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MANUFACTURING PROCESSES, Second Edition

J.P. Kaushish

© 2010 by PH I Le arning Pr iva te Limi ted , Ne w Delhi . Al l ri ght s re se rv ed . No part of thi s bo ok

may be reproduced in any form, by mimeograph or any other means, without permission in

writing from the publisher.

ISBN-978 -81 -203 -408 2-4

The export r ights of th is book are vested solely with the publ isher.

Second Printi ng (Secon d Edit ion) ... ... August , 2010

Publ ished by Asoke K. Ghosh. PHI Learning Pr ivate Limited. M-97, Connaught Circus,

New Delhi-110001 and Pr inted by Mohan Makhi jani at Rekha Printers Pr ivate Limited,

New Delhi-110020.



28. In centrifugal castings, impurities are:

(a) uniformly distributed in casting

(b) forced towards outer surface

(c) collected close to centre of casting

29. Centrifugally cast products have

(a) large grain structure with high porosity

(b) fine grain structures with high density

(c) fine grain structure with low density

(d) segregation o f slag towards the outer skin of casting

30. In green-sand molding process, uniform ramming leads to

(a) less chance o f gas porosity

(b) uniform flow of molten metal into the old

(c) greater dimensional stability of casting

(d) less sand expansion type of casting defect

A n sw ers o f ob jective type questions

1. (a) 2. (b) and (c) 3. (c) 4. (d) 5. (d)

6. (a) 7. (c) 8. (a) 9. (b ) 10. (a) 11. (c)

12. (c) 13. (d) 14. (a) 15. (b) 16. (b) 17. (c)

18. (b) 19. (c) 20. (b) 21. (c) 22. (b) 23. (c)

24. (d) 25. (c) 26. (b) 27. (e) 28. (c) 29. (b)

30. (c).



*u •

Metal Machining Processes and Machine Too

6 . 1 I N T R O D U C T I O N

Various manufacturing processes used for transforming metals into some usable products are

based on basic pro perti es of meta ls , fo r example, the pro cess o f casting is based on the

property o f ‘feasib il it y’ (or melt in g), forging on the property of ‘malleability’, and rolling or

forming on the property of ‘ductility’. Likewise, the process of machining is based on the

pro perty o f ‘divis ib ili ty ’, whic h is th e capabil ity o f metal fo r gett in g div id ed in to small bi ts

and separated from the workpiece in the form of chips. Blank is the piece of metal out ofwhich a product or a component o f some use is machined out. Machining consists o f forcing

a cutting tool of harder material through the excess (or surplus) material on the workpiece

blank; the excess mate rial bein g pro gre ssiv ely separa te d from th e bla nk in th e fo rm of chips

because o f th e rel ative moti on main ta in ed between the tool an d th e workpie ce. The opera tion

finally results into a transformed product machined to the desired shape and size.

Metal machining or metal cutting comprises those processes wherein removal of material

from a workpiece is effected by relative motion between the cutting tool and the workpiece.

The cutting tool may be (a) single-point cutting tool as used for turning on lathe or shaping

or (b) m ulti-point cutting tool as used for drilling or milling operations. Basic elements of a

machining operation include (a) workpiece, (b) tool and (c) chip. Workpiece provid es the

pare nt meta l from whic h unwante d meta l in th e fo rm o f chip is removed by th e cutting act ion

of tool for getting the desired shape and size of the manufactured product. The machining

operation is greatly affected by the chemical c omp osition and physical properties of workpiece

metal. Tool material and tool geometry pl ay an im portant ro le in machin ing effe ctiv ely and

economically. Similarly, type and geometry o f chip are affected by metals of workpieces and

tool, geometry of tool and cutting fluid. The process of machining has gained importance as

it successfully and economically meets the basic objectives of manufacturing a product,

such as highe r metal removal rates, high class finish on the wo rkpiece, p roduction of

componen ts of intricate shapes, less power consum ption in comparison to m any o ther production



methods, etc. However, one major drawback of machining process is loss of material in the

form of chips. Metal cutting processes are performed on metal cutting machines or machine

tools using different types of cutting tools.

6 .1 .1 C l a s s i f i c a t i o n o f M a c h i n i n g P r o c e s s e s

Machining processes can be broadly classified as follows:

(a) Metal cutting processes using (i) single-point cutting tool include turning, boring,

threading, shaping, planing and slotting and (ii) multi-point cutting tool include

drilling, milling, tapping, broaching and hobbing.

(b) Grinding processes include surface grinding, cylindrical grinding and centreless

grinding.

(c) Finishing processes include lapping, honing and super-finishing.

(d) Unconvent ional machining processes include e lect ro-discharge machining,

ultrasonic machining, electroche mical m achining, electron beam machining, laser beam machin in g, etc.

Selection of a suitable machining process depends on workpiece material, shape, size

and quantity of product to be made, expected degree of accuracy in the dimensions of

pro duct, re quire ment of surf ace finish and fina lly th e cost o f produ ction.

6 .2 C U T TIN G TO O L S A N D T H E IR N O M E N C L A T U R E

As already mentioned that during machining a workpiece, a cutting tool of harder material

is forced through the surplus material of the workpiece blank, the surplus material being

pro gre ssiv ely separa te d from th e bla nk in th e fo rm of ‘chip* because o f the rel at ive motion

maintained between tool and workpiece. The cutting tools are made from high strength and

harder materials such as high carbon steel, high speed steel, cemented carbide, etc. Various

cutting tool materials have been described under Section 3.18.

It may be noted that a cutting tool never peels the material

away from the workpie ce like a knife does. The tool has a ‘cutting

ed ge ’ which is blunt and n eeds su fficient force to pry the chip

from the job (Fig. 6.1). In fact, the cutting edge causes the internal

shearing action in the metal such that the metal below the cuttingedge of the tool yields and flows plastically. First of all, the

compression of the metal under the tool edge takes place [Fig. 6.2(a)]

which is followed by the separation of the metal in the form of

chip [Fig. 6.2(b)] when the compression limit of the metal just

under the tool edge has been exceeded. The cutting tools as used

on lathes have only a ‘sin gle c utting edg e ’ or 'point ’ at one end

of its body, it is then called ‘sing le-p oin t to o l’. The ‘point’, which

is wedge-shaped portion, forms the cutting part of the tool. There

are multi-point cutting tools’ also as will be discussed in the

following.

Fig. 6.1 Turning with a

single-point tool.



Fig. 6.2 Showing the principle of metal cutting with a single-point tool: (a) Compression of metal

under tool edge and (b) The cutting edge causes internal shearing action in the metal. The

metal below the tool edge yields and flows plastically, which is followed by the separation

of sheared metal in the form of a chip.

6 .2 .1 C l a s s i f i c a t i o n o f C u t t in g T o o l s

All cutting tools can be broadly classified as:

(i) Single-point cutting tools having only one cutting edge. These tools find wide

applications for lathe, shaper, planer, slotter, boring machine, etc.

(ii) Multi-point cutting tools have more than one cutting edge such as twist drills,

reamers, taps, milling cutters, broaches, etc. A multi-point cutting tool may differ

in overall appearance and purpose but each cutting edge of the tool acts as and has

its basic features of a single-point cutting tool. Also, the cutting process performed

by mult i-poin t cu tt ing tools close ly re semble s machin in g as perform ed by sing le-pointcutting tools.

Cutting tools are some times classified based on their motion during cutting, for example,

linear motion tools as that of lathe, shaper, planer and slotter; rotary motion tools as milling

cutters and grinding wheels; rotary and linear motion tools as twist drills, reamers, honing

tools, etc.

Besides above, a tool may be a solid or forged tool (Fig. 6.3) made from high carbon

steel or high speed steel. Cutting bits or inserts made o f high speed steel, stellite or cemented

carbide are available, which can be brazed on a high carbon steel shank and tools thus made

arc called brazed tools. The cutting bits can be held with the tool shank with some clampingsystem. The tool bit is inserted in a slot (in the tool holder) made at 15° to the base, thus

reducing the effective clearance angle and increasing the top rake angle by 15°. Tool bit is

less expensive than solid tool. Also, the tool can be adjusted to the correct height easily by

adjusting the position of the tool bit in the slot. Regrinding of tool is easier as only the end

cutting edges are required to be ground. It is very easy to withdraw or replace the tool bit

without disturbing the setting.

Terms relating to the geometry of single-point tool: Important terms relating to the

geometry of a single-point cutting tool are explained in the following with reference to

Fig. 6.4.



Clamping screw

Shank

' ' ' ' -V (To ol ho lder)

(iv)

Fig. 6.3 Different types of lathe tools: (i) Solid or forged tool, (ii) Brazed tipped tool, (iii) Mechanically

held tool tip or insert and (iv) Tool bit held in a tool shank.

Shank

End-cutting

edge angle (C* )

Side rake angle +

Auxi l iary cutt ing edge

or end cutt ing edge

Front or

auxil iary f lank

Main cutting edge or

side cutting edge

Back rake angle (a j

Main f lank

Side relief angl e ( 9S)

or side clearance angle

Side cutting

edge angle (0$)

Front clearance or

end rel ief angle (0 J

Fig. 6.4 Geometry of a single-point cutting tool.

Shank is the body of the tool and is usually rectangular in cross-section. Face is the

surface against which the chip slides upwards. Flank (main) is that surface which faces the

workpie ce. It is the surfac e adjacen t to and below the main cutting edg e whe n the tool lies

in horizontal position. Heel is the lowest portion of the side-cutting and end-cutting edges.

Nose or point is the wedge-shaped portion and is the conjunction of side- and end-cutting

edge. Base is the underside of shank. Rake refers to the slope of the tool top away from the

cutting edge . Tool has side rake and ba ck rake.

Besides the body parts of the tool as me ntioned above, the tool geom etry also includesvarious tool angles which have been explained in the following.



6 .2 .2 A n g l es o f a S i n g l e -p o i n t C u t t in g T o o l

Angles of the tool play a significant role in efficient and economical machining of different

metals. These tool angles vary according to the metal to be machined and the tool material.

A change in the chief angles of cutting tool will correspon dingly cha nge the forces due to the

cutting action as also the conditions for heat transmission through the cutting elements of thetool. Thus, the tool angles of a cutting tool influence its performance and life. Important

angles of a single-point tool are discussed in the following with reference to Fig. 6.5.

• Side cutt ing ed ge angl e (CJ Approach angle

as =U °T

V an9*e \

0, = 6°

8 14 6

ab Back rake

a , Side rake

0„ End relief

0SSide relief

Cb End cutt ing edge

Cs Side cutt ing edge

R Nose radius

Too l designation

20 15

Fig. 6.5 Important angles and cutting tool signature of a single-point cutting tool.

1. R ake angle is the rake or slope of the tool face and is formed between tool face and

a plane parallel to its base. When this slope is towards the shank, it is called back rake

or top rake and when measured towards the side of the tool, it is called side rake. Rake

angle has the following functions:

(i) Allows chips to flow in a convenient direction away from the cutting edge.

(ii) Redu ces chip pressure on tool face and provides keenn ess to the cutting edge and

consequently improves finish on the workpiece.

(iii) Redu ces cutting forces required to shear the metal and thus helps increasing tool life

and reduces power consumption.

Provision of rake angle depends upon following main factors:

(i) Workpiece materials as harder materials (cast iron) need smaller rake angle than

softer materials such as aluminium or steel.



(ii) Tool m aterial , for examp le, ceme nted carbide permits mach ining at very high cutting

speeds with little effect of rake angle on cutting pressure and hence rake angle in

such cases may be reduced to zero or even negative rake may be provided to

increase tool strength.

(iii) Depth o f cu t, for example, higher depth of cut (as in rough cutting) gives severecutting pressures on tool and hence rake is decreased to increase tip angle that

results in strong cutting edge.

Front rake is important when tool removes metal from its front cutting edge (a parting-off

tool).

Side rake influences machining when tool removes metal on its side cutting edge only.

Side rake allows chips to flow by the side of the tool and away from tool post. Since the

single-point tools generally remove metal both on its end and side cutting edges, a slope on

the face of the tool is given suitably combining the front and side rake together, and this

resultant slope is called true rake.The rake or slope of the face of the tool may be positive, zero or negative as shown in

Fig. 6.6.

(a) Positive rake (c) Negative rake

Fig. 6.6 Positive, zero and negative rake. Note the position and direction of thrust on the tool in each

case. R—Rake and T—Thrust.

Positive rake: A tool has positive rake when face of the tool slopes away from the cutting

edges and also slants towards the back (shank) or side of the tool [Fig. 6.6(a)]. A rake angle

specifies the ease with which a metal is machined. The higher the rake angle, the better is

the cutting and less are cutting forces. Since an increase in rake angle reduces the strength

of tool tip, heat dissipation and tool life, it is, therefore, usually kept not more than 15° (for

high speed steel tool).

Zero rake: A tool has zero rake when no rake is provided on tool, i.e. the tool face has noslope and is parallel to the upper surface of the tool shank [Fig. 6.6(b)!. A zero rake increases

tool strength and avoid s digging o f the tool into the w orkpiece. Brass is turned well with tools

having zero rake angle.

