Turning Drilling Milling Shaper Grinding

8/4/2019 Turning Drilling Milling Shaper Grinding

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TURNING AND BORING AND RELATED PROCESSES

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Dimensional Accuracy

Dimensional Accuracyin turning operation is controlled by many factors, includingthe wear at the nose of the tool [figure 23-7]. Precision is influenced by deflection

due to the cutting forces and surface roughness. Tool wear causes the work piecedimension to change from the initial diameter when the tool is sharp to the diameter

obtained after the tool has worn. The cutting forces increase as the tool wears,

resulting in an increased deflection between the work piece and cutting tool. Built-up

edge (BUE) may form at the tip of the cutting tool.

The BUE has a tendency to change the actual diameter of the workpiece. Thus to hold close tolerances, the size of the wear land, themagnitude of the radial [thrust] force, and the elimination of the BUEshould be taken into account.

Turned surfaces display characteristic turning grooves that are produced by the feed

and the tool tip corner radius as shown in figure 23-7. The roughness resulting from

feed marks from round-nosed tool can be approximated by the formula:

CR

ffCRCRy r

r

84

222

=

=

Wherey is the roughness height, CR the corner radius of insert, and frthe feed rate

(mm/rev.)

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To improve the surface finish, reduce feed and increase the corner radius. Other

factors like BUE formations, cutting-edge sharpness, and tool wear grooves in theflank wear area also affect the surface finish in turning. Flank wear and BUE can

combine to affect both surface finish and accuracy as shown in figure 23-7.

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DRILLING AND RELATED HOLE-MAKING PROCESSES

Introduction

In manufacturing it is probable that more holes are produced than any othershape, and large proportion of theses are made by drilling. Of all the machining

processes performed, drilling makes up to 25%. Consequently, drilling is a very

important process. Although drilling appears to be relatively simple process, it is

rea lly c om plex proc ess. Most d rilling is done with a too l having two c utting ed ge s

or lips as shown in figure 24-1. This is a tw ist d rill, the most c ommon d rill geometry.

The c utting ed ge s a re a t the end of rela tively flexib le to ol. Cutting a c tion takes

p lace inside the wo rk piec e. The only exit fo r the c hips is the hole tha t is filled by

the d rill. Fric tion results in hea t tha t is add itiona l to tha t d ue to c hip fo rma tion. The

c ounte r flow of the c hips ma kes lubric a tion a nd c oo ling d iffic ult.

There are four major actions taking place at the point of the drill.

1. A small hole is formed by the web-chips are not cut here in the normal sense.2. Chips are formed by the rotating lips.3. Chips are removed from the hole by the screw action of the helical flutes.4. The drill is guided by lands or margins that rub against the walls of the hall.

Figure 24-1 Nomenclature and geometry of conventional twist drill. Shank style dependsupon the method used to hold the drill. Tangs or notches prevent slippage:

a) Straight shank with tang, b) Tapered shank with tang,c) Straight shank whistle notch, d) Straight shank with flat notch.

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FUNDAMENTALS OF THE DRILLING PROCESS +Summary of drilling parameters in table.

Refer to Text book:Materials and processes in manufacturing, E. Paul Degarmo

Two-flute drills are available that have holes extending throughout the length ofeach land to permit coolant to be supplied, under pressure, to the point adjacent toeach cutting edge. These are helpful in providing cooling and also in promotingchip removal from the hole in drilling to moderate depths. They require specialfittings through which the coolant can be supplied to the rotating drill, and theyare used primarily on automatic and semiautomatic machines.

Larger holes in thin material may be made with a hole cutter(Figure 24-8), whereby the main hole is produced by the thin-walled, multiple-tooth cutter with sawteeth. Hole cutters are often called hole saws.

When starting to drill a hole, a drill can deflect rather easily because of the "walking"action of the chisel point. Hole location accuracy is lost. Consequently, to assure that

a hole is started accurately, a center drill (Figure 24-9) is used prior to a regular

chisel-point twist drill.

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Spec ia l c om bination d rillscan drill two or more diameters, or drill andcountersink and/or counter bore, in a single operation (Figure 24-10).

