Download - Unit5 Power Press Machine

J3103/5/1POWER PRESS MACHINE

General Objective: To understand the use of presses and press tools in coldmetal working.

Specific Objectives : At the end of the unit you will be able to;

Ø Know the types and function of presses and presstools.

Ø Sketch and know the parts of presses and press tools.

Ø Elaborate on the methods of cold working in sheetmetal i.e. shearing, bending and drawing.

UNIT 5

OBJECTIVES

POWER PRESS MACHINE

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5.0. INTRODUCTION

In the manufacturing section of the engineering industry, metal articlescan be worked to shape either by metal cutting, or by metal forming. Metal

cutting can often be wasteful because, on the average, 40% of the originalcomponent material is removed by expensive machining operations to becomescrap. This scrap material will then on the average be worth 5% of its originalvalue as raw material. In many cases machining operations can be moreeconomically carried out by metal forming.

It is interesting to carry out a break-even cost analysis upon amanufacturing operation where metal cutting or metal forming is possiblealternative processes. Often the forming process requires expensive dies andfixed costs are higher; material wastage is negligible, labour costs are low andhence variable costs are lower. The metal cutting process is often lower on fixedcosts, but higher on variable costs. Hence, the forming process will be more

economical when quantities required are large.

In this chapter we shall be concerned solely with the use of presses andpress tools ( Fig. 5.1 (a), (b) and (c) ) as a means of cold working metal objectsinto shape. Most of this type of work is carried out upon ductile metal in sheetor strip form of relatively thin section. It represents an important part of the

INPUT

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manufacturing industry, being used for the cheap production for large quantitiesof components, such as motor car bodies, electric motor parts, domestic electricalarticles, etc.

Figure 5.1. Press machine

(a)

(b)

(c)

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The metal if in coiled strip form, may be fed automatically into the presstool by power rolls or power slide, or may be hand fed by operator. If the metalis in some other form, such as a sheet or partially formed shape, it may be

located in the tool by mechanical hands which have gripping fingers or locatingpans which drop the metal drop part into the correct position. Again, anoperator may hand feed the part. The mechanical feed or location devices mustof course be synchronized to operate every time the press ram lifts the top toolclear of the bottom tool. Where the press is hand fed, stringent safetyprecautions must be taken to ensure that the operators' hands cannot be trapped

in the press tool. Efficient guards must be provided which are completely foolproof, and it should always be remembered that a press is potentially a verydangerous machine.

The presses mi which press-tool work is done may be divided into (a)hand-operated presses, and (b) power-operated presses. Hand operated presses

are very simple in construction and are generally used, only for small batches ofsimple work; an example is shown in Fig. 5.1 (a)

Power presses may be subdivided into (1) Single-acting presses, (2)Double-acting presses, and (3) Triple-acting presses. Single-acting presses areessentially similar to the simple hand-operated press inn that they have only

one ram; the means of operating the ram are various. These means may be acrank, an eccentric or a toggle-lever mechanism In Fig. 109 is shown a typicalcrank-operated single-acting press. The ram is guided in the frame and isactuated by a crankshaft through the medium of a connecting rod. The press isprovided with a fly-wheel and may be driven either by a belt or by an electricmotor. A clutch enables the flywheel to be coupled to the crankshaft when the

press is required to work; this clutch may be arranged to disengageautomatically when the crankshaft has made one complete revolution and the

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ram has reached the top of its stroke, a brake being used to prevent overrunningor, alternatively, the crankshaft may be allowed to keep on revolving as long asmay be desired. In the latter case the feed to the stock must be automatic and

the press must have additional mechanism to provide this automatic feed duringthe time that the ram and punch are clear of the work. Blanking, piercing, andtrimming operations on strip material will generally be done with a continuouslyrunning press and automatic feed, but second operation work must generally beput into the die by hand and then the clutch must be operated every time theram is required to make a stroke.

Because of the great danger to an operator's hands which arises from thestarting of a press cycle before the operator's hands are clear of the dies allpresses must, by law, be fitted with guards. These are arranged to push theoperator's hands away (if they are not already clear) as the press ram begins todescend; the guard is clearly visible in Fig. 5.1 (c). In addition some presses are

arranged so that two levers have to be moved simultaneously before the presswill start and the levers are situated so that the operator must use both hands tomove them. This,, however, generally slows down production.

