesof constructing the outline of thesharpener. Construction lines whichare no longer required have been omit-ted from successive diagrams.

In stage (1) of Fig. 4. 21, thebasic 17 mm square has been drawn andinside it are four lines parallel tothe sides and 1 mm away. The intersec-tions of these lines fixed the centresof the 1 mm radius arcs. It is astraightforward matter to construct the11 mm line and the 8 mm radius arc.

Stage (2) shows how the inside 19mm arc is constructed. The arc mustpass through point A and therefore itscentre lies on an arc, centre A, radius19 mm. The arc in question and thelower left 1 mm radius circle must havea common normal and so the centre ofthe arc must lie on an arc, centre B,radius 19 - 1 = 18 mm. Where the twoarcs intersect is the centre of thenecessary 19 mm radius arc.

Stage (3) shows how the 3 mm arcis constructed. This arc has commonnormals with both the 8 mm arc and thelower right 1 mm arc. Where two arcs,one centre C, radius 8 + 3 = 11 mm, andthe other centre D, radius 1 + 3 = 4 mm,intersect is the centre of the necess-ary 3 mm radius arc.

The centre of the other 19 mm arcis found in exactly the same way as thecentre of the 3 mm arc . This has beendone in stage (4) .

The constructions described aboveoccur quite often in engineering draw-ings and are worth remembering.

EXERCISE 4.3

Without reference to Fig. 4. 21 orthe text relating to it, draw out theoutline shown in Fig. 4.20 using a scaleof 5/1. Be careful to keep the linesand the arcs uniformly thick.

37

GENEROUS SPACINGFOR CLARITY C

SHORT"EXTENSIONSMALLGAP

-DIMENSIONLINE

.PROJECTIONLINE

FIG. 5.1CORRECTDIMENSIONING

65

4823

—$ $-—

^

FIG. 5.3

INCORRECTDIMENSIONING

FIG. 5.2

B

FIG. 5.4

Dimensioning

The purpose of an engineering draw-ing is to enable a component to be manu-factured or a set of components to becorrectly assembled. In the formercase, it is necessary to define the c<3m-

ponent completely and unambiguously; itis essential to state every measurementrequired in the clearest possible way.Furthermore , the way in which the dim-ensioning is done should take accountof the methods by which the object isto be manufactured. In this Chapter,these two aspects of dimensioning willbe considered separately, as far as ispossible

.

General principlesThe dimension is stated next to a

thin dimension line which has arrow-heads at either end. The arrowheadstouch either the appropriate featuresof the object or thin projection linesextended from these features. Figure5.1 shows the recommendations of B.S.308.

Projection lines enable the dimen-sions to be placed outside the L-shapedoutline. The projection lines start

just clear of the outline and extendjust beyond the arrowhead. Paralleldimension lines are well spaced forclarity. Dimension lines may be placedwithin the outline, if there is somegood reason for doing so. Arrowheadsshould be slim and have a minimumlength of about 3 mm.

Figure 5.2 shows the correct andincorrect ways of using projection andcentre lines for dimensioning. Theincorrect dimensioning is manifestlylacking in clarity.

Where several dimensions arestated from a common datum line, themethod shown in Figure 5.3 is recommen-ded. Notice that the dimension figuresare placed near the appropriate arrow-head.

EXERCISE 5 .1

Draw a complete, clear set -of pro-jection and extension lines for the out-line shown in Figure 5.4. Do not addany figures alongside the dimensionlines. All vertical dimensions are tobe taken from a common datum linethrough B-B.

39

7629° 45'

42

FIG. 5.5

150°

ir ir ir i*4-25n ^ O

T

012^ ^

FIG. 5.10

022

FIG. 5.6

81

5643

I I

u

FIG. 5.7

^777^777777777P^(~R&

i/-R3

7777777777}

x*

FIG. 5.11

I

4 »-

I

L5_98

y°

*—*

;

90

VV 32-fr^

FIG. 5.8

THIRD ANGLE PROJECTIONXr*

E- » «,

SECTION X-X (U

4 HOLES10 THRO

6 HOLESEQUI-SPACED

\k>—mm

i^HOLE-ffi< 06X10^ DEEP 08

II

FIG. 5.12

FIG. 5.9

Arrangement ofdimensions

Figures 5.5 to 5.9 show variousmethods of dimensioning: each one ill-ustrates some important point or points

.

In figure 5 .5 , the numbers areorientated so that they may be readfrom either the bottom or the right-hand edge of the drawing.

Figure 5.6 shows how narrow spacesare dimensioned by using inward pointingarrowheads

.

In Figure 5.7, notice how the par-allel dimension lines have the dimen-sion figures staggered for clarity.

Figure 5.8 illustrates the advan-tage of placing overall dimensions out-side intermediate dimensions: if theintermediate ones were outside, therewould be a confusing crossing of lines.

Dimensions of diameters should beplaced on the most appropriate view, toensure clarity. The set of concentriccircles in the right-hand view of Fig-ure 5.9 is difficult to understand butthe sectional view makes it clear whateach circle means. The dimensions arebetter placed on t^ie sectional view.B.S.308 recommends that a diameter isindicated by, for example, 22 (as inthe figure) but 22 DIA is used by manydraughtsmen

.

Circles, radiiand holes

Circles are dimensioned by one ofthe methods shown in Figure 5.10, whileFigure 5.11 shows that the radii ofarcs are dimensioned by a line lyingalong a radius. If the centre of thearc need not be located, it need not beindicated. If the centre must be loc-ated but lies in an inconvenient place,a fictitious centre may be created andthe true position specified; the partof the dimension line that bears thearrowhead is along a true radius

.

Holes are dimensioned by one ofthe methods shown in Figure 5.12. Iden-tical holes may be specified by a leaderline that points to only one of theholes but specifies the number of suchholes. When the depth of a hole isstated, but not drawn, the depth refersto the cylindrical part of the holeonly. The conical end formed by the118° (or thereabouts) angle at the tipof a drill is not included in the depth.Holes whose centres lie on -the circum-ference of a circle and subtend equalangles at the centre may be specifiedfor position in the way which is illus-trated .

41


SCALE 1 /2

CIRCULARHOLE

—

k.

l

i

\ f1

k 1

i ,i

i

t

1

FIG. 5.13

WASHER

FIG. 5.14

EXERCISE 5.2

Two views of a bracket are givenin Figure 5.13. By scaling from thedrawing (a practice to be deplored butwhich is necessary for this exercise)draw out the given views and a view asseen in the direction of arrow X, inFIRST angle projection. Allow amplespace between the views . Add all nec-essary dimensions and labels.

Choose the dimensions carefullyand use only the methods described inthis Chapter. Always ask yourself: isthis the right view on which to statethis dimension? All dimensions shouldbe stated in millimetres and a note tothis effect should be inserted on thedrawing

.

Dimensioning for manu-facture and function

Figure 5.14 is a pictorial drawingof an exploded collection of parts

which assemble to form a simple rotat-ing joint between two rods of circularcross-section. Only part of the rod isshown on each end. The five componentsare assembled by inserting the plainend into the fork end so that the holesare aligned, pushing the pin throughthe holes, placing the washer on theend of the pin, and dropping the splitpin through the hole in the pin. Theassembly is kept together by opening upthe split pin, in the usual way.

Figures 5.15.1 and 5.15.2 show twoalternative ways of dimensioning theplain end and Figures 5.16.1 and 5.16.2show two alternative ways of dimension-ing the fork end. (N.B. The designsof the components do not represent goodengineering practice because there aretoo many sharp edges; they have beensimplified in order to reduce the num-ber of dimensions in the figures.)Both sets of dimensions would enablethe ends to be manufactured as each de-fines the geometry in a logical andpractical form.

43


ALL DIMENSIONS ARE IN MM

rrVv

-^

SCALE1/2

To

HOLE <t> 10

^>\rX\

(0CM

I

CO

~~I

,-=J. *26 „|

X:

HOLE <f> 10

FIG. 5.15.1 FIG. 5.15.2

,-H--ki^v

T^ A"

DRILL* 10

THRO

penFIG. 5.16.1

i

/i" ! \o

/

\- £ If)

1

1

__l'

DRILL 4> 10THRO

FIG. 5.16.2

A B

FIG. 5.17.1

C E F

1

35

I-

YZ

FIG. 5.17.2

Now consider the effect of a smallerror on some of the sizes. For con-venience, it will be assumed that eachdimension can be held to a tolerance of+ 0-1 mm. This would mean that, in Fig.5.15.1, the worst thing which couldhappen is for the 30 mm dimension to be0*1 mm oversize and both of the 7-5

dimensions to be 0-1 undersize. Thethickness of the plain end would thenbe

(30 + 0-1) - 2(7-5 - 0-1) = 15*3 mm

Turning now to Fig. 5. 16.1, the 35

mm dimension will be 0*1 mm undersizeand both of the 10 mm dimensions willbe 0-1 mm oversize. The width of theslot in the fork end would then be

(35 - 0-1) - 2(10 + 0-1) = 14-7 mm

The net result is that the plainend is 0*6 mm thicker than the slotinto which it is supposed to fit (andbe free to rotate) and the assemblywill not function as intended. Giventhe tolerances of ± 0-1 mm on all dim-ensions, it is equally possible thatthe plain end would be 0-6 mm thinnerthan the slot, but the cumulative erroris much larger than it need be.

Now consider Figs. 5.15.2 and5.16.2. The worst condition is thatthe plain end is 0-1 mm oversize andthe slot is 0-1 mm oversize. The netresult is that the interference, theexcess of material size over hole size,is 0*2 mm, or one third of the previouscase.

Clearly, the dimensions used inFigs. 5.15.2 and 5.16.2 take account ofthe functioning of the assembly, as

well as the manufacture of the individ-ual components, and are preferable tothe other method of defining the geo-metry .

As a general guide , components

should be dimensioned so that matingsizes are specified directly and notindirectly. This may not always bepossible (as the next example illust-rates) but it is usually possible tominimize the cumulative error. The com-ponents considered above could alwaysbe assembled if the width of the plainend were dimensioned as 14-9 ± 0-1 andthe width of the slot as 15-1 ± 0-1. Atone extreme there would be a clearancespace of 0*4 mm and, at the other, a

perfect fit.Now consider the assembly of the

pin, fork end, washer and split pin(the plain end does not affect thisarrangement) . The pin is shown in Fig-ure 5.17.1: how should it be dimension-ed so as to minimize the cumulativeerror?

In this case the mating size is BCbut this dimension should not be statedas it would make the manufacturing pro-cess the subject of same arithmetic.The hole can only be drilled by firstlocating its centre, and there is alwaysa chance of a mistake when calculatingBC + ^CE.

Figure 5.17.2 shows how the twomating distances are composed. The cum-ulative error will be minimized bystating the dimensions as shown. Oneother point should be noticed: it wouldbe possible to manufacture sizes XY andYZ very accurately. Starting from anexcess of material, it is a simple oper-ation to remove a small amount from aflat surface. On the other hand, thehole in the pin requires some accuratepositioning of the drill and, once thehole has been made, no correction ispossible. As the mating dimension isdifficult to produce when it is knownin terms of two specified dimensions,there is all the more reason to expressit as directly as possible.

45


BOSS WITHRADIAL HOLETAPPED M4XO-7

2 HOLES <t> 8STRAIGHT THRO

FIG. 5.18

Note. The metric thread designation M4 * 0*7 means:International Standards Organization metric tnreadwith a nominal diameter of 4 mm and an axial pitchof 0*7 mm

.

'Oi ^T I€

THIRD ANGLE PROJECTIONSCALE 1 /2

-/^X\

^-f&X

FIG. 5.19

EXERCISE 5.3

Figure 5.18 is a pictorial viewof a depth-setting component for acircular saw. Many of the dimensionsare missing. You are asked to preparea dimensioned orthographic drawing ofthe object. Which views would mostclearly show the object?

The component is fixed to the sawhousing by means of two screws passingthrough the 7 mm wide slots; whenthese screws are tightened the depthof the cut is set. The two 8 mm diam-eter holes are for rods along which canslide the width setting component.

Figure 5.19 shows three views ofthe depth-setting component. These arethe views that would be seen in thedirections of arrows X 2 , Y 2 an<^ z

iof

Fig. 5. 18. These views have been chosen

because they reveal more details thanthose in the directions of arrows Xi

,

Yx and Z 2 . Students requiring practicein the use of drawing instrumentsshould redraw these views, scale 2/1;the non-circular curves in the upperand left-hand views should be sketchedin as neatly as possible. Alternatively,there is sufficient space around theviews in Fig. 5. 19 for dimensions to beinserted.

Completely dimension the three-view drawing, scaling distances fromFig. 5. 19 when necessary. Be careful totake the function of the component intoaccount and check that all necessarydistances and angles have been included.Threads should be designated as shownin Fig. 5. 18. Add all necessary titlesand labels.

47

FIG. 6.1


ALL DIMENSIONS ARE IN MMSCALE 1 /2

REFERENCE PLANE

FIG. 6.2


REFERENCE

2 HOLES 08

PLANE

VIEW IN THEDIRECTION OFARROW A

ALL DIMENSIONS,

ARE IN MM LSCALE 1/2 f

'

l : '

FIG. 6.3

€&mi>ra d

Auxiliary Projections

Many engineering components havesurfaces which are inclined to the mainplanes of the object. Figure 6.1 is apictorial view of such a component; thebase (through which pass two holes) de-fines the three directions of any stan-dard orthographic drawing. Figure 6.2is a drawing, in third angle projection,of the same component. The upper viewis a view along arrow Z (of Fig. 6.1),the lower right-hand view is a viewalong arrow X and the lower left-handview is a view along arrow Y. None ofthe views shows the true shape of theinclined surface.

Notice that the dimensions whichrelate to the inclined surface have tobe placed on adjacent views. The true18 mm distance can only be shown on thelower right-hand view. It would be ad-vantageous to have the three dimensions,which relate to the inclined surface,grouped together. This could be doneif a view were drawn of what would beseen along arrow A of Fig. 6.1, wherearrow A is perpendicular to the inclinedsurface. Such a view is known as anAUXILIARY VIEW.

Figure 6.3 is an orthographic draw-ing showing views along arrows X, Z andA of Fig. 6.1. The first thing to not-ice is that the rule relating to (inthis case) third angle projection isstill followed. The view along arrowX is the link view (see Chapter 1) ; andwhat would be seen from vertically above

it is drawn vertically above it. Whatwould be seen looking from above view Xand perpendicular to the sloping surfaceis drawn above view X and projected ina direction perpendicular to the slopingsurface.

As the auxiliary view is ratherspecial, it has an arrow to indicatethe direction in which it is viewed anda title which refers to the arrow. Theauxiliary view takes all the dimensionsrelating to the sloping surface and thismakes the drawing easier to follow.

Now consider the construction ofthe auxiliary view. Fig. 6.1 has a ref-erence plane which has been indicated,as a line, in the appropriate views ofFigs . 6.2 and 6.3. Looking first atFig. 6. 2, it is clear that distances per-pendicular to the reference plane willappear to have the same true lengthwhether viewed along arrow Y or arrow Z

.

For example, the 24 mm width of the com-ponent is the same in views along Y andZ . In other words , rotating the viewingdirection through 90O, about the X arrow,has no effect on distances parallel toX. The same is true if the viewing dir-ection is moved from Y to A: distancesparallel to X remain the same.

A convenient way to remember thisrule is that DIMENSIONS, ALONG ALL SETSOF PROJECTION LINES RADIATING FROM ACOMMON VIEW, ARE IDENTICAL. Thus, inFig. 6. 2, there are two sets of imagin-ary projection lines, one horizontal

49


FIG. 6.4

REFERENCE PLANE

VIEW IN DIRECTIONOF ARROW A

1' 2

FIG. 6.6


VIEW IN

DIRECTIONOF ARROW A

FIG. 6.5

and the other vertical, radiating fromthe lower right-hand view. The refer-ence plane can be drawn in at any con-venient distance from the common view,and then dimensions, along both sets ofprojection lines perpendicular to thereference plane, must be identical.

The same is true of Fig. 6. 3: thereare two sets of imaginary projectionlines, one vertical and the other at 30°above the horizontal, radiating fromthe lowest view. Dimensions, alongboth sets of projection lines, are iden-tical.

This important principle is furtherillustrated in Figures 6.4 and 6.5. Ahexagonal prism is cut at an angle, asshown . A view along arrow X shows theangle at which the prism is cut. Aview along arrow Y shows the cross-section of the prism. An auxiliaryview along arrow A is needed to showthe true shape of the inclined surface.

Imagine that the views alongarrows X and Y are the only ones drawnin Fig.6.5> the auxiliary view is yetto be drawn. A perpendicular to thesloping surface specifies the directionfor a new set of projection lines. Theview along arrow X will then have twosets of projection lines radiating fromit. In this example it is convenientto use a plane of symmetry as the ref-erence plane. It can be drawn in, per-pendicular to the dotted projectionlines, at any convenient distance fromthe common view. The various pointsaround the object are numbered for easeof identification and the perpendiculardistance from the reference plane istransferred from the left-hand view tothe auxiliary view. The appropriatelines are then drawn in and the surface1-4-5-9-10 appears in its true shape inthe auxiliary view. One other import-ant fact is well illustrated in Fig. 6.5;notice that the sense of the transferreddistances remains the same. For exam-ple, in both the left-hand and the aux-iliary view, points 8, 9 and 10 are thefurthest from the common view, whereaspoints 2 , 3 , 4 , 5 and 6 are the nearest

to the common view.Figure 6.6 is a further example of

the process of producing an auxiliaryview that shows the true shape of asloping surface. Basically, the objectis a pyramid with a regular pentagonalbase. It has been cut to produce thesloping surface shown in the lower view.The two right-hand views have been pro-duced by constructing the complete pyr-amid and then locating the five pointsof the sloping surface by the intersec-tion of the appropriate lines. For ex-ample, point 1' must lie on line O-l inboth the upper and lower views . Point1' is easily located when the slopingsurface is drawn on the lower view andthen the intersection of a vertical pro-jection line up from 1' and the line0-1, in the upper view, locates theposition of 1' in the upper view.

Like Fig. 6. 5, the pyramid has aplane of symmetry and it is convenientto use it as the reference plane forthe auxiliary view. A perpendicular tothe sloping surface gives the directionfor the projection lines to the auxil-iary view. There are two sets of pro-jection lines (indicated by dottedlines) from the lower view, and dimen-sions along these lines must be identic-al . Therefore the perpendicular dis-tances from points 1, 1', 2, 2', 3, 3',4 ,

4' , 5 ,

5' to the reference plane in

the upper view are transferred to theauxiliary view. Once again, the senseof the dimensions is not affected bythe transfer; for example, point 3 is,in both cases, the nearest point to thecommon view.

EXERCISE 6.1

Figure 6.7 has two views of acruciform section bar. With the aidof the 5 mm spaced guide lines, drawan auxiliary view as seen in the direc-tion of arrow A, to show the true shapeof the sloping surface. Do not showhidden detail. Label the auxiliaryview. When you have finished, compareyour solution with Figure 6.8.

51

THIRD ANGLE PROJECTIONTHIRD ANGLE PROJECTION


FIG. 6.7FIG. 6.9





VIEW IN DIRECTIONOF ARROW B

FIG. 6.8 FIG. 6.10



FIG. 6.11


VIEW IN DIRECTIONOF ARROW C

DISTANCES1-11,2-10 ETC AREIDENTICAL


FOR CLARITY,ONLY ONE SURFACE OFTHE HOLE IS SHOWN

FIG. 6.13

HOLE <t> 50 THRO

100)

L_J

00 k ^T26

,

18|, 94 28.



SCALE 1/4

FIG. 6.14



FIG. 6.15

EXERCISE 6.2

Figure 6.9 shows two views of asquare section block with V grooves

.

Project an auxiliary view as seen inthe direction of arrow B, to show thetrue shape of the sloping surface • Nodimensions or hidden details are re-quired. Label the auxiliary view. Whenyou have finished, compare your solu-tion with Figure 6.10.

EXERCISE 6.3

Figure 6.11 gives two views of apyramid with a regular three-armedbase. The pyramid is to have its topremoved by cutting at the angle shown.Redraw the given views, scale 1/1,showing the effect of cutting the pyra-mid, and project an auxiliary view toshow the true shape of the sloping sur-face. Show all hidden detail but donot add dimensions. Label the auxili-ary view and state the type of projec-tion used. When you have finished,compare your solution with Figure 6.12.

CirclesOne of the disadvantages of auxil-

iary projection can be seen by referringback to Fig. 6. 3 (p. 48). The two circul-ar holes appear as ellipses in the aux-iliary view and they have been construc-ted by a process that is rather time-consuming. When drawing an auxiliaryview it is necessary to ask yourself:what purpose does the view serve? If,as in Fig. 6. 3, the important part ofthe view is the sloping surface , thenonly that part need be shown and theimaginary break can be indicated withan irregular thick line. If the holeshave to be shown, as might be the caseif only two views were to be drawn, amethod of construction is shown in Fig-ure 6.13. A different object has beenused in order to illustrate a number ofprinciples.

The view labelled X can be complet-ed first as there are no unusual feat-ures. It is then possible to draw the

straight lines of view Z and the smallhole, but the large one cannot be drawn.In the view labelled A, it is possibleto draw the large hole but not thesmall one; the circular outline of thelarge hole has been marked with twelveequally spaced, numbered points. Aline from each point is projected backto the view X and another line is pro-jected from view X to view Z. As always,distances across the object, measuredalong two lines radiating from the samepoint on view X, must be the same andso the 6-12 line may be located in viewZ and the perpendicular distances fromthis line to the other ten points aretransferred from view A to view Z. Thesymmetrical arrangement of the pointsreduces the amount of work to be done.The points are joined by using a curvetemplate or sketching.

The same method is used to trans-fer distances from the small circle inview Z to construct the ellipse in viewA. Normally the projection linesshould be erased.

The view labelled Y hasbeen addedto illustrate the importance of projec-ting the ellipses from the correctplanes. In this view the complete ell-ipse comes from the right-hand surfaceof view X and the partial ellipse comesfrom the left-hand surface. Noticealso that the numbered points now appearanticlockwise. The symmetry of theellipse disguises this inversion but itis bound to arise if the ellipse isproperly constructed by maintaining thesense of the dimensions (for example,in views A, Z and Y, point 3 is alwaysthe one nearest to view X)

.

EXEkCISE 6.4

Figure 6.14 shows a three-viewdrawing of a bracket; two of the viewsare incomplete . Redraw the views

,

scale 1/1, and complete both the auxil-iary view and the upper right-hand view.Do not add dimensions or hidden detail

.

Show all necessary titles and labels.When you have finished, compare yoursolution with Figure 6.15.

55

FIG. 6.16

FIG. 6.18

THIRD ANGLE PROJECTIONTHIRD ANGLE PROJECTION

FIG. 6.17

VIEW DIRECTION \

OF ARROW A"

FIG. 6.19

Double auxiliaryprojection

The sloping surfaces examined inthis chapter so far have been inclinedto only one of the projection planes.In other words, there has always beenone view in which the sloping surfaceappeared as a straight line. Figure6.16 shows an object with a triangularplane (1-2-3) which is inclined to twoprojection planes. The projectionplanes of Fig. 6. 16 are those normal tox, y and z, defined by directions 6-5,2-6 and 6-7 respectively. The plane1-2-3 will not appear as a straightline when viewed in any of these direc-tions.

Figure 6.17 demonstrates thispoint. Look first at views X, Y andZ; the plane appears as a differentshape of triangle in each of them.Furthermore, it is impossible to drawan auxiliary view which will show thetrue shape of the triangle. An auxili-ary view has been projected from viewX in a direction perpendicular to line2-3 (direction "a" of Fig. 6. 16); itshows the true length of line 2-3 butthe other lines are foreshortened. Anauxiliary view has been projected fromview Y in the direction perpendicularto line 1-3: it only shows the truelength of line 1-3. Similarly, theview in the direction of arrow C, pro-jected from view Z, shows only the truelength of line 1-2.

Clearly, it is not possible to pro-duce, directly, an auxiliary view show-ing the true shape of the triangularsurface. What is missing is a view inwhich the triangular surface appears asa straight line . Such a view may beobtained by an auxiliary projectionalong rather than perpendicular to oneof the lines of the triangular surface.For example, a view along arrow A' (ofFig. 6. 16) will make points 2 and 3appear coincident and superimpose lines1-2 and 1-3. This auxiliary view hasto be projected from view X since it isthe view where line 2-3 appears In itstrue length. Thus, view A' providesthe first stage in the production of aview of the true shape of the triangularsurface.

Figure 6.18 illustrates the sameobject, differently orientated, andwith a reference plane shown which isperpendicular to line 2-3. Views X andA' from Fig. 6. 17, have been enlargedand redrawn in Figure 6.19. Try tothink of tnese views as the originalviews of the object and forget theother views of Fig. 6. 17. The lowerviews of Fig. 6. 19 contain all the infor-mation needed for the required view.

Referring now to Fig. 6. 18, it ispossible to see, whether one looks alongarrow X or arrow A" because they areboth parallel to the reference plane,that distances perpendicular to thereference plane must remain the same.The auxiliary view along arrow A" isdrawn by the usual method of projectingin a direction perpendicular to line1-2, drawing the reference line in aconvenient position and parallel toline 1-2, and transferring dimensionstaken along. the projection lines onview X to the auxiliary view. Thisview, along arrow A", shows the trueshape of the triangular surface.

The process of drawing one auxili-ary view and then projecting anotherauxiliary view from it is known asDOUBLE or SECOND AUXILIARY PROJECTION.Apart from finding the true shape of asurface not appearing as a line in anyof the original views, the process hasa number of other uses and some of themwill be examined below.

It may be shown that there is notheoretical limit to the number of aux-iliary views that could be drawn. Thereare problems where Treble AuxiliaryProjection is a convenient method ofanalysis but, for all practical purposes,there is no need to use anything morethan a Double Auxiliary Projection.

Now examine the process applied toan engineering component rather than ageometrical block. In the lower right-hand corner of Figure 6.2 are twoviews of a simple bracket (for clarity,any holes have been omitted) . Neitherview shows the true shape of the in-clined surface and the first step to-wards producing such a view is to drawan auxiliary view that shows the sur-face as a straight line.

This can be done by selecting aviewing direction along any line in the

57

VIEW IN DIRECTIONOF ARROW A'

Y THIRD ANGLE9^ \i3 PROJECTION

*V\&/

- .<<$0K

VIEW IN DIRECTIONOF ARROW A"

FIRSTREFERENCE 6 9 PLANE

FIG. 6.20


SECTION X-X

FIG. 6.25

surface which appears in its truelength. In this case, lines 1-4 and2-3 in the lower view, or lines 2-3 and6-4 in the upper view, are equally suit-able but the upper view has been usedin order to fit all the views into theavailable space

.

A view along arrow A' is drawn inthe usual way and dimensions perpendic-ular to the First Reference Plane inthe lower view are transferred to theappropriate projection in the FirstAuxiliary View. The surface 1-2-3-4appears as a straight line.

A second auxiliary view is projec-ted, perpendicular to plane 1-2-3-4.The first auxiliary view is now thecommon view from which two sets of pro-jection lines radiate; two new refer-ence planes are introduced, perpendic-ular to the two sets of lines. Dimen-sions are transferred from the upperview to the second auxiliary view. Inthis example, only visible lines havebeen shown but it is necessary to loc-ate point 13 so that the 10-13 line isin the correct direction

.