Negative rake: A tool has negative rake when the tool face slopes away from the cutting

edge and slants upwards towards the side or back of the tool [Fig. 6.6(c)]. Negative rake is

used on cemented carbide or ceramic tools. Negative rake results into a tool with reduced

keenness but stronger cutting edge (and hence stronger tool) or tool tip. Carbide tools with

negative rake are used for machining extra hard surfaces and stronger materials in mass

pro duction.



Cutting action o f a tool with positive and negative rake is shown in Fig. 6.7.

Built-up edge Cutt ing edge

(a) (b)

Fig. 6.7 Showing the cutting action of a tool with positive rake (a) and negative rake (b). Note that

in positive rake cutting, there exists a tendency for the metal to build up and also more

pronounced crater formation. In negative rake cutting, the tendency of crater formation is less

and the cutting edge in the process gives a burnishing (polishing) effect on the machined

surface of workpieces. The thrust of cut shown by arrow passes through the cutting edge of the tool at (a) and thus introduces a bending load at the cutting edge, whereas at (b) the

thrust passes through the tool shank and this gives a compression load on the stronger

portion of the tool.

Advantage of using negative rake on tool

(i) Negative rake gives larger tip angle and hence a stronger tool.

(ii) In case of tipped tools, an advanta ge with nega tive rake is that there is a tendency

of the chip pressure to press lip against the body of tool, a favourable factor since

carbide tips are very good for compre ssive loads. Negative rake on these tools varies

from 5° to 10°.(iii) Th e point of application o f cutting force is altered from cutting edge (a wea ker tip)

to a stronger section.

(iv) Very high cutting s peed s can be used for faster metal removal.

(v) Tool wear is decreased and hence tool life is increased.

(vi) Heav ier depth o f cut can be taken as negative rake increases tip angle of the tool.

There are certain limitations of using negative rake, for example, higher cutting speed

should be kept to take full advantage of negative rake; rigidity o f the machine tool must be

ensured against higher cutting speeds and vibrations; high heat generated by negative rake

turning must be taken care of for better tool life and higher powe r requirement, above 10 to

15% more than what required for positive rake machining.

2. Clearance angles: Clearance angle is the angle between the machined surface and the

Hank faces (Fig. 6.4) of the tool. It helps preventing the flank of the tool from rubbing

against the surface of the workpiece, thus allowing the cutting edge of the tool only to

come in contact with the workpiece, for example, front clearance angle (also called

end relief angle) pre vents the fron t or auxil ia ry flank o f th e tool from ru bbin g against

the finished surface of the workpiece. In case the angle is too small, the tool will rub

on the surface of the job and spoil surface finish. Too large end relief angle may give

tool digging tendency and may chatter. The side clearance angle (or side r elief angle)



prevents th e side or main flank of th e tool fr om rubbin g against the work pie ce under

longitudinal feeds. Values of these angles for turning tools vary between 5° and 15°.

3. Side cutting edge angle: Side cutting edge angle is the angle betw een the side cutting

edge and the longitudinal axis of tool. Its com plimentary angle is approach angle,

(Fig. 6.5) which is between feed direction and side cutting edge. Side cutting edge anglehelps providing a wider cutting edge and thus an increased tool life as cutting force,

distributed on wider surface, provides greater cutting speeds and quick heat dissipation

and gives a better finish on work surface. It controls direction of chip flow. Too large

side cutting edge angle produces chatter. It is usually kept around 15° although in

turning tools, it varies from 0 to 90°, for example, a knife edge turning tool has 0° side

cutting edge angle and its cutting edge is perpendicular to the work surface and such

a tool is used for turning slender workpiece as no bending stress is produced when tool

is fed. A square n ose tool with sid e cuttin g edge angle 90° is used for finish turning.

4. End cutting edge angle: It prevents the trailing end of the cutting edge of tool fromrubbing against the workpiece. A larger end cutting edge angle weakens the tool. It is

usually kept between 8° and 15°.

5. Lip angle: Lip angle o r cutting angle depends on the rake and clearance angle provided

on tool and determines the strength of cutting edge. The lip angle is maximum when

rake (positive) and clearance angle are minimum. But in negative rake, lip angle increases

as rake increases. A larger lip angle permits machining of harder metals, allows heavier

depth of cut and increases tool life and better heat dissipation. This simultaneously calls

for reduced cutting speeds, which is a disadvantage.

6. Nose radius: While greater nose radius increases abrasion, it also helps in improvingsurface finish, tool strength and tool life. Large nose radius may cause chatter. For rough

turning, it is kept about 0.4 mm and for finish turning, 0.8 to 1.6 mm.

Average recom men ded tool angles for ma chining different metals are given in Table 6.1.

TABLE 6.1 Recommended angles for high carbon and high speed steel turning tools

Material Front rake, deg Front clearance, deg Side rake, deg Side clearance, deg

Mild steel 10-12 6-8 10-12 6-8

Stainless steel 5-7 6-8 8-10 7-9

Aluminium 30-35 8-10 14-16 12-14

Brass 0-6 8-10 1-5 10-12

Cast iron 3-5 6-8 10-12 6-9

Copper 14-16 12-14 18-20 12-14

6 .2 .3 N o m e n c l a t u r e o f a L a t h e T o o l

Nomencla tu re o f a cutt in g to ol means systematic naming of various parts and angles of the

tool. Complete nomenclature of various parts of a single-point tool is shown in Fig. 6.4 and

Fig. 6.5 which includes shank, face, flank, heel, nose, base, back rake, side rake, side clearance,



end clearance, end cutting edge, side cutting edge and lip angle. These elements define the

shape of a cutting tool.

Cutting tool signature: The cutting tool signature (or tool designation) is a sequence of

numb ers listing various angles, in degrees and the size of nose radius. The American Standards

Association (ASA) has standardized the numerical method of tool identification. The sevenelements comprising the signature of a single-point tool are always written in the following

order: back rake angle, side rake angle, end relief angle, side relief angle, end cutting edge

angle, side cutting edge angle and nose radius.

Example: A tool shape specified as per ASA system is given below (Fig. 6.5):

8-14-6-6-20-15-4

has back rake angle 8°, side rake angle 14°, end relief angle 6°, side relief angle 6°, end

cutting edge angle 20°, side cutting edge angle 15° and nose radius 4 mm.

Besides the American Standards Association (ASA) System, also called coordinate

system (or X-Y-Z Plane System) which has been described in the above, the other systems

of tool designation include British System, Continental System and International System

(or Orthogonal Rake System).

In Orthogonal Rake System (ORS) or International System, main parameters of a

single-point tool are designated in the following order: inclination angle (X), orthogonal rake

angle (O'), side relief angle ()), end relief angle (yx), auxiliary cutting angle (0,), approach

angle ( 0O) and n ose radiu s (/?). For exam ple, a cutting tool des igna ted as 0-10-5-5-7-90 -1 will

have the following values of its parameters.

X = 0° (inclination angle)

a = 10° (orthogonal rake angle)

Y = 5° (side relief angle)

Y\ = 5° (end relief angle)

0> = 7° (auxiliary cutting angle)

0Q = 90° (approach angle)

R = 1 mm (nose radius)

6 .3 M E C H A N IC S O F M E T A L C U T T IN G

The topics generally covered under the treatment on mechanics o f metal cutt ing include basic

mechan ism o f metal cutting and shear zone, formation o f chip, orthogonal and oblique cutting,

forces on chip (Merchant’s Analysis), etc. These are discussed in the following.

6 .3 .1 F o r m a t i o n o f C h i p

To understand clearly the fundamentals of the mechanism of metal cutting on machine tools,

let us first try to understand a simple case of cutting with an ordinary hand tool, say a flat

chisel, under the blows of hammer because the cutting principle as applied to any hand tool

used in bench working or a cutting tool used on a machine tool is the same.



Refer Fig. 6.8 wherein shearing action of a cold chisel is shown during the process of

cutting surplus metal from a workpiece under the blow of a hammer. The chisel is shown flat

on the workpiece surface without any clearance angle, primarily to ensure that depth of cut

can be maintained and secondly, the clearance angle takes no actual part in the cutting or

shearing action of the chisel. Note that the force (F) of the hammer blow is transmitted atapproximately 90° to the cutting face AC, and this sets up shear stress across a narrow region

in the workpiece say the shear plane AB. Under the effect of heavy blows of hammer, the

metal ahead of the cutting edge of chisel will shear across the shear plane and moves up the

chisel face AC in the form of a ‘segment of chip’. Since the energy required to shear or

rupture the metal will be the shearing force along the shear plane AB, this shearing force will,

therefore, be proportional to the length AB. Hence, the smaller the rake angle of chisel, the

greater will be the length (AB) of shear plane and the larger will be the energy required to

shear the metal.

Fig. 6.8 Illustrating the shearing action of a cold chisel.

Chip formation may be compared to the movement of card stack when pushed along the

tool face. The consecutive displacements of lamellae of forming chip are depicted in Fig. 6.9

wherein the segmen ts of the chip numbered from 1 to 6 earlier occupied the positions shown

by th e dott ed lines. W hen th e tool advances, the segment 7 slips a fini te dis ta nce re la tive to

the uncut metal. As the tool advances further, the next segment 8 slips similarly and previous

segment 7 moves over the tool as a part of the chip. Although the card model is a little over

simplification of what happens during metal cutting, it does illustrate some of the major

considerations in the metal cutting process.



The basic mechanism o f chip formation, therefore, consists of a deformation of metal

lying just ahead of the cutting edge of tool, by process of shear, in a narrow zone (called

shear zone or primary deformation zone) extending from the cutting edge of the tool

obliquely up to the uncut surface of workpiece in front of the tool (Fig. 6.10). During metal

cutting, the metal in the area in front of the cutting edge of the tool is severely compressed

causing high temperature shear stress in the metal, the shear stress being maximum along a

narrow zone or plane called the shear plane (Fig. 6.11). When the shear stress in the workpiece

metal just ahead of the cutting edge of tool reaches a value exceeding the ultimate strength

of the metal, particles of the metal start shearing away (or rupture) and flow plastically along

the shear plane, forming ‘segments of chip ’ whic h flow upw ards along the face of the tool.

In this way, more and more new chip segm ents are formed and the cycle o f compression ,

pla sti c flow and ru ptu re is re peate d re sult in g in to th e bir th o f a contin uously flowin g chip .

Since the width of shear zone is small, chip formation is often described as a process of

successive shears of thin layers of workpiece metal along particular surfaces. Chips are highly

compressed body and h ave burnished and deformed underside (due to deformation at secondary shear zone on account of friction between chip and tool face). The primary shear zone

deformations are required for the formation of chip, whereas deformations in secondary shear

zone are secondary deformations which, in fact, are disturbances and are not required.

Fig. 6.11 Illustrating the shear zone (ABDC), shear plane and shear angle (<>).



speed, excessive feed, smaller rake angle and poor lubrication or cooling of tool during

cutting. Besides giving rough machine d surface and fluctuating cutting force and tool vibration,

built -u p edge also carr ie s away some mate ri al from the tool le ading to th e fo rm ation o f a

crater which resu lts in tool wear. Formation o f a built-up edge can be avoided by (i) reducing

friction at chip tool interface by means of polishing the tool face and use of adequate supply

of lubricant, (ii) keeping larger rake angle and (iii) maintaining low feeds and higher cuttingspeed as the latter generates high temperature which reduces weld strength and reduces

possib il it y o f form ation o f buil t-up edge through welding.

Besides the above types of chips, homogeneous strain chips are also there which are

pro duced in machin ing meta ls like ti tan iu m alloys and oth ers suff ering a mark ed decre ase in

yield strength with temperature and poor thermal conductivity. Such chips are banded with

regions of large and small strains.

6 .3 .3 C h i p C o n t r o l a n d C h i p B r e ak e r s

Machining of specially high tensile strength metals at higher speeds generates chips that needto be handled with care, particularly if the carbide tools are used. Higher speeds generate high

temperatures and continuous type of chips with blue colour which get collected in the shape

of a coil. Large continuous coils (if allowed to be formed) may prove quite dangerous as they

may engage the entire machine and workpiece and give a lot of difficulties in their removal.

Besides this, cutting edge of the tool is spoiled due to crater formation. The finish on the

workpiece is poor. If the chip gets curled around the revolving workpiece or the tool, it may

be a hazardous situation fo r th e operator. W hen bra ss an d ca st iron are machin ed, they do not

generate continuous chips of the type as generated in case of high speed machining of high

tensile strength metals. Chip breakers are, therefore, used with the tool which help inbre aking th e chips in to small pie ces (as it is easy to bre ak the ch ips whic h are work -h ardened

during the chip formation). A few simple chip breaking methods are shown in Fig. 6.15.

(a) Groove type (b) Step type (c) Clamp type

Fig. 6.15 Different types of chip breakers (or chip breaking method).

breaker

insert (tip)

6 .3 .4 O r t h o g o n a l a n d O b l iq u e C u t t i n g

There are two basic methods of metal cutting with a single-point tool: (i) orthogonal cutting

(or two-dimensional cutting ) and (ii) oblique cutting (or three-dimensional cutting).

Orthogonal cutting takes place when the cutting face (or cutting edge) of the tool

remains at right angles to the direction of cutting velocity or work feed [Fig. 6.16(a)].

Oblique cutting takes place when the cutting face or cutting edge of the tool is inclined

at an angle less than 90° with the direction of tool feed or work feed, the chip being disposedoff at a certain angle [Fig. 6.16(b)).