The use of an eccentric instead of a crank enables greater forces to beapplied to the ram for a given diameter of shaft and is consequently found in

presses for very heavy work. Very large presses, such as are used for theproduction of motor-car body panels, sometimes have four eccentrics, one at eachcorner of the "ram," so as to ensure an even pressure and to eliminate tilting.When very heavy forces must be used as, for example, in coin embossing, togglemechanism is sometimes used, but this form of operation generally necessitatesa comparatively short working stroke.

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Single-acting presses are made either open-fronted as Shown in Fig. 5.1(b), the frame being C-shaped, or double sided as in Fig. 5.1 (c). They are ofteninclinable so that gravity may be used to discharge the job clear of the dies.

The double-acting press has two reciprocating parts, an inner memberactuated usually by a crankshaft and connecting rod and an outer memberactuated usually by cams carried by the crankshaft. Double-acting presses areused chiefly for drawing operations and the outer member is used to actuate theholder or pressure-plate, while the inner member carries the drawing punch.

The use of cams makes it easy to arrange that the holder descends ahead of thepunch so that the blank is gripped before the drawing starts and also to keep theholder at rest during the drawing. Double-acting presses do not usually runcontinuously.

A triple-acting press is similar to a double-acting press but has, in

addition, a third reciprocating member carried in guides in the base of themachine so that a second punch can be made to draw the blank upwards into asuitably shaped recess formed in the top punch. They are used only for largework such as motor-car body panels.

In order to appreciate press tool design, it may help to briefly reconsider

the elementary principles of metal plasticity. Standard tensile or compressiontests which cold work the specimen being used are an ideal means of obtainingdata about the plastic range of metals.

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Consider Fig. 5.2, which shows the results of a tensile test upon arelatively ductile material, such a low carbon steel.

Figure 5.2 Tensile test of a Ductile Metal

The metal is elastic up to point A and will return to its original size if theforce is withdrawn. However, if the force is increased to point B, before beingwithdrawn, the force extension graph follows the line BC, parallel to line AO, asthe force is removed. The test piece will then be permanently extended byamount OC, and will not return to its former size. CD represents the elasticcontraction ( recovery ) which occurs as the force is removed.

Area OABD represents the work required to cause deformation OC.

If the overstrained material is again subjected to a tensile force upon atesting machine, we shall plot an entirely different force-extension graph thanwe first derived. This second graph will now have its origin at C ( instead of O),its yield point approx. at B ( instead of A ), and its breaking point at approx. the

same point as would have occurred if the first test had been completed to failure.In effect the original piece of metal in being cold-worked to point B well above

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the yield point acquires a new set of properties. These new properties result in adifferent force-extension graph being derived if the metal is reworked. This isthe most important first effect of cold working, which means that a cheaper

material can be specified for a cold forming operation, the component finishingwith new superior properties comparable to a more costly material.

The results of cold working a metal well within the plastic range of themetal can be summarized as follows:

a) The yield point, and hence the stress at yield point is raised, where

stress at the yield point = sy =areasectionalCross

pointyieldatForce

b) The ductility is lowered, and hence the elongation % is reduced,

where elongation % =lengthoriginal

extension x 100

Should high ductility be the important property, such as in a deep

drawing operation which must be carried out in several stages, then the metalmust be annealed after cold working to restore it to its original state. Fromfigure 5.2. it can be seen that no annealing takes place, then between points Aand B, each successive increment of elongation will require an increasingincrement of work. In other words, as cold working proceeds, the resistance todeformation rises steadily.

In cold working there will be minor changes in dimensions of the workpiece when the work is removed from the tool. This is the elastic -recovery of thematerial shown in the graph due to the release oil stresses causing thedeformation. The work after removal from the press tool will spring back from

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the die shape to take up a different shape. This must be allowed for in the tooldesign.

One other factor to consider in press tool work is the direction of rolling of

the strip to be worked in the press tool. Cold rolled sheet or strip is used whichhas been rolled in a certain direction thus giving the strip directional properties.It will be found that bending can be carried out more successfully across thegrain (direction of rolling), than along the grain. Components which areproduced from tools which effect bending along the direction of rolling willalmost certainly crack during the bending operation.