Countersinking and counter boring usually follow drilling. A step d rillhas

a single set of flutes and is ground to two or more diameters. Sub land

d rillshave a sep a ra te set of flutes on a single bod y for ea c h d iame ter

or op era tion; they p rovide b ette r c hip flow , and the c utting edges c an

be ground to give proper cutting conditions for each operation.

Combination drills are expensive and may be difficult to regrind but

can be economical for production-type operations if they reduce

work hand ling , setups, or sep ara te m ac hines and operat ions.

FIGURE 24-10 Special purpose subland drill (above),

and some of the operations possible with such drills (below).

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REAMING Reaming removes a small amount of material

from the surface of holes. It is done for two purposes: to bring holes

to a more exact size and to improve the finish of an existing hole.

Multiedge cutting tools are used as shown in Figure 24-23. No special

machines are built for reaming. The same machine that was employed

for drilling the hole can be used for reaming by changing the cutting

tool. To obtain proper results, only a minimum amount of materials

should be left for removal by reaming. As little as 0.13 mm is

desirable, and in no case should the amount exceed 0.4 mm. A

properly reamed hole will be within 0.0254 mm of correct size and

havea fine finish.

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Figure 24-24 Types of reamers: (top to bottom)

Straight-fluted rose reamer, Straight-fluted chucking reamer,Straight-fluted taper reamer, Straight-fluted hand reamer,

Expansion reamer, Shell reamer,Adjustable insert-blade reamer

MILLINGINTRODUCTIONMilling is a basic machining process by which a surface is generated by

progressive chip removal. The workpiece is fed into a rotating cutting tool.

Sometimes the workpiece remains stationary, and the cutter is fed to the work.

In nearly all cases, a multiple-tooth cutter is used so that the material removal

rate is high. Often the desired surface is obtained in a single pass of the cutter

or work and, because very good surface finish can be obtained, milling is

particularly well suited and widely used for mass-production work.

Many types of milling machines are used, ranging from relatively simple andversatile machines that are used for general-purpose machining in job shops and

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tool-and die work (these are NC or CNC machines) to highly specialized

machines for mass production. Unquestionably, more flat surfaces are produced

by milling than by any other machining process.

The cutting tool used in milling is known as a milling

cutter. Equally spaced peripheral teeth willintermittently engage and machine the workpiece. This iscalled interrupted cutting.

FUNDAMENTALS OF MILLING PROCESSES

Milling operations can be classified into two broad categories called peripheral milling

and face milling. Each has many variations. Inperipheral millingthe surface is generat-

ed by teeth located on the periphery of the cutter body (Figure 25-1). The surface is

parallel with the axis of rotation of the cutter. Both flat and formed surfaces can be

produced by this method, the cross section of the resulting surface corresponding tothe axial contour of the cutter. This process, often called slab milling, is usually

performed on horizontal spindle milling machines. In slab milling, the tool rotates at

some rpm (Ns)while the work feeds past the tool at a table feed rate fm in mm per

minute, which depends on the feed per tooth, ft.

As in the other processes, the cutting speed Vand feed per tooth are selected bythe engineer or the machine tool operator. As before, these variables depend uponthe work material, the tool material, and the specific process. The cutting velocity isthat which occurs at the cutting edges of the teeth in the milling center. The rpm of

the spindle is determined from the surface cutting speed, where D is the cutter ofdiameter in [mm] according to:

Ns = 1000 V/ D

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The depth of cut, called DOC in Figure 25-1, is simply the distance between the oldand new machined surface. The width of cut is the width of the cutter or the work,in [mm], and is given the symbol W. The length of the cut L is the length of thework plus some allowance LA,for approach and over travel. The feed of the tablefm, in [mm per minute], is related to the amount of metal each tool removesduring a revolution, the feed per tooth ft, according to

fm= ftNsn where nis the number of teeth in the cutter (teeth/rev.).The cutting time Tm= L + LA/ fm

Wherethe length of approach = LA =2

2

)2

(4

DOCDD

= DOCDt (

MRR = volume / Tm = L W DOC / Tm = W fm DOC m m3/ min

Ignoring LA, Values for ft are given in Table 25-1 along with recommended cuttingspeeds in m / min.