The great difficulty with thistype of problem is visualization. Theprinciples are extremely simple; it isthe odd angles and orientations of theobject that create challenges for eventhe most experienced draughtsman.There are three points to remember:first, the basic rule, DIMENSIONS,ALONG ALL SETS OF PROJECTION LINESRADIATING FROM A COMMON VIEW, AREIDENTICAL. Second, points on viewsprojected from a common view always re-tain their relative distances from thecommon view. Third, it is very help-ful to number the important pointsaround the object, for ease of identif-ication. If you keep these things inmind, you should be able to solve mostdouble auxiliary projection problems,after some practice.

EXERCISE 6.5

Figure 6.21 is a pictorial viewof a component. Draw, scale 2/1, inthird angle projection, views as seenin the direction of arrows Y and Z

.

(N.B. Lines 1-2, 3-4, 5-6, 7-8 and9-10 are all parallel.) Hence, drawan auxiliary view in which surface5-6-7-8 appears as a straight line,and a second auxiliary view showingthe true shape of this surface. Do notadd hidden details or dimensions. In-sert all appropriate titles and labels.When you have finished, compare yoursolution with Figure 6.23.

EXERCISE 6.6

Figure 6.22 is a pictorial view ofa bracket. Draw, scale 2/1, in thirdangle projection, views as seen in thedirection of arrows X and Y. (N.B.

Points P, Q, R and S are all 40 mmabove the base plane . ) Hence draw anauxiliary view in which the L-shapedsurface appears as a straight line , anda second auxiliary view showing thetrue shape of this surface. Do not adddimensions and show only the hiddendetails required for the constructionof the auxiliary views. Add all appro-priate titles and labels . When youhave finished, compare your solutionwith Figure 6.24.

Surfaces with allsides foreshortened

In the above examples of doubleauxiliary projection, at least one sideof the sloping surface has appeared inits true length. Such a side can beused to determine the direction for afirst auxiliary view which will showthe surface as a straight line. A moredifficult problem arises when none ofthe sides appears in its true lengthand the first task is to find the direc-tion in which to project the first aux-iliary view.

Figure 6. 2 5 contains such a sur-face, 1-2-3-4-5-6, and shows no sidein its true length in either of thetwo left-hand views. Consider a horiz-ontal section through point 1 - SectionX-X. By projecting vertically upwardsfrom the point 0, where this sectionplane cuts the line 4-5, the sectionX-X can be drawn; line 0-1 must be ahorizontal line on the sectional viewwhich passes through two points on thesloping surface. An auxiliary view,in the direction of arrow A', along 0-1,will make the whole surface, 1-2-3-4-5-6,appear as a straight line.

This auxiliary view has been drawnin the usual way, and a second auxili-ary view can then be drawn as seen inthe direction of arrow A"

, perpendicularto the line 6-5-1-4-2-3. The view inthe direction of arrow A" shows the trueshape of the sloping surface.

The most important new principlehere is the method of finding the direc-tion in which to project the first aux-iliary view. As in the previous exam-ples, a line in the surface of the planeis required which appears in its truelength. The upper left-hand view ofFig. 6. 25 will show only horizontallines in their true length and so itis necessary to create a horizontalline in the lower view and to projectit back to the upper view.

EXERCISE 6.7

Figure 6.26 is a pictorial view ofa component with a special surface,1-2-3-4. Using double auxiliary projec-tion, construct the true shape of thespecial surface

.

59


V «k

FIG. 6.21


FIG. 6.22

VIEW IN

DIRECTIONOF ARROWA'


VIEW IN

DIRECTIONOF ARROW A"

FIRST

REFERENCE PLANE

FIG. 6.23




FIG. 6.24


•P.


HOLE 4MO

FIG. 6.28


THIRD ANGLE9/ >,3 PROJECTION

9^10VIEW IN DIRECTIONOF ARROW A"

REFERENCE

FIG. 6.29

VIEW X


FIG. 6.27

VIEW IN

DIRECTION OFARROW,

Pi

PERPENDICULARN&DISTANCE FROM \^POINT R & P2 TO LINEOi02 IS SHORTESTDISTANCE. vBETWEEN A.LINES ;/ / A


VIEW INDIRECTION OFARROW A'

FIG. 6.31

Circles in doubleauxiliary projection

In principle, there is no addition-al difficulty with circles when morethan one successive auxiliary views areinvolved. Figure 6.2 7 shows, on theright-hand side, two views of an irreg-ular block with a trapezium-shapedbase; these views will be consideredto be the original views and have beenlabelled VIEW X and VIEW Y.

The drawing is to show a circularhole, drilled with its axis perpendic-ular to the inclined quadrilateral sur-face. Like Fig. 6. 25, the first step isto discover the direction of an auxil-iary view that will show the slopingsurface as a straight line. This hasbeen done by drawing a vertical linethrough Q (in VIEW X) , to cut line SRat 0; is then projected horizontallyto VIEW Y, in which QO gives the direc-tion for a first auxiliary view.

The first auxiliary view is drawnin the usual way and then a second aux-iliary view is projected from it in adirection perpendicular to the lineRQPS. In the view in the direction ofarrow A", the surface PQRS appears inits true shape and it is now possibleto draw a circle to represent the holein the required position.

The circle is marked with twelveequally spaced points and the line 9-3is parallel to the second referenceplane. The twelve points are projectedback to the view in the direction ofarrow A*, to meet the line RQPS. Forclarity in this example, the depth ofthe hole will not be considered.

The next step is to draw the ell-i'pse in VIEW Y, and this is done injust the same way as in Fig. 6. 12. Thedimensions from the "2nd Ref. Plane" toline 9-3, and the perpendicular distan-ces of the other ten points from line9-3, are identical in both the viewsprojected from the view in the direc-tion of arrow A'

.

Finally, the hole is shown as ahidden shape in VIEW X. Each of thetwelve points is the same perpendiculardistance from the "1st Ref. Plane" inboth the views projected from VIEW Y.

It can be seen that it is more com-plicated to draw circles in double aux-iliary projection, but no new principlesare involved.

EXERCISE 6.8

Figure 6.28 shows the same basicobject as Fig. 6. 20; a hole has beenadded and its axis is perpendicular tothe surface 1-2-3-4. Redraw the fourviews of Fig. 6. 20 showing the visibleand hidden edges of the hole in eachview. Add all necessary titles andlabels but do not insert dimensions.When you have finished, compare yoursolution with Figure 6.29.

Projective geometrySo far in this Chapter, double

auxiliary projection has been used asa method of finding the true shape ofplane surfaces: this is not only auseful process in engineering drawings,but can also be used for analysing thegeometry of proposed designs . For ex-ample, the true shape of a surfacegives all the correct distances andangles on that surface . A view inwhich two intersecting surfaces bothappear as straight lines gives theangle between the surfaces.

The subject is so vast that onlya separate book could do justice to it.However, the following example is aparticularly useful application ofdouble auxiliary projection to an ess-entially geometrical problem.

Figure 6.30. shows two perpendicularboards; the surface of the thin one isa vertical plane and the surface of thethick one is a horizontal plane . Twostraight lines, P1P2 and QiQ 2 , are de-fined by the positions of Pi, Qi and P 2 ,

Q 2 in the vertical and horizontalplanes, respectively.

The line Q1Q2 is shown with a coilaround it for ease of identification.The four points have been projected hor-izontally or vertically on to planesperpendicular to those in which theylie

.

The two lines do not intersect andit is required to find the shortestdistance between them. Such a problemfrequently arises when power lines andother cables are in close proximity, orwhen the locus of a moving object bringspart of it close to a stationary compon-ent.

Figure 6. SI shows, on the right-hand side, views of the two boards andlines, as seen in the direction ofarrows Y and Z, in third angle projec-tion. Notice that the horizontal boardhas a corner removed to produce a ver-tical plane, parallel to the planeP!P{P 2 .

The shortest distance between PiP 2and QiQ 2 will appear when either lineis viewed in a direction that makes itstwo points coincide. Arrow A" (in Fig.6.30) is such a direction, and thefirst step in constructing such a viewis to draw an auxiliary view in whichline,

P

XP 2 appears in its true length.

Arrow A' is perpendicular to theplane P

1P}P 2 and an auxiliary view in

the direction of this arrow has beendrawn in the usual way. The surface ofthe horizontal board is used as the"1st Ref. Plane" and dimensions perpen-dicular to this plane are transferredfrom the view in the direction of arrowY to the view in the direction of arrow

63

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A', in which line P XP 2 appears in its

true length.The vertical board will now be ig-

nored as its existence would hide Pj

and P 2 from the viewer who looked inthe direction of arrow A". The secondauxiliary view is drawn by using thevertical plane through point Q 2 as the"2nd Ref . Plane" and dimensions perpen-dicular to this plane are transferredfrom the view in the direction of arrowZ to the view in the direction of arrowA". Line P1P2 now appears as a pointand the shortest distance between thetwo lines is the perpendicular distancefrom that point to line Q!Q 2 .

If required, the point on Q1Q2/closest to PiP 2 , can be located and pro-jected back to the view in the direc-tion of arrow A' , in which view thecorresponding point on PiP 2 can be loc-ated. Both points can then be projec-ted back to the two original views onthe right-hand side.

EXERCISE 6.9

Figure 6.32 shows a cable, PiP 2 /

passing close to the edge of a roof,

QiQ.2- Use double auxiliary projectionto determine the shortest distance bet-ween PiP 2 and QiQ 2 . Locate, in allviews, the points on PxP 2 and Q1Q2 whichare closest together. Compare yoursolution with Figure 6.33.

Isometric projectionThe method for producing pictorial

drawings, known as "Isometric Drawing",is examined in Chapter 8 . It is conven-ient to discuss the origin of the term"Isometric" in this chapter, as it in-volves a process of double auxiliaryprojection.

Figure 6.3 4 is a drawing of a cube;a pictorial view of the cube is inset atthe top left and its corners are letteredfor ease of identification. In addition,two of the three visible planes areshaded in a distinctive way.

At the top of the orthographic draw-ing are views as seen in the directionsof arrows Z and X. The auxiliary viewshave been drawn so as to produce a viewof the cube as seen in a direction from

to P. The first auxiliary view hasbeen drawn to show the diagonal PO inits true length and the second auxiliaryview is projected along PO and gives.the desired result.

It may be shown that all the vis-ible and hidden square surfaces of thecube become congruent rhombuses in theview in the direction of arrow A". Theacute angles of the rhombuses are 60°.

The edges of the cube appear shorterthan their true length by a factor of/2//3.

Hence, an "isometric" (meaning"Equal- length") projection is a viewlike one along the long diagonal of a

cube, when equal lengths in three mutu-ally perpendicular directions, OX, OYand 0Z , have equal apparent lengths inthe view that is constructed. As theeffect of an isometric projection is

now known, a pictorial view of any ob-ject can be produced without usingdouble auxiliary projection. The iso-metric projection can be produced direc-tly by reducing distances in the direc-tions OX, OY and OZ by a factor of /2//3and setting these distances out alongaxes with angles of 60° between them.Indeed, if a picture of the object isall that is required, the factor /2//3can be ignored and the only effect willbe that the object will appear ratherlarger than life

.

Figure 6.35 illustrates the directconstruction of an isometric drawingand the construction of the same view(with, of necessity, a different orien-tation) by the use of a double auxiliaryprojection. In the isometric drawing,the dimensions in the three directionsOX, OY and OZ have been transferred,without any reduction, from the twoupper views of the orthographic drawing.The four views in the orthographic draw-ing are arranged in exactly the sameway as the views of Fig. 6. 34. Followthrough the successive auxiliary viewsand make sure that you understand howthey have been constructed.

Isometric drawing is treated indetail in Chapter 8.

EXERCISE 6.10

Make an isometric projection fromthe two views of the object shown inFig. 6. 8

.

65

FIG. 6.32

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DIRECTIONOF ARROW A'


SCALE 1/200

FIG. 6.33

FIG. 7.1

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Developments

The development of a three-dimensional surface is the two-dimen-sional shape which can be convertedinto that surface. For example, Figure7.1 shows a cylindrical surface ofheight H and diameter D . Clearly, thedevelopment of this surface is the rect-angle of height H and length tiD; apaper rectangle of this size could beconverted into the cylindrical surface.

There are two important points tonotice about the surface and its devel-opment. Firstly, if the solid surfacewere placed on a plane surface, therewould be line contact between the twosurfaces. Secondly, the shortest dis-tance between any two points, measuredon the solid surface, is the same onthe development

.

Figure 7.2 illustrates three typesof solid surface. The cube consistsentirely of flat surfaces and has nocurvature; it can be developed and onepassible development is shown. Thecylinder has line contact with a planesurface; it can be developed and thisis shown. The cylinder has singlecurvature , the test for which is theline contact with a plane surface. Thesphere has point contact with a planesurface and this is the test for doublecurvature; it cannot be developed.

There are two practical uses forthe idea of development of surfaces.In a manufacturing process, the develop-ment may be cut out of a sheet of mater-ial and then the material is bent andjoined to produce the solid surface.In a design situation, it may be necess-ary to examine the geometry of pointsand lines on a solid surface . If thesurface can be developed, then thedesign problem is easier to handle andto understand. Navigation is a diffic-ult subject to most people because theearth is very nearly spherical and aspherical surface cannot be developed.The type of projections used for mapscannot produce a development of thesurface, and distances between any twopoints do not always correspond withthe true distances. How much easierthings would be if we lived on thesurface of a cylinder'.

Figure 7.3 shows an amusing ideal-ised problem with some practical implic-ations. The surfaces F1F2F3F4 andCJC2C3C4 represent the floor and ceilingof a room, respectively. What is theshortest length of cable that could beused to connect point A with point B?The cable must remain in contact with awall or the ceiling along its entirelength

.

At first sight, it seems that theshortest distance between points A andB is 12 >75 m, the cable following ahorizontal path along three walls.Such a path would be represented on adevelopment of the room's surfaces bya horizontal line on Figure 7.4. How-ever, this is only one of many ways ofdeveloping the room (the floor surfacehas been omitted as it is not the bestplace to run a cable and, in fact, isnot needed for a solution) and you areasked to look at alternative develop-ments to see if the length of cable canbe reduced.

A second development is shown inFigure 7.5. If the cable followed theroute shown, the length required wouldbe exactly 12-5 m. Notice that theline AB crosses none of the discontin-uities in the development and so thedistance is the true length of a cablethat crosses the edges at points P, Q,R and S.

A third development is shown inFigure 7.6; the length of cable re-quired is slightly less than the lastcase, 12-476 m. Once again the line ABcrosses no discontinuities and the dis-tance is the true length of a cablethat crosses the edges at points T, Uand V. The three routes are shown pic-torially in Figure 7.7. Distances suchas CiQ, C3V, etc., could be scaled fromthe developments.

The above problem demonstratesthat information can be more easily ob-tained from the development of a sur-face than from the surface itself, pro-vided the various arrangements of thedevelopment are fully examined. Asolid of this rectangular form is easi-ly developed but the developments ofthe following examples are progressivelymore complex

.

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DEVELOPMENT

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12' 1'2' 3' « ' 10' 11' 12' 1' 2' 3' 4' 5' 6' 7' 8' 9'

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8 yi THIRD ANGLE PROJECTION7 '

5

FIG. 7.8

VIEW IN

DIRECTIONOF ARROW X

FIG. 7.9

Modified cylinderFigure 7.8 shows the type of joint

frequently used for two tubes which forma right-angled connection. The surfaceof the vertical tube has been developedby the following method:

(1) Divide the circular end view ofthe tube into 12 equally spacedpoints. Label the points 1 to 12.

(2) Project vertically upwards fromeach of the twelve points, acrossthe upper view, to meet the inter-face of the two tubes.

(3) Set out a base-line for the devel-opment by making distances 9' -10',10' -11', etc., equal to chords9-10, 10-11, etc. (This resultsin a small error as the base ofthe development will be 120.sinl5°= 3-106C in length, instead of tiD,

a reduction of about 1% .

)

(4) Project upwards from each point onthe base-line and across from thecorresponding point on the inter-face. Join the points so foundwith a smooth curve.

This method produces a reasonablyaccurate development of the surface.Distances parallel with the axis of thetube are true distances which stay thesame in the development. Any part of acylinder can be developed by this meth-od, provided it is possible to draw aview showing distances parallel to theaxis. The spacing along the base-lineneed not be uniform, but chords subten-ding an angle of more than 30° at thecentre of the circle are not recommended.

Right coneFigure 7.9 contains a pictorial

view of a right cone (i.e. the apex isperpendicularly above the centre of thecircular base) . A view as seen in thedirection of arrow X is the isoscelestriangle A-3-9, and this is also thetriangle produced by the intersectionof the cone and the reference plane, inthe pictorial view.

Notice that only the generatorsA-3 and A-9 appear in their true length.The generator A-5 is foreshortened, butits true length can be obtained by pro-jecting horizontally across to points 3

or 9, as all generators are of length L.

The surface of the cone has beendeveloped by drawing a circular arc,centre A, radius L. Only half of thebase is shown, using the line 3-9 as anaxis of symmetry. The straight linedistance between two points, such as 4

and 5, is transferred from the view ofhalf the base to the development. Theerror introduced depends on the propor-tions of the cone but is rarely of anysignificance

.

In effect, the cone has been divid-ed into twelve small triangles with acommon apex. The sides of the trianglesare generators of the cone. The develop-ment is shown as a sector because anypoint on the base is distance L from theapex.

Figure 7.10 shows a right cone cutby a cylinder, the axis of which is par-allel to arrow X of Fig. 7. 9. The bottompart of the cone has been developed bythe following method:

(1) Draw a development of the completecone (by the method illustrated inFig. 7. 9)

.

(2) Where the cylinder intersects agenerator, e.g. point 5' on lineA-5|, project horizontally to meetline A-3 at 5". Just as A-3 is

the true length of line A-5 , soA-5" is the true length of lineA-5".

(3) With centre A, radius A-5", drawan arc to cut the A-5 line of thedevelopment . (This is also takento the A-l line as points 5 and 1

are arranged symmetrically.)(4) Repeat the process for the inter-

section of the cylinder and theother generators. Join the pointswith a smooth, symmetrical curve.

EXERCISE 7.1

Figure 7.11 is a pictorial view ofa T-connection between two tubes of dif-ferent diameters. The transition pieceis part of a right cone. Draw, inorthographic projection, views as seenin the directions of arrows X and Y.Choose any suitable scale and make thetubes, away from the transition piece,of any convenient length.

Develop the surface of the coneand, by considering lines such asO1-P-Q-R-S-O2 in both views, developthe hole in the 52 mm diameter tube.When you have finished, compare yoursolution with Figure 7.12.

71


FIG, 7.11

DEVELOPMENT OFHOLE IN CYLINDER

FIG. 7.12


I2

APEX OFOBLIQUE CONE

FIG. 7.15

SCALE 1 /20

FIG. 7.16

EQUAL CHORDLENGTHS

TRUE LENGTHSCONSTRUCTION

TRUE LENGTHS

FIG. 7.14

Oblique coneFigure 7.13 contains a pictorial

view of an oblique cone (i.e. the apexis offset from a perpendicular throughthe centre of the circular base) . Aview as seen in the direction of arrowX is the triangle A-3-9, and this isalso produced by the intersection ofthe cone and the vertical referenceplane in the pictorial view.

Notice that only a line such asA-9, in the reference plane, appears inits true length. The line A-5 is fore-shortened, its apparent length appearingas A-5'. The true length may be ob-tained by constructing the right-angledtriangle A-A'-5; a convenient way ofdoing this is shown in the figure.

Only half of the base is shown,using the line 3-9 as an axis of symme-try. The apex A is projected on to theplane containing the base, to give pointA'. With centre A', radius A'-5, thedistance A' -5 is set out at 90° to theright of AA' : the hypoteneuse of tri-angle A-A'-5 is the true length of lineA-5 . The same construction process isrepeated, to draw a complete set oftrue length lines.

The surface of the cone has beendeveloped by starting at line A-9, whichalready appears in its true length, anddrawing the triangle A-9-10. Line A- 10comes from the set of true lengths andthe side 9-10 equals the chord 9-10,from the circular base. Triangle A-10-11comes next and the other triangles fol-low, in the appropriate order. The num-bered points are joined with a smoothcurve

.

Notice that the position of the'

discontinuity in the development is ofsome importance when the manufacture ofsuch a cone is being considered. Thenumber of developed shapes that can becut from a given size of sheet materialwould be affected by the position of thejoin. For example, a join along lineA-3, instead of A-9, might enable moreshapes to be cut from the same size ofsheet.

Figure 7.14 shows an oblique cone,

cut by a plane surface PQ; the inter-section of this plane and the referenceplane (in the pictorial view) producesthe line P-Q. The lower part of thecone has been developed by the followingmethod:

(1) Draw a development of the completecone (by the method illustrated inFig. 7. 13)

.

(2) Where the plane PQ intersepts aline such as A-5 1 (point N'), pro-ject horizontally to meet the truelength line A-5 at point N. Justas A-5 is the true length of lineA-5 ' , so A-N is the true length ofline A-N'

.

(3) With centre A, radius A-N, swingan arc to cut the A-5 line of thedevelopment. (This arc is alsotaken to the A-l line, as points 5

and 1 are arranged symmetrically.)(4) Repeat the process for the inter-

section of the plane PQ and theother lines on the surface of thecone. Join the points with asmooth, symmetrical curve.

EXEkCISE 7.2

Figure 7. IS is a pictorial view ofthe most common application of obliquecones in piping. Two small diameterpipes are joined to form a single, largediameter, pipe; the axes of all threepipes are parallel. The transitionpiece takes the form of two intersectingoblique cones with a common base. Whenviewed in the direction of arrow Z , onehalf of the transition piece is themirror image of the other

.

Draw a view of one oblique cone,as seen in the direction of arrow Z

,

and hence develop the surface of onehalf of the transition piece. When youhave finished, compare your solutionwith Figure 7.16. (It may be helpfulif the development in Fig. 7.14 is traced,cut out and joined up to give a trunc-ated oblique cone, which may be usedfor an aid in this exercise. Indeed,any of the solutions, in this chapter,may be used in this way if difficultiesare encountered.)

75

FIG, 7.17

,0800


FIG. 7.18

SCALE 1/2012

FIG. 7.19

TriangulationThe oblique cone was developed by

a process of triangulation. This is a

process in which the true lengths oftwo intersecting lines, on the solidsurface, are determined and drawn outas two sides of a plane triangle; thethird side comes from a convenient shortline that lies, as nearly as possible,on the solid surface. The same processis particularly useful for transitionpieces, such as the one shown in Figure7 .17 , where a change from a polygonalsection duct to a closed-curve sectionduct is involved.

Views , as seen in the directions ofarrows X and Z , have been drawn in ortho-graphic projection. The true length ofa line, such as P-5, has been construc-ted by the same method that was used foran oblique cone. The complete develop-ment would consist of four isoscelestriangles (one of which appears in itstrue shape in the view in the directionof arrow X) and four shapes, like P-4-7,which has been constructed by treatingthe curved surface as three adjacenttriangles. Only half of the development

is shown in Fig. 7. 17.In general, the true length of a

line is constructed by drawing, ineffect, an auxiliary view as seen in adirection perpendicular to the line, asit appears in one of the views of theorthographic drawing. Of course, theauxiliary view need not be indicatedand labelled as shown in Chapter 6; itis only a step in the construction ofthe development. Nevertheless, it is a

convenient way to work out how to deter-mine a true length; the constructionshown in Fig. 7. 17 is based on the prin-ciple illustrated in Fig. 6. 30, wherethe first auxiliary view shows the truelength of line P

1-P 2 .

EXERCISE 7.3

Figure 7.18 is a pictorial view ofa transition piece for a wind tunnel,in which the section changes from arectangle to a circle. N.B. The rect-angle and the circle are not coaxial

.

Using a suitable scale, constructa development of the surface of thetransition piece . When you have fin-ished, compare your solution with Figure7.19.

77

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Pictorial Drawings

Previous Chapters have been mainlyconcerned with orthographic projection:a drawing system that is particularlysuitable for detail and general arrange-ment drawings that have to provide pre-cise manufacturing instructions . Indesign, training and sales work it isoften useful to be able to representobjects pictorially without going tothe extent of making a proper perspec-tive drawing. Figure 8.1 shows fourwell-established methods of doing thiswith the aid of only the usual drawingequipment

:

(1) Oblique drawing(2) Isometric drawing(3) Dimetric drawing(4) Trimetric drawing.

Most people find that the methodsin the above list are in order of easeof making the drawing and in reverseorder of pleasing results . This Chap-ter is devoted mainly to isometric draw-ings as they are not too difficult toproduce and have a fairly pleasingappearance

.

Isometric projectionFig. 6. 34 (p. 6 4) shows the meaning

of isometric projection; a view of acube along the long diagonal shows theobject with all its sides having equallength and the angles between them areeither 60° or 120°. Fig. 6. 35 shows howa pictorial view of an object may beproduced either by isometric projection

or by isometric drawing. In the formercase, double auxiliary projection is

used and, in the latter, three-dimen-sional Cartesian coordinates are trans-ferred to three coplanar axes with 120°between them.

Isometric drawingFigure 8.2 is the same object as

Fig. 6. 35; the isometric drawing pro-cess is shown in more detail. The iso-metric grid, in the centre, is ruledout with vertical lines and lines at 60°either side of the vertical. Each lineintersects the other lines at intervalsof 5 mm. The object would fit into arectangular block 55 mm high, 45 mmlong and 35 mm wide . This block hasbeen drawn out on the grid.

On the lower grid, all verticalheights have been transferred to thevertical set of lines , measuring fromthe lower horizontal plane, ABCDE . Alllengths have been transferred to theset of lines inclined at 60° clockwiseto the vertical , measuring from thevertical plane EFG. All widths havebeen transferred to the set of linesinclined at 60° anticlockwise to thevertical, measuring from the verticalplane containing G and A.

It is convenient to think of thisprocess as a method o£ plotting outPOINTS. It is always possible to reador to construct the Cartesian coordin-ates of all the points contained in acomplete detail drawing. After thesepoints have been plotted on the isometric

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grid it is possible to decide the cor-rect ways of drawing in the lines.

Sloping surfaces and curves oftencause some difficulties. Sloping sur-faces are plotted out by locating thepoints that form the plane of the sur-face, e.g. points J, K, L, M in Fig. 8. 2.

Sometimes the coordinates of thesepoints are known from the dimensionsgiven on the drawing but it is oftennecessary to construct (or calculate)part of the geometry of the object inorder to determine the necessary dimen-sions.

Curves can be dealt with by treat-ing them as a finite set of points;each point has to be plotted on the iso-metric grid. The set of points is thenjoined up by careful sketching or theuse of french curves or a flexicurve

.

Circular arcs and complete circles thatlie in one of the isometric planes maybe dealt with by special techniques des-cribed below.

It is not essential to have an iso-metric grid in order to make an isomet-ric drawing: a 60° set-square and a

scale may be used with ordinary drawingpaper . However , for quick isometricsketches and for learning the techniqueof isometric drawing, a grid is veryuseful: pads of isometric grid paperare on the market.

The object appearing in the lowergrid of Fig. 8. 2 looks larger than itdoes in the orthographic drawing at thetop of that figure . The actual dimen-sions were used along the isometricaxes whereas any line which is riot

viewed along a line perpendicular to it-self should be foreshortened. It wasshown, in Chapter 6, that the objecthas been magnified by a factor of /3/V2.It would be possible to correct for thisby having an isometric scale incorpor-ated in the grid, e.g. 2 divisions wouldrepresent 10 mm but the actual distancebetween the divisions would be 10/2//3mm. In most practical applicationsthere is no need to correct for the fore-shortening effect, as it affects theaxes equally. The slight magnificationthat results from the use of actual dim-ensions can be accepted.