There are three different ways of cold working sheet metal in press toolstools. These are:1) Shearing. In this case the required shape of work is sheared from themetal strip, the metal being deformed to shear failure. There are threevariations of shearing, viz.,

a) Blanking, in which a blank is punched from the strip, the blank

removed by the punch being the required article. The metal left iswaste. The die is made to the required shape and size, i.e., the punchis made smaller than the die by the amount of clearance required.

b) Piercing, in which the blank punched or pierced from the metal strip iswaste, the hole left in the strip being required. The piercing punchmade to the required shape and size, the clearance being added to the

die.c) Cropping, in which the piece blanked from the strip is waste,

left with the cropped ends being the required part. A cropping tool isin principle, a blanking tool.

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Each of these operations is shown at Fig 5.3(a), (b) and (c) respectively

In Fig 5.3 (b) it can be seen that the operation is one of piercing threeholes followed by the strip moving forward one pitch. Then a blanking operationfollows to give a blanked and pierced component. Piercing is most commonlycarried out in conjunction with blanking.

Figure 5.3 (c) shows a cropping operation which is used when thecomponent is relatively long, and the width is sufficiently accurate without

blanking. This is economically good sense where it can be done, as the tool ischeaper than a full blanking tool.

2) Bending. This is carried out on blanks, strip, sheet, rod or wire and consistsof local deformation, as opposed to a change of shape of the complete article.Forces must be high enough to cold work the material within the plastic range.

Strip feedRequired part

b) Piercing

Piercing Waste

Figure 5.3. Press tool shearing operations

Required part

a) Blanking

Strip feed

Waste

c) Cropping

Waste

Required partStrip feed

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3) Drawing. This is carried out on blanks, and involves considerabledeformation or a complete change of shape of part. Again the deformation mustbe carried out in the plastic range. As stated earlier, deep drawing may require

several drawing stages with interstate annealing.In the next section we will consider each of these methods of press work,

and the principles of design of a typical tool of each type.

5.1. SHEARING

Figure 5.4 shows a blanking and piercing press tool. This is a simpleshearing tool, and the punches and die can be of any required profile. The toolshown has the main features of any press tool which we will examine in moredetail.

Figure 5.4. Blank and pierce tool

Pressure plate

Blanking punch

Shank

Top set

Punch plate

Bottom set

Stripper

Piercing punch

Strip feed

Press ram

Press bed

Die

Pilot

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5.1.1. Punch

Made from a non-shrinking, non-distorting alloy tool steel, such as

a high carbon, high chromium alloy steel. Punches are hardened andground, and are held in a punch plate. The shanks may be a drive fit inthe punch plate as shown. Alternatively, where there are many piercingpunches, say, which require precise location in the punch plate to matchthe die, the punches may be held in place in the punch plate by Cerromatrix or some other low melting point alloy. This method greatly eases

the problems of manufacture of the press tool.

5.1.2. Die

Made from the same metal as the punch, and like the punch isretargeted by grinding the top face. Complex die shapes may necessitate

the die being made in more than one piece. The die profile is ‘backed off’with taper as shown to allow the blanks and piercing slugs to easily fallclear into tote boxes positioned under the press bed.

5.1.3. Stripper

This may be made out of mild steel and is a clearance fit for thepunches at the top, and the strip at the side which it guides into thecorrect position under the punches. The stripper prevents the metal striplifting up with the punch as the ram returns. It also provides a means ofhousing the stops.

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5.1.4. Stop

There are many forms of stops, the one shown being a simple spring

loaded type. As the strip is pushed under the stop it rides up over the stripto drop into the space left by the blank. If the strip is then pulled backagainst the stop (against the direction of feed), the strip is correctlylocated for the next shearing operation, leaving the minimum of wastemetal between the blanked holes. This feeding operation up to a stop canbe carried out at high speed by the operator, and a well designed stop

must be simple and efficient in use.More complex tools may require more than one stop, and

arrangements can be made to operate stops by the movement of the pressram if desired. Automatic feed devices do not require soon as the strip isautomatically moved along the correct pitch length, between each strokeof the press.