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UP VERSUS DOWN MILLING

For either slab end or face milling, surfaces can be generated by twodistinctly different methods [figure 25-4]. Up milling is the traditional way tomill and is called conventionalmilling. The cutter rotates against the directionof feed of the workpiece.

In climb or down milling, the cutter rotation is in the same direction as thefeed rate. The method of chip formation is completely different in the twocases.

In up milling, the chip is very thin at the beginning where the tooth first contactsthe work, then it increases in thickness, becoming a maximum where the toothleaves the work.The cutter tends to push the work along and lift it upward from the table. This ac-tion tends to eliminate any effect of looseness in the feed screw and nut of themilling machine table and results in a smooth cut. However, the action also tends

to loosen the work from the clamping device; therefore, greater clamping forcesmust be employed with the danger of deflecting the part.

In addition, the smoothness of the generated surfacedepends greatly on the sharpness of the cutting edges. Inup milling, chips can be carried into the newly machinedsurface, causing the surface finish to be poorer (rougher)than in down milling.

In down milling, maximum chip thickness occurs close to the point at

which the tooth contacts the work. Because the relative motion tendsto pull the work piece into the cutter, any possibility of looseness inthe table feed screw must be eliminated if down milling is to be used.

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It should never be attempted on machines that are not designed forthis type of milling.

Virtually all modern milling machines are capable of doing downmilling and it is a most favorable application for carbide cutting

edges. Because the material yields in approximately a tangential di-rection at the end of the tooth engagement, there is less tendency(than when up milling is used) for the machined surface to showtooth marks, and the cutting process is smoother with less chatter.Another advantage of down milling is that the cutting force tends to hold thework against the machine table, permitting lower clamping forces. However,the fact that the cutter teeth strike against the surface of the work at the

beginning of each chip can be a disadvantage if the workpiece has a hard sur-face, as castings sometimes do. This may cause the teeth to dull rapidly.Metals that readily workharden should be climb milled.

MILLING TOOLS AND CUTTERS

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Figure 25-10 The chips are formed progressively by the teeth of a plain helical-toothmilling cutter during up milling.

SHAPER AND PLANERINTRODUCTION TO SHAPING AND PLANING

The processes ofshaping andplaning are among the oldest single-point machining

processes. Shaping has largely been replaced by milling and broaching, as a

production process while planing still has applications in producing long flat cuts,

like those in the ways of machine tools. From a consideration of the relativemotions between the tool and the workpiece, shaping and planing both use a

straight-line cutting motion with a single-point cutting tool to generate a flat

surface.

In shaping, the workpiece is fed at right angles to the cutting motion

between successive strokes of the tool, as shown in Figure 26-1, where fcisthe feed per stroke, V is the cutting speed, and t is the DOC. For eithershaping or planing, the tool is held in a clapper box which prevents thecutting edge from being, damaged on the return stroke of the tool.

In addition to plain flat surfaces, the shapes most commonly produced onthe shaper and planer are those illustrated in Figure 26-2. Relatively skilledworkers are required to operate shapers and planers, and most of theshapes that can be produced on them also can be made by much more

productive processes, such as milling, broaching, or grinding.

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Figure 26-2 : Types of surfaces commonly machined

by shaping and planing

In shaping, the cutting tool is held in the tool post located in the ram, whichreciprocates over the work with a forward stroke, cutting at velocity Vand aquick return stroke at velocity VR. The rpm of drive crank (Ns) drives theram and determines the velocity of the operation. See Figure 26-1d. Thestroke ratio

Rs = cutting stroke angle / 360 = 200 / 360 = 5 / 9

The tool is advancing 55% of the time. The number of strokes per minute isNs, determined by the

rpm of the drive crank. Feed,fcis in mm per stroke and is at right angles to the cutting direction.