EXERCISE 8.1

Use the grid provided to constructan isometric drawing of the object

shown in Figure 8.3. Choose the viewcarefully so that as much of the detailof the object is revealed as possible.Do not use the isometric scale (/2//3)and do not add dimensions. Make thevisible lines much bolder than the gridlines.

Sloping surfacesIn Exercise 8.1, it is fairly easy

to locate all the eight points lying onthe sloping surface, but the followingexample shows how it is often necessaryto construct some of the geometry ofthe object before commencing the isomet-ric drawing. Consider a regular hexagonof 50 mm across flats, cut from a platethat is 15 mm thick. Figure 8.4 showsthe steps in the construction of an iso-metric drawing of such an object. In

steps 1 to 4, a scale drawing of thehexagonal shape has been made . Steps 5

to 8 are stages in the construction ofthe isometric drawing itself. The stepsare as follows:

(1) Draw two parallel lines, AB and ED,50 mm apart, to subtend an angleof 60° at a point 0, midway betweenthem.

(2) Draw a line through 0, parallel toAB and ED.

(3) Locate point C by drawing a linethrough B at 60° to the horizontal.

(4) Locate the sixth point F by draw-ing a line through A at 60° to thehorizontal. Complete the outlineof the hexagon. Choose axes Oxand Oy.

(5) Construct isometric axes Ox, Oyand Oz at the correct angles forthe isometric drawing. Let point

correspond to the point at thecentre of the hexagon. Points Fand C lie on the Ox axis and dis-tances OC and OF are made equal inthe drawings for step (4) and step(5) , i.e. the isometric scale isnot used.

(6) Locate point B as shown, using dis-tances obtained from the drawingfor step (4) . On the isometricdrawing, distances are always mea-sured parallel to either Ox, Oy orOz .

(7) Locate A, E and D in exactly thesame way that B was located. Com-plete the hexagon.

81



30

FIG. 8.8

FIG. 8.6

THIRD ANGLE PROJECTION60 10


CENTRES OF FOURARCS SHIFTEDALONG AXIS sT ZX70

FIG. 8.7

(8) Locate the visible points at theother end of the plate by construc-ting lines parallel to Oz

.

AA' = BB' = CC = DD' = 15 mm.Complete the drawing.

The need to produce a scale drawingof part of an object before commencingthe isometric drawing can sometimes beavoided when the nature of isometricdrawing is fully understood. However,the beginner would be well advised towork from scale drawings at first.

EXERCISE 8.2

Use the grid provided to producean isometric drawing of the objectshown in Figure 8.5. The viewing dir-ection should reveal as much detail aspossible and, without using the isomet-ric scale, the object should appearabout twice full size. Do not dimension.

CirclesAny circle may be treated as a fin-

ite number of points and these can beplotted out on the isometric drawing.The set of points is then joined with asmooth curve. This process is verytime-consuming and is not always neces-sary, since the isometric drawing usu-ally serves as an illustration, and onlya rough approximation to an ellipse isrequired. (Strictly speaking, when theisometric scale is not used, a circleof diameter d which lies in one of theisometric planes will appear as an ell-ipse with major axis d/3//2 and minoraxis <i//2.)

A very useful approximate methodfor isometric circles is known as the"Four Arc" method. This is describedbelow and shown in Figure 8.6; the iso-metric scale is not used.

(1) Locate the centre of the isomet-ric circle by drawing two linesparallel to the isometric axesthat define the plane in which thecircle lies, and in the appropriateposition for the centre of thecircle.

(2) Draw two perpendicular linesthrough to bisect the isometricaxes for the plane in which thecircle lies. One of these lineswill be the third isometric axis,Oz

.

(3) Lightly draw a circle, centre 0,radius equal to that of the requir-ed isometric circle, to intersectOz at P and Q

.

(4) Draw two lines through P (or Q) at30° to Oz which cut the circle atM and N on the isometric axes afterintersecting 0a at R and S.

(5) Construct two arcs, centres P andQ, radius PM which are bounded bythe circle.

(6) Construct two arcs, centres R andS, radius RM which are bounded bythe circle.

(7) The four arcs produce a shape whichclosely follows the correct ellipse.All construdtion lines have beenremoved

.

(8) The same construction is shown forthe other two isometric planes.

Figure 8.7 shows the same processas some of the steps in the making ofan isometric drawing of a rectangularplate with a circular hole . Notice howthe centres of some of tne four arcshave been displaced along an isometricaxis to facilitate the drawing of anisometric circle in a plane parallel tothe first one. The Four Arc method re-quires a lot of construction lines; theyshould be drawn very faintly for easyerasure.

EXERCISE 8.3

Using a drawing board and instru-ments, make an isometric drawing of thecomponent shown in Figure 8.8. Withoutusing the isometric scale, the componentshould appear about full size. Hint:It is possible to use the Four Arcmethod for the hole as well as the com-plete circles; it can be treated as acomplete circle cut by two parallellines.

83

THIRD ANGLE PROJECTIONALL DIMENSIONS ARE IN MM


,.15 5

ABCDEFGHJ<$> 16

FIG. 8.10

Four arc methodgives curves thatdo not blend

FIG. 8.9 Ml ong the i sometri c

axes the distancesbetween letters equalthose in orthographicdrani ng

>( V-. w Pin-H

73m71

-or->z.m

Y uc z

VIEW IN


THIRD ANGLE PROJECTION VIEW IN

DIRECTIONOF ARROW A"

FIG. 8.12

FIG. 8.13.1 FIG. 8.13.2

Blended arcsThe four arc method cannot be used

when a number of circular arcs blendtogether (with a common normal at thetransition point). Figure 8.9 showssuch a system and an attempt to use thefour arc method. Because the ellipsesare not reproduced accurately, espec-ially at the ends of the major axes,the arcs will not blend. It is some-times possible to manipulate the arcsand achieve satisfactory blending but,as in the successful pictorial drawingin Fig. 8. 9, it may be just as quick toplot the arcs out as a finite number ofpoints

.

Dimetric projectionIn Chapter 6, Fig. 6. 34, it was

shown that an isometric view is a viewalong the long diagonal, OP, of a cube;all the edges are equally foreshortened.In Figure 8.12, the first auxiliaryview is in the same direction as thatin Fig. 6. 34 but the second auxiliaryviaw is not along OP. The result is

that only two of three intersectingedges are equally foreshortened: thisis known as a Dimetric View. By varyingthe direction of arrow A" , the angle 6

can be changed; this affects the ratioof the lengths of the edges UZ and VZto the length of OZ

.

EXERCISE 8.4

Make an isometric drawing, abouttwice full size, of the cam shown inFigure 8.10. The surface marked H isto be visible. Use the four arc methodonly where it is appropriate

.

Small circlesMany engineering components have

fillet radii and other small curvesthat would take a great deal of time toconstruct in an isometric drawing.Small circles (with a radius of lessthan about 5 mm) may be drawn by locat-ing the four points which lie on theisometric axes and sketching an ellipsethrough these four points. A satisfac-tory ellipse can be sketched, providedthe axes of symmetry are taken into acc-ount. It may be seen in Fig. 8. 6 thatthe axes of symmetry always bisect theisometric axes for the plane in whichthe circle lies.

EXERCISE 8.5

Make an isometric drawing, aboutfull size, of the component shown inFigure 8.11. Show as much detail aspossible by carefully selecting theviewing direction.

Dimetric drawingA dimetric drawing can be made dir-

ectly, in the same way as an isometricdrawing, but the process is more complic-ated. In an isometric drawing the sidesof a cube are in the ratio 1:1:1 andthe angles between the vertical sidesand the horizontal sides are both 60°.

Figure 8.12.1 shows the correct anglesfor a dimetric drawing of a cube inwhich the vertical sides are half thelength of the horizontal sides. Thisview is far from pleasing, but it canbe reorientated, as in Figure 8.13.2,so that one of the longer sides is vert-ical; the appearance is greatly imp-roved .

Figure 8.14 shows how the objectillustrated in Fig. 8.1 is drawn as a

dimetric view with ratios 1:1:%. Thecorrect angles for other ratios can bedetermined by an analysis of the geom-etry of Fig. 8. 12.

EXERCISE 8.6

Make a dimetric drawing of the ob-ject shown in Fig. 8. 5, using ratios of1:1:%. Make your viewing directionabout the same as the solution to Exer-cise 8.2 and compare the two pictorialviews

.

85


FIG. 8.11

35

20

C E

20

' K/ML

B D

18

45


41° 25' D

The dimensions shown are tneactual distances used for theconstruction of the DimetricView

17° 10'


FIG. 8.14

wp v.-

-)

JOm71

TJr>2m

u z o O

VIEW IN



17° 07 \24°46'

\23° 16'

FIG. 8.15 FIG. 8.16

The correct angles to be used with convenientratios may be calculated from;

/'°^V j. f0Y¥ - 1 + tane tan» .

\ozJ KozJ 1 - tan9 tan*

/OXV f2XY = tan ; 9 - tan 2 *uz ; - Voz; tan2e+ tan2„

Tne dimensions shown are the actual distances usedfor the construction of the Trimetric View.

^^12-28'23° 16'

FIG. 8.17

FIG. 8.18 FIG. 8.19

»\A,

FIG. 8.20 FIG. 8.21

28

axio

/24

242

FIG. 8.22.1 FIG. 8.22.2

Trimetric projection Oblique drawingA double auxiliary view of a cube

that shows unequal foreshortening ofthree intersecting edges , is known as aTrimetric View. Figure 8.15 shows howboth arrows A' and A" are in differentdirections from the corresponding arrowsin Fig. 6. 34. There is a unique rela-tionship between the angles 9 and <(> andthe ratio of the lengths UZ:VZ:OZ.

Trimetric drawingFigure 8.16 shows the correct ang-

les for two trimetric views of a cubethat have convenient ratios. Figure8.17 shows how the trimetric view con-tained in Fig. 8.1 was constructed.

Methods of producingdimetric and trimetricdrawings

The construction of these pictor-ial views may be facilitated by the useof:

(1) Grid paper, like that used forisometric drawings, with the linesruled at the appropriate anglesand spaced according to the corres-ponding ratios

.

(2) A special template scale havingslots at the appropriate angles,the edges of the slots being grad-uated according to the ratios ofthe dimetric/trimetric scale.

Circles are troublesome in bothdimetric and trimetric drawings: thismay account for the lack of popularityof these methods.

EXERCISE 8.7

Make a trimetric drawing of theobject shown in Fig. 8. 5, using eitherof the sets of ratios shown in Fig.8.16. Make your viewing directionabout the same as the solution to Exer-cise 8.6 and compare the two pictorialviews

.

The great advantage of obliquedrawing is that one set of parallelplanes of the object appears in its trueshape. If the planes concerned containcircular features then the constructionof the pictorial view becomes very mucheasier than for any of the other threemethods described in this Chapter.

Figure 8.18 shows an oblique draw-ing of the same object which appears inFig. 8. 7. The angles used are oo and 30°and the ratios are 1:1:1. It is notpossible to produce such a view bydouble auxiliary projection; an obliqueview is simply a convenient, fictitiouspictorial representation of the object.

The appearance of an oblique draw-ing may be improved by changing theratios to 1:1:%; this has been done inFigure 8.19. Moreover, it is possibleto use almost any angle for the obliqueaxis. Figure 8.20 has ratios 1:1:1 andthe oblique axis is at 45°; this isknown as a "Cavalier" oblique drawing.Figure 8.21 has ratios 1:1:% and theoblique axis is at 45°; this is knownas a "Cabinet" oblique drawing.

The oblique drawing contained inFig. 8.1 is also a "Cabinet" obliquedrawing.

Figure 8.2 2.1 illustrates the cor-rect use of oblique projection wheredrawing time is saved by showing theblended arcs (from Fig. 8. 9) in theirtrue shape. There are two objectionsto Figure 8.22.2: it is more difficultto draw and the result looks ratherpeculiar.

EXERCISE 8.8

Make an oblique drawing of the ob-ject in Fig. 8. 8. Use any suitable ang-le and ratios.

EXERCISE 8.9

Make a "Cabinet" oblique drawingof the cam shown in Fig. 8. 10. Showthe surface labelled H.

88

afsinga ®

Mating Parts

The effects of small differencesbetween the size stated on a drawingand the manufactured size are brieflyexamined in Chapter 5 . In practice itis always necessary to allow a "toler-ance" on every dimension: even if itwere possible to manufacture with com-plete accuracy and to verify this byperfect measurement, the cost would beuneconomic. The "tolerance" is thedifference between the maximum accept-able size and the minimum acceptablesize

.

The correct functioning of an ass-embly of parts is the responsibility ofthe design department of an organiza-tion. It follows that tolerances shouldbe selected by the design personnelrather than those responsible for themanufacturing processes

.

A tolerance will depend on whetherand how a feature from one part is tofit into (or "mate" with) another part.Figure 9.1 is a set of detail drawingsof four of the five parts shown in Fig.5.14 (p. 42). Clearly, the dimensionsA and B on the pin do not affect thefunctioning of the assembly and a gen-eral tolerance, say 1-0 mm, might beacceptable. However, the dimensions Xand Y do affect the functioning andgreater accuracy is required. Further-more, it is important to ensure thatthe pin can be inserted in the hole,i.e. the effect of the tolerances onthe diameters of the pin and the holeis that the former does not signific-antly exceed the latter; a slightinterference or "push" fit might beacceptable. The nature of the fit dep-ends upon the "deviation" or difference

between the actual size and the basicsize" (10 mm) of both the pin and thehole

.

Tolerances, deviationsand limits

Figure 9.2 illustrates the meaningsof the terms "tolerance", "deviation"and "limit". For convenience, any twomating parts will be referred to as ashaft and a hole regardless of theiractual form. A positive deviation meansthat the size is larger than the basicsize. Thus, for the dimension shown inFigure 9.3,

the basic size is 10 mm,

the upper deviation is + 0*006 mm,

the lower deviation is - 0-005 mm,

the tolerance is + 0*006 - (-0-005)= 0-011 mm,

the maximum limit of size is10-000 + 0-006 = 10-006 mm,

the minimum limit of size is10-000 - 0-005 = 9-995 mm.

The location of the limits of sizewith respect to the basic size is ex-pressed in terms of the "fundamental"deviation. The deviation that relatesto the limit which is nearer to thebasic size is termed the "fundamental

"

deviation. Most systems of limits andfits are based on standardized toler-ances and standardized fundamental dev-iations .

100069-995

FIG. 9.3

89

© PLAIN END

* 10000+ 0-015


FORK END

1X45 x <J> 9-995C\

|

37-500 K>025 - 0-009,

(j> 20

PIN WASHER

®,,1'50 (

,1-50

<f>20

006

TOLERANCEEXCEPT WHEREOTHERWISESTATED ±0-5

ALL DIMENSIONSARE IN MMSCALE 1/2

10-000+ 0-015

_<p 10000+ 0-015

FIG. 9.1

MAXIMUM LIMIT OF SIZE

BASIC SIZE

HOLE

MINIMUM LIMIT OF SIZE-

MAXIMUM LIMIT OF SIZE-

UPPER DEVIATION (shaft) es

LOWER DEVIATION (shaft) ei

TOLERANCE (shaft)

TOLERANCE (HOLE)

LOWER DEVIATION (HOLE) EI

UPPER DEVIATION (HOLE) ES

FIG. 9.2

1. Both limits of size aredirectly specified; themaximum limit is givenfirst

59-970

59-940

60-04660-000

The 1 iniit of size formaximum material isspecified first, followedby the tolerance with theappropriate sign.

36000

<ZZZ>-—\

36033.* »-0-016

3. The mean size is given,followed by'

1

± half theto! erance .

70-0161 0-007

FIG. 9.4

NEGATIVE DEVIATIONS POSITIVE

SHAFT

Ic3"

—

O

|e hirvj

if

A KK*\Vt

HOLE BK\\NK\\\\W\\\Mcimvwwwwvww^

C Dm\\V\\\,\\VVV\\\\\V7

D|^k\\\\\\\\\\\\\\\\\W

emwvwwvwwvwvw^EFk\^K\\\\V\V\\\\\\\\\\\^

Ftm\\v\ww\wwww?^7

Js

\\\\\\\\\\\\\\\\\S\SSS3

k^ {K\\\\\\V\\VV\VV\\\\\^^

"MT^E

~N^g \\\\\\\\\\W\\\\\V\\^n7

Pl^'t'raw-^m^wwwvwwwwiS ^k\\\^

Tt^KWW X\\VSSkV\\\\\\\\\\\\\\\\\\A

mmww^^^VIMkWVW^^vww

Yt^^K\\\\\\^3^ \\\\\\\\\^

2 K>^S\V\\\\\\\\\\\ ^^\\\\\\\\\\^

%zg^^^^^^^^^^^^^^^^^^^^

^w^wwww^

tzc^\\\\\\\^\\\^^^^

FIG. 9.5

MINIMUMCLEARANCE

MAXIMUMCLEARANCE

MINIMUMINTERFERENCE

FUNDAMENTAL DEVIATION(HOLE) _ .

TOLERANCE (shaft)

MAXIMUM"CLEARANCECLEARANCE FIT

FUNDAMENTAL DEVIATION(shaft)

TOLERANCE (HOLE)MAXIMUMINTERFERENCE

MAXIMUMINTERFERENCE

TRANSITION FIT INTERFERENCE FIT

FIG. 9.6

FIG. 9.7

BRITISH STANDARDSELECTED ISO FITS : HOLE BASIS

ES = Upper deviation (HOLE) EI = Lower deviation (HOLE)

Tolerancesto scale

Clearance f its

HOLE

C11 d10 e9 f7 g6

r | i | |

^^ shaft ^ 1 f^l k\\\s

^ ^H11

ESEI

esei

H9ESEI

esei

H9ESEI

esei

H8ESEI

esei

H7ESEI

esei

Over To Hll ell H9 dlO H9 e9 H8 f7 H7 g«

mm mm

3

0001 mm

+ 60

0001 mm

-60- 120

0001 mm

+ 25

0001 mm

-20-60

0001 mm

+ 25

0001 mm

- 14- 39

0001 mm

+ 14

001 mm

-6- 16

0001 mm

+ 10

0001 mm

-2-8

3 6+ 75 -70

- 145+ 30 -30

-78+ 30 -20

-50+ 18 - 10

- 22+ 12 -4

- 12

6 10+ 90 -80

- 170+ 36 -40

-98+ 36 -25

-61+ 22 - 13

-28+ 15 - 5

- 14

10 18+ 110 - 95

-205+ 43 -50

- 120+ 43 -32

- 75+ 27 - 16

-34+ 18 - 6

- 17

18 30+ 130 - 110

- 240+ 52 -65

- 149+ 52 -40

-92+ 33 -20

-41+ 21 - 7

-20

30 40+ 160 - 120

-280 + 62 - 80- 180

+ 62 - 50- 112

+ 39 -25-50

+ 25 -9-25

40 50+ 160 - 130

-290

50 65+ 190 - 140

-330 + 74 - 100-220

+ 74 -60- 134

+ 46 -30-60

+ 30 - 10-29

65 80+ 190 - 150

-340

80 100+ ,220 - 170

- 390 + 87 - 120-260

+ 87 - 72- 159

+ 54 - 36-71

+ 35 - 12- 34

100 120+ 220 - 180

-400

120 140+ 250 -200

-450+ 100 - 145

- 305+ 100 - 84

- 185+ 63 -43

-83+ 40 - 14

- 39140 160+ 250 - 210

-460

160 180+ 250 -230

-480

180 200+ 290 -240

-530i 115 170

3551 115 - 100

- 215+ 72 - 50

-96t- 46 - 15

-44200 2251 290 - 260

- 550

225 250+ 290 -280

- 570

250 280+ 320 -300

- 6201 130 190

-400+ 130 - 110

- 240+ 81 - 56

- 108

+ 52 - 17- 49

280 315+ 320 - 330

- 650

315 355+ 360 - 360

- 720 ^ 140 - 210- 440

+ 140 - 125- 265

+ 89 -62- 119

+ 57 - 18- 54

355 400+ 360 -400

- 760

400 450+ 400 -440

- 840 + 155 - 230480

+ 155 - 135 + 97 - 68- 131

+ 63 20- 60

450 500+ 400 - 480

- 880

- 290

Extracted from BS 4500:1969Reproduced by permission

es = Upper deviation (shaft) ei = Lower deviatio n (shal

Over

Transition fits Interference fits

h6 k6

Jn6

1

P6 s6

1 I 1 I (

IP ^ RS idiH7ESEI

esei

H7ESEI

esei

H7ESEI

esei

H7ESEI

esei

H7ESEI

esei I

H7 h6 H7 k6 H7 n6 H7 p6 H7 s6 To

0001 mm

+ 10

0001 mm

-6

0001 mm

+ 10

0001 mm

+ 6+

0001 mm

+ 10

0001 mm

+ 10

+ 4

0001mm

+ 10

0001 mm

+ 12

+ 6

0001 mm

+ 10

0001mm

+ 20+ 14

mm mm

3+ 12 - 8 + 12 .+ 9

+ 1

+ 12 + 16

+ 8+ 12 + 20

+ 12+ 12 + 27

+ 19 3 6+ 15 -9 + 15 + 10

+ 1

+ 15 + 19

+ 10+ 15 + 24

+ 15+ 15 + 32

+ 23 6 10+ 18 - 11 + 18 + 12

+ 1

+ 18 + 23+ 12

+ 18 + 29+ 18

+ 18 + 39+ 28 10 18

+ 21 - 13 + 21 + 15

+ 2+ 21 + 28

+ 15+ 21 /+35

+ 22+ 21 + 48

+ 35 18 30

+ 25 - 16 + 25 + 18

+ 2+ 25 + 33

+ 17+ 25 + 42

+ 26+ 25 + 59

+ 43

30 40

40 50

+ 30 - 19 + 30 + 21

+ 2+ 30 + 39

+ 20+ 30 + 51

+ 32

+ 30 + 72+ 53 50 65

+ 30 + 78+ 59 65 80

+ 35 -22 + 35 + 25+ 3

+ 35 + 45+ 23

+ 35 + 59+ 37

+ 35 + 93+ 71 80 100

+ 35 + 101

+ 79 100 120

-25 + 40 + 28+ 3

+ 40 + 52+ 27

+ 40 + 68+ 43

+ 40 + 117+ 92 120 140

+ 40 + 40 + 125

+ 100 140 160+ 40 + 133

+ 108 160 180

- 29 + 46 + 33+ 4

1 46 f 60+ 31

f 46 f 79+ 50

+ 46 + 151

+ 122 180 200+ 46 + 46 + 159

+ 130 200 225+ 46 + 169

+ 140 225 250

+ 52 - 32 + 52 + 36+ 4

+ 52 + 66+ 34

+ 52 + 88+ 56

+ 52 + 190+ 158 250 280

+ 52 + 202+ 170 280 315

+ 57 - 36 + 57 + 40+ 4

+ 57 f 73

+ 37+ 57 + 98

+ 62

+ 57 + 226+ 190 315 355

+ 57 + 244+ 208 355 400

+ 63 -40 + 63 + 45+ 5

+ 63 + 80+ 40

+ 63 + 108+ 68

+ 63 + 272+ 232 400 450

+ 63 + 292+ 252 450 500

to the selection of fits

Hll-cll

Slack runningfit

H9-dl0

Loose runningfit

H9-e9

Easy runningfit

H8-f7

Normal runningfit

H7-g6

Sliding andlocation fit

H7-h6

Location fit

Exampl es

Used to give flexibilityunder load, easy assembly ora close fit at elevatedworking temperatures.

Used for gland seals, loosepulleys and very largebearings .

Used for widely separatedbearings or several bearingsin line.

Suitable for applicationsrequiring a good quality fitthat is easy to produce.

Not normally used for con-tinuously running bearingsunless load is slight.Suitable for precisionsliding and location.

Suitable forassembl ies .

many non-running

I.C. engi neexhaust valvein guide

12 MM DIA H11-C11

44 MM DIAH9-d10 idler gear on

spi ndle

80 MM DIA

Camshaft inbeari ng

18 MM DIAH8-f7

Gearbox shaftin bearing

6 MM DIA H7-g6

Valve mechan-ism link pin

12 MM DIAH7-h6

Valve guidein head

H7-k6

Push fit

Used for location fits whenslight interference, whicheliminates movement of onepart relative to the other,is an advantage.

20 MM DIAH7-k6

CI utch memberkeyed toshaft

H7-n6

Tight assemblyfit

Used when the degree ofclearance that can resultfrom a H7-k6 fit is notacceptable .

=f80 MM DIAH7-n6 Commutator

shell onshaft

H7-p6

Press fit

Ferrous parts are not over-strained during assembly ordi smantl ing

.

200 MM DIAH7-p6

Split journal

§H bearing

H7-S6

Heavy pressfit

Mainly used for permanentassembl ies

.

100 MM DIAH7-S6 Cylinder

liner inblock

FIG. 9.8

EXERCISE 9.1

State the upper and lower devia-tions, the maximum and minimum limitsof size, the tolerance and the funda-mental deviation for the dimensionsshown in Figure 9.4. Answers are atthe end of the Chapter.

Notice that a dimension may betoleranced in three distinct ways.

Selection of fitsThis is a task which requires con-

siderable design experience. However,there are certain applications wherethe choice of fit is very limited andthese, together with an introduction tothe International Organization for Stan-dardization (ISO) system of limits andfits, are treated in this Chapter.

British standard4500:1969

The ISO system has been acceptedin Britain, the U.S.A., Canada and manyEuropean countries. The system is in-corporated in B.S.4500 of 1969 which iswholly metric and replaces the earlierB.S.1916. At first sight, the vastrange of tolerances and deviations canbe rather bewildering; in fact, thesystem is very straightforward as thereare only three principal variables:

(1) the basic size of the two matingcomponents

,

(2) the grade of the tolerance (thesmaller the tolerance, the lowerthe grade ) and

(3) the fundamental deviation.

The scope of B.S.4500 is very wideand only a small part of the range offits would be used for one particularassembly.

The range of basic size extends upto 3 150 mm but, for most applications,a smaller range to 500 mm is sufficient.There are 18 grades of tolerance, des-ignated by the symbols IT01, ITO, IT1,etc., up to IT16. The lower grades areused only for tools and gauges; the

upper grades are general tolerances fornon-mating parts. In Fig. 9.1, the gradeof the tolerance on the pin diameter isIT6 and, for the hole in the fork end,it is IT7. There are 28 symbols forfundamental deviations, both for shaftsand holes. An upper-case (capital)letter code is used for the holes and alower case letter code is used for theshafts. Figure 9.5 illustrates therange of fundamental deviations; thetolerance is represented by the shadedrectangles.

A hole is designated by (using thefork end of Fig. 9.1 as an example) 10 mmdia H7; i.e. the basic size is 10 mm,the fundamental deviation is H, and thetolerance grade is IT7 . The mating pin(shaft) is 10 mm dia h6. The fit bet-ween two mating parts is designated

10 mm dia H7/h6, or 10 mm dia H7-h6.

Figure 9.6 shows the relationshipsbetween fundamental deviations, toler-ances, clearances and interferences.There are three ways of producing anyparticular clearance or interference;taking a clearance as the example, theparts could be:

(1) Hole larger than basic size; shaftsmaller than basic size.

(2) Hole larger than basic size; max-imum limit of shaft equal to basic'size. This is termed a unilateralshaft basis system.