5.1.5. Pilot

These are held in the blanking punch and are a clearance fit for thepierced hole. As the ram descends, the pilot locates the previously piercedhole and positions the strip more precisely under the blanking punch.

Pilots are used where the relationship of pierced holes to a blank profilemust be precise; therefore a stop initially positions the strip, but the finalpositioning depends upon the pilot. It can be seen then that thistechnique is ideal for a roll feed press which has no stops upon the too].

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5.1.6. Sets

These may be made from mild steel as they are bolsters to which

the die and punch assemblies are screwed. The top set holds the shank bymeans of which the punch assembly is located and held in the press ram.The bottom set is bolted or clamped to the press bed in the correctrelationship to the top set so that the axes of the punches tied die holesare in line.

Cast iron die sets are commercially available which are made

complete with guide puts and bushes. Hence the top set can always belocated in exactly the same position relative to the bottom set. The presstool maker then ensures that the punch and die are always a perfect fitwhen the tool is closed. This makes the setting operation on the pressquick and easy, because the complete tool is simply mounted and fastenedon the press, no locating between punch and die being necessary.

5.1.7. Pressure Plate

This is a hardened and ground steel plate which is insertedbetween punch and top set, in order to take the impact on the head of thepunch as it shears through the strip. Let us now consider some other

important features of sheet metal shearing, which affect the design of thetool.

5.2. CLEARANCE BETWEEN PUNCH AND DIE

When the punch hits the work metal strip, it penetrates a certain

percentage of the strip thickness before the metal shears and the whole blankruptures. The depth of penetration depends upon the hardness of the work

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material, being greater for a soft material, and can be seen as a polished rimaround the edge of the blank.

The amount of clearance allowed between the punch and die also affects

the appearance of the blanked edge and the accuracy of the finished blank. Theclearance varies with the thickness and the hardness of the metal being shearedand will be some value up to 10% of the strip thickness. The edge surfaceappearance and quality produced upon a standard tool will not be as good as amachined surface, but can be adequate if the correct clearance is chosen. Figure5.5 shows the effect on the blank edge of allowing insufficient clearance. Note

that the clearance is the gap between the adjacent walls of punch and due, i.e.,radial clearance for circular punches.

Fig. 5.5. Effect of Insufficient Clearance

5.3. FORCE AND WORK DONE REQUIRED FOR SHEARING

It is necessary to know the force for a particular shearing operation inorder to choose a press of adequate capacity for the press tool. The shearingforce varies during blanking (piercing or cropping) because of the nature of theprocess as just described. This is illustrated in fig. 5.6 which shows a forcepenetration graph for a blanking operation in which the correct amount of punch

and die clearance is allowed.

Ruptured surface

Polished rim

a) Correct clearanceb) Insufficient clearance

Blank

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It can be seen from the graphcurve that the force reaches the maximum (Fmax) as the punch penetrates themetal, then falls away rapidly as the metal ruptures. The graph shown is for afairly hard steel where, say, c = 15%. For soft steel, c might equal 40% but Fmax

would be much lower. This maximum punch force depends upon the edge area

to be sheared and the shear strength of the metal, therefore:

Fmax = (ultimate shear stress of material) x (shearing area) = t x material thickness (t) x work profile perimeter.

= t x t x x

The area under the force-penetration curve is equal to the work doneduring the shearing operation, therefore:

Work done = ( maximum punch force ) x (% penetration) x

(material thickness)= Fmax x c x t.

% Penetration (c)

Punch force(F)MN

Fig. 5.6. Force penetration

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This is an estimate of course, and depends upon the assumption thatmaximum punch force is sustained during complete punch penetration of thework metal.

Example 5.1

SolutionWork profile perimeter, x = p (44-45 + 22-23)

= 209-5 mm.

Fmax = t. t . x

= 0-432 x 1-59 x 209.5= 0-144 MN

Work done =Fmax . c . t =144 x 0-25 x 1-59 = 57 J.