Fig. 6.16 Orthogonal and oblique cutting.

In machining with same depth of cut and feed by the above two methods, the cutting

force that shears the metal acts on a larger area in the case of oblique cutting. It results in

smaller heat developed per unit area due to friction along the tool-workpiecc interface and

consequently longer tool life.

Chip flow in orthogonal and oblique cutting is shown in Fig. 6.17. In orthogonal cutting

at (a) where cutting edge of the tool (OC) is at right angle to relative velocity V of the work,

the chip coils in a tight, flat spiral. In oblique cutting at (b) where cutting edge of the tool

is inclined at an angle (/), the chip flows sideways in a long curl. The inclination angle (/')

is the angle between the cutting edge and the normal to the direction of the work velocity ( V ).

The chip flow angle ( t jc) is the angle measured in the plane of the cutting face between the

chip flow direction and the normal to the cutting edge. In orthog onal cu tting, / = 0 and rjc = 0.Main features of orthogonal cutting and oblique cutting are summarized in Table 6.2 with

reference to Fig. 6.17.

TABLE 6.2 Features of orthogonal and oblique cutting

Orthogonal cutting Oblique cutting

Cutting edge remains normal to the direct ion of Cutting edge remains inclined at an acute angle towork feed (or velocity V). the direction of work feed.

Direction of chip flow velocity is normal to the Direction of chip flow velocity is at angle (rjc). cutting edge.

Angle of inclination (/') is zero.

Chip flow angle (rjc) is zero.

Cutting edge is larger than the width of cut.

Cutting edge inclined at an angle (/) with normalto work feed (or velocity 10-

Three mutually perpendicular components of cuttingforces act at the cutting edge of the tool.

Cutting edge may or may not be longer than thewidth of cut.



Fig. 6.17 Direction of chip flow in orthogonal cutting (a) and oblique cutting (b). Inclination angle (/)

and chip flow angle (rjc) are shown at (c).

Bulk metal machining carried out in shops is through oblique cutting method only; the

orthogonal cutting is confined mainly to such operations as parting off, facing, knife turning,

broaching, slo tt ing, etc . Orth ogonal cutting bein g the sim plest type is consid ere d in the major

pa rt o f th is ch ap te r. However, the princip le develo ped for orthogonal cutt in g applie s gen era lly

to oblique cutting also.

6 .3 .5 C h i p T h i c k n e s s R a ti o (o r C u t t in g R a t io )

It is observed during practice that the thickness of the chip produced is more than the actualdepth of cut. The reason is that a chip flows upwards at a slower speed than the velocity of

cut. The velocity of chip flow is directly affected by the shear plane angle (0); the smaller

this angle, the slower will be the chip flow velocity and thicker will be the chip.

Refer Fig. 6.18. Let

t - chip thickness prior to deformation

= depth o f cut, which in a turning operatio n, is ‘feed ’ per revolution

/. = chip thickness after deformation

Then, chip thickness ratio (r) = — (6.1)*c

Further, the reverse of V is called chip reduction ratio or coefficient (K ).

Then, chip reduction ratio (K ) = —= — (6.2)r t

Since is always more than 7 \ chip thickness ratio (r) is always less than unity. The

higher the value o f V \ the better will be the machining operation.

Since orthogonal cutting is being cons idered, width of chip equals to width o f cut.

Taking volume of chip produced equal to volumeof m etalcut, andwidth andspecific gravity

of metal being same for both cases,

/ • / = v / c



Chip

t = Original dept h of cut

tc = Thick nes s of chip

Fig. 6.18 Illustrating shear angle (0). shear plane and rake angle (a ) of the tool. Note that the

thickness of chip (/c) is more than the depth of the cut (/).

where / = length of chip before cutting = nDI rev

lc = length of chip

Chip thickness ratio (or cutting ratio) r m a y a l s o b e d e f i n ed a s t h e r a t io o f ch ip

veloci ty ( Vc) to the cut t ing speed ( V) (Fig. 6.19).

V = Cutt ing veloci ty; Vc = Veloci ty of chip; Vs = Veloci ty of shea r; a = Too l rake angle; ^ = Shea r angle

Fig. 6.19 Velocity relationship in orthogonal cutting.

or (6.3)

Length o f chip cut

Length of chip before cutting

(or uncut chip length/rev)

Then, (6.4)r lc t

C



In Fig. 6.18, 0 = shear angle and a - tool rake angle

depth of cut = / = ML sin 0

and chip thickness = tc = ML cos ( 0 - a)

Then, chip thickness ratio (r):

/ ML sin 0 s in0 s in0r = — = = = :------------- = ----------:----- (6.5A)

tc ML c os (0 - a ) cos^co sor + sin^sinfl ' c o s ( 0 - a )

(Dividing numerator and denominator by sin 0)

_ 1 ____________

cot^co sor + sin a

or r(cot 0 cos a + sin a) = 1

1-/ • sin aor cot 0 cot a - ---------------

r

r C 0 S a c ’ r* \or tan 0 - ---------------- (6.5B)1 - r sin or

Hence.

shear angle (0 ) = tan-1r cos a

1- r sin a(6 .6 )

6 .3 .6 V e l o c i t y R e l a ti o n s h i p in O r t h o g o n a l C u t t in g

The following three velocities are involved in orthogonal cutting [Fig. 6.19(a)].

V = cutting velocity or velocity o f tool relative to w ork

Vc = velocity of chip How or velocity of chip flow relative to tool

Vg = velocity of shea r or velocity of displacement of the chip along the shear pla ne

relative to work

The cutting velocity (F) and rake angle ( a ) are known. The following approach is

undertaken to find V; and F and the relationship between the three velocities.

Refer Fig. 6.19(b) which show's the velocity diagram wherein:

v = vc + vs

By applying sine rule.

F FT c _______________ S

sin 0 s in [ (9O°-0) + (0-<2 ')] s in ( (18 O - (9O -0 + 0 - a r + 0))

F F FTc r s '

or ^ 7 -------------- — —— :sin 0 cos a c os (0 - a )

Hence.

F • sin 0velocity of chip flow (FJ =--------------

(6.7)c cos(0 - a )



And

V •COS OLvelocity of shear (V ) = ---------1----- (6.8)

c o s ( 0 - a )

Since, r - — — , thencos( 0 - a )

velocity of chip flow (Vc) = cutting velocity (V) x chip thickness ratio (r),

or Vc = V r (6.9)

6 .3 .7 F o r c e s A c t i n g o n C h i p in O r t h o g o n a l C u t t in g (M e r c h a n t ’s A n a ly s i s )

M erchant established relationship between various forces acting on the chip during orthogonal

metal cutting but with the following assumptions:

(i) Cutting velocity always remains constant.

(ii) Cutting edge of tool remains sharp always during cutting with no contact betweenworkpiece and tool flank.

(iii) Chip does not flow sideways.

(iv) Only continuous chip is produced.

(v) There is no built-up edge.

(vi) No consideration is mad e o f the inertia force of the chip.

(vii) Width of tool is greater than w idth of cut.

(viii) Behaviour of the chip is like that of a free body which is in the state of a stable

equilibrium due to the action of two resultant forces which are equal, opposite and

collinear.

Because of a number of flaws and practical difficulties, the above assumptions were

modified later.

The forces acting on the chip in orthogonal cutting are as a result o f the cutting force (/?)

(Fig. 6.23) applied through the tool. These forces are given in the following with reference

to Fig. 6.20(a).

Fs = Shear force or metal resistance to shear during chip formation. It acts along shear

plane.

Fc = Backing up or compressive normal force exerted by workpiece on the chip. It acts

normal to shear plane.

N = Force exerted by tool on the chip. It acts normal to the tool face.

F = Frictional force (or p N ) or resistance of the tool against the chip flow.^It acts along

tool face. Here ju is kinetic coefficient of friction between tool face and chip and

^i - F I N = tan /?, where /? is angle of friction.

Figure 6.20(b) shows the free-body diagram of chip. Forces F and Fc have their resultant

force R whereas forces F and N have their resultant force R '. The re su ltant fo rces R a nd R'

are equ al in magnitude, op posite in direction a nd collinear. The chip can, therefore, be

regarded as an independent body held in mechanical equilibrium by the action of two equal andopposite forces R which the workpiece exerts on chip and R' which the tool exerts on the chip.



Fig. 6.23 Merchant’s circle diagram.

For the convenience of further relationships between various forces, the two triangles of

forces of the free-body diagram of chip [Fig. 6.20(b)] have been considered together in

Fig. 6.23, called the Merchant circle diagram. For the sake of simplicity, the cutting forcesare plotted at the tool point instead o f their actual point of application and a com posite cutting

force circle (Fig. 6.23) is obtained wherein d iameter o f the circle is R (note that R = /?'). From

this diagram, various force relationships can be obtained.

The cutting force (Ff) and feed force (F , or Fg) can be found with the help of force

dynamometer. When laid as in Fig. 6.23, resultant R can be found easily. Knowing the rake

angle (or) of the tool, forces F and N can be determined. Shear angle (0) can be found as:

cos a

t a n0 = F ^ k T a <6 1 0 >

where a = rake angle

K = chip reduction coefficient

Chip thickness (t c)

Uncut thickness (or feed in tumingXO

Knowing above, all other component forces on the chip may be determined from the

geometry of Fig. 6.23 wherein

Ft - Cutting force

F j = Feed force(forces acting on the tool and measured by force dynamometer)



F c =

F,= F =

N =

Now,

or

Then,

or

Further,

or

Then,

or

an d

Also,

and

or

and

or

or

(forces exerted by workpiece on the chip)

the chips)

Compressive or normal force

on shear plane

Shear force on the shear plane

Frictiona l force along rake face o f the tool I (forces ex erted by tool on

Norm al force at the rak e fa ce o f tool

F = AQ + QB = AQ + DC (as QB = DC)

F = Ft sin a + Ff cos a

N = PQ - PD = Ft cos a - Ff sin a

N = Ft cos a - F f sin a

Fs = AO - O K = A O - P E = Ft cos 0 - F f sin 0

Fs = Ft cos 0 - Ff sin 0

Fc = CK = CE + EK

= CE + PO (as EK = PO)

Fc = F f cos 0 + F( sin 0

Ft = R cos ( fi - a)

F f - R sin - a)

F = R cos (3 - a + 0)

F, R cos(/? - a ) cos(/? - a )

F s R cos( p - a + 0) cos(P - a + 0)

F = F l t s

c ospg - a )

cos (P - a + 0)

F Ft s \ n a + F f C o s a

N Ft c o sa - F j sin a

F Ff + F. tan a — = -L ------= i z n pAr F, - F f tan a

— = tan P - n (kinetic coefficient of friction) N

where P angle of friction = tan-1 /7

Also,CP Pf

tan PAC = tan ( p - a ) = —- = — A P F,

(6 . 11)

(6.12)

(6.13)

(6.14)

(6.15)

(6.16)

(6.17)

(6.18)

(6.19)

or - f = t an ( /? -t f) (6 .20 )



Merchant developed a relationship between shear angle (0), angle of friction (ft) and

tool rake angle (a ) as follows:

2 <f> + f t - a = C (6.21)

where C is a machine constant which depends on the rate of change of shear strength of the

workpiece metal with applied compressive stress as also the internal coefficient of friction.

6 .3 .8 S t r e s s a n d S t r a in o n t h e C h i p

Chips are produced due to the plastic deformation of the metal; they experience stress and

strain. As shown in Fig. 6.20(a), two forces F : and Fs (perpendicu lar to each other) act at the

shear plane. Now refer Fig. 6.24 wherein:

A = cross-sectional area of uncut chip = h x t

where b = width of cut and t = depth of cut or uncut chip thickness

A y = shear plane area

or A = A sin <p

Chip

A Shear

\ pl an e

t

-H b H -Workpiece

Fig. 6.24 Geometry of chip formation in orthogonal cutting.

Mean normal stress ( o)

a = = F- = F' s m ‘t‘ (6.22) A A!s\n<p A

Putting the values of F. = Ff cos <p + Ft sin <p

(F sco sfl + F, sin 0) sin 0Mean normal stress (o ) = — ------------------------------ (6.23)

A

Mean shear stress (t)

F F. sin <pT = — = — -

As A

or Fs = ^ ~ - ~ (6.24)s in^



It can be shown that

^ cos s in (p ( F , c o s 0 - / y s i n 0 ) s i n 0(6.25)

Shear strain (£)

Shear strain is given by the following expressions:

e - cot <p + t an(0 - a)

and also

€ =cos a

(6.26)

(6.27)sin <f>cos(<p- a )

6 .3 .9 F o r c e s o f a S i n g l e -p o i n t T o o l

During metal cutting, the workpiece metal offers resistance to the cutting tool. This resistance

is overcome by the cutting force applied through the tool. The work done by this force in

cutting is spent in shearing the chip from the wo rkpiece, deforming the chip and overcom ing

the friction of the chip on the tool face. The magnitude of cutting force depends on

workpiec e m aterial, feed, depth of cut, cutting speed, tool angles and lubricant or coolant

used.

Forces acting on a single-point turning tool in oblique or conventional cutting are

shown in Fig. 6.25 and these are:

Fa = Axial feed force or thrust force acting in horizontal plane parallel to the axis of

work but in the direction opposite to the feed.

Fr = Radial force acting in horizontal plane along a radius of work, i.e. along the axis

of tool.