(3) Minimum limit of hole equal tobasic size; shaft smaller thanbasic size. This is termed a uni-lateral hole basis system.

The first system is not practicalbecause of the lack of uniformity. Thesecond system is particularly usefulwhen stock bar material is used for theshaft, or if many components are moun-ted on a common shaft. The third sys-tem is recommended for most other app-lications as it is usually convenientto make a standard size of hole (with adrill or a reamer) and then produce theshaft to suit it. It may be seen, fromFig. 9. 5, that holes suitable "for a uni-lateral hole basis system have theletter code H.

95

Fits for generalengineering products

B.S.4500 suggests that a selection-of only ten fits, with a unilateralhole basis, will prove suitable for thegreat majority of applications. Thesefits are illustrated and the completelist of deviations are given in Figure9.7. The range of basic sizes is up to500 mm, as anything in excess of thisis too large to be considered a generalengineering product and special problemsarise with thermal expansion.

The industrial user who standard-izes on only the ten fits in Fig. 9.

7

(many users need far less) enjoys aconsiderable economic and engineeringadvantage. For any particular basicsize, the number of tools and gaugesrequired is reasonably small but, ifone of the above fits is not suitable,it is very probable that another combin-ation (without the need of additionalgauges) will be acceptable. For exam-ple, a H8-f7 fit could sometimes givetoo much clearance but the H7-g6 fitwould be too expensive; from the samestock of gauges, a H7-f7 fit might bejust right.

EXERCISE 9.2

Write out toleranced dimensionsfor Holes and Shafts with the followingfits:

100 mm dia H7-h6;

20 mm H7-k6;

50 mm dia H9-e9;

150 mm H7-s6.

Typical applicationsof hole basis fits

The ten selected ISO fits arebriefly described in Figure 9.8. Thenames given to them are only intendedto give some indication of the natureof the fit. The illustrated examplesare typical applications but the choiceof fit is not automatic. For example,the valve mechanism link pin is .shownwith a H7-g6 fit with the bush whereas

a suitable fit with the fork end couldbe H7-h6. Unless the assembly is re-designed, the pin cannot change from g6to h6 deviations along its length. Thedesigner would have to decide whetherto accept relative movement between thepin and the fork because of the use ofH7-g6 or to eliminate it by using h6for the pin and a positive fundamentaldeviation for tne hole in the bush.

Remember that Fig. 9. 8 is only aguide to selection; there are no hard-and-fast rules

.

EXERCISE 9.3

Figure 9.9 shows part of a crank-shaft. Which dimensions should be in-dividually toleranced? Select a poss-ible fit for the 50 mm dia shaft endand draw a new, suitably toleranceddimension on the figure.

Other manufacturingtolerances

Apart from errors of size, thereare two further classes of error likelyto arise in any manufacturing process

.

A simple example is a hole in a pieceof sheet metal; not only can the sizeof the hole be incorrect, but it is alsopossible for its centre to be in thewrong place (an error of position) andthe hole to be non-circular (an errorof form). B.S.308 defines a geometric-al tolerance as the maximum permissibleoverall variation of form or positionabout that shown on the drawing* Thereare recommended methods of specifyingstraightness , flatness, parallelism,squareness, angularity, concentricity,symmetry, and position.

It is not always necessary to spec-ify geometrical tolerances as, for ex-ample, when the designer is familiarwith the quality of components manufac-tured in his employer ' s machine shop

.

When there is a lack of direct contactbetween the design and manufacturingpersonnel (sub-contractors, manufactureoverseas, etc.) it is worth consideringspecific geometrical tolerances.


Shaft Hole Hole Shaft

Upper deviation -0.030 +0-046 +0-025 +0-033 +0-023

Lower deviation -0-060 +0-017 +0-009

Maximum limit of size 59*970 60-046 36-025 36-033 70-023

Minimum limit of size 59-940 60 - 000 36-000 36-017 70-009

Tolerance 0-030 0-046 0-025 0-016 0-014

Fundamental deviation -0-030 +0-017 +0-009

96



UNSPECIFIED RADII 5

FIG 9.9

22-225DO NOT SCALE THIS DRAWING

UNLESS OTHERWISE STATED, DIMENSIONS ARE IN MM

23-5 25-5

THIS DRAWING IS THE PROPERTY OF MAGPETE ENG. CO LTD

13 AMP FUSED PLUG

18-25

SCALE S/S Torn p.cItrp. d,v,q.|chd, r.c. |app. d.l. |date. 15.7. 70

MATERIAL

SEE PRODN. LAYOUT

. op*

> t/)OJ» yry_

Ulu.

GA 1962

FIG. 10.1

DO NOT SCALE THIS DRAWING

_|UI

11

t5 *

« P) 7a o a

U Li

THIS DRAWING OR DESIGN IS THE PROPERTY OF MAGPETE ENG.CO LTD

ASS. OF TERMINAL PLATESCALE SfS IDRN.E.W, ITRD, J.D. ICHD, AC IAPP. M.J.C. [DATE. 22^71

MATERIAL

SEE PRODN,LAYOUT

AS 33

FIG. 10.2

DO NOT SCALE THIS DRAWINGALL DIMENSIONS UNLESSOTHERWISE STATED TO BEWITHIN +0-12

-0-12

3DIA

2-8 4-3 DIA ROD

— 0-1

7-1

30002-925 DIA

45°

UNLESS OTHERWISE STATED, DIMENSIONS ARE IN MMTHIS DRAWING OR DESIGN IS THE PROPERTY OFMAGPETE ENG. CO LTD

RIVETSCALE axl |DRN H.P.M, |TRD. D.V.O, |CHD. P.C. |APP. M.J.C-PATE 4. 4, 71

MATERIALBRASS RODB76FINISH

SEE PRODN. LAYOUT

1)654

FIG. 10.3

ciKwra w

Engineering Drawingsin Industry

Most of the drawings shown inearlier chapters illustrate particularpoints about engineering drawing: theycould not be used in an industrial sit-uation because they lack certain essen-tial features . It has been stated al-ready that the function of an engineer-ing drawing is to facilitate the manu-facture of a component, or the correctassembly of a set of components; thereis one other important function, namelyto illustrate. These three functionsare treated in this chapter.

General arrangementdrawings

Figure 10.1 is a General Arrange-ment Drawing of a 13 amp plug: a famil-iar object that serves to illustratethe differences between each type ofdrawing. Only certain important dimen-sions and the major components appearin this drawing; its function is toillustrate the general appearance andbasic geometry of the object. A gener-al arrangement drawing does not necess-arily show every component; Fig.lO.lgives little indication of, for example,the method by which the cable is to beclamped

.

Assembly andsub-assembly drawings

Figure 10.2 shows a sub-assemblyof a terminal plate for the 13 ampplug . All the four parts that make upthe sub-assembly are shown and each isgiven an item number (in a circle, fromwhich a leader line indicates the appro-priate part and terminates in a dot)and a part number (which often refersto a detail drawing) adjacent to theitem number. Some companies have aprinted box on their drawing sheets inwhich the separate items are listed,together with their part numbers andthe quantity required for one of theassemblies shown. Other companies pre-fer to list the items on a separate"Production Layout" or "Bill of Material".

An assembly drawing of the com-plete plug would have to contain atleast one sectional view in order toshow all the items. On the assemblydrawing of a typical domestic plugthere are seventeen items : two of themare sub-assemblies. One sub-assemblyhas four items and the other has two,so even such a simple object containstwenty-one components and requirestwenty-four engineering drawings formanufacture and assembly. This willgive the student some indication of thenumber of drawings that would be re-quired for an internal combustion eng-ine or a machine tool. Much of thecost of an engineering product consistsof design and drawing costs; the act-ual material costs are often very low.

Most drawings are modified in thecourse of time and the changes arelisted on the drawing. The box on theright-hand side of Fig. 10. 2 has beenincluded for this purpose and many com-panies have some similar system.

Detail drawingsFigure 10.3 is a detail drawing of

Item No. 3 from Fig. 10. 2. Its func-tion is to provide the appropriate de-partment with precise, complete inform-ation for the manufacture of a singlecomponent. The drawing number (D654)

has been referred to in Fig/10. 2. Bydefinition, any detail drawing contains:

1. All necessary views of the objectin an identifiable type of ortho-graphic projection.

2. All necessary dimensions.3. Tolerances.4. An indication of the material from

which the component is to be made.

Depending on the policy of theorganization using the drawing it mayalso contain:

1. Manufacturing instructions.2. Surface finish instructions.3. Special treatment instructions.

99


8 SCREW 6-7857 27 PIN 04 233 712 26 KEEP PLATE 03 001744 1

5 SIDE PLATE 'B' 03 001743 1

4 SIDE PLATE 'A' 03 001742 1

3 NUT 6-2139 1

2 SUSPENSION BOLT 04 233711 1

1 MAIN CASTING 09 211136 1

ITEMNO DESCRIPTION PART

NOQTYREQD

FIG. 10.4


SCALE 1 1

A

04 DRILL ON ASSEMBLYWITH PART #2 TO MARKCENTRE

I12. 16 1208 /-v

.HOLE 03DRILL WITH

J, Jfi PARTS#3&4Jfl-M-J- CLAMPED TOGETHER

y„utr

HOLE 03-C'BORE06x6 DEEP

1 IN 15

04 DRILL AFTERASSEMBLY WITHPART#1

?l i. 26. QCO

I

!

V"—>K*/'

o j

R 12,7s R24V«

Js

76

FIG. 10.5

Conventions inassembly drawings

In Chapter 3 (Sections) it wasstated that, as well as ribs, there area number of other common parts that arenot shown in section when the sectionplane passes longitudinally throughthem. Figure 10.4 shows three views ofan assembly in which a section planepasses longitudinally through two pinsand two screws; these fasteners arenot shown in section as their internalshape is of no interest to the readerof the drawing, indeed it may be con-fusing. The other common parts treatedin this way are bolts, nuts, rods,rivets, keys, shims and washers.

Fig. 10. 4 also shows the conven-tional method of using section liningto distinguish between adjacent parts;the direction and the spacing of thelining is varied . The spacing shouldtake account of the size of the sec-tioned part; the large main castinghas lining with spaces of about twicethe width of the small keep plate.

EXERCISE 10.1

Figure 10.5 details eight parts ofa heavy duty joint. Part No. 3 fitsinto the 8 mm diameter hole in part No.6; part No. 4 follows it and is wedgedinto place by part No . 7 . Part No.

5

slides over part No. 3 and part No. 4 andthen part No. 2 passes through all ofthem. Part No.l fits on the end ofpart No. 2 and is held in place by partNo. 8.

Draw three views of the completeassembly; one view is to be a sectioncorresponding to section X-X of part No.6 . The other views should be selectedto show as much detail as possible.Ignore the intersection lines thatappear on parts 5 and 6; they can beindicated by thin straight lines in theform of a V.

Label and list the parts as hasbeen done in Fig. 10. 4. Show only ess-ential hidden detail. Do not add anydimensions. Insert all necessary titlesand labels

.

All the parts shown in an assemblyor a sub-assembly drawing require eithera detail drawing, a designation thatidentifies a. stocked item, or some in-formation about the source of supply.The parts that are most commonly stockedare fasteners with screw threads suchas nuts, bolts and screws.

ISO metric threads were brieflyintroduced in Chapter 5 . They are des-ignated as shown in Figure 10.6. B.S.3643 specifies the thread form and twoseries of diameter-pitch combinations.The two series, one with coarse and theother with fine pitches, are the sameas those scheduled in ISO RecommendationR 262. A selection from the full rangeis shown in Figure 10.7; the "FirstChoice" of basic major diameters (themaximum material diameter of a bolt)are those that would normally be stocked,whilst only larger organizations wouldstock the full range of "Second Choice"diameters. B.S. 3643 also has a list of"Third Choice" diameters.

The various types of fasteners arespecified in the following BritishStandards:

B.S. 4190 ISO metric black hexagon bolts,screws and nuts.

B.S. 3692 ISO metric precision hexagonbolts, screws and nuts.

B.S. 418 3 Metric machine screws andmachine screw nuts.

The essential difference between a boltand a screw is that only the latter isthreaded right up to the head.

All ISO metric hexagon fastenersare chamfered at 30° to the top of thehead and this produces the character-istic shape shown in stage (7) of Figure10.8 which is rather awkward to draw

.

The other views in Fig. 10. 8 show thesteps in the construction. If the hex-agon shape appears in one view, it maybe constructed as shown in stages (1)

and (2) . A view showing the distanceacross the corners is begun in stage(3) and completed, with the aid of anarc template, in stage (4). HeightAB = CD = EF = GH. A view showing thedistance across the flats is begun instage (5) by transferring the height ABfrom stage (3) and completed, againwith the aid of an arc template, instage (6) . A bolt with a full bearinghead is shown in stage (7) ; it has beenconstructed by using only the coeffic-ients for d that are shown in the vari-ous stages.

Actual sizes of ISO metric nuts,bolts and screws are given in Figure10.9.

EXERCISE 10.2

Make a general arrangement drawingfrom the assembly drawing shown in Fig.10.4. Choose suitable dimensions forsizes that are not shown. Add onlymajor dimensions to the general arr-angement drawing.

101

—THREAD SYSTEM SYMBOL- ISO METRIC-,—NOMINAL DIAMETER IN MILLIMETRESr-PITCH IN MILLIMETRESTHREAD TOLERANCE CLASS SYMBOL-

M12X1-25-6H

g3ee§

FIG, 10.6

"M 10 x 1-5- 5 g

The thread tolerance classsymbol (B.S.4500) is onlyincluded when required.The absence of an indicationof pi tch imp] ies that acoarse thread is specified.

FIG. 10.7

ISO Metric screwthreads

.

Selected Coarseand Find seriesfor screws,bolts and nuts.

Dimensionsmi 1 1 imetres

.

in

\/sk/

,90°

Basic majordiameters

First Secondchoice choice

2-5

3

BASIC FORMOF ISO METRICTHREAD

AXIS OF THREAD

Coarseseries

0-35

0-35

0-4

0-45

0-45

0-5

0-6

0-7

0-75

0-8

1

1

1-25

1-5

1'75

2

2

2-5

2-5

2-5

3

3

3-5

3-5

4

4

Fineseries

Surface texture

.

Any material surface appears roughwhen examined under a microscope . Someassemblies will only function correctlyif the degree of roughness does not ex-ceed certain limits; these must bespecified on the detail drawing. Figure10.10 illustrates the recommendationsof B.S.308 with regard to symbols forthe control of surface finish.

The roughness numbers shown in Fig.10.10 are explained in B.S.1134.

EXERCISE 10.3

Figure 10.11 shows two views of acam; surface textures have been indic-ated but the notes are not clear.There are other errors and omissionsin this drawing.

Redraw the two views and insertall dimensions, tolerances and surfacetexture symbols in accordance with therecommendations of B.S.308. (Refer toFigs. 5.10, 5.11, 5.12, 9.4 and 10.10.)

the tolerances that are indicated.Make a new detail drawing of the objectin a form that would be suitable forissue to a machine shop, i.e. all thedata required for machining and inspec-tion should be present on the drawing.

Modifications todetail drawings

Figs. 10.2 and 10.3 show how modif-ications are recorded on both assemblyand detail drawings . Modifications maybe made for many reasons ; the compon-ent is not strong enough, not rigidenough, is difficult to assemble, wearsaway too quickly, etc. All too oftenthe task of modifying a detail drawingis given to a junior employee with vagueverbal instructions about what is re-quired. If the component gives anyfurther trouble, the diagnostic processis often hampered by poor records ofthe modification process. Ideally, therevision should be recorded as fully aspossible both on the drawing and in theoffice records and memoranda.

EXERCISE 10.4

Make detail drawings of items 1 to7 from Fig. 10. 4. Choose suitable dim-ensions for those not shown. Matingsizes should be toleranced after selec-ting suitable fits from Fig. 9. 8. Indic-ate the surfaces that are to be mach-ined but do not show figures for sur-face texture

.

EXERCISE 10.5

Figure 10.12 shows two views of adie casting; some dimensions are mis-sing. After casting, some machining isdone to the surfaces that are indicatedand certain dimensions are to be given

EXERCISE 10.6

Figure 10.13 is a pictorial sketchof a boss for a centrifuge which shearedapart after about 100 hours of running".The component is to be strengthened bythe incorporation of six ribs arrangedradially on the centre lines of the sixholes . The ribs may extend as far asthe hole bosses in the radial directionand to within 3 mm of the surface A, inthe axial direction. Make a detaildrawing of the modified component. Listthe revision in a suitable revisiontable, with a succinct description ofthe changes made. Write a brief note,in the form of a memorandum, describingthe modification in more detail.

103

NOMINAL THREADDIAMETER d

WIDTH ACROSS FLATS.OR 1-5d

1-155XWIDTH ACROSSFLATS.OR 1-75 d

THICKNESS OFNUT, OR 0-8 d

HEIGHT OF BOLTHEAD, ORO-65d

PROJECT FROMHEXAGON OR d hzzcch]

30° CHAMFERNOT SHOWN

ABOUTd10

ARC TEMPLATEOF SELECTEDRADIUS

FIG. 10.8

lih T


<t> 50 FINISH TO 1-6/fm

GRIND TO0-4 yUm

POLISH TOOBTAIN FINISHOF O-OSyttm

MACHINE ONLY WHERE STATED


FIG. 10.11

ISO Metric hexagonal nuts,bolts and screws

Dimensions irl millimetres

Thread Basic width Hei ght Thi ckness Designation

di a across flats of head of nut

H

Q

^

DOUBLE

WASHER

CHAMFERED

FACED

(see

Fig. 10.

7

for

pi tches)

PS

PWASHERFACED

DJ:

FULLBEARING

-MKr- -JmN-

Normal Smal 1

3 5-5 2 2-4 M3

4 7 2-8 3-2 M4

5 8 3-5 4 M5

6 10 4 5 M6

7 11 5 5-5 M7

8 13 12 5-5 6-5 M8

10 17 14 7 8 M10

12 19 17 8 10 Ml 2

14 22 19 9 11 M14

16 24 22 10 13 M16

18 27 24 12 15 Ml 8

20 30 27 13 16 M20

22 32 30 14 18 M22

24 36 32 15 19 M24

27 41 36 17 22 M27

30 46 41 18 24 M30

33 50 46 21 26 M33

36 55 50 23 29 M36

39 60 55 25 31 M39

FIG. 10.9

Machining and surface texture symbols

I'lachining symbol

Used to indicate that a surface is to bemachined, without defining either thesurface texture grade or the process tobe used.

The symbol is applied normal to thesurface or a projection line from thesurface .

General instructions

Used when most, or all surfaces are tobe machined.

. ALL OVERV EXCEPTAS STATED

Surface texture symbol

Used to indicate (in micrometres) themaximum acceptable roughness of thesurfaces that are to be machined.

0-8V

Particular process requirement symbol

Used when no other manufacturing processis to be used. °'°V

LAP

Optional process symbol

Used when a particular surface texture isrequired but machining is not essential.

6-4,

Prohibition of machining symbol

Used when a particular surface texture isrequired and the surface must not bemachined .

3-2, DO NOTv MACHINE

FIG. 10.10

5 HOLES <t> 8EQUI-SPACED

135 PCD



1*

UNSPECIFIED RADII 3

FIG. 10.12SECTION A-A

6-HOLES <peEQUI-SPACED ON64 P.C.D.

ZINC ALLOYDIE -CASTING

FIG. 10.13


oGO

R

i. ..

20

25 »

40


30'

100

FIG. 11.1

FIG. 11.2

ALL DIMENSIONS ARE IN MM020

Sketching

At the design and development stageof any engineering product, a great dealof communication takes place by means ofsketches: both freehand and made withthe aid of a straight edge. The sket-ches may be of views in orthographicprojection, or pictorial drawings madeby one of the methods treated in Chapter8. Graph paper and, for example, iso-metric ruled paper are very useful aidsto sketching but they tend to make thesketch less clear and are not alwaysreadily available . Students are advisedto practice sketching on plain paper andto use an H or HB pencil with a longconical point. Engineering sketches donot require artistic talent; they dorequire a methodical approach and quitea lot of practice.

When sketching vertical lines theforearm and elbow should rest on thedrawing surface; for horizontal linesthe most comfortable action is a slidingaction of the forearm, lightly sketchingsegments about 50 mm long at a time.Final lines are strengthened by applyingmore pressure. Most people find thathorizontal lines are the easiest to drawso, whenever possible, the sketchingsheet should be rotated to convert anysloping lines, and especially one down-wards and to the right, into a horizon-tal line. Orthographic views should belined in by first drawing the rectanglesthat will fit around the views. ' Oncetwo or more rectangles are in projec-tion, it is not so difficult to maintainproportions and alignment.

EXERCISE 11 .1

Sketch the two views shown in Fig-ure 11.1. Make the views about halffull size: it is more important to main-tain the correct proportions than tohave a precise scale

.

Sketching circlesCircles may be sketched using the

steps shown in Figure 11.2. Step 1 isto sketch a faint square, of side equalto the diameter of the circle. Step 2

is to draw in the bisectors and thediagonals of the square. Step 3 is tolocate points on the diagonals at thecorrect radius from the centre : thiscan be done by eye or by using a scrappiece of paper if a particularly neatcircle is required. Step 4 is to sketcha thin arc through about three adjacentpoints. Step 5 is progressively torotate the paper to a comfortable pos-ition and to continue the arc. Step 6

is to strengthen the arc carefully tothe required thickness of line and toerase any unwanted construction lines.

EXERCISE 11 .2

Sketch views of the object shownin Figure 11.2, as seen in the direc-tions of arrows X and Z . Use any suit-able scale but try to keep to the pro-portions of the figure.

109

THIRD ANGLE PROJECTION ALL DIMENSIONS,Ji, ARE IN MM

SCALE 1 /2

2 / /

FIG. 11.4

oto

40

110

30



FIG. 11.5

FIG. 11.6

FIG. 11.8

FIG. 11.7

THIRD ANGLE PROJECTION ALL DIMENSIONSARE IN MM

2 HOLES TAP M12 X1-75-22 DEEP

5, 28,

18,

|

R2'

—ihHr~

S2j !

KR2

i

i

i

18

FIG. 11.9

Pictorial drawings- oblique sketches

A sketch of this type shows thefront face and surfaces parallel to itin their true shape (see Chapter 8) andis particularly suitable for objectshaving circular or other curved featuresin these planes. The general methodfor sketching pictorial drawings isfirst to sketch a rectangular block ofmaterial of length, breadth and heightequal to those of the object. The det-ails can then be located on the appro-priate planes and the block is, ineffect, cut away until the desired res-ult is achieved.

Figure 11.4 shows a detail drawingof a component and the various stagesin the production of a "Cavalier" ob-lique sketch.

EXERCISE 11 .3

Make an oblique sketch of the cone-clutch component shown in Figure U.S.The hexagonal hole should be sketchedby first drawing a circle of diameterequal to the size across flats; theside of the hexagon is then constructedequal to the radius of the circle.

Pictorial drawings-isometric sketches

An isometric sketch begins withthe three isometric axes, one sketchedvertically and the others at 60° fromit. As with oblique sketches, thefirst stage is to sketch a rectangularblock of material of the appropriateproportions. Thereafter, any detail is

inserted by locating the required iso-metric plane and placing the particularlines, polygon or curve in this plane.Figure 11.6 shows the stages in thesketching of an isometric drawing of a

slotted regular hexagonal prism.Circles in isometric planes are

sketched by the method shown in Figure11.7. Step 1 is to enclose the requiredcircle with an isometric square (a

rhombus) with sides approximately equalto the diameter of the circle . Step 2

is to bisect the isometric axes inorder to locate the major and minoraxes of the ellipse. Step 3 is to drawa curve, symmetrical about the minoraxis and tangential to the isometricsquare. Step 4 is to mirror this curveabout the major axis. Step 5 is todraw a third curve, symmetrical aboutthe major axis and tangential to theisometric square. Step 6 is to mirrorthe third curve about the minor axis

.

In step 7, the unwanted constructionlines are removed and the ellipse isboldly sketched in

.

Figure 11.8 shows the stages inthe production of an isometric sketchof the same object for which an obliquesketch was made in Fig. 11.

4

EXERCISE 11 .4

Make an isometric sketch of thecone-clutch component shown in Fig. 11. 5.

Refer to Fig. 11. 6 for the constructionof the hexagon.

EXERCISE 11 .5

Sketch, in first angle projection,views of the component shown in Figure11.9, as seen in the direction of arrowX and on section plane A-A.

113


RlO

FIG. 11.10

4 HOLES 010(CENTRES ATCORNERS OFSQUARE OFSIDE 40)


FILLET RADII 4

#20 STRAIGHT THRO'

SURFACE S

FIG. 11.11

Hinge, for Solar Pane.

I

On Communication Jatcllite.

Jolar lanel-

Housing forLocking pin

and dampingSystem.

Satellitem/att

Longeron.

FIG. 11.12

EXERCISE 11.6

Make an oblique sketch of the sec-tioned component shown in Figure 11.10.

EXERCISE 11.7

Sketch a new isometric drawing ofthe bracket shown in Figure 11.11 so asto reveal the underneath surface S.

The sketching of ideasThe most useful function of sket-

ching' is the communication of an ideafor the solution of a particular prob-lem. Many students are reluctant tomake sketches because they have greatdifficulty with the double task ofworking out the idea and putting it onpaper in a reasonably presentable form.This is a skill that will only be ac-quired after plenty of practice butthose who already possess this skillare too often unwilling to admit thatthey ever make intermediate sketches

.

Figure 11.12 shows the sort of superbsketch often displayed with a mixtureof pride and surprise that the studentcannot achieve the same standard. Manypeople tend to develop the idea in aseries of sketches (which are laterdestroyed) like those shown in Figure11.13. Students are advised to makeintermediate sketches and to be mostsceptical of anyone who implies thatthey are unnecessary.

Intermediate sketches also helpthe designer to clarify the problem inhis own mind. Very few people can plan,visualize and then sketch a solution toa problem without first producing an in-ferior sketch which triggers a new ideaand enables a better sketch of an imp-roved design to be made.

EXERCISE 11 .8

Figure 11.14 shows a 13 amp plug(fused for 5 amps) with the cover re-moved. The normal method of retainingthe cable in such a plug is illustrated:a fibre strip is cramped down on to thecable by means of two self-tappingscrews. The function of the cable-retaining device is to prevent thetransmission of tension in the cableto the electrical connections.

Devise and sketch an improved sys-tem of cable retention.

EXERCISE 11 .9

Figure 11.15 shows two views of amagnetic tape spool. A storage rack isrequired for twelve such spools: eachspool is to have its own compartment

.

When the spool, wound with tape, is in-serted into the compartment the end ofthe tape is to be gently held so thatit cannot come unwound when the rack isbeing transported. The tape will bewound to a radius of between 55 and 62mm.

Make sketches of one compartmentand its tape-holding system.

115

tSolctr foments -for Commu-nic cLtions Sctte.CCit*

Spring cybanat&d, I>anybed /.oafcingr &/ng<z

r //7ner- Potnet

•SateUitevatts

Compression

Spring

Satellite

fZ/fower

Damjb'ng/nediv/n

fldjustino srretv

cum could beincorpora tednith locking

mechanism

Jp&ntnosbrino could

be. made, part of/necJiar>is/r>

Sorter

Mumin/'um -

honeftomb (food/efeatab/lity

Crul/>ed to S*e- STMprovide. re/><"-t T^/b/WtO)damf/ng~

Combine. Springpin and datnib/ng

cy/inder/n one housing

Mount housingon &ate(iita

longe-ron

lonoe

FIG. 11.13

LIVEEARTH

FUSE

NEUTRAL

FIBRE STRIP

CABLE

FIG. 11.14


:445'

3 SLOTSEQUI-SPACED7 RC.D.

4>18

X1 DEEP

R1-5

ENLARGED VIEWOF HOLE SCALE 2/1


HOLE *6ON18RC.R.