5.4. SHEAR

It can be seen from the above expression for work done, that if the work isspread over a greater stroke movement of the punch, then Fmax will be reduced.Hence the tool could be used upon a smaller capacity press (assuming the pressshut height and stroke are satisfactory) which could be both convenient and

Calculate the maximum punch force necessary to blank a steel washer 44-45mm OD x 22.23 mm ID x 1.59 mm thick, if t = 432 N/mm2. Estimate the work

done if % penetration is 25

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more economical. This effect can be achieved by grinding either single or doubleshear upon either the punch or die. Shear is usually applied when thick metal isbeing blanked, or if the work has an extensive contour. Examples are shown atFig. 5.7.

Shear is applied to the punch { Fig.5.7 (a) } when piercing or cropping,because the slug which is punched out will be deformed.

Shear is applied to the die { Fig 5.7 (b) } when blanking, because the flatface of the punch produces a flat blank without distortion.

The amount of shear (s) to be ground upon the tool depends upon thereduction in punch force required. By a consideration of the amounts of workdone with, or without shear, the following expression can be deduced:

s =F

txcxFF -max

This is true for single or double shear.

PunchPunch

Depth of shear (s)

a) Punch withsingle shear

b) Die with doubleshear

Die Die

Figure 5.7. Shear applied toa press tool

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Example 5.2

Solution

s =F

txcxFF -max

=60

59.125.0)60144( xx-

= 0.553 mm

5.5. BENDING

Figure 5.8 shows a bending tool for producing a simple component.

Fig 5.8 Bending Tool.

In Example 5.1 calculate the amount of shear which must be ground upon thetool, if the maximum punch force is to be reduced to 0-06 MN.

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The metal blank ready for bending is positioned on the die in the locatingblocks. The punch descends, and the work is held between the punch face andthe spring loaded pressure pad. The die and punch corner radii should be as

large as possible to assist forming, and prefer any not less than twice thethickness of metal. As with shearing, the punch force varies as the operationtakes place. The pressure pad ejects the component after forming.

5.5.1. Force Required for Bending

Allowing for friction, a satisfactory estimate can be obtained or themaximum punch force necessary for bending, by assuming that thebending stress is half the shear stress for the material.

Referring to Fig 5.9 the maximum punch force for completing onebend of the component shown is:

Maximum punch force = (Bending area) x (Bending stress)

\Fmax = l x t x sB

For bending the whole component,

Fmax = 21.x t.x sB

Figure 5.9. Component

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Where t, = metal thickness l = bend lengthsB = bending stress

=2t

Bending forces are less than shearing loads, and the above approx-imation gives a generous allowance. However, a more precise value ofpunch forces could be obtained by a detailed consideration of the bendingmoments involved.

5.5.2. Planishing

An important consideration which often arises when bending is thatof planishing the component. If this is required, the wall clearancebetween punch and die must be made less than t, and the bottom positionof the press ram stroke must be set such that the bottom clearance in thedie is less than t. Then the component will also be ironed out, or

planished, as well as being formed to shape, and the material must bestressed above yield point to achieve this.The additional punch force necessary for planishing is given by:Planishing force = (planishing area) x (yield stress)

\Fp = l x J x sy

The effect of planishing is to smooth and flatten the work and is done toimprove the finish and set the bends.

5.5.3. Allowance for Bending

When bending takes place, the outside layers of the work are undertension and are thus lengthened, and the insides layers are under

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compression, aid are thus shortened. As the volume of metal remains thesame, the width on the compressed side of the bend increases, while thaton the stretched side of the bend decreases. This gives the characteristicdeformed shape on bend sides that one gets if bending a flat plate in a

vice. This deformation leads to displacement of the neutral axis towardsthe compressed side, because the neutral axis coincides with the centre ofgravity of the work.

To calculate the length of blank required for bending, we must firstcalculate the length of the neutral axis of the finished formed component.In practice an allowance is then added for bending to compensate for the

shift in the neutral axis. These allowances will be found tabulated in toolreference and design books.

Example 5.3

Solution

a) Fmax = 21. t. sB where sB =2t = 193 N/mm2

= 2 x 38.1 x 3.18 x 0.193 = 0.047 MN

The component shown in Fig 5.9 has the following dimensions: J = 76.2 mm, I

= 38.1 mm, t = 3.18 mm, r = 6.35 mm and h = 31.75 mm. Calculate (a) themaximum punch force for bending the complete component, (b) the force ifplanishing is carried out, and (c) the strip length necessary to make the part(ignoring bend allowances). Take t as 386 N/mm2 and sy as 278 N/mm2.