Ft = Cutting force or tangential forc e acting in vertical plane and tangential to the

work surface.

Fig. 6.25 Showing cutting forces in conventional (oblique) turning process, /?—resultant force;

F — axial feed force; F—radial force; F,—cutting force.



In the above three forces, Ft is the largest in magnitude and Fr the smallest. Fo r turning

operation, Fa varies between 0.3 Ft and 0.6 F( and Fr between 0.2 Ft and 0.4 Fr The components

Ft and Fa arc determin ed easily with the help o f suitable force d ynamometer. T he resultant

force ( R ) can be computed as below:

R = ^ F 2 + F 2 + F 2 (6.28)

In orthogonal cutting, only two forces (Fa and Ff) come into play and Fr is zero

(Fig. 6.26). The resultant force ( R ) is as follows wherein Fa and Ft are axial (feed) force and

cutting force (or tangential force), respectively.

R = sjFa2 +F,: (6.29)

Fig. 6.26 Forces acting on a cutting tool in orthogonal cutting.

(a) Torque (T) developed on workpiece Negle cti ng F q and Fr being very small,

Torque (T) = ^ D (Nm)2 x 1 0 0 0

where D = dia. of work, mm, Ft = cutting force, N

(b) Heat produced ( H)

Heat produced = work done in metal cutting

/ / = 5 - - -T kN m/s o r kW or kJ/s6 0 x 1 0 0 0

where V = cutting speed, m/min, Ft = cutting force, N

Heat produced is also equal to the following where Ft is in kgf and V is m/min

F V

(6.30)

(6.31)

427, kcal/min (6.32)

(c) Power required (F)

P =F. -V

60 x 1000 x i]

, kW (6.33)



where F t - cutting force, N

V - cutting speed, m/min

11 = efficiency (say 80 to 90%)

(d) Metal removal rate (MRR)

= V • b • /, cm 3/min (6.34)where V = cutting velocity, cm/min

h = width of cut (cm) or feed rate, cm/rev.

/ = depth of cut or uncut chip thickness, cm

Maximum metal removal rate (MRR) max.

Max. powe r available at machine spindle (kW) , v= -------- — --------------------------------------------- - , (cm /min) (6.35)

Power required (kW/c m' /min)

6 .3 .1 0 P o p u l a r T h e o r i e s o n M e c h a n i c s o f M e t a l C u t t i n g

Various relationships have been derived earlier for shear angle ( <p), friction angle (/?) and rake

angle (a). Several investigators have carried out a lot o f work to establish realistic relationship

among a , <pand /? and d eveloped several theories with slight variations in their assum ptions

and results. More important theories include:

(i) Eamst-Merchant Theory

(ii) Merchant Theory

(iii) Stabler Theory

(iv) Lee and Shaffer’s Theory

The following two theories are more popular among metal cutting engineers.

Ea r n s t -M e r c h a n t t h e o r y

This theory is based on the principle of minimum energy consumption and implies that during

cutting, the metal shear should occur in that direction in which energy requirement for

shearing isminimum.Further, the behavio ur of metal beingmachined is like that of an ideal

pl ast ic . Also, th e shear str ess is maxim um and consta nt at shear pl ane an d independent o f

shear angle <p. They came up with the following relationship:

6 = -----

— + — i f i - a ) (6.36) y 4 2 2 4 ’

Lee and Sha f f e r ’s t h eo r y

In this theory, the process of orthogonal cutting has been analyzed by applying theory of

pla sticity fo r an idea l rig id pla sti c mate rial. Followin g assumpti ons are made:

(i) Workpiece metal ahead of the cutting tool behaves like an ideal plastic material.

(ii) Deformation of metal takes place on a single shear plane.

(iii) Chip does not get hardened.

(iv) Chip separates from the parent metal of work at shear plane.



They derived the following relationship:

(p = + a - = 45° + a - {)

or 0 + P - a = 45° (6.37)

This was further modified as:

0 = — + a + 6 - p (6.38)4

where 0 covers the changes in different param eters because of the formation of built-up edge.

6 .4 H E A T IN M E T A L C U T T I N G

When a metal is deformed plastically as in metal cutting, most of the energy used is converted

into heat. The energy available at the cutting edge in a metal cutting process is converted intoheat, mostly in frictional heat as also the heat caused by destruction of molecular or atomic

bond s in metal in the shear plane. The ma in sources of hea t are: (i) the she ar zo ne where the

main plastic deformation takes place, (ii) the chip-tool interface zone where secondary plastic

deformation takes place due to friction and (iii) the work-tool interface where frictional rubbing

takes place (Fig. 6.27). As a result of this heat, high temperatures are generated in the region

of tool cutting edge which have a controlling influence on the rate o f wear o f tool and on the

fr ic tion between chip and tool; for example, the temperature plays a major role in the formation

of crater on the tool face and leads to failure of tool by softening and thermal stresses.

Fig. 6.27 Regions of heat generation in metal cutting include: (1) Primary shear zone, (2) Secondary

shear zone and (3) Work-tool interface zone.

The three main regions of heat generation in metal cutting shown in Fig. 6.27 are

discussed in the following:

I. The shear zone or prima ry deform ation zone No. 1 is the region in which plastic

deformation of metal occurs during machining. Due to this deformation, about 80%o f total heat is generated in shear zone. A port io n o f th is heat (about 75% ) is ca rr ie d



6 .4 .2 F a c t o r s A f f e c t in g T e m p e r a t u r e i n M e t a l C u t t in g

All the above discussed three zones of heat generation in metal cutting lead to temperature

rise at tool-chip interface. The temperature plays a major role in the formation of crater on

the tool face and leads to the failure of tool by softening and thermal stresses. Factors

affecting temperature in metal cutting are given in the following:

(a) Materials o f workpiece and tool: These affect temperature in metal cutting since

materials with higher thermal conductivity are responsible for production of lower

temperatures at cutting edge.

(b) Tool geometry: If the rake angle is increased in positive direction, both cutting

force and amount of heat generated are reduced but too large rake angle weakens

the cutting edge and reduces the heat conducting capacity of tool.

(c) Cutting conditions: Cutting speed has grea t influence on the production of temperature.

Since frictional heat increases with cutting speed, the tool-chip interface temperature

increases with cutting speed. The tool-chip interface tem perature rises but less rapidlythan for a rise in cutting speed.

Changes in depth of cut have little effect on temperature. Less heat is generated when

higher feed rates are used but surface quality is adversely affected.

6 .4 .3 M e a s u r e m e n t o f C h ip - to o l In t e r f ac e T e m p e r a t u r e

A number of methods are available for the measurement of chip-tool interface temperature

and these include tool work thermocouples, embedded thermocouples, infrared photographic

technique, temperature sensitive techniques, etc. Tool work thermocouple technique is most

widely used.

6 .5 C U T T I N G F L U ID S

Cutting fluids (or metal working fluids) are those materials which are applied to the tool and

workpiece to facilitate the cutting operation by removing heat and reducing friction. These are

also known as coolants when cooling quality of the coolant is more and lubricants when

lubricating properties are greater. Cutting fluids are available in the form of liquid, gas and

solids to suit different cutting conditions. The use of cutting fluids should be economically

ju sti fi ed consid erin g its cost o f pumpin g, collection and fi lt er ing of the cutt in g fluid. Promin en t

metal w orking processes involving the use o f these fluids include machining, grinding, lapping,

honing, forging, rolling, extrusion, drawing, etc.

6 .5 .1 P u r p o s e o f U s i n g C u t t in g Fl u i d s

Lubrication or cutting fluid action is not fully understood. The hydrodynamic fluid film

lubrication is not possible in metal cutting due to the presence of high pressure and temperature

and higher velocities at cutting point. However, some lubricant fluid may reach the tool point

due to surface tension of the fluid through the capillaries formed by minute hills and valleys

of the chip and tool surface against the outward motion of the chip. The mechanism of



boundary lubrication or extreme pressure (EP) lubrication has helped in explaining the

reduction of kinetic coefficient of friction between chip and tool. In the presence of high

temperature and high pressure, the highly clean reactive surface of the chip reacts with the

special cutting fluids used and forms compounds having low shear strength and layered

structures which are easily sheared by sliding action of chip on the tool face and this results

in preventing metal to metal contact by keeping the chip and tool apart. EP additives in

cutting fluids provide better surface finish, improved tool life with reduction in cutting forces.

Compounds of sulphur and chlorine like chlorinated paraffin or sulphurized fats are often

used as EP additives in cutting fluids. Although all cutting fluids provide cooling and lubricating

action, the heat transfer from the cutting zone depends on the rate of fluid flow, its thermal

conductivity, etc. The cooling effectiveness does not depend mainly on thermal properties of the

fluid but it also depends on the wetting action and vapour formation for quick removal of heat.

Cutting fluids are used for the following purposes:

(i) Cooling o f tool which is necessary to prevent metallurgical damage and to assist in

decreasing friction at tool-chip interface and tool-w orkpiece interface. Reduce d friction

results in increased tool life, less power consumption and good surface finish. Cooling

action of the fluid is by direct carrying away of the heat. A high specific heat and

high heat-conductivity together with a high film-coefficient for heat transfer are

necessary for a good coolant. Water happens to be a very effective coolant but it

may lead to corrosion o f the wo rkpiece and is not very effective in reducing friction.

(ii) Cooling o f workpiece is required to prevent its excessive thermal distortion.

(iii) Lubricating and reducing fric tion results in many advantages, for example, power

consum ption in metal cutting is reduced; abrasion o r wear on the tool is reduced and

hence tool life increased; lubrication helps generating less heat at tool tip givinglonger tool life; chips are helped out of the flutes of drill, tap, dies, etc.; reduction

in built-up edge and consequent reduction in friction at tool-workpiece area.

(iv) Improving surface finish.

(v) Protecting machined surface against corrosion.

(vi) Breaking chips into sm all pieces and washing them away from tool.

6 .5 .2 P r o p e r t ie s o f C u t t in g F lu i d

The good cutting fluid should possess the following properties:

• High heat absorption for quickly absorbing the heat.

• Goo d lubricating property for producing low coefficient of friction.

• Neutral so as not to react chemically.

• Stability again st oxid izing in air.

• High flash point to avoid fire hazards.

• Odourless, harmless to the skin of operator and bearings of machine.

• Non-corrosive to the work and machine.

• Transparent so that cutting action o f tool is observedclearly.

• Low viscosity permitting free flow of cutting fluid.• Low price in view of minimizin g production cost.



6 .5 .3 T y p e s o f L u b r i c a n t s

Cutting oils, mainly used as lubricants, are classified into the following main groups:

(i) Aqueous solutions include water, either plain or containing alkali like borax, sodium

carbonate or salt or water soluble additives. These are cheap and have high specific

heat and high heat conductivity. These are used where mainly cooling and washingaway o f chips is required as these are likely to develop rust on machine tool element.

For mainly cooling, the cheapest and best solution is soda or borax in water.

(ii) Soluble oils or conventional emulsions contain up to 80% of water, fatty acids,

mineral oils and soap acting as an emulsifying agent which breaks the oil into

minute particles so that these are dispersed thro ughout the water. The w ater provides

cooling effect and oil provides lubricating effect and freedom from rusting. These

cutting fluids are cheap and used where cooling is the prime requirement. Most of

the cutting and grinding involves the use of such emulsions.

Compound emulsions arc emulsions compounded with some special additives. Emuls ion denotes the solution made by diluting the soluble o il in water. Compound

emulsions are good lubricants, coolants and wear-resisting fluids.

(iii) Mineral oils are essentially hydrocarbons such as paraffin and naphthalene. The

paraffin hydrocarb ons are hig hly oxid ation resi stant at elevate d te mpe ra tu res. Minera l

oils, however, do not find much favour in boundary lubrication as in deep drawing

and extrusion processes. It is with this reason that mineral oils are normally used

in compounded form as discussed in the following.

(iv) Fatty oils and acids are preferred most for bounda ry lubrication as friction-reducing

agents under extremely high pressures. Fatty oils (such as lard oil and tallow) and

acids are used in various forms, for examp le, in the form of greases made by mixing

with straight mineral oils and used for lubrication in wire drawing.

(v) Compound mineral oils are actually mineral oils compounded or mixed with

substances such as fatty oils and acids. As pointed out above that straight mineral

oils do not work satisfactorily in boundary lubrication as encou ntered in metal

working processes such as forming, deep drawing, extrusion, etc. Mineral oils

compounded with sulphurized fatty oils give a very good lubricant for use under

excessively high pressures and excessive friction as in drawing, grinding, extrusion

and forming. Sulphurized mineral oils are used for machining tough low carbon

steels. These oils further compounded with chlorine give chlorinated compounds,use of which in metal cutting promotes anti-weld characteristics and hence reduced

problems o f buil t-up ed ge.

(vi) Waxes are generally derived from petroleum and used as lubricants in rolling,

drawing and tinning. Various waxes include paraffin wax, natural bees wax, etc.

Waxes when compounded with fatty acids and soap work well under high pressures.

(vii) Graphite suspensions made by mixing graphite powder in oil or water are used in

forging, foundry works, extrusion, wire drawing, etc.

(viii) Minerals including salt, metals, refractory materials, etc. are used as lubricants or

coolants. Salt is a common quenching media (coolant) in heat-treatment processes.