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The length of stroke l must be greater than the length of the workpiece [or length of

cut] L, since velocity is position variant. Let L= twice the length of the block being

cut or 2L. The cutting velocity Vis assumed to be twice the average forward velocityV, of the ram. The general relationship between cutting speed and rpm is

V = D Ns/ 1000 m/min whereD is the diameter (of rotational member) in mm.

For shaping, the cutting speed V= 2l Ns / 1000 Rs

Once a cutting speed (V) is selected, the rpm, Ns, of the machine can be calculated.

Tables for suggested feed values, fc, in mm per stroke (or cycle) and recommended

depths of cut are also available. The maximum depth of cut is based on the power

available to form the chips. This calculation requires that the metal removal rate

(MRR) be known. The MRR is the volume of metal removed per unit time:MRR = L W DOC / Tm mm

3/ min

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where W is the width of block being cut and L is the length of block being cut, so

volume of cut = WLDOCwhere DOC is the depth of cut and Tm is the cutting time in

minutes to cut that volume.

In general, Tm is the total length of the cut divided by the feed rate. For shaping,

Tm is the width of block divided by the feed rate fc of the tool moving across the

width. Thus, for shaping, Tm= W / Nsx fcAlso, Tm = S / Ns where the number of strokes for the job is S = W / fc for a surface

of width W.

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Abrasive Machining ProcessesINTRODUCTION

Abrasive machining is a material-removal process that involves the interaction of abrasive grits with the workpiece at

high cutting speeds and shallow penetration depths. The chips that are formed resemble those formed by other

machining processes. Unquestionably abrasive machining is the oldest of the basic machining processes.

The results that can be obtained by abrasive machining range from the finest an smoothest surfaces produced by any

machining process, in which very little material is removed, to rough, coarse surfaces that accompany high material-removal rates. The abrasive particles may be (1) free (see Chapter 31); (2) mounted in resin on a belt (called coa

product); or most commonly (3) close packed into wheels or stones, with abrasive grits he together by bonding material

(called bonded productor a grinding wheel). Figure 27-1 shows

Compared to machining, abrasive machining processes have three

unique characteristics. First, each cutting edge is very small, and

many of these edg es c an cut simultaneously. When suitab le mac hine

tools are employed, very fine cuts are possible and fine surfaces and

close dimensional control can be obtained. Sec ond, bec ause ex-

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tremely hard abrasive grits can be produced, including diamonds,

very hard materials, such as hardened steel, glass, carbides, and

ceramics, can readily be machined. As a result, the abrasive

machining processes are not only important as manufacturing

processes, they are indeed essential. Many of our modern products,

such as modern machine tools, automobiles, space vehicles, andaircraft, could not be manufactured without these processes. Third, in

grinding, you have no control over the actual tool geometry (rake

angles, cutting edge radius) or all the cutting parameters (depth of

cut). As a result of these parameters and variables, grinding is a

c omplex p roc ess.

ABRASIVES

An abrasive is a hard material that can cut or abrade other substances. Naturalabrasives have existed from the earliest times. Early grinding wheels were cut fromslabs of sandstone, but because they were not uniform in structure throughout, theywore unevenly and did not produce consistent results. Emery,a mixture of alumina(Al203) and magnetite (Fe3O4), is another natural abrasive still in use today and isused on coated paper and cloth (emery paper). Corundum (natural Al203 ) anddiamonds are other naturally occurring abrasive materials. Today, the only naturalabrasives that have commercial importance are quartz sand, garnets, anddiamonds. For example, quartz is used primarily in air blasting, but artificialabrasives are also making inroads in these applications.

Hardness,the ability to resist penetration, is the key property for an abrasive. Theparticles must be able to decompose at elevated temperatures. Two otherproperties are significant in abrasive grits, attrition, and friability. Attritionrefers tothe abrasive wear action of the grits resulting in dulled edges, grit flattening, andwheel glazing. Friability refers to the fracture of the grits and is the opposite oftoughness. In grinding it is important that grits be able to fracture to expose new,sharp edges.

Artificialabrasives called [silicon carbide(SiC)]. As can be seen in Figure 27-2, theresulting grits, or grains, are regular in shape, with cutting edges having everypossible rake angle. Silicon carbide crystals are very hard (Knoop 2480), friable,and rather brittle. This limits their use. Silicon carbide is sold under the trade namesCarborundum and Crystolon.