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b) Fp =l. J. sy , where sy = 278 N/mm2

= 38.1 x 76.2 x 0.278= 0.806 MN

c) Length of' vertical sides.= 2(h - t - r)=2(31.75 – 3.18 – 6.35) = 2 x 22.22 = 44.44 mm

Length along bottom= J - 2r= 76.2 - (2 x 6-35) = 63-5 mm

Length of bends

= p (r +2t )

= 3-142 (6-35 + 1-59) = 24-95

\Total developed length = 44-44 + 63-5 + 24-95

= 132-89 mm

In this example, right angled bends are required. If the bend is formed ina right angled die, then elastic recovery, or spring back will occur after bending,which can cause difficulty in ejection. Also the corner angle will be greater than90º. Different techniques are used to overcome this such as planishing andsetting the bend, or overbending for example.

Other standard forms of bending tools are shown at Fig 5.10, the methodused at (b) being similar to a press brake technique.

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5.6. DRAWING

Figure 5.11 shows a typical drawing tool in which a blank is being drawninto a cylindrical cup. The work is shown in a partially drawn state, the punchnot yet having 'bottomed'.

Fig 5.10. Press Bending Tools

Pressure pads

Bottom set

Work

Die

Punc

h

Figure 5.11. Drawing tool

Punch Work piece

Die

Work piece

Punch

Pressure pad

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The spring (or rubber) loaded pressure pad on the punch keeps the metaltight against the die face, and the blank is ironed out as it is drawn over theradiuses edge of the die hole. This prevents wrinkling of the cup and keeps the

finished edge of the rim straight. The pressure which is applied by this pad isimportant; no pressure gives heavy wrinkling and excessive pressure results inthe bottom being pressed out of the cup. The optimum pad pressure giving, bestresults varies with the type of work, but will be to the order of 30 to 40% of thedrawing pressure. Figure 5.12 shows the stresses which Occur as drawing takesplace.

As the blank is crowded into the die with a consequent reduction indiameter, the compressive stress in the undrawn flange will cause wrinkling ifthis stress exceeds the tensile stress. The correct use of a pressure pad preventsthis. If, on the other hand, the force on the blank is such that the tensile stressin the wall exceeds the ultimate tensile stress (su) of the material, then the walls

will crack and rupture. However, referring to Fig 5.2, the force must be of suchmagnitude that the component is worked within the plastic range of the

material. This leads to the conclusion that, for deep drawing particularly, the

material must be very ductile having a low ratio:u

y

ss

Fig 5.12. Drawing Stresses.

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The die pressure pad shown in Fig 12.11 supports the work and also actsas an ejector.

The restriction mentioned above on tensile forces means that there is a

limit to the amount of drawing which can be attempted in one operation. It has

been found that the drawing ratiodD should not exceed a value of 2 ( where D =

blank diameter and d = cup diameter) for most materials, although this dependsupon the value of su for the material. A deep drawn cup may have to be

completed in a series of operations.

5.6.1. Blank Size

In order to calculate the blank diameter D to produce a cup ofdiameter d, it is necessary to equate the surface area of each. This leadsto the well known expression:

D = dhd 42 + Where h = cup height

This makes no allowance for a corner radius, and in practice onemay finally adopt a trial and error technique; in drawing work experienceis at a premium.

5.6.2. Force Required for Drawing

Again, the force will vary as the operation proceeds, but areasonable approximation for the maximum punch force can beestablished using the expression:

Fmax = p. d. t. su where su = the ultimate tensile stress of the material.

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Example 5.4

Solution

D = dhd 42 +

= )0.382.764(5800 xx+

= 116005800 +

= 17400

= 132 mm

Drawing ratio =dD =

2.76132

= 1.73\assume the cup can be drawn in one operation.Fmax = p. d. t. su

= 3.142 x 76.2 x 0.8 x 0.432 = 0-083 MN

5.7. COMBINATION TOOLS

It is often possible where batch quantities justify it to incorporate twooperations into the one too] such that the operations are carried out incombination. This increases the production rate hence reducing the variablecosts with little increase in fixed costs. The dual operation is usually blank anddraws and is very successful on thin materials where the total force required is

not impracticable, and the component can be drawn to full depth in the oneoperation. Figure 5.12 shows the principle of operation of such a too].