Various chlorides and hydroxides of brine and caustic soda are used as quenching

media for rapid cooling. Metals such as lead and copper coatings on steel wires for

drawing. Graphite, bentonite, lime, etc. arc other lubricants.

(ix) Chemical compounds consist mainly of a rust inhibitor, such as sodium nitrate,

mixed with good amount of water. These are used in grinding and where corrosionis to be avoided.

(x) Solid lubricants include various powders, vitreous materials, pastes, greases, dry

film, etc. Stick wax and bar soap are sometimes used to lubricate the tool.

(xi) Gaseous fluids such as air (still or compressed) arc used as blast or suction where

fluids cannot be used. Refrigerated compressed air, nitrogen and carbon dioxide

have been used with advantage. Carbon dioxide is effectively used with carbide

tools for reducing crater wear when machining titanium alloys. Liquid carbon dioxide,

argon, oil mist, etc. have also been used. In order to reduce adve rse chemical effects

of using some cutting fluids, liquid nitrogen as a coolant in machining and grinding

is used by injecting at the tool-workpiece interface in cryogenic machining.

6 .5 .4 T y p e s o f C u t t in g F lu i d s

Cutting flu ids are the coolants and lubricants which find extensive use in metal machining,

grinding, honing and lapping. Cutting fluids can broadly be classified as follows:

Active cutting oils contain such constituents which can react chemically with work

surface to help machining operations. These include highly sulphurized mineral oils, fatty

oils, sulphurized and chlorinated fatty oils, etc. Active cutting oils are used for machining

generally ferrous metals.Inactive cutting oils are, in fact, straight mineral oils or these oils are mixed with neat

fatty oils or sulphurized fatty oils. Fatty oils used include lard oil, tallow and fatty acids. The

inactive cutting oils are used as cutting fluid in machining non-ferrous metals such as copper

and copper alloys which usually get discoloured with the use of active cutting oils.

The Indian Standards (IS: 1115-1973 and IS: 3065-1970) provide specifications for

soluble cutting oils and neat cutting oils (or straight oils).

6 .5 .5 O i ls a n d C o m p o u n d s S u g g e s t e d f o r U s e f o r D i f f e re n t M e ta ls an d

M a c h i n i n g O p e r a t i o n s

(i) Lo>v carbon steels

For turning , soluble oils, straight mineral oils, or lard oil

For drilling and milling, soluble oil

For tapping, active mineral oil

For grinding , soluble oil, active type mineral oils and compounds

For broaching, active mineral oil or soluble oil emulsion

For threading, soluble oil

For milling, sulphurized mineral fatty oils



results in setting up of thermal stresses due to which cracks are developed in the tool known

as thermal cracks. Under high temperatures, when tool loses strength, some of its metal may

flow plastically und er pressure resulting into edge depression and localized bulging (Fig. 6.29).

Fig. 6.29 Showing edge depression and localized bulging of tool because of high temperatures involvedin metal cutting.

2. Excessive stress and mechanical chipping take place at the nose or cutting edge of

the tool. When the tool is acted upon by an excessive force or stress, its cutting edge may

fail by crushing or chipping off of the nose mainly due to lack of tool strength. Common

reasons for this type of failure are too high cutting pressure, too high vibrations or chatter,

mechanical impact, excessive wear and weak tip or cutting edge.

3. Gradual wear takes place when a tool is used for some time and its wear is judged

by loss o f weig ht or mate ri al from th e tool . Thus, tool wear can be defined as the loss of

weight or mass that accompanies the contact of sliding surfaces. The following two types ofwear are found to occur in cutting tools.

(i) Crater wear is the gradual or progressive wear that develops on the rake surface

of the tool and the region where wear takes place in a cutting tool is its face, at a

small distance (say ‘a ’) from its cutting edge o r nose (Fig. 6.30). The wea r gradually

takes place while machining ductile materials such as steel in which continuous

chips are produced. Main feature of this wear is formation of a crater or depression

at the tool chip interface (or tool face). It is due to the high pressure of the hot chips

sliding up the face of tool, as a result of which some metal from the tool face is

supposed to be transferred to the sliding (or outgoing) chips (through the mechan ism

of diffusion) and the result is the formation of a crater or depression on the tool face

(the crater wear). Crater modifies the tool geometry and increases the cutting forces

and softens the tool tip. A continued growth of crater results in the cutting edge of

the tool beco ming weak and may finally fail. Higher feeds and lack of cutting fluid

increase the rate of crater wear.

(ii) Flank wear takes place as a result of friction or abrasion between the progressively

increasing contact area on the tool flank and the newly machined workpiece surface

(Fig. 6.30). Excessive heat is generated as a result of this. Abrasion action is added

by the hard mic ro -c onsti tu ents of the cut meta l as also th e broken parts o f buil t-up

edge (if it is there). This type of wear is more pronounced while machining brittlematerials like cast iron or when feed is more than 0.15 mm/rev.



Fig. 6.30 Principal types of tool wear. The principal region where crater wear takes place is the tool

face, at a small distance (a) from the cutting point of tool. Crater width is shown by (b) and

depth by (d).

Flank wear is a flat portion worn behind

the cutting edge. The worn region at the

flank is called w e a r l a n d (Fig. 6.31). The

flank wear occurs on the tool nose and

front and side relief faces and its magnitude

mainly depends on the relative hardness

of the workpiece and tool material and also

the extent of strain hardening o f the chips.

The effect of flank wear is expressed in

terms of width (or height) o f wear lan d Flank wea r land

(denoted by V B or WL). It is sug geste d Hg. 6-31Hankwear,

that the tool be reground before the flank

wear reaches its limiting value , VB = 0.6 to 2 mm for HSS tool. Increased wear land

means that frictional heat will cause exces sive temperature at the cutting point of the

tool, resulting in rapid loss of tool hardness which may lead to the catastrophic tool

failure. The burnishing action of the tool at its wear land will result in poor surface

finish on the workpiece.

6 .7 M E C H A N IS M O F T O O L W E A R

As already mentioned, tool wear is associated with loss of weight or mass of the tool.

Although the wear mechanism of cutting tools is a very complex phenomenon, the common

mechanisms thought responsible for causing wear are given in the following. Wear seldom

involves a single unique mechanism.

The wear mechanisms associated with progressive tool wear include the following.

WL o r VB



6 .7 .1 A b r a s i o n W e a r

Abrasion wear is a type of mechanical wear which occurs when hard constituents of one

surface plough through the material o f the other surface. U nder this mechan ism, hard particles

(harder than tool material) on the underside of sliding chip plough into the relatively softer

material of tool face and remove metal particles (from tool) by mechanical action. Thematerial of the tool face is softened because of high temperature.Hard pa rt ic le s present on

the underside o f the chip may include fragments of hard tool material, broken pie ce s o f strain

hardened built-up edge, extremely hard constituents such as carbides, oxide, nitrides, etc.

present in th e workpie ce mate rial .

6 .7 .2 A d h e s i o n W e a r

Adhesion wear is due to excessive cutting pressure

that results in generation of a lot of friction between

chip and tool face and consequently extremely highlocalized temperature which causes metallic bond

be tw een tool material and chip. Microscop ical ly rough

surfaces exist under the chip and on the tool face

because o f which in place o f true surf ace contact,

only point contact (Fig. 6.32) takes place between

chip material and tool material. Under the effect of

very high temperature at chip-tool interface, a metallic

bond ta kes pla ce (a t conta ct points ) between the

materials of chip and tool in the form of small spotwelds which keep breaking when the chip slides.

During the process, a small portion of the welded

tool contact is also carried away by the sliding chips.

In this way, small particles of tool material from the

tool face continue to be separated and carried away

by th e chip by adhesion to its underside. Amount of

material so transferred from tool face to the chip

depends on the contact area and relative hardness of

the chip and tool materials.

6 .7 .3 D i f f u s i o n W e a r

Diffusion wear occurs by a solid-state diffusion mechanism which consists of transfer of

atoms in a metal crystal lattice. The transfer of atoms takes place at elevated temperatures

from the area of high concentration to that of low concentration. In such a situation, metal

atoms are transferred at the points of contact from tool material to the chip material. It

weakens the tool and may ultimately lead to tool failure. The amount of diffusion depends

on temperature at the contact area between ch ip and tool face, perio do f contact between chip

and tool and the bonding affinity between materials of tooland chip. Wear of carbide tools

by dif fu sion is a well- known phenomenon.

Fig. 6.32 Showing point contacts and

metallic bonds (welds) formed between

mating surfaces of chip and tool.



6 .7 .4 C h e m i c a l W e a r

Chemical wear occurs when a cutting fluid used during machining is chemically active to the

material of tool. Hence, only a suitable type of cutting fluid should be used to increase the

tool life.

6 .8 T O O L L I FE

Tool life is defined as the time interval fo r which tool works satisfactorily between two

successive grindings o f resharpenings o f the tool. Thus, tool life is basically a functional life

o f tool. Tool life can be used as the basis for evaluating the performance of a tool material,

assessing machinability of workpiece material and knowing the cutting conditions.

Tool life is expressed in the following ways:

(i) Time period in minutes between two successive grindings of the tool.(ii) Num ber o f components machined between two successiv e grindin gs.

(iii) Volume o f metal removed between two successiv e grindings.

Volume of metal removed per minute ( V m):

Vm = JtD • / f • N, mm 3/m in (6.40)

where D - dia of workpiece, mm

t = depth of cut, mm

/ = feed, mm/rev

N - N umb er of revolution o f job per minute

If T be th e time fo r tool fa ilure in minute s, th en to ta l volume of meta l re moved up to

tool failure

= xD t f - N T, mm3 (6.41)

We know that

. xD N .cutting speed — V — , m/min

or xD N = 1000F

Substituting this in Eq. (6.41), in terms of total volume of metal rem oved to tool failure,

Tool life ( T l ) = m O V f - T t (m m 3) (6.42)

6 .8 .1 F a c t o r s A f f e c t i n g T o o l L i f e

Various factors which affect tool life are discussed in the following:

(i) Cutting speed has greatest effect on tool life. Higher the cutting speed, smaller is

the tool life. Reduction in tool life corresponding to an increase in cutting speed is

para bo li c (Fig . 6.3 3). Based on work o f F.W. Tay lor, th e rela tionship between cutting

speed and tool life can be given as

V • r = C (6 .43)



Fig. 6.33 Showing parabolic reduction in tool life with increase in cutting speed.

where V - cutting speed, m/minT = tool life, min •

n = an exponent (also called tool life index)

depending largely on tool material

and n - O.l to 0.15 for HSS tool

= 0.2 to 0.5 for cemented carbide tool

= 0.6 to 1.0 for carbide tools

C = machining constant which is equal to cutting speed (m/min) that will

give a tool life of one minute

Use of proper cutting speed and feed considerably effects the rate of production,surface finish and production cost. Rate of feed depends on depth of cut used.

Highest permissible feed with finish in view should be adopted as it directly effects

the machining time. The cutting speed to be used will be governed by tool material,

workpiec e material, pow er of machine, finish on the job . etc. Average cutting speeds

to be used for different tool materials and w orkpiece m aterials for various machining

operations are given in Table 6.3.

TABLE 6.3 Average cutting speeds in metres per minute

Operations and tool material Material to he machined

HSS tools Cemented carbide tools

Ceramic tools

Straight

turning

Drilling Reaming & threading

Roughing Finishing Roughing Finishing

Mild steel 50 45 10 140 180 400 500

Free cutting steel 60 45 15 150 200 420 600Cast steel 20 15 5 50 100 150 200

Stainless steel 20 25 5 40 70 120 200

Cast iron 25 25 5 75 140 200 350Aluminium 200 150 25 500 600 — —

Brass 120 90 30 300 400 — —



Feed is usually kept 0.2 to 0.8 mm/rev. Depth of cut for rough turning is kept 2 to

5 mm and fo r finishing, 0.5 to 1 mm.

(ii) Feed and depth of cut have similar effect on tool life. An increase in feed or depth of

cut will result in reduced tool life but not nearly as much as an increase in cutting speed.

(iii) Tool geometry, part ic ula rly to ol angle s, in fluences the perform ance an d li fe of tool .

An increase in rake angle (within limits) increases tool life by reducing cutting force

and heat generated. But very large rake angle w eakens the cutting edge. The optimum

value of rake angle varies between -5 and +10°. the minus sign indicated -ive rake

which gives a stronger tool as cemented carbide and ceramic tools have -ive rake

and operate effectively at higher cutting speeds.

Relief angles or clearance angles are provided on cutting tools to prevent rubbing

of tool flank against the machined surface of the work. This way, these angles

reduce heat generation and consequ ently inc rease tool life. But large relief angles

give a weaker tool. Simultaneously an increase in cutting edge angles (both front

and end) up to a limit improve s tool life. Nose radius helps increasing surface finishand a stronger tool.

(iv) Tool material: An ideal tool material is the one which will remove the largest

volume of material from the job at all speeds. But it is not possible to get an ideal

tool material. Hence, it can be said that the higher the hot hardness and toughness

o f a tool material, the longer the tool life.

(v) Work material and its microstructure play an important role on the tool performance;

for example, the presence of free graphite and ferrite in cast iron and steel imparts

softness to them. The increase in cutting temperature and power consumption varies

directly as hardness o f workpiece material. The higher is the hardness of work material,the greater will be the tool wear and shorter will be the tool life. Type of surface of

work material (scaly or smooth) also influences machining operation and the tool life.