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Aluminum oxide (Al203 ) is the most widely used artificial abrasive. Althoughaluminum oxide is softer (Knoop 2100) than silicon carbide, it is considerablytougher. Consequently, it is better general-purpose abrasive.

Diamonds are the hardest of all materials. Those that are used for abrasives areeither natural, off-color stones (called garnets)that are not suitable for gems, orsmall synthetic stones that are produced specifically for abrasive purposes.Manufactured stones appear to be somewhat more friable and thus tend to cut

faster and cooler. Diamond abrasive wheels used extensively for sharpeningcarbide and ceramic cutting tools. Diamonds also used for truing and dressingother types of abrasive wheels. Diamonds are usually used only when cheaperabrasives will not produce the desired results. Garnets are used primarily in theform of very finely crushed and graded powders for fine polishing.

ABRASIVE GRAIN SIZE AND GEOMETRYTo enhance the process capability of grinding, abrasivegrains are sorted into sizes by mechanical sievingmachines. The number of openings per linear inch in a sieve

(or screen) through which most of the particles of aparticular size can pass determines the grain size (Figure27-3).

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A No. 24 grit would pass through a standard screen having24 openings per inch but would not pass through onehaving 30 openings per inch. Commercial practicecommonly designates grain sizes from 4 to 24, inclusive, ascoarse; 30 to 60, inclusive, as medium; and 70 to 600,inclusive, as fine. Grains smaller than 220 are usuallytermed powders. Sizes from 240 to 600 are designated asfloursizes. These are used primarily for lapping, or in finehoning stones for fine finishing tasks.

The grain size is closely related to the surface finish andmetal removal rate. In grinding wheels and belts, coarsegrains cut faster while fine grains provide better finish, asshown in Figure 27-4.

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SURFACE TREATMENTS, FINISHING, AND INTEGRITY

INTRODUCTION

Many manufacturing processes influence surface properties which in turn may significantly affect the way thecomponent functions in service. The demands for greaterstrength and longer life in components often depend onchanges in the surface properties rather than the bulkproperties. These changes may be mechanical, thermal,chemical, and/or physical and therefore are difficult todescribe in general terms.

Many metal-cutting processes intended to produce a specific geometry can oftenhave the effect of producing alterations in the surface material of the component,which in turn, produces changes in performance.

The term "surface integrity" was defined in reference to the nature of the

surface condition that is produced by the manufacturing process.If we view the

process to have five main components (workpiece, tool, machine tool,

environment, and process variables) we see that surface properties can be

altered by all of these parameters by producing:

. High temperatures involved in the machining process

. Plastic deformation of the work material (residual stress)

. Surface geometry (roughness, cracks, distortion)

More specifically, surface integrity refers to the enhanced surface condition of a

component or specimen which influences its performance. Change in surfacecan lead to failure in base metal.

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Surface integrity has two aspects: topography characteristics and surface layer

characteristics. Topography is made up of surface roughness, waviness, errors of

form, and flaws (Figure 31-2).

A typical roughness profile includes the peaks and valleys that are considered sep-arately from waviness. Flaws also add to texture, but should be measured

independent of it. Changes in the surface layer, as a result of processing, include

plastic deformation, residual stresses, cracks, and other metallurgical changes

(hardness, over aging, phase changes, recrystallization, intergranular attack, and

hydrogen embrittlement). See Figure 31-2.

The surface layer will always sustain local surface deformation due to a machining

pass, a traditional process.

Surface integrity has become the subject of intense interest because the traditional,non-traditional, and post treatment methods used to manufacture hardware can

change the material's properties. Although the consequences of these changes

become a design problem, the preservation of properties is a manufacturingconsideration. Designs that require a high degree of surface integrity are the onesthat:

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. Are highly stressed

. Employ low safety factors

. Operate in severe environments

. Must have prime reliability

. Have a high surface area to volume ratio

. Are made with alloys that are sensitive to processing

Turning Drilling Milling Shaper Grinding

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