A cup is to be drawn to a diameter of 76.2 mm x 38-0 mm deep in 0.8mm thickmaterial. Estimate the blank diameter and the maximum drawing force if su

= 432 N/mm2

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The punch blanks upon diameter D and draws on diameter d. The punchand die pressure pads are spring loaded. As the punch descends in the guide, itshears the blank from the strip, and the blank will be gripped between thepunch face and the pressure pads. The punch continues to descend, and the

blank is drawn over the drawing die. The drawn cup is in an inverted position tothat shown in Fig 5.12.

As the punch withdraws, the pressure pads act as ejectors so that the cupdoes not stick to the die or punch. The punch guide can also be arranged to actas a stripper, so that the strip is removed from the punch.

5.8. PROGRESSION TOOL

The combination tool described in the previous section combines morethan one press operation at the one tool station, and the punches and dies arepositioned on the same axis. A progression tool, by contrast, has more thin onepress operation positioned at separate stations oil the press tool set, i.e., there is

Fig 5.12. Combination Too].

Punch (blanking& drawing)

Drawing die StripPressure pads

Fixed punch guide

D

d

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one station for- each operations these following on behind each other. The stripis therefore progressively, worked upon as it is fed through the tool. Simple toolsof this type, such as the tool shown at Fig 5.10 are called follow-on tools, and areusually blank and pierce tools.

Large progression tools can be very complex and expensive to make,therefore they are only suitable for high quantity production, and the strip feedwill be automatic. Complex progression tools may include a combination typeoperation at one of the stations.

Figure 5.13 shows the principle of operation of a progression tool.

Assume the component being produced in the progression tool shown issome type of locking wisher having formed tabs upon either side. The washer isblanked and pierced at station 1, and finished formed at station 3. In all, fivestations are provided, spaced an equal pitch apart, the strip being roll fedthrough the guide and moving one pitch forward each time the puncheswithdraw. Stations 1, 3 and 5 are working stations, and stations 2 and 4 are idle

Strip feed

Strip guide Stripper plate

Station

Pitch

Fig 5.13. Progression Tool.

Die

Punch plate

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stations. This is done to provide room for the punches, and to strengthen the dieblock. The blanks and formed washers are carried in the strip from station tostation before the finished washer is finally pushed out of the strip by the ejectorpunch at station 5. Let us examine each operation at each station in turn.

Station 1Blank and pierce operation. The fixed piercing punch in the bottom tool

pierces a slug as the top tool descends. The slugs are pushed up the centre of theblanking punch to finally fall out through the exit hole in the punch plate. Thespring loaded pressure pad in the bottom tool supports the blank and pushes it

back into the strip after blanking and piercing are complete.

Station 2Idle operation. The stripper holds the blank and strip as the top tool

descends.

Station 3Forming operation. Bending of tabs takes place on the washer being

supported by a pressure pad. Again, the pressure pad pushes the formed washerback into the strip to allow it to be carried to the next station. A pilot in thepunch locates the pierced hole ensuring the blank is accurately positioned forforming.

Station 4Idle operation. The stripper plate has a clearance slot cut in it so that the

washer tabs are not damaged as the stripper plate grips the strip.

Station 5

Ejection operation. The finished washer is pushed out of the strip to fallclear of the tool into a hopper. These tools are very fast in operation and the

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amount of scrap strip left to deal with can create problems. On some tools thescrap strip is cut up by the tool to be ejected off the die face using compressed airfor example. Sometimes a jet of compressed air is used for ejection purposes

upon a press tool.

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5.1. By considering the costs involved, compare the cold working of a component to

shape, with machining of a component to shape.

5.2. Name and write short notes on three types of cold working sheet metal works.

5.2. What is the difference between blanking and piercing?

5.3. Draw and label a schematic diagram of a Blanking and Piercing tool.

5.5. Clearly show the difference between combination tools and progression tools, and

sketch an example of each type.