A / 3 SPOKESEQUI-SPACED

SECTION A-A

FIG. 11.15

€MAG» 1

Introduction to Synthesis

The earlier chapters of this bookare concerned with the engineering draw-ings and sketches that, apart fromwords, are an engineering designer'smain method of communication. In thefollowing chapters a simple design pro-ject will be examined from its begin-nings to the detail drawing stage.

Design is a difficult word to de-fine and means different things to diff-erent people. For these reasons, theterm "synthesis" has been introduced toemphasize that the process to be exam-ined is the very opposite of analysis.Synthesis will be taken to mean thebuilding up of a concept of a piece ofhardware which will economically fulfila well-defined function. Synthesis isa truly creative activity in which some-thing that meets certain requirementsis gradually evolved. It is this creat-ivity that gives so much satisfactionto the successful designer.

It is possible to identify variousphases of a design project but theycannot be clearly defined and, in prac-tice, they overlap and interact to agreat extent. However, it is conven-ient to treat these phases in separatechapters, drawing attention to theareas where they are related. Thephases are:

1. Discovering the true nature of theproblem posed.

2

.

Determining whether the task isone that the organization will beable to carry out.

3

.

Examining the factors that willinfluence the design.

4. Finding possible solutions and sel-ecting the most suitable one

.

5. Preparing an overall design anddividing up the work that it willentail

.

6. Making decisions on items of in-creasing detail, in consultationwith the other people affected.

7. Directing the manufacture of therequired quantity.

In the following chapters these phasesare illustrated by looking at the des-ign of an automatic toothbrush: a sur-prising example, at first sight. How-ever, it has four advantages which, inthe author's view, outweigh any objec-tions that it is not a serious engin-eering project.

1. Virtually everyone is familiarwith the operation that the deviceis to perform.

2. It is simple enough to treat fullyin the available space and yetsufficiently complex to illustratemany important points

.

3. As the parts are normally lightlyloaded, the student does not re-quire a knowledge of anything morethan elementary dynamics and stressanalysis and the project is suit-able for the early stages of acourse.

4. A teaching institution could pur-chase some example (s) of such aproduct and generate added inter-est in the project.

The exercises at the ends of the follow-ing chapters form a continuing develop-ment of the project. The dialoguepassages may be read or recorded forplayback to students.

118

CHAraa m

Problem Definition

The first phase in any design pro-ject is the process of discovering thetrue nature of the problem posed. Pro-blems originate either inside or out-side the organization which is to under-take the design: they come from a"customer" or from another departmentwithin the organization. For conven-ience, the originator of the designproject will be referred to as the"customer", regardless of their rela-tionship to the organization that isto carry out the design.

One of the major factors influen-cing the way in which a design problemis tackled is the way in which the pro-blem is formulated. Many wrong decis-ions have been made and a considerableamount of time has been wasted by apoorly defined or downright misleadingstatement of the problem. A classicexample is the "gas cylinder" story.The oxygen used in a small factory wasdelivered in liquid form in the usualheavy steel cylinders . Due to a reorg-anization, the process for which theoxygen was required was transferredfrom the ground floor to the secondfloor of the building. At first thecylinders were raised, by an improvisedlifting tackle, up the outside of thebuilding and into a second floor window.Following an accident, it was foundthat the cost of installing a. properlift was prohibitive and it was decidedto design a "walking-upstairs" carriagefor the cylinders.

Fortunately, at this stage someoneasked the question: "Is it liquid orgaseous oxygen that we require on the

second floor?" The answer was "Gaseousoxygen", and to design and install suit-able valves and piping was a cheap andstraightforward exercise.

In the above' example, the error inthe original definition of the problemis fairly obvious and was easily uncov-ered; unfortunately this is not alwaysthe case the the incorrect definitiongoes unchallenged. It is extremelyimportant to start a design with thetrue needs of the situation. established.No amount of ingenuity or technicalskill can remedy the situation if thedesign starts off on the wrong footing.

The situation is often complicatedby the fact that the ultimate user ofthe design either does not know hisexact requirements or has difficulty incommunicating them to the designer.For example, the average motorist canusually describe his requirements inonly a negative fashion - the featuresof a particular vehicle that he doesnot like. His communication with thedesigners is through the indirect tech-niques of market research and the uncer-tainties of the mass media. Admittedly,this example is rather special but thesituation is not necessarily betterwhen the design is tailored for a part-icular customer. A company that sellsinternal telephone systems has frequen-tly found that its customers have avery poor idea of which of their employ-ees need to be able to contact others

.

Therefore, the first step in thedesign process is to ascertain, so faras is economically possible, the trueneeds of the particular situation.

PHASE 1 Discovering the True Nature of the Problem Posed

The following discussion illustra-tes how one organization might tacklephase 1 of the automatic toothbrushproject.

Make notes of the important pointsthat emerge from the meeting. Then,read through the comments printed initalics.

Compare your notes with those pro-duced by the "Director of New Projects"(.Figure 12.1) .

(To aid identification, the peoplepresent at meetings concerned with thisproject use each other's first name;the attendance lists are always arrangedin alphabetical order .

)

119

*Z)*ZU«**ils s£r £onAU>K«. - S*j(^' '72.

FIG. 13.1

Notes made by the 'Director of new projects'

Scene: An engineering company's conference room

Present : AndrewBrianCharlesDonEricFred

Director of new projectsMarket researchIndustrial designMechanical engineeringElectrical engineeringProduction engineering

Notice which specialists have been in-vited to the meeting . As will emerge,even at such an early stage they allhave something to contribute.

ANDREW. Now you're all here, let'smake a start. The purpose of thismeeting is to discuss an enquiry we'vehave from Halworths about an automatictoothbrush. I think they'll ask us totender and it would be a very usefulorder if we got it. Brian and I wentto see their Marketing Director to findout what they've got in mind so I'llask him to give you the details - Brian?

Andrew deliberately refers to the pro-ject as "an automatic toothbrush" . Theadjective "electric" would channeleveryone's thoughts towards one partic-ular solution which the customer has notspecifically requested. Halworths maybe content with any satisfactory devicethat replaces the manual brushing action.

BRIAN. Just a few words about Halworthsfirst: as you know they have retailbranches in most large towns and theysell most things for the home , the mainexceptions being furniture and the moreexpensive electrical goods. They sella lot of things with their own brandname "Magpete". Their profits havebeen a bit static of .late and so theywant to expand the cosmetic and toiletpreparations side of their operations.

It is important that the designer hasnot only general but also special, up-to-date information about the customer

.

FRED. Hence the toothbrush?

BRIAN. Right. Now their MarketingDirector is a chap called Cabot - he'snew and he's very energetic. He's hada survey done and he reckons that theautomatic brushes on sale at the momentare too expensive and very poorly adver-tised.

CHARLES. What does he call "expensive"?

BRIAN. The cheapest is about threepounds but you can pay over twelvepounds

.

ERIC. Is that for one with a batteryrecharging unit?

BRIAN. That's right. There are aboutten models on the market - they all run

off batteries but two of them have therechargeable type

.

ANDREW. That's really why I invitedyou Eric - there wouldn't be much elec-trical engineering involved in anyother type

.

ERIC. Well, if your Mr. Cabot wants tosell at a really low price, I think youcan forget rechargeable batteries: thenickel-cadmium cells alone would pushthe cost well up.

ANDREW . No , I want us to keep openminds at this stage. I'm not even con-vinced that the brush should be batterypowered; but we'll get to that later.

There is always a tendency to startthinking about possible solutions tothe problem. To a certain extent thisis necessary but it can tend to limitcreative thought which is why Andrewhas decided to discourage it.

BRIAN. Cabot reckons, and I've checkedon this, that all the research done bydental health experts leads to the con-clusion that automatic toothbrushesare no better for your teeth and gumsthan ordinary brushes, so he wouldn'tbe able to plug the improved healthangle in his advertising. However,kids love 'em, they're fun to use, andthey certainly don't harm teeth. Sowhat Cabot wants is something mainlyfor the child under twelve, at a lowprice, but better than a toy so thatthe Mums and Dads will use it too.

CHARLES. Thank goodness for that 1 Ishan't have to start designing MickeyMouse battery holders.

ANDREW. I thought Dougal and Zebedeewere the current favourites! Pleasecan we postpone a decision on the powersupply though. Carry on Brian.

BRIAN. Well, I got my staff to look atsales of automatic toothbrushes and I'mnow inclined to agree with Cabot. Whenyou look at British homes, even thosewith comparatively modest incomes,they've often got televisions, transis-tor radios, fridges and even electricrazors, but less than one per cent haveautomatic toothbrushes. People eitheraren't aware that they exist, or thinkthey're very expensive, or can't seeany particular advantage in buying one.

121

A significant number of homes wherethere is an electric toothbrush got themas Christmas presents.

It is risky to believe everything thatthe customer says; some of it may bepure speculation and so Brian has usedhis resources to make a check. Thereis, of course, an economic limit to howfar this may be done .

DON. How does that help us?

BRIAN. The point is that, at the moment,most people don't actively want to ownan automatic toothbrush, but I thinkthat Halworths could interest childrenin them and their parents would buythem if the price is low enough andthey can use it too.

CHARLES. So we'd have to produce some-thing that appeals to children, but notexclusively so, and with interchangeablebrushes so the whole family can have ago?

BRIAN. Exactly, and the selling pricemust be considerably less than threepounds

.

FRED. Plastic mouldings - a metal caseis right out. What quantity wouldCabot want?

ANDREW. He hedged, but I think weshould think in terms of about fiftythousand a year.

FRED. I see - that's fair enough. Now,that's a production rate of about twohundred a day - quite a lot of our cap-acity. How soon does he want to startselling them?

A design of which only one will be manu-factured poses a very different problemfrom one which will be made in largequantities . Fred's question is farfrom selfish.

ANDREW. I got the impression that,other things being equal, the orderwill go to the firm that can let himhave a reasonable supply to start testmarketing next Autumn - about ninemonths, in other words.

Similarly , although there is clearly aminimum time required to produce any-thing at all to solve the problem, thelonger the time available the greaterthe number of alternative designs thatcan be considered.

BRIAN. I told you he was energetic.

DON. Looks as if we shall have to betoo. How badly do we need this contract,Andrew?

ANDREW. Well, we only want it if theprofit margin is right. Cabot wantssomething really cheap, but we want ournormal profit on the deal - if .not

,

then there are plenty of other new pro-jects in the pipeline. Any other ques-tions? No? Now, I'd like -us to meethere again at the same time next week.Don, Eric and Fred, will you let mehave your comments on the project then.I'd also like ideas from you Charlesabout how we make it appeal to children.Meeting closed.

notice that the nature of the problemposed depends on the organization under-taking the design as well as on thecustomer . This company can easilyafford to lose the order; to a companythat could not the problem would bevery different

.

The attendance at this and subse-quent meetings is not intended to ill-ustrate good or even typical design pro-ject organization. A particular "char-acter" is present if it would be poss-ible (not necessarily desirable) forhim to work on the project at thatstage

.

122

a annas* w*

Problem Acceptability

The second phase of a design pro-ject is a process of determining whetherthe task is one that the organizationconcerned will be able to carry out.This will depend upon the answers toseveral questions, for example:

What will we be expected toproduce?

Is the project commercially attrac-tive to the company?

Are there any special advantagesto be gained by undertaking this project?

Do we have or can we acquire thenecessary technical expertise for thedesign?

Do we have or can we acquire thenecessary equipment for the developmentand manufacture?

Do we have or can we acquire thenecessary skilled manufacturing personnel?

Do we have sufficient time inwhich to produce what is required fromus?

Some of these questions will bevery difficult to answer until somesort of solution to the problem hasbeen examined. Unfortunately, the pro-cess of finding any solution will costmoney and there is a limit to what canbe spent. If the project is very largethe customer often agrees to pay forthe preliminary work involved: a designor feasibility, study. With smallerprojects the initial cost may have tobe borne by the company, and managerialskill is needed in order to decide whatshould be spent so as to have a chanceof winning a profitable order.

At this stage the engineering de-partments of the organization can givevaluable advice to the management, pro-vided the latter supplies sufficient,up-to-date information. Often, thecustomer's ideas and requirements aregradually evolved during this periodand the technological staff need toknow the relevant changes that haveoccurred.

PHASE 2 Determining whether the task is one that the organization will be able tocarry out

This exercise follows on from thePhase 1 exercise; the same organiza-tion has now reached Phase 2 of theautomatic toothbrush project. TheDirector of New Projects has arrangeda meeting.

Make notes of the important pointsthat emerge from the discussion.

Read through the comments thatappear in italic

.

Compare your notes with those pro-duced by the Director of New Projects(Figure 14.1) .


Present : AndrewBrianCharlesDonEricFred

Director of new projectsMarket researchIndustrial designMechanical engineeringElectrical engineeringProduction engineering

This meeting was arranged at the end of the Phase 1 meeting.

ANDREW. The first thing I want to sayis that we have received an invitationto tender for the Halworth's AutomaticToothbrush. I don't like abbreviationsas a rule but I can ' t keep saying Auto-matic Toothbrush, so I suggest we referto this project as the ATB. Now

Halworths want us to quote for a testmarketing batch of ten thousand: thecost of basic tooling will have to berecovered with that number. They alsowant figures for possible future produc-tion at the rate of fifty thousand andone hundred thousand a year. Some of

123

£/•) /^-t&^&g Ss^rr~*4fy, t^*****^

^-ST*<3» .*c

.

*S) £.& 0CH? 6***£H^ ^^£S(.

FIG. 14.1

Notes made by the 'Director of new projects'

that would be for associated retailersin Germany and Holland.

This new -information is part of the"problem definition" rather than the"problem acceptability" phase. Thedata for a design project never arrivein a neat, tidy package: the customer '

s

requirements tend to alter and otherinformation has to be laboriouslyunearthed.

FRED. Your Mr. Cabot thinks big.

ANDREW. Brian thinks the market isthere too.

BRIAN. Yes - I've been having a lookat some surveys made in the U.S.A. andCanada. There seems to be a kind ofsnowballing of demand after about fiveyears of serious sales promotion

.

CHARLES. Isn't it serious at the mom-ent?

BRIAN. Hardly. When and where did youlast see one advertised?

CHARLES. There was a display packagein a Chemists I noticed last Saturday -

but then I was looking for one.

BRIAN. Exactly. Even then you have togo to the larger Chemists and depart-mental stores

.

ANDREW. Don, can I have your commentson feasibility?

DON. Well, trying to keep an open mindabout the source of power, let's assumethat we have to convert rotary motionto oscillating motion at the actualbrush.

BRIAN. All the models now on the markethave that. The bristles oscillate in avertical plane - when the thing's beingused normally.

DON. Well, that shouldn't be too diff-icult to achieve . The power requiredis very low. The motion doesn't haveto be precise . The other mechanicalaspects are sealing the mechanism ag-ainst water, saliva, etc., and providingsufficient lubrication for the life ofthe ATB . I'm assuming that the consumercan't be expected to oil the thing - oreven to clean it, however infrequently?

BRIAN. I agree: the sort of customerthat Cabot has in mind will either takeit to pieces or almost completely neg-lect it.

DON. In any event, the sealing is sim-plified if no extra lubricant is re-quired. Now the other mechanical pro-blem will be the casing, and the great-est loads are almost certainly going tobe due to dropping the thing on thebathroom floor. I think we could pro-vide something strong enough to toleratea fall of one metre on to say, a coveredwooden floor; but if someone knocksthe thing off a shelf that's about threemetres above a stone or concrete floor -

well, that'll be the end of it.

Don is speculating about the greatestloads. He has not mentioned a fall intoa hard bathroom basin and no one raisesthis point. Children do not stand verymuch above such a basin and so Don'sidea of the greatest load could be thelogical one.

BRIAN. I think that's perfectly accept-able.

Don has succeededthe problems thatbe solved regardlthe source of powtant part of Phas.uneconomic to proproblem and thenis acceptable to

The people avisualize the proenergy flow chart

in idenwill press of aer. Thie 2; itduae soldeci de i

the compt th%s mduct in

tifying one ofobably have tony decision ons is an impor-is usually

utions to the

f the p-roblemany .

eeting maythe form of an

ANDREW. Don, what about the actualbrushes?

DON. I'm going to assume that we willsubcontract the brush heads. The spin-dle from the ATB will have to have asquare end, or something of the sort,and the brush suppliers will have tohold the square hole at the end of thebrush to certain limits, so there's aslight push fit.

acquireenergy

regulatesupply

convertinto

mechanicalwork

transmitwork tobrush-head

Examples

Battery Switch

Head of water Tap

Spring Brake

Motor

Turbine

Motor

Gearing

Linkage

Chain

FIG. 14.2

125

This is a very important part of Phase2: the company needs to recognize theparts of the project that it should nottackle. A plastic moulding containingnylon bristles is clearly a task for aspecialist company

.

FRED. That's easy enough with the sortof plastic mouldings that they use.

ANDREW. Right. What's your verdictthen, Don?

DON. Go ahead. It's certainly somethingwe can tackle with our present resources- except for the brushes.

ANDREW. Fine. I'll make a note tohave a word with Halworths about that.Eric, can I have your comments?

ERIC. If the ATB is to be electricallypowered and sell at the sort of pricethat Cabot has in mind then we can for-get both mains operation and recharge-able batteries. For one reason, mainsinsulation and motors are too costlyand so are nickel-cadmium batteries.For another reason, a more importantone in my view, would you buy somethinglike a child's toy which is pluggedinto the mains and the other end goesinto the mouth?

Eric has had to come out strongly ag-ainst certain possible solutions beforehe can advise on the acceptability ofthe problem. Unlike don, the electricalengineer' s tasks (see flow chart, page

) are greatly affected by the natureof the solution and he is endeavouringto narrow the field.

ANDREW. Excellent point Eric: technic-ally it could be quite safe but psycho-logically, it's all wrong. What haveyou got against rechargeable batteriesthough, apart from cost?

ERIC. We've just agreed that the ATBis going to suffer considerable neglect.My information is that you need to re-charge at least every two days . Thetwo things are incompatible.

ANDREW. Fair enough. Now, what sortof areas could that leave for electricalengineering?

ERIC. The only viable electrical powersource is a subminiature motor drivenby a consumable zinc-carbon battery.Now there is absolutely no point inmaking the motor ourselves - we couldbuy one from Japan or Switzerland at afraction of the cost of making it here.

When Eric ' s arguments against certainsolutions are accepted, he is able toanalyse the problems that remain inmuch the same way that Bon did. Again,there is a recognition of the work that

126

the company should not tackle.

ANDREW. Do you agree, Fred?

To Andrew, the possibility of his owncompany making the motor is still alive,it is the sort of work that he feelscould be acceptable and so he takes asecond opinion

.

FRED. Certainly - the Japanese haveswamped the market with these littlemotors - they retail at about fifteennew pence, some of them.

ERIC. That would just leave us withthe battery connections , leads andswitch. The only minor difficultywould be sealing the switch againstwater and dribble. This is certainlya job we can cope with.

ANDREW. Thank you. Fred, any problemson the production side?

FRED. Nothing too serious. I'm alittle unhappy about the size of thefirst batch. If we ever get up to ahundred thousand a year , we could aff-ord a lot of automation but if every-thing came to a halt after the firstten thousand it would mean a lot ofeffort and disruption for a very smallreturn.

Fred's attitude to the problem is, nec-essarily, completely different . Hedoes not need to concern himself withsolutions at this stage but he is inter-ested in the production processes andhence needs to know if the quantity canbe changed.

ANDREW. Would it be better if we off-ered Halworths an attractive option ona second batch of ten thousand?

FRED. From my point of view, certainly.With that sort of number it's a goodproposition, but it's feasible evenwith ten thousand.

ANDREW. Thank you. Now Charlesyou make it child-orientated?

can

CHARLES. A qualified "yes", I think.It's going to need quite a lot of workon this aspect. At this stage I'd beinclined to go for colours and texturesthat appeal to children - but not onlyto children. I think the idea of aspecial shape in the form of an animalor say, a space rocket, is out: itwouldn't be taken seriously by mostadults

.

Charles , too, is not greatly affectedby particular solutions to the engin-eering problems. He has given somethought to the aesthetic qualities ofall solutions.

BRIAN. I'd agree about the animal butI'm not so sure about the rocket,Charles; that might have possibilities.

CHARLES. Obviously, Market Researchand my department will have to cooper-ate very closely on this point. As soonas Don's people can let me have someidea of sizes, we'll produce somethingfor Brian to use in his surveys

.

Hotioe the natural desire for more factsabout the problem as it affects onedepartment. Andrew discourages thisline of though as it can lead to pre-mature decisions

.

DON. Can't you measure some existingproducts?

ANDREW. Wait please! We're moving ontoo fast. Charles, is this a projectthat we are capable of carrying out?

CHARLES. Certainly.

ANDREW. All right. Now, don't takethis personally Charles: is it econom-ically viable for us to do the indus-trial design or should we hand it overto someone with more experience of des-igning for children?

Again the problem arises: shall we doit ourselves or hire a specialist?This time a design rather than a manu-facturing process is involved, but thequestion is just as important and needstactful handling

.

CHARLES. If we do that, I think wecould end up with something that isvery attractive to children but won'tsell because adults would regard it asan expensive toy.

BRIAN . That ' s a good point , Andrew

.

ANDREW. Yes, I accept that. So, itlooks as if we go ahead with a compet-itive tender, bearing in mind the pointsthat have emerged from this meeting

.

Anything we haven't dealt with?

CHARLES . The toothbrush I saw lastSaturday was stored in quite an elabor-ate stand which also took the brushheads. Is a stand to be part of ourdesign?

Notice how "problem definition" is stilloccurring , even at this point in time.

BRIAN. No. It'll keep the cost down.All Halworths want from us is the ATBand four brush heads. They will organ-ize the packaging.

ANDREW. Right. Now our next meetingwill be to discuss the requirements indetail. That's the last meeting thatBrian and I will attend. I think mostof you know George, one of the AssistantChief Designers: he'll be coordinatingthe project from then on and Harry willbe the design engineer in charge. I'lllet them have notes of our meetings todate and then arrange another meetingfor all eight of us that'll be - assoon as possible.

The project is now gathering mom-entum and more people are to be invol-ved. They will have to be briefed aboutwhat has happened so far, and short,accurate notes are still one of the bestways of doing this.

127

mm ^

Design Factors

Once a decision to proceed furtherwith the project has been taken, a thirdphase is in progress: a detailed exam-ination of all the factors that will in-fluence the design. Many companiesadopt a systematic approach to this ex-amination; some go so far as to usespecially prepared forms which becomepart of the records of the project , andto award points to each factor accordingto its relative importance. Otherorganizations feel that they can managejust as well with less paper work, andtheir examination is an unstructuredprocedure

.

There is no ideal method; whatsuits the particular company is rightfor that company. However, with theincreasing complexity of human societyand the products that it requires,

there has been a trend towards a moresystematic approach not only to designfactors but also to the whole designprocess.

One useful method of examining thefactors that will influence the designis to use a check-list containing key-words that can be expected to triggerappropriate questions. The answers tosuch questions are noted down and, ifdesired, the factor can be rated accor-ding to the influence it will have onthe design.

The answers to the questions be-low are translated into "requirements"during this phase of the design project.Students are advised to answer the act-ual questions when using the check-listfor the first time; later, the factorsmay be expressed as "requirements".

CHECK-LIST OF DESIGN FACTORS

KEYWORDS

1 Primary functions

2 Mechanical loads

3 Ergonomics

4 Safety

5 Physics

6 Chemistry

7 Geometry

8 Maintenance

9 Overhauls and Repairs

10 Life and Reliability

11 Running costs

128

TYPICAL QUESTION

What is the product supposed to do?

To what loads will tne product besubjected?

What type of person will use theproduct?

How will the product affect the personusing it?

What is the physical nature of the en-vironments to which the product will beexposed?

What is the chemical nature of the en-vironments to which the product will beexposed?

Into what spaces will the product go?

What routine tasks will the user beprepared to perform in order to ensurethat the product continues to function?

How will the user react to a seriousdeterioration in the performance of theproduct?

For how long will the user expect theproduct to perform satisfactorily?

How little will be user expect to payfor operating the product?

12 Effects on environment

13 Appearance

14 Laws, Regulations, Conditions andStandards

15 Quantity

16 Cost

17 Delivery date

What should the product not do to theenvironment where it is operated?

To what extent is the appearance of theproduct important?

Has society implemented any decisionsthat will in any way affect this product?

How many are to be produced?

What are we prepared to spend in orderto produce the above quantity?

When is the product required?

The difference between a "designfactor" and a "requirement" may be ill-ustrated by considering the design of afiling cabinet. The question is posed:what type of person will use the pro-duct? If the answer is: "female cler-ical staff", then this is a design fac-tor which may be taken into account byframing a requirement such as: "theeffort needed to open and close a fully-loaded drawer should not exceed thecapabilities of an average employee ofthis kind".

It is usually possible to dividethe requirements into three main groups:

1. Vital: such requirements must becompletely satisfied.

2. Important: such requirements mustbe satisfied but some concessionsmay be made

.

3. Worthy of attention: such require-ments may be satisfied if someadvantage is gained thereby.

To continue the filing cabinet example,it could be said to be: vital to con-tain papers and documents, important tomake it difficult to break open, worthyof attention that the eyes of the aver-age user are 1*5 metres above the baseof her footwear.

In the next chapter, the use thatcan be made of these groupings is exam-ined. It is often difficult for two ormore people to agree upon the import-ance that shall be attached to a certainrequirement: they interpret the wordingof the requirement in their own way.For this reason it is suggested that,although it is useful to have severalminds working on the identification of

the design factors, only one personshould have the responsibility of con-verting the design factors into require-ments and deciding on their importance

.

The final stage of Phase 3 of thedesign process is that of using therequirements to write a "specification"of the product.' A design specificationis the means of communicating the re-quirements to the other people who areto be involved. The specification isusually far more technical and precisethan the listing of design factors. Thespecification writer will decide upontarget figures whenever he can, so thatit will be possible to measure how wellthe product fulfils the specification.Considering the filing cabinet exampleonce more, it is a requirement that theeffort needed to open and close a fullyloaded drawer shall not exceed the cap-abilities of the average female clericalemployee. This could appear in thespecification as: "it shall be possibleto completely open or close a drawercontaining 15 kg of material by exertinga horizontal force on the handle not inexcess of 30 N".

The writing of a specification isa nice mixture of art and science and adetailed treatment of the subject isoutside the scope of this book. A veryuseful booklet entitled Guide to thePreparation of Speai.f-iaati.ons, PD 6112,is published by the British StandardsInstitution.

Students are advised to try theirhand at the writing of a specificationby working through the check-list ofdesign factors in the order shown. Thespecification can then be tidied up byidentifying related requirements andgrouping them together

.

Phase 3 Examining the factors that will

This exercise follows on from thePhase 2 exercise; the same organiza-tion has now reached Phase 3 of theautomatic toothbrush project.

1. Referring back to the Phase 1 andPhase 2 exercises, write down the des-ign factors by working through theseventeen keywords and questions shownin the check-list.

The Director of New Projects has a

influence the design

meeting to examine the design factors

.

Compare your completed check-list withthe points that emerge from the discus-sion and make any necessary alterationsor additions.