(vi) Nature of cutting, whether continuous cutting or intermittent cutting, affects tool

life. In intermittent cutting, tool is subjected to repeated impact loading. The continuous

cutting is better for prolonged tool life.

(vii) Rigidity of machine tool and work is important for avoiding unwanted vibrations

during cutting.

(viii) Cutting fluids of proper type and in adequate quantity help increasing the tool life

by keepin g the cutting edge of tool cooler and lubricated.

6 .9 C O S T C O N S ID E R A T IO N IN M A N U F A C T U R I N G

It is always attempted to produce an acceptable product at the minimum possible cost because

the m anufacturing cost of a product plays an important role in the marketing of the product

successfully. The cost o f any new product m ust be competitive with that o f the similar

pro ducts alre ady existing in the mark et. Ir re spective o f how wel l the new pro duct meets the

design specifications and quality standards, it has to be the criterion of the econom y in its cost

for being comp etitive in the market. It may also be noted that in order to earn m ore profit,

it is required that the production or man ufacturing cost should be kept lower.



6 .9 .1 E l e m e n t s o f C o s t

The term cost represents the expenditure incurred on ma nufacturing a product. The three main

elements of cost arc given below:

(i) Material cost may be direct material cost and indirect material cost. Direct material

cost is for material purchased from market as raw material and consumed in

manufacturing the product. Indirect material cost is the cost of all other materials

such as gas, oil, coolants used, etc. in manufacturing the product but they are never

the part of the product.

(ii) Labour cost may be direct labour cost and indirect labour cost. Direct labou r cost

includes expenditure on workpiece that is directly involved in manufacturing the

pro duct. In direct la bour cost is in curr ed on all oth er pers onnel who are indirec tly

connected with manufacturing the product such as foremen, supervisors, maintenance

staff, staff for support services, stores, time office, etc.

(iii) Expenses may also be direct expenses and indirect expenses. All expenditure incurred

in manufacturing a product, other than direct material and direct labour, falls in the

category of expenses. Direct expenses or chargeable expenses are those which can

be dir ectly attr ib ute d to manufactu ri ng o f th e pro duct , fo r example , expendit ure on

patt ern s, tools, jigs and fixtu res, di es , te sting, etc . all used in manufa ctu re o f a

part ic ula r pro duct only. Indi rect expenses or overh eads or on costs are those expenses

which include all the costs which cannot be directly and exclusively attributed to

manufacturing a particular product, for example, factory expenses include indirect

material and indirect labour cost, taxes, rent of buildings, depreciation, electric

bills, etc. Administrative expenses include salaries of office staff, travel bills, insurance,telegram and fax charges, stationery, legal affairs, etc.; selling expenses include

salary of sales personnel, advertisement and publicity, commissions on sale, etc.

Overheads should be reduced as far as possible to get better margin for profit.

Overheads are charged as percentage of direct labour cost or percentage of direct

labour manhours or machine hours.

Cost elements discussed above are combin ed to obtained the following cost structure.

(i) Prime cost or direct cost:

Prime cost = Direct material + Direct labour + Direct expenses

(ii) Factory cost or works cost:

Factory cost = Prime cost + Factory expenses

(iii) Manufacturing cost or production cost:

Manufacturing cost = Factory cost + Administrative expenses

(iv) Total cost:

Total cost = Manufacturing cost + Selling and distribution expenses

(v) Selling price:

Selling price = Total cost + Profit



At break-even point,

or

or

^ti ~ Cfi

*71 + ^bcp Cv, = C/2 + p c v2

C f l - C f \

CvI -C ,(6.46)

v2

The above equation thus helps in determining the break-even point quantity ( N ^ ) .

Using the break-even analysis, one can decide as to which process (1 or 2) should be used

for production of a given quantity in the most economical way. As shown in Fig. 6.34, if the

quantity to be produced is more than A ^ , then the manufacturing process 2 will be economical

to use.

Besides comparing economy aspects of two or more processes, the break-even point

analysis also helps in deciding the size of batch or quantity of product to be produced from

pro fi ta bil ity poin t o f vie w as demonstr ate d in th e fo llowing:

Let It = total income from the sale of N units with selling price Ps per unit = N Ps.

Total cost of product (C,) = Cy+ N Cv (6.47)

The plots of /, and C, are shown in Fig. 6.35. Break-even point is at the crossing of two

lines of /, and Cr The W. is the quan tity prod uce d by the proces s und er consid eration when

there is no profit at all. Hence to gain profit, the quantity of product to be produced should

be more th an N, bcp-

Quanti ly N ►

Fig. 6.35 Break-even chart for a single product for assessing the minimum quantity of product to be

manufactured.

At break-even point, ,

or

v c v

c r yV = L __

^ P. - C„(6.48)

The value of break-even point should preferably be maintained as small as possible by

reducing fixed cost and variable cost and by increasing the selling price.



6 .1 0 E C O N O M IC S O F M E T A L M A C H I N IN G

As already mentioned the manufacturing cost of a product should be attempted to keep low

to make the product accep table in the market and also to earn mo re profit. In metal machining,

attempts have been m ade in different ways such as optimizing tool life to minimize production

cost or maximizing production rate to lower down the production cost. But if cutting speed

is reduced to increase tool life, metal rem oval rate is reduced and if cutting speed is increased,

tool life is reduced with increased tooling cost. A balance has to be struck and an optimum

cutting speed be determined corresponding to which an economical tool life may be ensured

to result in economical production.

W.W. Gilbert evaluated tool life for (a) minimum cost per piece and (b) maximum

pro duction ra te, as dis cussed in th e fo llowin g.

6 .1 0.1 M i n i m u m C o s t p e r P i e c e

Total production cost of a product per piece comprises:

(a) Machining cost per piece

(b) Tool changing cost per piece

(c) Tool grinding cost per piece

(d) Idle cost or non-production co st per piece is on acco unt of time lost in replacing and

regrinding of worn-out tools, loading and unloading the workpiece and for other

items during which the machine remains idle.

Let K . - Direct labour cost + overhead charges (Rs.)

K2 = Cost of tool per grind (Rs.) L = Length o f machining, mm

D = Dia. of workpiece machined, mm

V - Cutting speed, m/min

/ = Feed rate, mm/rev

Ti = Idle time per piece, minutes

Tc = Tool changing time, minutes

(a) Machining cost per piece = (direct labour cost + overheads) x machining time per piece

. . . . . . length of machiningAnd machining time/piece = — ---------------------

feed rate x rpm

Tooling cost per piece

f x N

t c DN 1000FIn turning operations: V = ------- or N =-----------

6 1000 71D

Putting the value of N,

L t t DL

Machining time/piece = f V X 1000 f V • 1000

t t D



Then, machining cost/piece = K ] x machining time per piece

or Machining cost/piece = K. x — — (6.49)1 / V 1000

(b) Tool changing cost per piece = (direct cost + overheads) x tool failures per piece

x tool changing time per failure

We know that as per Taylor’s tool life equation,

/ n

v r = C o r 7 = y l/n

Total number of tool failures ( T ) per piece is given by

Y _ m achining time per piece

tool life (T )

or we can write. n D L i

T _ IQOO-/-* ' _ xD L {V )"

C Un 1000 / C 1'" y l /n

(6.50)

Hence,

‘- i

Tool changing cost/piece = K. • - • T (6.51)* * F 1 1000 / •C

(c) Tool regrinding cost per piece = cost of tool per grind (K 2) x total number of toolfailures per piece (Tx)

Hence,

i - i^ 71 D L - (V ) n

Tool r egn nd ing cost per piece = A , • -------------- r— (6.52)2 1000 f -Cx,n

(d) Idle or non-productive cost per piece = (direct labour cost + overheads) x idle time

Hence,

Idle cost per piece = K x x Tj (6.53)

Now, total production cost per piece (K) = sum of above four costs (a to d)

k DL J t D LA V y K DL ( V) n o r A — A . • -------------------------+ A | -------------------------- T7— Tr + a ^ ------------------------ —— + A | • T» ( 6 . 5 4 )

1 1000 f V 1000 / C 1 1 0 0 0 / C 1'"

The total production cost per piece and its component cost given above are plotted in

Fig. 6.36. It may be noted that the tooling costs increase while the machining cost decreases

with increase in cutting speed. The point 4 A ’ on the total cost per piece curve shows the

minimum cost of production. The cutting speed (V J corresponding to the point 'A ' on the total

cost curve gives optimum cutting speed for economical production and the tool life corresponding

to this optimum cutting speed ( Vo) will be the most economic al tool life. The pro duction cost

per pie ce (Km) corresponding to the point 4 A ’ is the minimum cost per piece.



Fig. 6.36 Effect of variations in cutting speed on various costs.

6 .1 0 .2 M a x i m u m P r o d u c t i o n R a te

Relation between production rate (or number o f p ieces produced per unit t ime ) and cutting

speed is shown in Fig. 6.37. It will be seen that at lower cutting speeds, the production rate

(or pieces produced in unit time) is also low. But when cutting speed is increased, production

rate also increases up to a point Pmx, whe re prod uction rate is maxim um . The corresp ondin g

cutting speed ( Vmxp) is the optimum cutting speed at which the rate of production is highest.

Any increase in cutting speed beyond V will lead to more wea r of tool, frequent changing

of tool, more down time and hence reduced rate of production. The point ‘P ' on the total time

curve gives minimum time taken for production of each piece. The cutting speed V is theoptimum value of cutting speed at which the total time taken in production of each component

will be minimum.Tot al t ime per piece

Cu t t i ng sp ee d ►

Fig. 6.37 Various time curves and maximum production curve.



The criteria of minimu m cost per piece or ma ximum production rate when taken individually

do not serve the purpose effectively as the ideal criteria should be: producing the components

at maximum rate and at minimum cost. Now let us plo t bo th th e min imum cost curve and

the maximum production rate curve in one diagram as shown in Fig. 6.38. The cutting speed

Vo at which production cost is minimum is not the same cutting speed at which production

rate is maximum which, in fact, is at cutting speed (Fmxp). The area between the two values

of cutting speed (V o and Fmxp) is known as high efficiency range (Hi-E range) because the

cutting speed lying in this range is either econom ical or productive and hence for efficient and

economical production, the cutting speed should be selected from within this range only.

Fig. 6.38 High efficiency range of cutting speed.

6 .1 0 .3 O p t i m u m C u t t i n g Sp e e d a n d O p t im u m T o o l L i fe f o r M in i m u m C o s t o f

P r o d u c t i o n a n d M a x i m u m P r o d u c t i o n R a t e

The following relationships can be used for calculating the above quantities.

For minimum cost:

In order to find optimum cutting speed and optim um tool life for ‘minimum cos t’, differentiate

total production cost (/Q with respect to cutting speed (V) and set it to zero, i.e. dKldV = 0.By doing so, we get:

Optimum cutting speed (Vn)

C

i - in

K ] x T c +K 2

(6.55)

Optimum tool life ( 7 ^ ) = (6.56)



For maximum production rate:

Total production time (Tp) = machining time + tool changing time + non-productive time

For Finding optimum cutting speed and optimum tool life for maximum production rate ,

differentiate total production time ( T p) with re spect to V and equate to zero, i.e.

^ = 0dV

By doing so, we get,

Optimum cutting speed (Fmxp) =

and

Op tim um tool life (7* ) =

where

= Operating cost (direct labour + overheads) (Rs.)

K 2 = Tool cost per grind (Rs.)

T = Tool changing time (minutes)

VQ= Cutting speed for minimum cost (mpm)

Tmc = Tool life for minimum cost (minutes)

Vmxp = Cutting speed for maximu m production (minutes)

Tmxp = Tool life for m aximu m production (minutes)

C = Machining constant

n = An exponent depending on cutting conditions

(6.57)

r- - l | r c (6.58)

6 . 1 1 M A C H I N A B I L I T Y

Machinability of a material refers to the ease with which it can be worked with a machine

tool. Ease of metal removal (or good machinability) implies:

(i) that higher cutting speed and lower pow er consumption in metal cutting can be expected.

(ii) that the forces acting against the cutting tool will be relatively low.

(iii) that the chip s will be brok en easily.

(iv) that a good finish will result.

(v) that the tool life will increa se reduc ing its frequ ent re-sha rpenin g orreplacement.

Ease of machin ing is affected by metal properties such as hardness, tensilestrength,

chemical composition, microstructure, degree of cold work and strain hardening. Machine

variables such as cutting speed, feed, depth of cut, tool material and its form, cutting fluid,

etc. also affect machinability.



In view of the fact that machinability depends on various variable factors, it is not

possib le to evalu ate th e same directly in te rm s o f some numerical va lue. The fo llowin g cri te ri a

or factors may be considered while evaluating machinability.

1. Tool life between grinds: The longer the tool life, the better the machinability.

2. Quality o f surface finish: The better the surface finish obtained, the higher themachina bility o f metal, i.e. machinability (or sometim es called fm ishability) signifies

how well a metal takes a good finish.

3. Pow er consumption: Low er power consumption per unit volume of metal removed

shows better machinability.

4 . Form and size o f chips.