ACTIVITY 5

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5.1. Machining – wasteful, 40% of the original component material is removed

by expensive machining operations to become scrap. This scrap material willonly be worth 5% of its original value as raw material. It is more economical tobe carried out by metal forming.

Although dies are expensive and fixed costs are higher; material wastageis negligible, labor costs are low and hence variable costs are lower. The metalcutting process is often lower on fixed costs, but higher on variable costs. More

economical when quantities required are large.

5.2.1 Shearing. The required shape of work is sheared from the metal strip, themetal being deformed to shear failure. There are three variations of shearing,viz., 1. Blanking 2. Piercing 3. Cropping

5.2.2. Bending -This is carried out on blanks, strip, sheet, rod or wire andconsists of local deformation, as opposed to a change of shape of the completearticle. Forces must be high enough to cold work the material within the plasticrange.

5.2.3. Drawing. This is carried out on blanks, and involves considerable

deformation or a complete change of shape of part. Again the deformation must

FEEDBACK ON ACTIVITY 5

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be carried out in the plastic range. Deep drawing may require several drawingstages with interstate annealing.

5.2. Blanking, in which a blank is punched from the strip, the blank removed

by the punch being the required article. The metal left is waste5.3. Piercing, in which the blank punched or pierced from the metal strip is

waste, the hole left in the strip being required.

5.4.

Blank and pierce tool

Pressure plate

Blanking punch

Shank

Top set

Punch plate

Bottom set

Stripper

Piercing punch

Strip feed

Press ram

Press bed

Die

Pilot

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5.5.

Strip feed

Strip guide Stripper plate

Station

Pitch

Progression Tool.

Die

Punch plate

Combination Too].

Punch (blanking& drawing)

Drawing die StripPressure pads

Fixed punch guide

D

d

Blanking die

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Combination.Tool - It increases the production rate hence reducing the variablecosts with little increase in fixed costs. The dual operation is usually blank and

draws and is very successful on thin materials where the total force required isnot impracticable, and the component can be drawn to full depth in the oneoperation. Note the drawing die and blanking die placed just below the strip.

Progression Tool. - It has more thin one press operation positioned at separatestations oil the press tool set, i.e., there is one station for- each operations these

following on behind each other. The strip is therefore progressively, workedupon as it is fed through the tool. Simple tools of this type, such as the toolshown at the above figure are called follow-on tools, and are usually blank andpierce tools. Note the number of stations in the figure above (5 stations)

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1. A steel blank 45 mm square having a 22mm diameter hole in the centreis to be blanked from 1mm thick sheet.(a) Calculate the maximum punch force necessary to shear the blank in

one operation if t =390 N/mm2.

(b) Calculate the work done if the % penetration is 20%(c) What will be the % reduction in punch force if 0.5mm double shear

is ground upon the tool?

2. A component is formed on a bending tool from 6 mm x 35 mm materialinto a ‘U’ shape 55mm wide, having unequal arms 62 mm and 48 mm longrespectively. The internal radius at each bend corner is 12 mm.Calculate:

(a) The maximum punch force for bending the part in one operation.

(b) The additional force if planishing is carried out

(c) The developed length of the part (ignoring bend allowances)

Take t as 400 N/mm2 and sy as 350 N/mm2.

3. Estimate (a) the maximum punch force, and (b) the blank diameter, for acup which is to be drawn to 62.3 mm dia. X 21.4 mm deep in 0.8 mm thickmaterial. su = 325 N/mm2.

SELF-ASSESSMENT 5

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4. Discuss the factor s involved in the cold pressworking processes and show theeffect of these factors on the design of press tool.

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1. (a) 0.097 MN, (b) 19.4 J, (c) 71.3%

2. (a) 0.084 MN, (b) 0.59 MN (c) 140.2 mm

3. (a) 0.051 MN, (b) 96 mm

4. (i) Plasticity – high ductility material used (in deep drawing, operations done in stages) –

metal must be annealed after cold working.

(ii) Elasticity – elastic recovering of the material due to the release of stresses cause

by/during deformation.

(iii) Direction of rolling of the strip to be worked in the press tool, e.g. bending – direction

of rolling across the grain is better than along the grain.

FEEDBACK OF SELF-ASSESSMENT 5

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