2. Work through the check list againand, in the light of what you know aboutthe organization working on this pro-ject, list the "requirements" that thedesign factors will impose and state

129

their importance

.

From this list of requirements pre-pare a specification for the automatictoothbrush in a form that could be circ-ulated to those present at the Phase 3

meeting. Try to quantify as many ofthe requirements as possible.

Compare your specification withthat produced by Harry, the designengineer.


Presenti Andrew - Director of new projectsBrian - Market researchCharles - Industrial designDon - Mechanical engineeringEric - Electrical engineeringFred - Production engineeringGeorge - Assistant Chief DesignerHarry - Design engineer

This meeting was arranged since the Phase 2 meeting.

ANDREW. There are just two things Iwant to report and then I'll hand themeeting over to George. First, Halworthshave changed their minds about the in-itial batch; they definitely want usto quote for twenty thousand. Second,the foodmixer order from Ideal PresentClub that we were hoping to get hasgone elsewhere, so we need this contractslightly more than I suggested at ourfirst meeting: we may have some extracapacity to use on it. George, themeeting is yours.

GEORGE. I think you all know the sys-tem that I like to use on these occas-ions. Harry will read through thetitles on our check-list and also readwhat he ' s been able to get down alreadyfrom Andrew's notes. I'd like you tochip in when there is something to beadded. Harry?

HARRY. Thank you. One other thing,perhaps you would also tell me whetherthe factor is vital, or important ormerely worthy of attention. First ofall "Function": that's vital, by def-inition. I've got two things down:"Brush teeth" and "Accept other brushes".-Second, "Mechanical loading": I

couldn ' t put anything down for thepower and the forces between the bris-tles and someone's teeth at the moment- they will have to be determined. Howimportant is it that the brush does notbreak if it ' s dropped?

BRIAN

.

Worthy of attention, I'd say.

DON. I suggested a fall of one metreon to a covered wooden floor

.

HARRY. Yes, I've got that. Third,"Ergonomics". "Will often be operatedby children under twelve . The moredifficult tasks such as changing brushesand, if battery driven, replacing batt-eries, will be done by adults with awide range of dexterity". What do wethink about making it difficult for achild to switch on?

ERIC. If you're thinking of a stiffswitch for adult use only, we triedthis once on a power tool and it's verydifficult. Some great strapping eight-year-olds could switch it on and someunfortified over-forties couldn't budgethe same switch.

CHARLES . I'd say make the switch quiteeasy to operate; after all, there isn'tmuch harm that the brush can do.

GEORGE. It's rather messy - if you'rein the line of fire.

BRIAN

.

You've got one?

GEORGE. Rather. They're all rightonce you've trained the grand-childrennot to switch on until it's in theirmouth

.

HARRY. O.K. I'll incorporate thesepoints in the specification. Fourth,"Safety". "Casing, switch and possiblycertain internal areas will be touchedby user ' s hands . Moving brush headwill go into user's mouth." Fifth,"Physical environmental conditions"."Will be used in bathrooms, washroomset cetera".

BRIAN. Kitchens too, I should think;the sort of price that Cabot has inmind will interest people who haven'tgot bathrooms

.

HARRY. Physically, there isn't muchdifference. They're all rooms withplenty of humidity and. quite a range oftemperature. Sixth, "Chemical environ-mental conditions". I've got: "Waterin liquid and vapour form. Air - alltypes from fresh to heavily polluted".

FRED. You get all sorts of medicinesand cosmetics around a bathroom.

GEORGE. We can't cater for everything.I think water is the only importantone

.

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HARRY. Right. Seventh, "Geometry ofenvironment". I think we can ignorethat one.

ANDREW. No. I think the possibilitythat people might want to store them ina bathroom cabinet is worthy of atten-tion.

HARRY. Yes, I'd overlooked that.Eighth, "Maintenance". "User will notbe expected to do any". Is that "vital"or "important"?

DON. Just a moment. If we use a batt-ery, then surely changing the batteryis "maintenance"?

HARRY. Right. I'll make that: "Userwill not be expected to do anythingmore than ensure continuation of powersupply".

BRIAN. I think it's rather academicwhether you call that "vital" or "imp-ortant". The switch could easily seizeup if muck isn't removed for five yearsor so . We can ' t ensure that that won '

t

happen.

HARRY. Point taken. I'll deal withthat in the specification. Ninth,"Overhauls and repairs". What doHalworths think about this?

ANDREW. Frankly, at the sort of pricewe're talking about, any major repairis going to cost nearly as much as a

new ATB. If we get the order, we wouldhave a special clause in it about rep-lacing worn or defective units . Thiscompany is not interested in doing rep-airs - it'll be cheaper to give dissat-isfied customers a completely new unit.

HARRY. What about spare brush-heads?

ANDREW. Ah, that's something I forgotto mention. I spoke to Cabot about thefact that we would almost certainly sub-contract the brush-heads. He suggestedthat we leave all that part of it tothem; then they can deal direct withthe brush manufacturers and get somepacked up as spares.

GEORGE. Good! We can treat the pro-jecting part of the spindle as an inter-face where our responsibility ends.

HARRY. Tenth, "Life and Reliability".We need not concern ourselves withbrush-head life now. I've put down afigure of two years ' continual use by afamily of four. Is that too modest?

BRIAN . I think that ' s something to beestablished during the development ofthe product. If we have to replace toomany when they are designed for twoyears' life, it ought to be possible to

improve the life with only minor modif-ications.

DON. The wearing parts will be thebearings of course, parts of the mechan-ism, seals and the switch I suppose.If we buy a sub-miniature motor fromJapan , we won ' t be able to increase thelife of its bearings.

ERIC. There would be wear at thebrushes and commutator as well but wecould probably get the Japanese to pro-duce something to our specification -

if we buy in the sort of quantitiesthat have been suggested.

GEORGE. Use your figures as a firststab then Harry, and add a note aboutdevelopment

.

HARRY. O.K. Eleventh, "Running costs".Can anyone help here?

BRIAN. I've got some figures here onthe use of batteries in some of theelectric brushes that are on the marketat the moment. Most of them cost abouta hundred and fifty new pence a year torun.

ANDREW. How much would it cost eachtime the batteries are replaced?

BRIAN. Depends on the batteries; sayten new pence, on average.

ANDREW. So it would be an excellentselling point if we could cut this down,or even eliminate it altogether?

BRIAN . Certainly

.

HARRY. All right, I've got that.Twelfth, "Effects on environment". I'velisted "noise" and "interference withradio and T.V." as possible things totake into account.

ERIC. The latter is unlikely to betroublesome. The sort of motors we'vebeen talking about only take a fewwatts - less than ten at a guess.

DON. The noise problem is closely tiedup with running cost, of course. Anoisy toothbrush is an inefficienttoothbrush.

HARRY . Thank you - that ' s a point I '

d

overlooked. Thirteenth, "Appearance".Charles, I've got down "Design mustappeal to children under twelve butmust not give the impression that it isjust a toy".

CHARLES. You might add that this com-pany will want to be concerned with theshape and colour of the brush-heads aswell as the hole for the spindle . Wedon't want an attractive design that isspoilt as soon as a brush is fixed on.

131

HARRY. Yes, got that. Fourteenth,"Laws, Regulations, Conditions and Stan-dards". I can't think of any laws,regulations or conditions that aregoing to affect the design - unless it'smains operated or recharged and thatisn't feasible.

FRED. What about paint on the casing?With children you'd have to watch thelead content

.

CHARLES. Most unlikely that we'd wantto use any.

GEORGE. It's a good point though. Ithink you had better add, under "Safety"that the casing could get bitten.

HARRY. Right. There are British Stan-dards for batteries - it's mainly thedimensions that would be of interest tous. Fifteenth, "Quantity". "initialbatch of twenty thousand. Possible fut-ure production at the rate of up to ahundred thousand a year". Sixteenth,"Cost". Andrew?

ANDREW. I'd like to see our unit costscoming out at about fifty new pence

.

That would mean that we ' d have to pro-duce the initial batch for ten thousandpounds. Now that is dependent upon theway in which the overheads are shared;if we buy the power unit there will benext to no overheads on that. As I saidearlier, this order is more importantto the company than appeared a fewweeks ago so I might be able to persuademy masters to favour it.

HARRY. I'll put down fifty pence forthe time being. Seventeenth, "DeliveryDate". What can we say about that?

GEORGE. The critical factors are likelyto be the tooling up time and trainingthe workers on the assembly line. Fred?

FRED. That's true, but if we can havea few prototypes by April, let's say,there will be enough spare capacity tostart production in the Summer.

HARRY. That's a bit vague, Fred.Could Halworths have anything at all bynext September?

FRED. Certainly - if we get prototypesthat are quite like the productionmodel by April.

GEORGE. Is it possible, Don?

DON. I can't answer that now; it dep-ends too much on the power unit. Ifit's a subminiature electric motor andwe only have to produce a simple mechan-ism and a case, then I'd say it wasdefinitely on. If we used - what shallI say - clockwork, for example - thenit's going to take a lot longer.

ERIC. I'd be inclined to agree. Wecould certainly produce a simple switchand the other connections in the timesuggested, but anything more sophistic-ated is out of the question.

CHARLES. The same is true of the gener-al appearance. I'd be very worried ifclockwork were used and we had to incor-porate a winder and so on.

ANDREW. Let me make the position quiteclear. We won't get this order ifHalworths can't have something by nextAutumn. Therefore the design must takethat into account, otherwise there isno point in our quoting. Nevertheless,I'm still not convinced that a battery-driven unit is the only way: I don'tlike the word but it would be an advan-tage if we had a gimmick. Can you putthat point in the specification?

I won't use thatHARRY. All right.word though!

ANDREW . Good '.

GEORGE. Is there anything else, gentle-men? Good - now when you get your copyof Harry's specification perhaps youcould let me have your comments quickly.I want to have a "Speculation session"to sort out possible solutions in abouttwo weeks' time.

132

Written by:Issued by :

SPECIFICATION

AUTOMATIC TOOTHBRUSH

H. A. MartinG. B. Bromley (Assistant Chief Designer)

Contents

21/12/71

1

.

Foreword2

.

Scope3

.

Definitions4. Related documents5

.

Conditions6. Characteristics7

.

Performance8. Life and Reliability9

.

Information and After-sales service

1. ForewordThis specification has been written inorder to set out the requirements ofHalworths Ltd. for a cheap, simple auto-matic toothbrush that will appeal mainlyto children. This document is to beused for the preparation of a tender toHalworths but is not to be a part ofany future agreement with them.

ScopeHalworths wish to retail a package con-taining a driving unit and four brush-heads. This specification appliesmainly to the driving unit; the brush-heads will be manufactured by a sub-contractor of Halworth's choice. Weare to be responsible for the method bywhich the brush-heads are fixed to thedriving unit and for ensuring that thedesign of the brush-heads is consistentwith that of the driving unit.

3. Definitions

3 . 1 TERMINOLOGY

"adult" - anyone over the age of 12years

"child" - anyone whose age is from4-12 years

"mouth" includes lips, teeth, gums andtongue

"shall" - present tense of verb meaning"is to, unless a change isagreed with the person issu-ing this specification".

3 . 2 ABBREVIATIONS

ATB = Automatic toothbrush (drivingunit + brush-head)

DU = Driving unit

3.3 MEASURING SYSTEM

Metric units are used throughout.Weights, i.e. forces due to gravity al-one, are expressed in kilogrammes (kg)

;

all other forces are expressed in new-tons (N) .

4 . Related documentsThis specification makes reference tothe following documents:

4.1 Drawing of Halworth's "Magpete"trade mark (Filed as D301 292)

4.2 British Standard 397

5. Conditions5.1 The DU will be operated in airunder the following conditions:Atmospheric pressure 99 - 105 kN/m 2

(990 - 1050 millibars)Temperature - 40°CRelative humidity 20 - 100%Domestic dust and grit in suspension.

5.2 The DU will be splashed with alltypes of water/saliva/toothpaste mix-tures at temperatures from - 100°C.

5.3 The ATB will be stored under theconditions described in 5.1 and willsometimes be in a bathroom cabinet ormedicine cupboard.

5.4 The ATB will be used by childrenand adults . The source of power willbe one that could reasonably be madeavailable in homes where the total in-come is about the national average.

5 .

5

The only routine maintenance thatwill be done is to ensure the continua-tion of the power supply. It will becarried out by adults.

6. Characteristics6.1 The brush-head shall be mounted insuch a way that it is convenient forall users to apply it to the mouth.

6.2 The tooth-cleaning action shall,within the limits specified in 7.1,give the best results when using nylonbristles of between 9 mm and 11 mm inlength.

6.3 Whatever part of the ATB is heldin the hand shall have a circumferencenot in excess of 120 mm at a convenientgripping position, and a weight not inexcess of 200 g. It shall be convenient

133

for both children and adults to grip andto operate any controls.

6.4 All parts of the ATB that can comeinto contact with hands, body, face ormouth shall have no sharp edges, con-tain no toxic materials, and be liableto cause no other type of injury to theuser.

6.5 The brush-head shall fit firmly onto the DU but shall be easy to change(see 7.6) .

6.6 The ATB shall appeal to childrenbut shall not give the impression thatit is a toy. The brush-heads and DUshall be visually integrated.

6 .

7

Halworth ' s trade-mark , "Magpete

"

(see 4.1) shall appear boldly on thedriving unit together with simple oper-ating symbols and maintenance instruc-tions in small, legible characters.Necessary additional instructions shallappear on any internal areas of the DUto which the user is expected to haveaccess

.

6.8 The DU shall be simple and cheapto produce. (The company may limit coststo £10 000 for an initial batch of20 000.) The ATB shall, if possible,have some unusual feature (s) that willtend to increase retail sales.

7. Performance7.1 When the ATB is operated normally,each bristle in the brush-head shalloscillate in a vertical plane and thetotal vertical movement of the tip ofan undeformed bristle shall be not lessthan 6 mm nor more than 8 mm: the locusneed not be a straight line. When thebristles are not actually brushing, thefrequency of the oscillations shall bebetween 20 and 30 Hz . At such frequen-cies the power available for brushingshall be at least 2 W and shall not ex-ceed 6 W. The brush-head shall stopwhen a torque of not more than 30 mN-mis applied while the rest of the DU isrigidly held.

7.2 90% of production driving unitsshall survive a fall of 1 m on to acovered wooden floor.

7.3 The running costs of the ATB (inaddition to new brush-heads) shall notexceed £1.50 per year at 1971 prices.

7.4 The free-running DU shall not inter-fere with a radio or television situatedin an adjacent room.

7.5 The noise produced by a free-runningDU shall be acceptable, in both volumeand quality, to a panel of users to beagreed with Halworths and/or their rep-resentatives in this matter.

7.6 The force required to fit or removethe brush-head shall not exceed 10 N inthe appropriate direction.

8. Life and Reliability8.1 90% of production driving unitsshall achieve a useful life of two yearswhen used for eight 5-minute periodseach day.

8.2 The DU shall be easily modified soas to improve on the figures given in8.1.

9. Information and After-sales service9.1 Halworths shall be supplied withinstructions for operating and maintain-ing the ATB, in a form that could easilybe incorporated in a leaflet for theretail package.

9.2 The instructions referred to in 9.1shall also include clauses dealing withthe non-availability of a repair serviceand the procedure to be adopted forcomplaints

.

9.3 Any items that the user will re-quire to maintain the ATB shall, when-ever possible, be referred to by thedesignation recommended in the appro-priate British Standard (e.g. batteries,see 4.2) .

134

AEftflaa

Problem Solving

The fourth' phase of the design pro-cess is that of finding possible solu-tions and selecting the most suitableone. The requirements that were dis-covered during Phase 3 may be classedas "vital", "important" or "worthy ofattention". A "possible solution" tothe design problem is any solution thatcould satisfy all the vital requirements:no other solutions are worthy of furtherconsideration. In practice, it is un-usual for such solutions to be proposed;the term "possible solution" has beenintroduced as a more convenient way ofdefining what is a "vital" requirement.To take an extreme example , a hydrogenballoon is not a possible solution tothe problem of getting a man to themoon because, by being unable to reachthe escape velocity, it does not satisfyone of the vital requirements.

Therefore, at first, the search forpossible solutions is focussed on the"vital" requirements. When these havebeen satisfied, the designers can turntheir attention to the "important" re-quirements. To continue with the moonjourney example, an (ironic) importantrequirement is that the cost shall notbe astronomical. Such a requirementmight rule out the possibility of usingatomic power.

All "possible" solutions will sat-isfy all the important requirements butthe extent to which they do so willvary. By intuition, experience or somesystem of points , someone has to decideupon which of the possible solutions isthe most suitable. If that cannot be

done, it is then necessary to look atthose requirements that are worthy ofattention. The most suitable solutionis always a compromise between conflic-ting requirements and the choice israrely obvious.

The brain can perform the mostamazing tasks of keeping all the require-ments in store, assessing their import-ance and hence judging the solutionspresented to it - but it is a humanbrain; it is apt to be prejudiced oncertain questions. A systematic app-roach to the selection of the most suit-able solution can help to counteractthis prejudice.

Possible solutions are found by avariety of methods but it is hard tobetter the human brain as a store, ormore accurately, a factory for such sol-utions. An individual mind often per-forms less well by itself and otherminds can act as a stimulus which imp-roves its performance. For these rea-sons, the search for possible solutionsis often done by a team at a meetingwhereas the selection of the most suit-able solution is often better left toone individual

.

The group search for possible solu-tions will be referred to as "Specula-tion". It has often been found thatsuch meetings give the best results if:

1. All evaluation is banned,2

.

The people present aim at qualityof ideas, not quantity, and

3

.

Ideas which closely resemble aprevious idea are still noted down.

Phase 4 Finding Possible Solutions and Selecting the Most Suitable One

own knowledge and the information con-tained in the specification. Rejectonly those proposals that are obviouslyunsuitable; do not attempt to selectone "ideal" method.

The exercises follow on from thePhase 3 exercises; the same organization has now reached Phase 4 of theautomatic toothbrush project.

1. Speculate about possible methods ofpowering the automatic toothbrush by con-sidering the question: in the placeswhere the brush is to be used, whatsources of energy could be made avail-able?

Evaluate these methods using your

2. Consider the question: How couldthe power be made to produce the typeof motion described in the specification,clause 7.1? Speculate about possibleways of converting and transmitting theenergy that is available (see Fig. 14.1).

135

Evaluate these methods using your ownknowledge and the information containedin the specification. Reject onlythose proposals that are obviously un-suitable; do not attempt to select one"ideal" method.

3. The Assistant Chief Designer has ameeting to consider the problems posedabove . Compare your speculation andevaluation with the points that emergefrom the discussion. Read through thenotes in italic.

Scene: The office of an Assistant Chief Designer

Present: George - Assistant Chief DesignerHarry - Design EngineerIan - Designer (Mechanical)John - Designer (Electrical)Ken - Production EngineerLawrence - Industrial Designer

The new personnel were chosen and the meeting was arranged in the later stages ofPhase 3.

GEORGE. I think we've now dealt withall the comments that were receivedabout the draft specification and you'veall got copies of the first issue, dated21st December. Now, let's do our firstbit of speculating: I think that thefirst question we must ask ourselves isin the places where the brush isgoing to be used, what sources of ener-gy could be available? Speculationonly please, I don't want any evaluation.

The way in whieh the question is posedhas a considerable influence on theanswers given.

Essentially , there are only three meth-ods of acquiring energy:

(1) create or use a chemical reaction(2) create or use a nuclear reaction(3) create or use a force field

(gravity 3 magnetism or electro-statics ) .

Electricity may be produced by any ofthese methods . At the present time,the many methods of turning heat intomechanical work are relatively complic-ated and inefficient

.

JOHN. Mains electricity and batteries.

KEN. A head of water.

GEORGE. I don't think so - Halworthswould probably accept something thatused body energy, as long as the userdoesn't actually have to agitate thebrush-head directly.

LAWRENCE. Then I suggest storingenergy in a spring.

IAN. Do you mean a metal spring?

LAWRENCE. Yes.

IAN. I'll add a fluid spring, then.

KEN. It's a very low power that's re-quired. I wonder if there ' d be enoughheat from a hand to drive it?

JOHN. I very much doubt it: you needa body surface area of . . .

GEORGE. Sorry John, but that soundslike evaluation to me. Any more spec-ulation please?

The -temptation to evaluate is alwayspresent . Although the criticism may beentirely justified, it does tend todiscourage other speculation. Converse-ly , a chairman who prevents evaluationwill tend to encourage further specula-tion.

IAN. Some convenient sort of hydro-carbon fuel, like a butane gas cartidgefor a cigarette lighter.

HARRY. Talking of cartridges - one ofthose soda syphon things with carbondioxide in it.

LAWRENCE. Can I just clear up onepoint? It's supposed to be an automatictoothbrush so does that mean that theperson using it must not supply theenergy in any way whatsoever?

Clarification is allowed during thespeculation stage. It is really a partof the process of defining the problem.

LAWRENCE. As I'm safe from- criticismat the moment, I suggest solar energy.

HARRY. Great: We could call it "TheSunshine Brush".

GEORGE . Any other sources of energy?No? Right, let's start some evaluation- would you like to comment Harry?

HARRY. If you look at item 6.8 in thespecification - "simple and cheap toproduce" - I think we must reject anysource of energy that has to be conver-ted from heat into work; however youdo it, it either means a lot of movingparts or some very accurate manufactur-ing process, or both.

136

Engines that convert heat into work arerarely economic at power outputs belowabout 10 kW. There are, of course,special applications where no othersource of power is suitable

.

GEORGE. I agree, so I'll cross out"butane gas", "body heat" and "solarenergy" unless someone wants to defendthem. Right?

HARRY. Now, when the feasibility ofthis project was discussed, I gatherthat Eric more or less convinced peoplethat mains electricity was unsuitable:cost and psychological reasons. Wouldyou agree , John?

JOHN. Certainly; when I suggested itI was simply ariswering the questionthat George posed.

Generally , it is better to pose thequestion in a way that encourages spec-ulation than to discourage it by statingin advance that certain solutions areunacaeptab le

.

GEORGE. That's all right, at least wehaven't overlooked it. So that leavesus with "batteries", "head of water","C0 2 cartridge", "metal spring" and"fluid spring".

HARRY. In retrospect, I'm not at allkeen on the CO2 cartridge; there wouldbe some very large pressures and forcesinvolved: we'd find it hard to make itsafe - at any price.

IAN. Hear, hear. If we want some sortof turbine, I think water is a muchbetter bet

.

GEORGE. I've crossed out C0 2 cartridge.Now, I think it would be convenient ifwe refer to the four suggestions wehave left as "electric", "hydraulic"and "spring" methods. Lawrence, yourboss was rather worried about incorpor-ating a winder for any clockwork mech-anism - would you agree?

Once the number of possible sol'utionshas been reduced, it is useful to adopttitles that do not restrict thought tosome particular interpretation of theproposed solution.

LAWRENCE. Not entirely, no. I gatherhe was thinking in terms of somethingthat looks like a key, but it need notbe. It could be a cylindrical end-capthat fits flush with the rest of thecasing - quite easy to incorporate.

The person who is to be more concernedwith the details of the product is oftenin a better position to evaluate thevarious proposals

.

KEN. Forgive me Mr. Chairman but I

think it's plain stupid to even considerclockwork. For one person to use itfor five minutes, that's clause 8.1,the stored energy would have to be aboutone kilojoule. That'll need quite aspring which '11 take either a largeeffort or a long time to wind up, orcompress. We couldn't make it econom-ically ourselves but a suitable unitwould be quite expensive to buy. Then,looking at 8.1 again, we'd have greatdifficulty in keeping the water out andsufficient lubricant in for a two yearlife. What's more, looking at 7.2, I

doubt if anything like 90% would survivea fall of one metre.

Apart from timing devices and toys,clockwork motors are used successfullyin cine cameras , dry razors, etc. Theadvantage of a systematic approach toPhase 4 is that people are expected toexplain their re'asons for rejecting a

particular solution and the prejudiced"it won't work" attitude can be overcome.

GEORGE. Thank you for being so frank.I think you've make some good pointsbut they do apply mainly to traditionalclockwork motors. Perhaps, with a fluidspring we could avoid these problems?

HARRY. Maybe, but of course the onething that I'm not supposed to put in atechnical specification is that Hal-worths won't give us the order if theydon't think that we could deliver bynext Autumn. I think that a novel typeof spring motor , that we ' d have to dev-elop if we can't buy one, would putthem right off.

In any design project it takes time andmoney to obtain data. It is rarelypossible to completely evaluate all theproposed solutions ; the very fact thatmore data are needed is a real argumentagainst adopting such a solution. How-ever, there would be no technologicalinnovation if lack of data were theonly deciding factor for rejection; thefuture prosperity of the organizationhas to be considered. In other words,there are long-term dangers involved ina policy of always selecting the moststraightforward solution.

LAWRENCE . Speaking as a parent , I'dsay that something spring-operated thatappeals to children would look verymuch like a toy to me.

GEORGE. Let's "recap". In favour ofan energy storing spring: it's novel,wouldn't cost anything to run, and thecontrols could be blended into the gen-eral shape. Against: expensive andrelatively delicate, lots of movingparts to give sufficient mechanical ad-vantage for children to put the energy

137

in, lubrication and sealing problems,and it would look very much like a toywhen we'd made it - which could welltake longer than other drives. I'd sayreject it. Any strong objections?

An important part of Phase 4 is the sum-mary or "recap" procedure . Unless some-one is taking notes, it is very easy toforget some of the points that havebeen made. A summary of the argumentsfor and against will ensure that nothingis overlooked.

IAN. As one of the people who suggestedsprings, I agree - the disadvantages areoverwhelming

.

LAWRENCE

.

Seconded.

At first, it is possible to reject someof the proposed solutions because theylack some of the qualities required. Itis then necessary to look more closelyat the quantities detailed in the spec-ification. One of the most importantof these is the power involved. Unlikequantities such as speed, torque, de-flection under load, weight, etc., thatthe engineer can readily manipulate,once power has been supplied to a sys-tem, it cannot be increased. Inevitablypower is lost until some fraction of itis used for the required purpose. There-fore, the power available is of greatimportance in the selection of a sourceof energy. Students would do well tofamiliarize themselves with the meansby which such powers are estimated.

cold water tank at 2*5 metres above thecold tap: if we could get a flow of 9litres in 30 seconds - I'm thinkingabout when I fill my watering can, al-though that ' s from a feeble kitchen tap- that's 0-3 kilogrammes of water persecond. The potential energy of onekilogramme will be 2-5 times 9-81 joules,say twenty five, and 0-3 kilogrammes persecond would give us 7-5 watts. It'stoo good to be true.

The water pressure is another fixedquantity, associated with the availablepower, that is bound to have an import-ant influence on the design.

HARRY. How much of that can you get tothe brush-head: that's the question?

GEORGE. We certainly can't reject iton the question of energy availablethough. What does worry me is how you'dmake the thing easy to fix on to amultiplicity of bathroom taps. TakingIan's figure of a head of 2-5 metres -

that's a pressure of about a quarter ofan atmosphere - not difficult to get apipe to stay on with that, but I thinkwe're more likely to get mains pressure.

KEN. We'd have to think in terms ofsome special adaptor that stays clampe.dto the tap and you fix the ATB to thatwhen you want to use it. There's alsothe problem of getting rid of the wastewater

.