5. Cutting forces: Reduce d cutting force is indicative of better machinability of metal.

6. Shear angle: Th e larger the shear angle, the better the machinability.

7. Rate o f m etal removal under standard cutting conditions.

The m ain factor chosen from above for judgin g machinability depends on the type of

operation and requirements of production. The factors often used to predict or evaluate

machinability include the tensile strength, hardness and shear angle.

6 .1 1.1 Im p r o v i n g M a c h i n a b i l i t y

The factors responsible for increasing or improving machinability are as follows:

• Chemical composition: The presence o f small amoun t of lead, manganese, sulphur

and phosphorus results in the improvement of machinability. Sulphur in the presence

o f mangane se forms m anganes e sulphide which in the form of brittle flakes is spreadthroughout the metal structure. Phosphorus also helps promoting brittleness in metal

and in turn gives ease in machining. The presence of these materials in the metal

enables the chips to break quickly due to brittleness, thereby providing ease in

machining and improved surface finish. Grey cast iron is much more machinable

than white cast iron because the former has carbon in free form as graphite flakes

which assist chips to break up easily. Also, graphite lubricates the tool durin g cutting.

The presence of carbon content below 0.3% and above 0.6% and the high alloy

contents in steel tends to decrease machinability.

• M icro stru ctu re: Metals with uniform microstructure having small undistor ted grains, lamellar structure in low and medium carbon steels and spheroidal structure

in high carbon steels ensure higher machinability rating. Non-uniform structure

with large and distor ted grains and presence of abrasive inclusions decrease

machinability.

• Treatm ent given to metal: Medium and high carbon steels are hard and their

machinability may be improved by their hot-working. Machinability of low carbon

steels can be improved by their cold-working. Certain heat treatment processes,

namely, annealing, normalizing and temp ering, also help increasing the machinability.

• Less hardness, less ductility and less tensile strength.



6 .1 1 .2 M a c h i n a b i l i t y In d e x

As mentioned above that the machinability of a material cannot be quantified directly in the

form of some absolute value, rather it is given in some relative form. Machinabilities of

different metals are compared (or given) in terms of their machinability index which is

defined as below:Machinability index %

_ C utting speed o f m etal invest igated fo r 2 0 min tool life ^ ^

Cutting spe ed o f a standard steel for 20 m in tool life

A standard steel is free cutting steel which is machined relatively easily and whose

machinability index is arbitrarily fixed at 100%. The SAE 1112 steel has been considered to

have its machinability index 100%. This steel has carbon contents 0.13 (max.), manganese

0.06.to 1.10 and sulphur 0.08 to 0.03%. Representative machinability index for some metals

are given below:

Low carbon steel 55 to 65%

Copper 70%

Stainless steel 25%

Brass 180%

Aluminium alloys 300 to 1500%

Magnesium alloys 500 to 2000%

It may be mentioned in general that (a) magnesium alloys, aluminium alloys and zinc

alloys have excellent machinability; (b) red brass, gun metal, grey cast iron, free cutting steels

and malleable cast iron have goo d machinability; (c) low carbon steels and low alloy steels

have poor machinability; and (d) stainless steel, sintered carbide, high speed steel and monel

metal have fa ir machinability.

6 .1 1 .3 M e a s u r e m e n t o f C u t t in g F o r c e s

1. Need for the measurement of cutting forces: Determination of cutting forces plays an

important role in the following areas:

(i) For analyzing the relationship among different forces acting during metal cutting,

determining temperature at the tool-chip interface, assessing tool wear and that wayclearly understanding the process of metal cutting.

(ii) Investigating machinability problems and tool life, wear, power requirement, etc.

(iii) For helping in the designing of proper tool to meet processes requirement.

(iv) Helping in designing the jigs and fixtures of adequate strength.

(v) Analyzing static and dynamic behaviour of machine tool and designing the proper

machine tool accordingly.

2. Cutting forces: Except some typical cases of metal cutting such as parting off, facing,

etc. involving action of only two component forces (i.e. orthogonal cutting), bulk of the

machining load involves oblique cutting wherein three component forces act as shown inFig. 6.25 for the turning operation. These forces are:



Fa = f e e d f o rce or thrust force acting in horizontal plane parallel to axis of work

Ft = cutting forc e acting in vertical plane and is tangential to work surface (also called

tangential forc e)

Fr = radial for ce acting in horizontal plane along the axis of the tool

The forces which are normally required to be measured are cutting force (F X feed

force (F J and shear force (Fs) as these are generally used in calculating other forces with the

help of various equations already discussed. The device used for measuring cutting forces is

called tool dynamometer or force d ynamom eter. The force in metal cutting is determined

by m easuring deflections or st ra in s in the ele m ents support in g the cutt in g tool . The design o f

the tool dynamometer should be such as to give strains or displacements large enough to be

measured accurately.

3. Types of tool dynam ometer: Tool dynamometers used for measuring cutting forces of

tool can be broadly classified as:

(a) Mechanical dynamometer

(b) Strain gauge type dynamometer

(c) Pneumatic or hydraulic dynamometer

(d) Electrical dynamometer

(e) Piezoelectric dynamometer

Only the mechanical type and strain gauge type dynamometers are discussed in the

following as these are more common in use.

Mec h a n i ca l d y n amom e t e r s

They often make use of sensitive dial indicators for direct measurement of tool forces as the

dial indicators are calibrated to show directly the magnitude of tool forces corresponding to

the deflections caused in the tool holder by these forces; for example, the cutting force (F,)

(Fig. 6.39) tends to deflect the tool and tool holder downwards, axial force (F a) acts along

the axis of workpiece (opposite to feed) and radial force (Fr) will push the tool away from

workpiece and may cause chatter. A typical two-dimensional dial indicator type mechanical

dynamometer is shown in Fig. 6.39 to measure force Ft and Fa.

or

cutting force (F,)

Fig. 6.39 A two-dimensional dial indicator type mechainical force dynamometer.



St r a i n g a u g e t y p e d y n amome t e r s

These are considered superior to mechanical dynamometers and are most widely used. A

typical cantilever type strain gauge turning dynamometer is shown in Fig. 6.40. The device

works in conjunction with a wheatstone flow bridge circuit. The forces being measured are

Ft and Fa. I t is well known that the electric resistance of a wire changes if it is stretched.Different pairs of strain gauges are cemented on the four flat surfaces (section at AA').

During the experimentation, different pairs of strain gauges are subjected to tension or

compression depending on the force applied and the surface on which they are mounted.

It will be seen that under the effect of cutting force (F,), strain gauges T, and T2 are

subjected to tension but the bottom strain gauges C, and C 2 will be su bjected to compression.

Likewise, force Fa puts strain gauges T3 and TA unde r tension and C 3 and C4 to compression.

Arrangement of one set of strain gauges (7*,, Tv C,, C2) in wheatstone bridge circuit is

shown in Fig. 6.41. The other set comprising gauges Ty 74, C3, C4 are arranged in another

brid e cir cuit . Strain gauges subje cte d to te nsio n sh ow an in cre ase in th eir re sis ta nce (due

to increase in length) and those subjected to compression show decrease in their resistance

(due to decrease in length). Changes in the resistance are measured by wheatstone bridge.

The strains and stresses corresponding to the changes in length (and so the resistance) in

the strain gauges can be found using standard formulae and so the corresponding forces

responsible for causing these changes.

Sect ion a t A-A '

(enlarged view)

Fig. 6.40 Using strain gauges for measurement of tool forces.

Fig. 6.41 The wheatstone bridge and the circuit showing arrangement of strain gauges.



6 .1 1 .4 N u m e r ic a l s o n M e c h a n i c s o f M e t al C u t t i n g

Some typical solved examples are given in the following to make the reader confident and

thorough with various metal cutting principles explained earlier in this chapter.

E xa m p le 6.1: Th e following relates to orthogon al turning o f a mild steel rod of 50 mm

diameter. Feed 0.8 mm; chip thickness 1.2 mm; work rotational speed 70 rpm. Calculate chip

thickness ratio (r), chip reduction ratio (K ) and total length of chip removed per minute.

feed rate mm/rev 0.8 ____ (Ans.)Solution:

then,

Now,

- = — = 0.66given thickness o f cut chip 1.2

K = - = — r 0.66

/ = JlDN, and

= 1.5 (An s.)

chip length

or

r = i = ---------------------

/ length of chip before cutting or uncu t chip length

lc = r I = 0.66 x 71- D N = 0.66 x 3.14 x 50 x 70 = 7523.4 mm (Ans.)

E xam ple 6.2: Du ring orthogon al ma chining, the tool had rake angle 10°, chip thicknes s

measured as 0.45 mm, uncut metal thickness 0.16 mm. Find (a) shear angle and (b) shear strain.

Solution: Given a = 10°; / = 0.16 mm; tc = 0.45 mm

r = l = M U .35

/, 0.45

Shear angle (0 ) = tan -l r cos a — 1 0.45 cos 10

1 - r sin a —lan

1 - 0 . 4 5 sin 10 J

Hence shear angle (0) = 25/54° (Ans.)

Shear strain (£) = cot 0 + tan (0 - a ) = cot 25.54° + tan(25.54 - 10) = 2.34 (Ans.)

E xa m ple 6.3: A bar of 85 mm diameter is turned down to 80 mm diameter. If meanlength

o f cut chip is 83 mm, rake angle 12° and cutting is orthogo nal, find cutting ratio and shear

angle.

Solution: Cutting ratio (r) = y when /. = 83 mm

f 85 + 80/ = /r-

or r =83

2

= 0.32

= 259 mm

25 9

Shea r angle (0) = tan -1

= 18.52° (Ans.)

(Ans.)

r cos a

1 - r sin a \ tan-l 0.32 cos

1- 0.32 sin

i 12°

in 12° J



Therefore, F = - ^ *sin 0

2 4 0 x 4

cos{ f i - a )

cos(<p + p - a )

x 0.161 f cos(36.87°-20°) 1

.119° J * |_ cos(16.119° +36.87° -20 °) Jsin 16

or Ft = 632.93 N (Ans.)

Feed force (F ^ can be calculated as follows:

f j r = * s i n t f r - * ) - a )

F, R c o s ( f l - a )

o r Fj = Ft - tan(/3 - a)

= 632.93 x tan(36.87° - 20°)

Hence, F/ = 179.93 N (Ans.)

Power consumption (P)

r - F ,x V _ 6 3 2 9 3 x 3 01000 x 60 1000 x 60

Hence, power (P) = 0.316 kW (Ans.)

E xam ple 6.6: A lathe has maximum spindle power of 5 kW and power required for turning

a steel rod is found 0.12 kW/cm 3/min. With cutting speed 35 m /min and feed rate 0.3 m m/rev,

calculate (a) maximum metal removal rate, (b) depth of cut, (c) cutting force and (d) normal

pressure on th e ch ip .

Solution: M ax. metal rem oval rate (M RR )max

Max. pow er available at spindle

Power required/cmVmin

= 41.66 cm 3/min (Ans.)0.12

Depth of cut (/)

(MRR)max = V t f here V = 35 x 100 cm/min

/ = 0 .3 /10 cm/rev

or 41.66 = 35 x 100 x / x 0.3/10

4 1 6 6 41-66 m o / a ^Hence, t - = --------- = 0.39 cm = 3.9 mm (Ans.)3 5 x 1 0 0 x 0 . 0 3 1 0 5

Cutting force (F f)

F V

' kW1000 x 60



Ft X 35or 5 = — ---------

1 0 0 0 x 6 0

5 x 1 0 0 0 x 60 A xor Ff = — ----- = 8571.4 N (Ans.)

Normal pressure on chip (P)

P = ----------5 -----------= 8571 4 = 7325 N/m m! (Ans.)chip area (/ x / ) 3.9 x 0.3

Example 6.7: In an orthogonal turning operation of a mild steel rod of 55 mm diameter,

cutting speed was 25 m/min, rake angle of tool 30°, feed rate 0.12 mm/rev, tangential force

2900 N, feed force 1200 N, length of continuous chip in one revolution 95 mm, determine

coefficient of friction, shear plane angle, velocity of chip along tool face and chip thickness.

Solution:Given:

D - 55 mm;

V - 25 m/min;

a - 3 0 ° ;/= 0.12 mm/rev;

Ft = 1200 N;

lc = 95 mm

^ v Ff +F, tan a 1200 + 2900 tan 30°Coefficient of friction (Li) = —— = — — — ——----- ——

Ft - Ff tan a 2900 -1 2 0 0 tan 30°

= 1.30 (Ans.)

Shear plane angle (0)

<f>= tan-1 [ r cos a

1 - r sin a

t I 95

But r = — = - f = ----------

, where I - JtD - n - 55 mm per rev = 0.55tc I t u x 55

And 0 = tan-10.55 cos 30

- = 33 .

U

28° (Ans.)1 - 0.55 sin 30

Velocity of chip along tool face (Vc)

We know, r = — V

Vc = r • V = 0.55 x 25 = 13.75 m/m in (Ans.)

Chip thickness (/c)

t feed r - — = ------ , since depth of cut (l) is feed in machining operation

feed 0.12 xor tc = = o ^ 5 = m m (

Example 6.8: The following observations were made in an orthogonal cutting operation.

Depth o f cut = 0.3 mm ; chip thickness = 0.5 mm; a = 20°; cutting v elocity = 100 m/min;

cutting force = 250 N; feed force = 110 N, find shear angle, shear strain, velocity of chipalong the tool face and work done in shear.