HARRY. The turbine doesn't have to be

D.C. electricity

Single phase A.C. electricity

Water supply

or

Heat

Input power (watts)

Current (amps) * Potential (volts)

Current (amps) * Potential (volts)x Power factor (ratio)

Head (metres) * Weight flow (newtonsper second)

Weight of head (newtons) x Velocity(metres per second)

Rate of production of heat minus Rateof loss of heat due to irreversibility(joules per second)

GEORGE. Good, that leaves us with astraight choice between "electric" and"hydraulic" methods. We must find outnow whether the latter could meet thespecification; I don't think there'smuch doubt that the former could. I'mtSiinking about the power required.

JOHN. That's quite easy: a high power1*5 volt battery could deliver up tofour amps. That would give us six wattsinput power - even allowing for the lowefficienOy of a subminiature motor, it'splenty.

IAN. I've been scribbling rapidly eversince Ken suggested a head of warter.At worst we might have a level in the

inside the part that's held in the hand.It could be by the tap and dischargethe water straight to the plug hole -

flexible drive to the brush-head.

IAN. Terrible sealing problems.

The tendency to define a problem, pro-pose a solution and criticize the pro-posal - all in rapid succession appearsto be a basic human characteristic.Only long schooling or a strong chair-man can prevent it and hence gain thebenefits of a more systematic approach.

GEORGE. Wait! Harry's idea soundedlike a good bit of speculating. Let'sevaluate it later. Can we have some

138

other speculation please about tap-driven systems? We must have high eff-iciency.

LAWRENCE . Use coaxial tubes - an innerone to feed the turbine and an outerone for the waste water.

JOHN. I think you could get a suffic-iently high efficiency from a hydroelec-tric system - turbine and generatorfixed to the tap and a flex from themto a motor in the hand unit

.

IAN. Ultimately, we don't want rotarymotion: why not a jet of water givingforced vibration to a spring at therequired frequency?

GEORGE. All done? Right- Harry?

evaluation

HARRY. How efficient would one of theselittle motors be, John?

JOHN. Could be about 50%, possibly thesame when you use one as a generator

.

HARRY. Multiply those two fifties bythe turbine efficiency and there's notmuch left, I'll bet. Ian's forced vib-ration idea sounds quite good exceptthat there will be quite high peakloads. We probably need the flywheeleffect of a turbine to cut down thespeed variation.

Another advantage of a systematic lookat solutions oan be that, onoe peopleare used to the procedure, they savetime by not overstating their case. Thehydroelectric system is rejected be-cause of the power loss involved - therewas no need to add, for example , thatit would double the cost of electricalcomponents compared with a battery-operated system.

KEN. I think the coaxial tube ideawould make it very easy to flood theturbine; water is going to be slow torun away on the downstream side.

GEORGE. It seems to me that we're leftwith turbine at tap, turbine in handunit and forced vibration in hand unitbut we reject electric transmission.I think the best thing we can do now iscompare those with battery operation,John?

JOHN. Far be it from me to run downelectricity but, when I first saw thespecification, it struck me that a sub-miniature motor was almost a foregoneconclusion and that depressed me becausewe would then have a product very muchlike everything else that's on the mar-ket. Our product would be inferior be-cause of its low price and that wouldbe its only selling point because,Ian' 11 correct me if I'm wrong, a cheap

mechanism is going to be inefficient andthe cost in batteries will be well overthe £1.5 per year that's stated inclause 7.3.

A battery -driven toothbrush is a demon-strably suitable solution to the problemposed by the specification; it couldsatisfy all the vital and important re-quirements. However , some of the re-quirements under consideration are un-usual and adversely affect the suitabil-ity of a battery-driven model.

IAN. I've done some work on that verypoint; I've got some sketches of poss-ible mechanisms that should be fairlycheap but they won't be any more effic-ient than existing ones. I doubt if wecan reduce the running costs of abattery-driven model so , as John says

,

there's no extra selling point.

GEORGE. This would be the right momentto look at your sketches, Ian. Apartfrom the forced vibration proposal weshall always have the problem of conver-ting rotation into the motion that'srequired.

IAN. If you look at this sketch, ATB 1,you'll see that it's a four-bar chain,ABCD. CD is in its extreme right-handposition; as AB rotates through 180°,CD will move to the left and move thetips of the bristles up through thespecified eight millimetres. I'm assum-ing that the roots of the bristles willlie in a plane through D: that waywe'll minimize the inertia of the brush-head. CD will return to the positionshown as AB goes from 180 to 360°. Thewhole thing could fit into the circum-ference that we have available forgripping. I was thinking, when makingthe sketch, that we'd have an electricmotor with its shaft roughly at thecentre of a cylindrical body . . .

GEORGE. Well, let's not get too immer-sed in detail. Any comments about thebasic idea?

HARRY. There'll be friction at thebearings supporting shafts A and D andat the pivots B and C but I don't seehow you can cut it down; unless youget the oscillating motion straightfrom the power supply, you've got tohave at least one additional shaft withits own bearings.

IAN. I think there'll be more frictionfrom the mechanism in this other sketch,ATB 2, although there's one less part.This is an inversion of the slider-crank chain: CD is driven down andthen up by the pin at the end of AB.Of course, this is a form of quick-return mechanism and CD completes itsdownstroke in just over half the timeit takes for the upstroke - but there '

s

139

nothing against that in the specifica-tion.

HARRY. I must admit it's something Ididn't think of, but I don't see thatit matters very much. The bristle tipspeed that's implied is already quitelow compared with some models on themarket.

Figure 14.2 can be used to indicatethat there is another area of the tooth-brush, -problem (converting rotary tooscillating motion) that is largelyunaffected by the source of energy. Byidentifying such areas and consideringpossible solutions at an early stage,it is possible to save time at designmeetings

.

Sketch ATB 1 is illustrated in Figure16.1.

A four-bar chain is any plane linkagewith four pivot-points . Two of thepivot-points are fixed in space and theother two move in complete circles orcircular arcs, depending on the geometryof the linkage.

Sketch ATB 2 is illustrated in Figure16.2.

A slider-crank chain is really a specialcase of a four-bar chain where one ofthe pivot-points moves in a straightline (a circular arc of infinite rad-ius) . In the form in which it is usedin an internal combustion engine, thestraight line (cylinder wall) is fixedin space; an inversion of this mechan-ism allows the straight line to move.

There are many advantages in having thewriter of the specification present atthis stage and fully involved in theproject. A remote figure of putativeinfallibility is apt to be misunder-stood.

KEN. It's a better mechanism from theproduction point of view than ATB 1 -

two fewer pivots to assemble.

LAWRENCE. For the industrial design,it might be nice to have the rotatingshaft and the oscillating spindle, co-axial. Could that be done?

The industrial designer can be employedin several ways: at one extreme he isordered to "put a pretty case aroundthis lot"; at the other he is treatedas an equal member of a team and hisviews are considered at all stages ofthe project. Not surprisingly, thesecond policy is nearly always the morerewarding

.

KEN. No kinematic problems. In bothmechanisms the cranks AB could be inte-gral with a gear wheel that's driven bya pinion rotating about D.

HARRY. More friction though - still,it would be a useful way to get AB run-ning at the specified speed.

GEORGE. Now, I think we've gone aboutas far as we usefully can for the mom-ent. The Director of New Projectswants to see an overall design quitesoon. I think we may have to submitbattery and tap-driven designs to himand let him find out what Halworthswould prefer. The choice can't be madeon economic and technical grounds alone;there's this requirement about extraselling points. John, can you start toproduce an overall design for a battery-operated brush, using Ian's mechanisms?

When there is no solution that appearsto be the most suitable, the choicewill depend upon the requirements thatwere "worthy of attention" . If thecustomer has made some ill-defined re-quests about characteristics that theproduct should have, a few definite pro-posals will usually produce reactionsthat clarify the situation.

JOHN. Yes, I think I've got all I need.

GEORGE. Fine, and Ian - will you gettogether with Harry and do the same fora tap-operated system?

IAN. Turbine or forced vibrations?

GEORGE. It's up to you; I want some-thing to show Andrew fairly soon and soI'll be chasing you for those schemes.Will you make a pictorial drawing sothat Halworth's people can understandit. Just show the moving parts andanything else of real importance . I

want you to agree on the same orienta-tion for both drawings so that they'reeasy to compare. It would be useful ifyou showed the same sort of spindle forfixing the brush-head to. We'll meetagain as soon as the overall design hasbeen fixed.

There is a considerable overlap betweenPhase 4 and Phase 5 as they are definedin Chapter 12 . For the toothbrush pro-ject, two overall designs are to beprepared before the most suitable solu-tion is selected.

140

Automatic Toothbrush Project

Sketch of sugge.ste.U

mechanism

ATB I

CO oscillates through about SO"

qiuinq a ve.rtic.al moi/eme-ntat the. tip of the. bristle, of 8mm

Scale

AJb/brox s/l

A6 rotates at

about /Sod rev/min

Brush-heacLfnounted anasp"ie/Ce. wicn w«through Dy

ue.

FIG. 16.1

Automatic Toothbrush Project

Sketch of suo/a/este.c/ mechanism

Jan. ftf&titSfue-

AB rotates at

about /Soo rev/mir>

Brc/sh-head

/nount&d on aSp/na'/e. w/tA aj</s

FIG. 16.2

CKwrma w

Solution Division

The fifth phase of synthesizing anew product is that of preparing anoverall design and dividing up the workthat it will entail. The overall des-ign may be prepared in any convenientform: for a machine it is essentiallya written and/or visual statement ofthe way in which energy is to be ac-quired, regulated, converted into mech-anical work and transmitted to theplace (s) where it is required. Theoverall design of a structure is astatement of the way in which loads areto be accepted and transmitted.

The purpose of the overall designis to facilitate a division of thetotal design work still to be done.Consider an ordinary typewriter: theoverall design will show that energy isto be acquired from the typist by theact of tapping on the keys and pushingthe carriage to the right. The latterenergy is stored in a coil spring andregulated by the depression of a key.(The energy has already been convertedinto mechanical work by a biochemicalprocess inside the typist.) Part ofthe energy supplied at the keys istransmitted to the type-bar as kineticenergy in order to squeeze the ribbonagainst the paper, another part isstored in a tension spring to returnthe type-bar to its original position,and another part is used to drive theribbon along. The picture is still in-complete but it illustrates the conceptof an overall design: the detail workto be done can be divided up as required.

The way in which the work is divid-ed will depend on the specialist skillsthe organization has available. Onecommon division is into electrical andmechanical areas of responsibility. Theelectrical work could itself be divided

into power systems and logic systems.The mechanical work might be dividedinto structural and vibration problems,mechanisms, fluid flow, thermodynamicsand heat transfer. There may be otherspecialist areas such as an opticalsystem or the industrial design.

Some organizations whose productsare less sophisticated prefer to divideup the design work according to theavailable man-hours; most of the staffhave sufficient expertise to tackle anypart of the design.

The physical divisions are moredifficult to settle. For example, ifone person is to design the carriage ofthe typewriter, does his responsibilityend with the moving parts or should itinclude the guide rails on which thecarriage runs?

The concept of an "interface" isvery useful here . Students who arefamiliar with the idea of a "free bodydiagram" from their study of dynamicswill readily appreciate that one compon-ent or subassembly can be thought of inisolation, provided the geometTy,forces, torques, heat and fluid flows,etc . , from and to neighbouring partsare taken into account at the "inter-faces".

Figure 17.1 illustrates the inter-faces between a typewriter key bar andthe other parts affecting it.

Once the work has been divided, itmay be necessary to establish a systemfor communication between the peopleinvolved, i.e. when who is to tell whomabout what. Most organizations traintheir staff to provide each other withthe necessary information but it is wisenot to trust too much to people ' s so-called commonsense.

142

Phase 5 Preparing an Overall Design and Dividing up the Work that it will Entail

EXERCISES

Phase 5 Preparing an overall design anddividing up the work that it will entail

These exercises follow on from thePhase 4 exercises; the same organiza-tion has now reached Phase 5 of theautomatic toothbrush project.

1. Prepare an overall design accordingto the instructions given to either Ianor John at the end of the Phase 4

meeting.

Figures 17.2 and 17.2 show theoverall designs that were prepared byIan and John respectively. Compare theappropriate one with your solution.

2. Fig. 17. 2 illustrates the overalldesign that is to be adopted for theHalworth's project. List the varioustypes of specialist knowledge that willbe required and suggest how the workmight conveniently be divided. Try todefine clearly where each person's res-ponsibility begins and ends. Compareyour proposals with the decisions madeby the Assistant Chief Designer at thefollowing meeting.


Present: George - Assistant Chief DesignerHarry - Design EngineerIan - Designer (Mechanical)John - Designer (Electrical)Ken - Production EngineerLawrence - Industrial Designer


GEORGE. A few pieces of informationfirst of all - for those of you whohaven ' t already heard on the grapevine

.

Ian produced this overall design, ATB 3,(Fig. 17. 2) which is essentially an im-pulse turbine driving the mechanism inATB 1. Why did you go for the turbinerather than the forced vibrations, Ian?

IAN. Harry and I agreed that forcedvibrations couldn't be very efficientwithout a lot of moving parts to producea push in two alternating directions.Even if you did that, there wouldn't beenough kinetic energy available to copewith sudden peak loads

.

GEORGE. Fair enough. John producedATB 4 (Fig. 17. 3) which is a singlecarbon-zinc cell driving a subminiaturemotor and the output is geared down todrive the mechanism in ATB 2 . Why thegearing, John?

JOHN. Mainly because there is a verycheap motor available that runs at fivethousand revs per minute. The differ-ence between its cost and the cost of aslower motor more than compensates forthe extra moving part. This designcould be cheap to buy but there is noother selling point.

GEORGE. And that's just what Mr. Cabotat Halworths thought, too. Andrew saidthat he was much more enthusiastic aboutdriving it from the tap - but he ' s nofool; he pointed out all the disadvan-tages , especially the problem of fittinga pipe to all sorts of different tapsand getting rid oifi the water that'sused. Halworths sell a hair washing

spray at the moment and they get a lotreturned by people who have trouble infitting them to their taps. Anyway,that's the sort of design that he wantsus to tender for. Thanks for your help,John, and for coming along today; Igather you weren't too keen on thisdesign in ATB 4 and Eric's found plentyfor you to do in any case?

JOHN . Oh yes , it would have been avery inferior product. I'll get backto something else. Cheerio.

GEORGE. Thanks again. Now, are thereany questions before we divide up thework involved in ATB 3?

KEN . Why have you used an impulse tur-bine rather than some other type?

IAN. Mainly for simplicity but thespecific speed is about right for theavailable head.

LAWRENCE. I'm rather lost, I'm afraid.This sort of arrangement is going to bequite awkward for me; do I understandthat there are other possibilities?

HARRY. As I persuaded Ian to use animpulse turbine, perhaps I should ex-plain. In this sort of turbine you letthe water come out of a nozzle into airat normal atmospheric pressure . Allthe energy the water gained as it camedown the pipe appears as velocity;there's no pressure left. The jet hitsthe turbine wheel and spins it roundbut the wheel must rotate in air, youcan't have water building up and gettingin the way. Now the other possibility

143

SHAFT TO CARRIAGEMECHANISM

ARM TO RIBBONMECHANISM

FIG. 17.1

WATER RUNSAROUND A CASING SQUARE END TO FIT

INTO BRUSH-HEAD

ATB 3

TURBINE WHEELROTATES AT1500 REV/ MIN

CRANKOSCILLATESTHROUGH 50°

i' WATER TO DRAIN

WATER FROMCOLD TAP

FIG. 17.2

BAR FROM BATTERY -VETO MOTOR TERMINAL

WIRE FROM BATTERY +VE

SWITCH

ATB4

WIRE FROMSWITCH TOMOTOR TERMINAL

1-5 V HIGH POWERBATTERY (E.G.

EVER READY HP2)

6W INPUTSUB-MINIATUREMOTOR. SPEED 5000 REV/MINAT NO LOAD

MOTORSHAFT

FIG. 17.3

GEAR WHEEL (46 TEETH)AND INTEGRAL PIN

SQUARE ENDTO FIT INTOBRUSH-HEAD

PINION (15 TEETH)DRIVEN DIRECTFROM MOTOR SHAFT

is to use some of the water's pressureto drive a wheel round; we call that a

reaction turbine. It's something likea propeller working in reverse: yousend water over it and make it go roundbut you need guide vanes to make surethat the water goes in in the right dir-ection and I think that will make thewhole thing too expensive.

LAWRENCE. What is the other thing Iansaid - specific speed?

IAN. It's a useful way of comparingdifferent turbines. If you could getjust one kilowatt out of a turbine froma head of exactly one metre of water,then the speed at which it was runningis the specific speed. We need muchless power from more than twice as muchhead so our turbine will be scaled downcompared with one working at the specif-ic speed. However, we know how to findthe speed that a scaled-up version ofour turbine would run at under the spec-ific conditions: it works out at abouttwenty eight revs per minute and thathelps us to choose the most suitabletype of turbine. I'm afraid there'snot much doubt that it should be an im-pulse type but don't forget that drawingATB 3 is highly diagrammatic. Forquickness the wheel has been drawn morelike a paddle wheel than an impulse tur-bine. The water must go in in a tangen-tial direction but it splashes about asit comes out.

KEN. I seem to remember that the littlebuckets that the water hits are quitecomplicated things. We could produce a

plastic moulding at a reasonable pricebut you wouldn't have much mass to givethe required flywheel effect.

IAN . That ' s what I thought ; it may benecessary to incorporate a metal ringto increase the mass

.

GEORGE. Can we move on then? Why thislinkage, Ian?

IAN. Less friction than ATB 2 and thespindle from the brush-head ends upnearly in line with the circumferenceof the turbine wheel. I thought thatwould make Lawrence's job easier.

LAWRENCE. Yes, I think it does.

GEORGE. Good. Now the other thingthat concerned me particularly was theergonomic problem with regard to theapplication of the brush-head to themolars on both the right and left sidesof the mouth. It's easy enough to movea battery-operated toothbrush from oneside to the other but we've got problemswith water inlet and outlet when we usea turbine. The inlet may get twistedup and the outlet may be held at the

top and the turbine will flood or soakthe user. Anyway, Harry's done someresearch so I'll let him tell you aboutit.

HARRY. I arranged for the MethodsStudy people to film two children andtwo adults using a battery-operatedbrush. I'd like you all to see thefilm but I think I can summarize theresults now as they're quite conclusive.We painted an arrow on the end of thecasing furthest from the brush-head andpointing away from it, and a star onthe casing at about the centre of grav-ity but on the area furthest away fromthe user's mouth. In all cases thearrow points slightly downwards for atleast ninety per cent of the time andthe star moves anywhere in a verticalplane within a circle of radius fiftymillimetres relative to the user's mouth.So, if we had water going in at the starand coming out parallel to the arrow, itwould be quite easy to handle

.

LAWRENCE. Does the arrow point upwardswhen the user is transferring the brushfrom the left side to the right side?

HARRY. Never. Even the younger child- about five he was - didn't attempt totransfer like that. It's most awkward:you'd have to raise your elbow so thatit was almost level with your mouth andchange hands. The arrow points slightlyupwards for a little while when adultsare brushing their front teeth - theincisors.

GEORGE. Would it be possible to usethe brush and always keep the arrowpointing down?

HARRY. Certainly. If you knew you weregoing to get a wet arm it would be poss-ible to drop your elbow a little andstill get at those incisors.

IAN. I can't see anything against thatarrangement for the working of the tur-bine .

GEORGE. Good. Now what about sealing.Do we need to keep water out of the mech-anism or can we use it as a lubricant?

KEN. Hard water is going to foul it upin a couple of years.

HARRY. That's all right as long as thelife can be increased by a simple modif-ication.

IAN. Complete immersion will waste a

lot of power.

GEORGE. All right, we'll have a wetarea round the turbine and a humid arearound the mechanism: water is the lub-ricant. Is there anything in the over-

145

all design that we haven't dealt with,apart from the connection to the tap?Good. Now, I think we must reject theidea of fitting the inlet pipe to anadaptor on the tap every time the brushis used: it's too inconvenient andexpensive. We'll go for a semi-permanentfitting to the tap and people can gettheir cold water from the turbine outlet.The tap fitting must be made withouttools of any kind; we'll use a large,easy-to-grip head on any screw that'sused.

HARRY. Shouldn't the fitting be fairlyeasy to take off, so the tap can stillhave a hair washing spray fixed to it?

GEORGE. Yes, I tnink so but, if that'snot possible, you might think aboutmaking it easy to fit something to theturbine outlet pipe. Now, how do youwant to divide the work up, Harry?

HARRY. I suggest four basic areas: one,tap to turbine inlet; two, turbinenozzle, wheel and outlet; three, mech-anism from turbine shaft to brush-headspindle; four, bearings and internaldetails of casing in consultation withLawrence on industrial design.

GEORGE. That seems sensible but I don'treally want to involve five differentpeople. Ian, you've done a lot of work

on the turbine and mechanism already:I ' d like you to carry on with those andcome to see me when you need some helpwith the detail design. There's a newchap called Mike Elrood who could doboth the tap to turbine inlet and thecasing, I think. Lawrence, you oughtto be consulted about the connection tothe tap as well, and if Mike does allthe non-moving parts you only have himto deal with about details. Harry?

HARRY. That'll suit me very well. I'llbrief Mike on what's happening. Areyou definitely assigned to this project.Ken?

KEN. Yes, I'd like to be kept informedof everything that affects the produc-tion. If we get this order, it's veryuseful if I already know exactly whatthe thing will look like. Fred's al-ready quite pleased that there are noelectrical components.

GEORGE. So am I; it cuts down the com-munication problem considerably. Thereare still only four people and myselfactively involved. Right, now to getthe tender out on schedule I'd like tosee some layout drawings by the end ofnext week please Ian. And Harry, willyou tell Mike the same? We'll have aprogress meeting about then.

146

ciiwra n

Solution Development

The sixth phase in the synthesisof a new product is that of making dec-isions on items of increasing detail

,

in consultation with the other peopleaffected. During this phase manythings are happening together and theexact nature of the work will dependvery much upon the commercial arrange-ments with the customer . The work en-tailed in the overall design was dividedup during Phase 5 , and several quitedistinct tasks have to be carried outbefore the preparation of the informa-tion required for the manufacture, ass-embly and testing of the product canbegin

•

The process described in Phase 4

is now carried out on a smaller areabut to a greater depth. Generally,there is a pattern of events that isrepeated as the work is progressivelyfocussed on different parts of the over-all design:

characteristic of experience is thehabit of mentally evaluating severalpossibilities before stating a proposedsolution.

An evaluation of the proposals forthe bicycle drive could lead to the dec-ision that gearing and shafting weretoo complicated, heavy and expensiveand that a belt was not strong enoughfor the power and speed involved. Thechain and sprockets must now be analysedso that details can be settled.

There is always some known datathat help to determine certain import-ant sizes, etc., but, almost invariably,there is no direct method of calculation.Design is not a process of substitutingvalues into a formula and calculatingthe required unknown quantity. Realdesign work is nearly always iterative

.

In other words, when a certain solutionis examined the analysis shows that itis not entirely satisfactory; the

Propose —> Analyse Modify —»- Check —*~ Inform

The person(s) concerned with a partic-ular part of the product will firstpropose an outline of a solution to theparticular detail problem on which theyare engaged. This proposal may takethe form of an idea in someone's mind,a verbal or written statement or a vis-ual presentation, or some combinationof these.

For example, whoever was first con-cerned with transmitting power from thefoot-operated cranks of a bicycle tothe back wheel might have proposed gear-ing, chain and sprockets, shafting,belt and pulleys, etc. Engineeringstudents should be able to visualizehow any of thes*e methods could be emp-loyed, although a sketch would make theactual proposal clearer to the "man^in-the-street". Once proposals have beenmade, any evaluation is usually (unlikePhase 4) inclined to be quantitative.An experienced designer or small teamwould only put forward proposals worthyof serious consideration. Indeed, the

first solution is modified in a waythat is expected to make it more satis-factory, and it is re-analysed. Theprocess is repeated until the result issatisfactory.

Quantitative analysis is usually aprocess of making certain simplifyingassumptions about the physical systemand then using either an exact or anumerical or a graphical method to det-ermine the required quantities. Alter-natively, measurements may be made ofthe behaviour of full size or dynamic-ally similar models. For example, alink in the bicycle chain may be anal-ysed by making the simplifying assump-tions that it is a perfectly uniformpiece of metal that is not strained be-yond the elastic limit and that thetensile force in the chain varies witha simple cyclic pattern. The linkcould then be checked to see thatstresses and deformations were accept-able and fatigue tests on similar spec-imens could be used to predict the life

147

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of such a link.Once a suitable design has been

established* the details must be commun-icated to the other people concerned;they may wish to comment on the propos-als before they are finally adopted.One designer's proposals can affectother people in much the same way thatthe design factors (investigated inChapter 15) affect the complete product.That is to say that there are some vitalproposals that will completely constrainsome aspect of another person's work,some important proposals that will stillleave him some room to manoeuvre, andsome proposals that may be taken intoaccount if some advantage is therebygained. It follows that someone musthave overall control of the project andmake decisions when the parties cannotagree amongst themselves.

There is a considerable overlapbetween this phase and phases 5 and 7

.

Some important details have alreadybeen decided when the overall design isproduced and some quantitative analysishas been done in order to choose themost suitable general solution to the

problem posed. In Phase 7 even themost minute details are settled and aconsiderable amount of quantitativeanalysis may be necessary, but it isduring Phase 6 that the product is re-fined into a shape that will easily berecognized in the final manufacturedproduct.

Once all the necessary decisionshave been made, tne product (or subass-embly or component) is quite well def-ined but cannot yet be manufactured.However, it may be possible to estimatecosts quite accurately. Some organiza-tions are able to make a good estimateof costs long before this stage isreached but this is usually becausetney have made similar products in therecent past. The less familiar theorganization is with the work that itis to undertake, the longer it takes toestablish costs accurately. This prob-lem is further complicated by technolog-ical innovation and, in recent years,by an increasing rate of inflation.

These problems should not be reg-arded as being outside the responsibil-ities of the engineer. Engineering isabout making things TO SELL.

Phase 6 Making Decisions on Items of Increasing Detail, in Consultation with theOther People Affected

This exercise follows on from the Phase5 exercises; the same organization hasnow reached Phase 6 of the automatictoothbrush project.

The Assistant Chief Designer has ameeting to discuss progress on the pro-ject. Study the calculations andsketches and note the decisions made atthe meeting.

Prepare a layout drawing of thecomplete driving unit, i.e. a drawingwhich indicates every part of the assem-bly, to scale as far as possible, butwith the less important details left tobe settled at the assembly and detaildrawing stage

.

Compare your solution with thoseshown in Chapter 19 {Figure 19.1).


Present: George - Assistant Chief DesignerHarry - Design EngineerIan - Designer (Mechanical)Ken - Production EngineerLawrence - Industrial DesignerMike - Junior Designer (Mechanical)


GEORGE. I thought it would be useful,now that we have a basic layout withsome definite ideas of sizes et cetera,to review the work that ' s been done andplan the remaining work. Where's thebest place to start, Harry?

HARRY. The turbine, definitely: itaffects everything else. Now neitherIan nor I could find anything useful inthe library on impulse turbines of thissize. There's plenty on propeller typereaction turbines as used in flow metersbut their main design criterion is touse as little of the available head aspossible; so that was no help. We dec-ided to use the analysis that works forfull size machines. We'll almost cert-

ainly have a much lower efficiency be-cause of manufacturing inaccuracies andour proportionally higher frictionlosses; on the other hand our nozzleshould be more efficient and give amore compact jet. I think Ian can bestexplain the details.

IAN. Here are my calculations. Youcan see on sheet 1 {.Figure 18.1) that,to get the necessary flow of water, ourjet diameter will have to be five pointthree eight millimetres. The pitchcircle diameter of the wheel, for thecorrect power at the design speed,should be forty millimetres.

149

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FIG. 18.2

LAWRENCE. Yes, you mentioned that theother day, but where did your figure ofseventy something come from?

IAN. We'll get there is easy stages.On sheet 2 (Figure 18.2) I've sketchedout the recommended shape of the littlebuckets that the water hits. Theyshould be about sixteen millimetres inthe radial direction so the outer dia-meter of the wheel comes out at fifty-six millimetres.

LAWRENCE. Is there no way of reducingthat?

HARRY. The short answer is "yes" butthat won't improve the performance orthe cost. The point is that the aver-age speed of a bucket ought to be halfthe speed of the jet in order to getthe best efficiency. We could have twojets and reduce the wheel diameter butthat would mean an increase in thespeed of rotation.

KEN. A smaller wheel would be more dif-ficult to manufacture as well.

LAWRENCE. Ah well, it'll be a challengefor me I suppose.

IAN . You can see the basic arrangementof the buckets at the bottom of sheet 2.Eighteen was the number that would beused on a full size turbine but wethought we would cut that down to six-teen to reduce the extent to which theymask each other. Now, you can see thegeometry of the nozzle at the top ofsheet 3 (Figure 18.3) and I've incorpor-ated that in a casing for the turbineat the bottom . That ' s where the seventyfour millimetres comes from, Lawrence.

LAWRENCE. Yes, I see why you need somuch radial clearance at the top andbottom, but do you need it at the sides?The toothbrush is never held so thatthe water could collect there.

HARRY. That's true. I think we couldgive you some space to use there

.

IAN. Right, I'll make a note of that.

MIKE. One other thing: does the inter-nal diameter of the water supply tubehave to be twenty millimetres all theway along?

HARRY. Ideally, yes Mike, but the tapitself isn't likely to be that size soI think we only need twenty millimetresnear the nozzle.

GEORGE. You'll have to try a lot ofthese things out on a prototype, if weget the order. What's next?

IAN. Sheet 4 (Figure 18.4) is just mythinking on paper really about the kine-matics of the mechanism. At the top ofsheet 5 (Figure 18.5) are four positionsof something like the final proposal,and you can see how I would fit it inat the bottom. At the top of sheet 6(Figure 18.6) is the proposed mechanism,full size. I just drew a circular cas-ing round it but only a quarter of thatarea is required.

KEN . What are those spring clips : arethey metal?

IAN. No, I've just drawn them like cir-clips for quickness. I'm no expert onplastics but presumably there is somesuitable fastener on the market.

KEN. It's not so easy but we couldsort it out with your detail designers.

IAN. Yes, as long as you're happyabout the relative positions of thelinks for assembly?

KEN. That seems all right.

HARRY. Good. Now Mike had a look atthe moving parts and made a few sugges-tions about supporting them. Is thatthe lower drawing on sheet 6?

MIKE. Yes, Ian and I thought we'd keepall this part of the design together.I must apologize for the casing: it'sjust a first rough drawing that was allI had time to do after finishing thetap connection. You see that we needanother join.

GEORGE. That's all right Mike; you'vegot time to knock it into a bettershape.

LAWRENCE. As you don't really need somuch clearance on either side of theturbine wheel, you could tidy that partup considerably. Presumably, the wateroutlet tube serves as the handle?

MIKE. That's the general idea; againwe need a model to establish the bestinternal geometry but I shouldn'tthink it will constrain you very much.

LAWRENCE. The centre line of the brush-head follows through to the water outletI see. Is that essential?

MIKE. Oh no, it's an ergonomic problemfor you; I had to show something and Ithought that the outlet pipe centreshould be on that side for easy hand-ling. For easy water outflow it wouldbe better nearer the wheel axis.

LAWRENCE. I'm sure we can find somecompromise solution.

151

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FIG. 18.3

KEN. Sorry if you've already though ofthis: the turbine shaft should have ashoulder on it in order to ensure thatthe wheel clears the end of the bearing.

MIKE. Thanks. As I said, it was avery quick drawing.

HARRY. Good. Now can we see yoursketches of the tap connection, Mike?

MIKE. Yes. I've labelled my sheetswith Roman numerals to distinguish themfrom Ian's. Sheet 1 {Figure 18.7} justshows my original ideas on how to fitall sorts of taps and how to hold thefitting in place. I've refined them onsheet 2 (Figure 18.8) and then sheet 3

{Figure 18.9) shows a layout drawing ofthe complete connection.

GEORGE. I see you've mentioned avoidinga possible patent for the clip - we'llhave to be careful that there are nopatents on a connection like this

.

KEN. I've got one on the end of mygarden hose that certainly resemblesthis but I wouldn't have thought thatthere ' s much you could patent

.

HARRY. I'll have a chat to our patentspeople, just in case.

GEORGE. Fine. Let's see what remains.Ian and Mike are going to tidy up thelayout on Ian's sheet 6 (Fig. 18. 6) and

Lawrence can do a few sketches now thatthe basic layout is settled. Assumingthat the tap connection is all right, I

think we can get that costed as itstands . I know what I wanted to askyou Lawrence: are you going to suggestany awkward colours or textures? Youknow the sort of things that put theco st up

.

LAWRENCE. I thought about using trans-parent material around the turbine. Itwould appeal to children to see it workworking

.

KEN. Brilliant idea - but it'll takethe cost up a bit I think.

HARRY. It's the first proposal thathas though, and it could just clinchthe order for us

.

GEORGE . I agree . Let me have a decentlayout drawing and the industrial des-ign sektches for the costing boys assoon as you can. Then Harry can writethe technical part of the tender andwe'll hand it over to the Director ofNew Projects. He's managed to persuadehis Masters, as he calls them, to lethim quote on the assumption that we pro-duce more than just the initial batch.So, with a toothbrush that uses nobatteries, has a fascinating turbinewheel whizzing round in full view, andcomes at a bargain price, I'll be verysurprised if we don't get the order.

153

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Solution Execution

The seventh phase of a design pro-ject is that of directing the manufac-ture of the required quantity. It wasstated (in Chapter 10) that the manu-facture of a component or the correctassembly of a set of components is fac-ilitated by engineering drawings. Thework involved in translating the layoutdrawings into working drawings is con-siderable in magnitude but often some-what routine in nature. Quite oftenthere are calculations still to be donebut they are likely to be rather lessinteresting than the earlier analysisthat established the major quantitiesinvolved in the problem.

For these reasons there is a tend-ency for the work to be handed over todetail designers and draughtsmen whohave not previously been involved inthe project and can only regard it asanother job, similar to others thatthey have seen in the past. The worstthat can happen is that the more seniormembers of the design team lose inter-est in some aspects of the problem atthis stage and, for various reasons,some apparently minor decision is takenby a junior employee which adverselyaffects the whole product.

Of course, there must be a sharingof responsibility at this phase; thework would take too long if other peoplewere not brought in. It is the super-vision of this work that is sometimesat fault. A good overall design can beruined by poor detail design. The mostsuccessful organizations are well awareof this and take steps to ensure that,at least, all detail work is properlychecked, at best that the new personnelare well-motivated by familiarizing themwith all those parts of the project thatcould be of interest

.

There are different ways of organ-izing the assembly and detail drawingwork . Some drawing offices prepare avery accurate assembly drawing which isthen translated into the required detaildrawings of the various components.Other organizations refine the layoutdrawings into something approaching thefinal assembly drawing, produce the det-ail drawings from this, and then make a

160

completely accurate assembly drawingfrom the detail drawings. Draughtsman'stime is valuable and the first methodmay produce a saving as long as theassembly drawing does not have to bedrastically modified before issue tothe manufacturing departments.

At this stage of the project theproduction engineers and developmentengineers may become deeply involvedand may suggest changes once prototypeshave been built .and tested. Simpleproducts are usually modified by issuinga revised drawing. More complex pro-ducts often have many completely differ-ent drawings for the prototype and pro-duction models.

The detail designer must have a

considerable knowledge of manufacturingprocesses and is sometimes made respon-sible for planning the manufacturingoperations. In other words, he decidesthe form of the raw material and exactlywhat is to be done to it in order toproduce the component shown on his draw-ing. Alternatively, the designer/draughtsman concentrates on drawingsomething that can be made but the act-ual production details are left to aspecialist who is attached to the manu-facturing departments

.

Manufacturing processes are an imp-ortant constraint on the designer/draughtsman and components are oftendrawn out in a particular way mainlybecause they are to be made by a certainprocess. Fundamentally, there are onlythree ways of shaping material:

1. Start with too much and cut awaythat which is not required,

2. Start with the right amount anddeform the material to give therequired shape,

3. Fill a space of the required shapewith a liquid material which willlater solidify.

The variations and combinations of thesemethods are legion but, generally speak-ing, they are in order of suitabilityfor small quantities of complex shapes.The detail design is greatly affected bythe quantity to be manufactured.

Finally, the detail designer isinvolved in the choice of materials tobe used. Whereas manufacturing pro-cesses have developed considerably in adecade, new materials have been intro-duced at a bewildering rate. Engineersdeal with two basic types of material:those with small compact molecules

,

such as metal and liquids, and thosewith long molecules (high polymers)such as rubbers and plastics.

The number of applications for thelatter type has increased rapidly and a

new outlook is spreading through indus-try: increasingly the question isasked "Is there a non-metallic materialthat it would be an advantage to use?".This certainly does not mean that metalsare obsolescent; there are many applic-ations where a metal is the only poss-ible material and this will be true forthe foreseeable future.

Phase 7 Directing the Manufacture of the

These exercises follow on from the Phase6 exercises; the same organization hashow reached Phase 7 of the automatictoothbrush project.

1. Referring to Chapter 18 only, makesketches of the external appearance ofthe proposed automatic toothbrush.

Compare your sketches with Figs.19.2, 19.3 and 19.4.

2

.

Suggest ways in which the detaildesign work involved in the layout draw-ings Fig. 18. 9 and Fig. 19.1 could beorganized.

Compare your suggestions with thedecisions taken at the meeting arrangedby the Assistant Chief Designer - Scene1.

3. Scene 2 is a short discussion onthe performance of part of the proto-type toothbrush. By picturing yourselfusing the complete product, list waysin which poor detail design could aff-ect its operation.

Compare your ideas with those ex-pressed by Andrew in Scene 3

.

Required Quantity

4

.

Make subassembly and detail draw-ings of the driving spindle and its 10mm radius link. Incorporate a modifiedend cross-section on which the brush-head will fit so that the bristles arealways pointing towards the user.

Compare your drawings with Figs.19.6 and 19.7.

5. The layout drawing in Fig. 19.1shows a system of dividing the casingwhich is, deliberately, far from ideal.Suggest a more suitable system to imp-rove appearance, function and assembly.

Compare your system with that pro-posed by Mike in Scene 3

.

Suggestions for further exercisesStudents who wish to proceed furtherwith the detail design of the automatictoothbrush will find sufficient inform-ation in Chapters 18 and 19 to tacklemany of the parts in the turbine wheeland mechanism subassembly.

The author has no doubts that thisparticular design could be made to workbut much of the material relating tothis project has been introduced toillustrate the process of synthesizinga new product; there is no suggestionthat a company would necessarily adoptthis solution.

Scene 1 : The office of an Assistant Chief Designer

Present: George - Assistant Chief DesignerHarry - Design EngineerIan -* Designer (Mechanical)Ken - Production EngineerLawrence - Industrial DesignerMike - Junior Designer (Mechanical)

GEORC . As I think most of you know,we won the order for an automatic tooth-brush for Halworths and we are justabout to initial a contract with themfor the supply of a test marketingbatch of twenty thousand . The contractis going to stipulate that delivery isto commence on the first of Septemberso that gives us just six months . NowI hope you've all had a chance to re-fresh your memories on the notes anddrawings that you produced for the ten-der. If you could find your final lay-out drawing, Mike we can see what's tobe done

.

MIKE

.

This one, ATB 5? [Figure 19.1)

GEORGE. That's it. Now I've jotteddown a few of the things we have todeal with. The first one is making aprototype; Halworths will want to com-ment on it before the production modelis finished. Will you give that toppriority please, Harry?

HARRY. Right. Are most of your ideasembodied in ATB 5 , Lawrence?

LAWRENCE. Quite a lot, but I'd stilllike to try an exit orifice that's

161



AUTOMATIC TOOTHBRUSH FORHALWORTHS LTD

SECTION D-D(MECHANISMONLY)

(SEE SHEET (3))

FIG. 19.1

directed slightly away from the user.And I'm not at all keen on that chimney-pot shaped nozzle in the top view.

HARRY. I'm inclined to agree aboutboth. Where are your designs?

LAWRENCE. There are four here. I usemy initials and a departmental numberingsystem to identify them. LB 0338 (Fig-

ure 19.2) and LB 03 39 (Figure 19.3) area couple of early ideas. LB 0340 (Fig-

ure 19.4) is a first accurate drawing,and LB 0341 (Figure 19. 5) is the pol-ished presentation that Halworths saw.The only other difference between thatand ATB 5 is the transparent insertsnear the turbine.

HARRY. I think the model shop can workdirectly from your drawings but weshall need some detail drawings of theturbine wheel and mechanism for thetool room. Who can I have to help,George?

GEORGE. Ian and Mike for the time be-ing, and there's Norman Cotton who'snearly finished another job: he'squite an expert on plastics in mechan-isms. Don't produce a tap connectionfor the prototype, use whatever you canthat's already available.

KEN . What ' s happening about that? Areyou thinking of subcontracting it?

GEORGE. Thinking, yes. I'd rathermake it ourselves but the Works Manageris concerned about the effect thatcould have on another order that usessynthetic rubber mouldings. We'll haveto wait and see . However , I think youcould go ahead and produce an assemblydrawing, Harry, so that it's in stock,so to speak. Now the next priority isproduction planning. Ken?

KEN. ATB 5 seems to be mostly plasticmouldings with odd pieces of metal for

the shafts. I'll arrange a meeting withthe appropriate people and see if theyspot any snags with this layout butpersonally I think it's just a questionof phasing the work to fit in with theother jobs. The most complicated partis the turbine wheel; is there a chancethat we could get to work on that quitesoon? It's a well-tested design; youshouldn't have to alter it when you testthe prototype

.

IAN. I'm not so sure. We've no inform-ation on how well one of this size willwork and we may need to increase themoment of inertia with a metal insertfor the plastic production model, it'salmost impossible to predict the loadsso we shall have to get a few people toclean their teeth and see how badly thewheel slows at peak loads.

GEORGE. Can you give that absolutepriority then, Harry. Get the toolroom on it this week, if you can, andwe'll find out if the turbine works soKen knows what he's going to deal with.

KEN. Thank you. Just one otner thing:who's going to make the inlet tube?

GEORGE. That's easy:reels and chop it upquired. Lawrence hasNow I ' 11 call anotheras there's somethingthe meantime, I'll beof events and targetstart to think aboutpower we shall need.

you buy that into the length re-the details. O.K.?meeting as soonto report and, inissuing a schedule

dates so we canhow much more man-

HARRY. I'll come and see you later,Lawrence and talk about materials andcolours for the casing. Ian, will youdrop everything and draw up a turbinethat the tool room can produce and a

suitable nozzle. Mike, give Ian a handwith the drawings when he ' s ready , butyou may as well start that fitting ass-embly while you're waiting.

Scene 2: A Testing Area

Present: George - Assistant Chief DesignerHarry - Design EngineerIan - Designer (Mechanical)Ken - Production EngineerNorman - Detail Designer

HARRY. Well, there you are. The turb-ine works all right but it's very ineff-icient. We seem to be getting just overtwo watts out of a theoretical six wattsthat's going in. When the turbine getsflooded or there is a big peak load it

can slow down to rest in just over asecond. I'd say that the moment ofinertia is about right.

GEORGE. Looks like it.of?

What's it made

IAN. That's epoxy resin. There's agreater mass there than we'd probablyhave in the production models.

NORMAN. It's not a very accuratelymade wheel. I think we'll get morepower out of a properly moulded one

.

KEN. Yes. So it looks as if there willhave to be a metal insert to get thesame moment of inertia?

163

' tdcC& out

LB0338

FIG. 19.2

jfatldorttis ATB

LB 0339

FIG. 19.3

Profiles

Handle, and- P*tj,/>'«>f,yl">e ?

outlet tube.

/nle-t tuSe.

"Concertino?ty/>*Cfrey).

^TnTp^Pr- ?

LB 0340

Mechanism cover

FIG. 19.4

//a (.worth's

WATsR OP£#AT£b.

LB 0341HALWORTHS LTDautomatic toothbrush

FIG. 19.5

HARRY. We won't if we can possiblyavoid it and keep the costs down. Doyou think you could redesign it Norman,so that more of the mass is away fromthe centre?

NORMAN. I'll try. The other thingthat's slowing it down is the bearingand journal materials. If we could usep.t.f.e. or something like it, it wouldbe a lot better.

HARRY. See what you and Ian can devise.We don't particularly want another partto insert.

GEORGE. Is the nozzle satisfactory forthe pressure you're using?

HARRY. Seems to be. It gives a niceclean jet and we should be able to getthe geometry just as accurate in poly-styrene and a more efficient turbinewheel in polyethylene; we may be ableto cut the flow down a bit. It strikesme as rather a lot to dispose of.

KEN. How soon could we see the detaildrawing of the wheel?

NORMAN. That's what I'm concentratingon at the moment

.

GEORGE. Let's say next week then, Ken.How's the rest of tne prototype, Harry?

HARRY. Tap connection and tube: noproblems. Same with the mechanism.What's happened to the casing, Ian?

IAN. The model people are also usingepoxy resin and wooden formers to getthe general shape. We hope to fit thisturbine wheel into it and bond on diff-erent nozzles to get the best compromise.

GEORGE. Good. Now, as soon as theprototype is working satisfactorilywe'll be able to get a few more draughts-men working on the details. Then Ithink it would be a good idea to reviewprogress once Halworths have seen theprototype, and get the Director of NewProjects along to help to put everyone,especially the new people, in thepicture.

Scene 3: The company's conference room

Present: Andrew - Director of new projectsGeorge - Assistant Chief DesignerHarry - Design EngineerIan - Designer (Mechanical)Ken - Production EngineerLawrence - Industrial DesignerMike - Junior Designer (Mechanical)Norman - Detail DesignerPaul - Tool Designer/DraughtsmanRichard - DraughtsmanSimon - Drawing Office Apprentice

HARRY. It's very pleasant to see allthe members of my present team gatheredtogether in one place; I'd normallyhave a long walk to see so many of them.

GEORGE. Yes. Well, I'm glad we couldfind a convenient time for the Directorof New Projects to give you some back-ground information, Andrew?

ANDREW. Our design won this ordermainly, I think, because it's different.Other companies showed Halworths battery-driven brushes which were quite goodand easy for children to use but theywere looking for something a bit morethan that. Our brush is, if we're hon-est, not so convenient to use but it'sgot tremendous potential child-appeal.Kids are used to playing with battery-driven objects these days so a littlewater turbine has a great advantage.The initial order is for twenty thousandbut I shall be very disappointed if wedon't get future orders of at leastfifty thousand a year. Whether we door not depends to a great extent on allof you. Now Halworths have seen the

prototype working and they only had oneor two minor complaints; George willdeal with those later I expect. How-ever, let me give you my private night-mare of the things that could go wrongat this stage. Just picture a littleboy or girl who's just been given oneof our toothbrushes. Father fixes iton the tap for the child, turns it onfairly gently, according to the printedinstructions and splosh! the fittingcomes shooting off producing dampnessand depression all round. He triesagain with the same results and thechild learns a few new words . Eventu-ally, being a bit of a handyman, hefinds a suitable clip and succeeds inmaking the tube stay on. He's alreadytoying with the idea of punching themanager of the local Halworths on thenose; fortunately for me he doesn'tknow how much I had to do with it - yet.The toothbrush starts working, we hope,but stops every time that the childapplies more pressure. Having brushedhis left side molars using the righthand, he changes sides and promptly getswater right up the sleeve of his pyjama

166

jacket. Eventually, after much stalling,the child's teeth are done and Fathersays: "Let me have a go". He tugs atthe brush-head which won't come off sohe tugs again and its stem breaks.Downstairs to the garage to put thebroken end in a vice and Father is feel-ing displeased with us once more. Hesucceeds in removing one now uselessbrush-head and repairs to the bathroomwhere he gingerly pushes a new head onto the spindle. This one is loose be-cause it's in a different position rel-ative to the spindle. Father adjustsit, turns on the tap and starts brushing.He doesn't think the action is suffic-iently vigourous so he turns the tap onmore and pushes the brush-head into thefurthest corner of his mouth. Somethingfails inside the mechanism and Fatheris drenched into the bargain. Now bythis time, he's not really interestedin having his money back; his one amb-ition is to see my head on a plate, andof course he won't be the only fatheraffected. I might be able to cope withone homicidal maniac storming into myoffice but I really don't want morethan a dozen: that would be too much.So please, when you're doing the detailwork on this project, think of me andmy nightmare : it need not come true

.

GEORGE. Of course, the other extremeis that the thing is so expensive toproduce that we don ' t make a profit onthe deal.

ANDREW. Don't! That's too horrible tocontemplate

.

GEORGE. Do you want to run through thecomments that Halworths made on theprototype?

ANDREW. No, you do that. I shall haveto be getting back. Is there anythingany of you want to ask me?

PAUL. This problem with water goingover people: don't you think they'llget used to using the brush in such away that it doesn't happen?

ANDREW. Oh yes, I'm sure they will butwhat we must do is make sure that it '

s

easy to do that; even if it means anextra part and a few extra pence.

GEORGE. Anything else? Well, thankyou very much Andrew. Now, can we havea progress report, Harry? I'll dealwith Halworth's comments as they arise.

HARRY. Right. If you look at the lay-out drawing , ATB 5 and Lawrence '

s

rough sketch, LB 0340, I'll run throughwhat's happened so far. Norman's fin-ished the detail of the turbine wheeland Ken's people have looked at thefirst issue.

KEN. There are a few details to clearup with Norman and then the tooling tobe considered. We could make it in ahigh density polyethylene and get aboutthe right mass.

HARRY. Mike made a preliminary assemblydrawing of the tap fitting and, now thatwe're manufacturing it ourselves,Lawrence has improved its appearance,and Mike and Simon are doing the produc-tion drawings.

GEORGE. Let's have a few prototypes assoon as you can. You saw how concernedAndrew was about drenching people

.

HARRY. Yes, that's in hand. We've gotsome of that concertina tubing thatLawrence suggested and it bonds on tosynthetic rubber very well. The mechan-ism: Norman's working on that now thathe's finished the turbine for the timebeing.

NORMAN. I've a couple of drawings here,actually. SA 7918 (Figure 19.6) is asubassembly of the driving spindle andits link. D 15770A (Figure 19.7) is adetail of the spindle.

KEN. That's quite useful; will theybe issued soon?

IAN. They're still to be checked andapproved but I ' 11 send you copies whenthey have been.

GEORGE. Halworths are going to buy thebrush-heads from the Porcupine brushcompany. They'll need copies of thosedrawings

.

HARRY. That's mainly why I got Normanworking on them at this stage. Lawrencealso needs to contact them about thegeneral shape and colours.

LAWRENCE. I've been in touch already.They use high impact polystyrene to setthe nylon bristles in; we could use asimilar material for the red parts ofmy sketch and keep the same surfacetexture

.

GEORGE. That sounds sensible. Now,Halworths pointed out that , with oursquare-ended spindle,- a child couldput the brush-head on so that it waswrongly aligned with the outlet pipe.Also with a square rigid end the matingsquare hole tends to enlarge and even-tually the brush-head won't stay on. I

see you've changed the cross -sectionthat fits into the brush-head so itcan only be aligned one way. It'llstill give problems with loose brush-heads .

NORMAN. The difficulty is that, witha new head and some sort of wedging

167

-#-£3-DO NOT SCALE THIS DRAWING

90° ±2°ON ASSEMBLY

25-000

ALL DIMENSIONS ARE IN MILLIMETRESITEMNO.

SPINDLE LINKDRIVING SPINDLE

DESCRIPTION

D15771ND15570A

NO.OFF,

THIS DRAWING IS THE PROPERTY OF POLYREETE CO.

DRIVING SPINDLE ASS.SCALE S/S |DWN. N.C.ITRD. |CHD. |APPD. |DATE

MATERIAL

FINISH

SEE PRODN.PLAN

SA 7918

FIG. 19.6

-0-^"DO NOT SCALE THIS DRAWING

TOLERANCE EXCEPT WHEREOTHERWISE STATED ±0-1

40-000

ALL DIMENSIONS ARE IN MILLIMETRESTHIS DRAWING IS THE PROPERTY OF POLYREETE CO.

DRIVING SPINDLESCALE 2/1 IDWN. N.C.ITRD. |CHD. |APPD. |DATE

FIG. 19.7

MATERIAL:STAINLESS STEEL

EN56AFINISH:DEBURR

SEE PRODN. PLAN

D15770A

action, it would be all too easy toexceed the ten newtons maximum forcethat's in the specification.

GEORGE. I see. Well, leave it likethat for the initial batch; we mayhave to change it when Halworths havesome customer reactions.

HARRY. O.K. Now I've divided the restof the work in two. First, those partswhich contain bearings, i.e. whatLawrence refers to in LB 0340 as the"mechanism cover" and "turbine cover".Second, the parts without bearings, i.e.the "outlet tube" and "end piece".

GEORGE. Halworths felt quite stronglyabout that. The prototype is very diff-icult to use without getting wet. Wesuggested a modified outlet tube butwhat they wanted, er: colour contrast.Perhaps Lawrence can explain . . .

LAWRENCE. The change in colour at theend gives a visual emphasis to the factthat something special happens there.I'm doing a coloured drawing at themoment, mainly for Halworths' benefit,but I don ' t think there ' s much doubtthat they'll insist on a differentcolour at the water outlet.

HARRY. Ah well, it's convenient inmany ways, I suppose. Now Ian and Paulare working on the mechanism and turbinecovers. How far have you got?

IAN. We want to change the way thatthey are divided. The division in ATB5 was fairly convenient for making theprototype but there's a better way.We'll have all the external surfacesand the left-hand bearing in one mould-ing; Lawrence is improving their appear-ance. The two right-hand bearings are

in a separate internal wall and the com-plete wheel and mechanism can be assem-bled with that. Then the subassemblycan be inserted into the combined mech-anism and turbine cover.

KEN. Good. That's a much betterarrangement for production.

PAUL. In fairness to Mike, he thoughtof it first but rejected it for theprototype

.

GEORGE. Now that will deal with anotherof Halworths' complaints. They thoughtthe covers looked too fussy.

LAWRENCE . I've smartened them up con-siderably in my colour drawing.

HARRY. That just leaves the outlettube and end piece. Richard's dealingwith that ..."

RICHARD. It seems quite straightforwardexcept for the material for the outlettube . Lawrence has suggested Polyprop-ylene. It's still a bit pricey.

GEORGE. Halworths thought the proto-type tube was unpleasant to handle soI'd like to see an improvement there.

RICHARD. I'll sort it out with Harrywhen we've got a better idea of costs.

HARRY. Well, that's about all I think.It ' s coming along quite well and thereare only a few minor snags

.

GEORGE. Good, let's hope it stays thatway! If Halworths sell as many ofthese as they think they will, our workwill soon be appearing in half thebathrooms in the country.

169

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