Solution: Given: depth of cut (/) = 0.3 mm; tc = 0.5 mm; a = 20°; V - 100 m/min;

F t = 250 N ; Ff = 110 N

Shear angle (0)

r cos a

<p - tan-1

= tan-1

1 - r sin a

0.6 x cos 20

1 - 0.6 sin 20

, where r - — = — = 0.6tc 0.5

= 35.3° (Ans.)

Shear strain (£)

e = cot 0 + tan (0 - a)

= cot 35.3° + tan(35.3 - 20) = 1.68 (Ans.)

Work done in shear (Ws)

w . = F , x V,

And Fs = Ft cos <p - F f sin 0

= 250 cos 35.3° - 110 sin 35.3° = 140.74 N (An s.)

V c o s a 100 x cos 20= 97.47 m/minvs =

cos (<p -a) c o s ( 3 5 . 3 - 2 0 )

Work done in shear (fVs) = Fs x Vs - 140.74 x 97.47

= 13,717.92 NM /min (Ans.)

Velocity of chip along the tool face (Vc)

y _ y s >n <P

c cos(<p-a)

100 x sin 35.3° 57.78 c o . . . /A x= ---------------------= ---------- = 59.9 m/min (Ans.)

cos(35.3 - 20) 0.964

E xa m ple 6.9: Given: chip length = 95 mm; uncut chip length = 245 mm; rake angle = 20°;

depth of cut = 0.5 mm

Horizontal and vertical components of cutting force are 2300 N and 250 N, respectively.

Considering orthogonal cutting, find out (a) shear plane angle, (b) chip thickness, (c) frictionangle and (d) resultant cutting force.

Solution: Given: lc = 95 mm; / = 245 mm; a = 20°; t = 0.5 mm; FH (= Ft) = 2300 N and

Fv(= F ) = 250 N

Shear plane angle (^)

r _ chip length (/c ) = 95 _ Q 3g

uncut chip length (/) 245

<p = tan-1r cos a

1 - r sin a= tan -i

0.38 cos 20

1 -0 .38 s in 20= 22.29° (Ans.)



_ . . F f cos a + F. sin aCoefficient of friction (/i) = — ------------------------

F, cos a - F f sin a

100 cos 10° + 180 sin 10c

180 cos 1 0 ° - 1 0 0 sin 10°

= 0.67 (Ans.)

Angle of friction (/?) = tan -1 p = tan-1 0.67 = 34° (Ans.)

Chip flow velocity ( V c):

Vc = Vr when V = cutting velocity = 280 m/min

or Vc = 280 x 0.78

= 218.4 m/min (Ans.)

E xam ple 6.11: A toolgave a tool life of one hour betw een re grindin g while ro ugh cuttin g

at 20 m /min. W hat will be its pro bable life when engaged in perform in g f in ishin g opera tion,

given n = 1/8 for roughing and 1/10 for finishing?

Solution: V T = C

For roughing, 20(7 r) ,/8 = C = 20(7^.),/10 for finishing

where Tr = tool life fo r rough ing an d 7^ = tool life for finishing

Hence. (Tr)m = (7) ) 1' 10

or (6 0) ,/8 = (7} ) ,/10

Hence, tool life for finishing (Tj) =167 min (Ans . )

E xam ple 6.12: Whilemachining at a cutting speedof 30m/min theuseful life o f an HSS

tool was foundone hour. Find out the tool life if the same tooloperates at acutting speed

of 40 m/min. Take n = 0.12 in Taylor’s equation.

Solution: According to Taylor’s equation, VT' = C

In the first case,

30 x (60)° 12 = C

or C = 49

In the second case,

4 0 (7 )° 12 = 49

r I 4 9 ^o r T = I —

1/ 0.12

or 5.5 min (An s.)4 o ;

E xam p le 6.13: A tool had a life o f 10 min w hen cutting at 200 m/min. Find the cutting

speed for the same tool to have a tool life of 150 min. Take n = 0.22 in Taylor’s equation.

Solution: V J f = C = V f l \ n

or V2 =

T V» / \0.22 / , \0.2211

101 l l 5 0 ,• V, =

1 V

•150 = | — J ■150 = 82.67 m/min (Ans.)



Solution: Let V* = Break-even speed at which both the tools give the same tool life.

il.032

Then

and

or40

v x

'4 0

also T

1/0.32

m

m

1/0.4

1/0.4

Tool A

Tool B

3.12585

Vx

i2.5

vx

101493 66611.2

or

or

or

yX ^3.125

71.5

vx

101493= 1.52

66611

( P ) 0-6 = 1.52

P = (1 .52 ),/0-6 = (1.5 2)166 = 2 m/s (A ns.)

E xam ple 6.18: Ta ylor’s tool life relation for HSS tool is given as VTW = C, and that for

carbide tool VTUS = C2 taking that at a speed of 25 m/min, the tool life was 160 min in each

case, compare their cutting life at a speed of 35 m/min.

Solution: For HSS tool, 25 x (160),/7 = C,

and for carbide tool, 25 x (160),/5 = C2

At a cutting speed of 35 m/min:

For HSS tool = 25(160),/7 = 35(T HSS) 1/7

7

x 160 = 15.17 minor

For carbide tool,

or

T =HSS

25

35

25(160)1/s = 35(7;) 1/5

carbide

25

35x 160 = 29.74 min

then r carbide

Th s s

29.74

15.17= 1.96 (Ans.)

E xam ple 6.19: In a turning operation , the following tool life was given:

V T 0,12 x / 07 x cP3 = C

At a cutting speed of 25 m/min, feed = 0.3 mm/rev and job dia 3 mm, the tool life was

one hour. Calculate the tool life if the cutting speed, feed and depth of cut are increased by25% individually and all taken together.



Solution: VT012 x / ° 7 x </°3 = C

Putting values of V, f and d,

C = 25 x ( 6 0 ) °12 x (0.3)° 7 x (3 )03

= 24.36

Now takin g in cre ase o f 25% in div id ually in V , / a n d d,

taking V = 1.25 x 25 = 31.25 m/s

( T ) 0 ] 2 = ------------------------- o J = 1 '331.25 x (0.3) ° 7 x (3)

o r T = (1.3)1/l2 = 8.89 min (Ans.)

Now feed i f ) = 1.25 x 0.3 = 0.375 mm/rev

(T)0A2 = ------------^ ------- — = 1.39

25 x (0.375) x (3)

T = (1.39) 1/012 = 15.53 min (Ans.)

Now dia m ete r = 1.25 x 3 = 3.75

(T ) ° 12 = ------------ ^ -------- o J = 1 ' 5 225 x (0.3) x (3.75)

o r r = (1.52)l/012 = (1.52)8333 = 32.75 min (Ans.)

Now ta king in cre asedvalue of V , / and d together,

V - 31.25 m /s , / = 0.375 mm/rev and d - 3.75 mm

(D 0-12 = ---------------- ^ ----------- o j = 1.0431.25 x (0.375)° 7 x(3.75)° 3

or T = (1.04),/0-12 = (1.04)833 = 1.38 min (Ans.)

E xam p le 6.20: Th e following data were obtained during machining on lathe:

Machining constant (C) = 80

Tool c ha ng ing time (7* ) = 5 min

Tool regrinding time = 3 min

Tool depression cost/grind = Rs. 1.20 per grind

Operating cost = 25 paise per minute

Labour + overhead per minute (AT,) = 20 paise

Work loading and unloading time = 35 sec

Feed = 0.25 mm/rev

Exponent (n ) = 0.25

Job length = 450 mm, machined all over in 4 passes

Cutting speed = 30 m/min

Work dia = 55 mm

Idle time per piece = 4 min

Tool grinding cos t = Rs. 1.20 per grind



Calculate:

(i) Optimum cutting speed for minimum cost

(ii) Tool life corresponding to above optimum cutting speed

(iii) Cutting speed for maximum production

(iv) Tool life for maximum production

(v) Machining cost per piece

(vi) Tool changing cost per piece

(vii) Tool life

(viii) Idle cost

(ix) Tool regrinding cost

(x) Total cost of producing 1500 components

Solution:

(i) Optimum cutting speed for minimum cost:

r I - iKn

( K J C + K 2 )

*1

But K 2 = total cost per grind = tool regrinding time x operating cost + tool depreciation cost

+ tool changing time x operating cost.

K 2 = 3 x 25 + 1.20 + 5 x 25 = 320 = 3.20 (Rs.)

80

or

then.

i

0.25- 1

^0 .2x 5 + 3.20

0.20

3.25= 50.5 m/min (Ans.)

(ii)

or

V T 025 = C or T =C

V

1/0.25r so i 1/025

[ 50 .5 J

T = 6.34 min (Ans.)

(iii) Cu tting speed for maxim um produ ction (F mxp):

80

IH rT [(e - " 5

-1O.25

or

(iv)

Vmxp = 47.6 m/m in (Ans.)

Tool life (T ) = N mxp 7 r i - r T = ' 1

c 1 0.25- 1 5 = 1 5 min (Ans.)

(v) For m achinin g cos t per piece, First let us find the ma chining time p er piece.



x . , . . . . xD L x x 5 5 x 4 5 0 ( 4 )Machining time per piece = --------------- = -------------------

f - V -1000 0.25 x 30 x1 00 0

= 41.44 min

Machining cost per piece = Xj x 41.44 = 0.2 x 41.44= 8.28 (Rs.) (Ans.)

(C \ lln (?>oV /0'25(vi) Tool life, T = = 1 ^ 1 = 1.28 min (Ans.)

(vii) Tool changing cost per piece

- K x x tool failures per piece x Tc

„ machining time ^ — K. x —— x T

tool life

= 20 x x 5 = 3237.5 = 32.37 (Rs.) (Ans.)1.28

(viii) Idle cost per piece = Kj x idle time/piece = 20 x 4 = 80

= 0.80 (Rs.) (Ans.)

(ix) Tool regrinding cost per piece

= Cost per grind x Number of tool failures per piece

mach ine time , 41.44= 1.20 x --------------------= 1.20 x --------

tool life 1.28

= 38.85 (Rs.) (Ans.)

(x) Total cost of production per piece (K)

= Idle cost per piece + Machining cost per piece

+ Tool changing cost per piece + Tool regrinding cost per piece

= 0.80 + 8.28 + 32.37 + 38.85 = 80.3

Manufacturing cost of 1500 pieces

= 1500 x 80.3 = 120,450 (Rs.) (Ans.)

6 .1 2 M A C H I N E T O O L S

The term machining (or metal cutting) includes a large number of metal cutting operations

such a s sawing , turning , facing,- boring , taper turnin g, threadin g, knurling , milling, slotting,

shaping, grinding, etc. All these operations are performed on various types of machine tools

as will be discussed in the following. The cutting tools used for removing the excess metal

may be operated by hand (as in case of hacksaw cutting or filing operation) or by machine

(i.e. power) (as in case of lathe or shaper). Machine or power actuated metal cutting tools and

systems (or machines) are called machine tools. The machine tools are provided with facility

for holding and rotating (or reciprocating or m oving) the workpiece or jo b as also for supporting,



guiding and feeding the cutting tool into the workpiece. They also have facilities for transmitting

pow er to th eir various sections to perform various opera tions.

More commonly used machine tools in a machine shop are as follows.

1. La the 2. D rilling ma chine

3. M illing ma chine 4. Shaper 5. Planer 6 . Grinding machine

A machine shop is the shop in a workshop where all the above machine tools (along

with other metal cutting machines) are installed for conducting various machining operations

with the purpose of giving the desired shape to the workpiece.

6 .1 2.1 F u n c t i o n s o f a M a c h i n e T o o l

A machine tool performs the following functions.

(i) Holding, supporting and rotating (or moving) the workpiece as desired during machining.

(ii) Holding, rotating and guiding (and feeding) the cutting tool in relation to the workpiece.

(iii) Providing power drives to the workpiece and the cutting tool as also to other components

of machine tools to help performing various metal cutting operations.

(iv) Regulating cutting speed, feed, etc. for various machining operations.

6 .1 2 .2 T y p e s o f M a c h i n e T o o l s

Machine tools can be broadly classified as (a) standard machine tools and (b) special purpose

machine tools. Standard machine tools are those which are capable of performing a number

o f machining o perations to prod uce a large variety o f job s with different shapes and sizes.These form part of any machine shop. Examples include lathe, milling machine, shaper,

planer, dri ll in g mach ine, etc. Special purpose machine tools are those which perform only

some specified machining operations so as to produce a large number of identical items, such

as automatic machines used for mass production. Transfer machines and numerical ly

controlled (NC) machines are also automatic machines. Transfer m achines consist of a group

o f machine tools arranged in a sequence to work as a single unit which is automated. Numerically

controlled (NC) machines have systems for controlling the relative movements of cutting

tools and workpiece, cutting speeds, feeds, depth of cuts, sequencing of proper tools for a

particular o peration an d all the machining parameters au tomatically with the he lp o f a prea rrang ed

pro gra m m e fed to the control unit. Much clo ser dim ensional to le ra nces on jo b an d hig her

pro ductivity are ensure d with these machines.

6 .1 3 P R O D U C T IO N O F G E O M E T R I C A L S H A P E S O N M A C H IN E T O O L S

The geometrical shape of the machined surface depends on the shape of the tool and its path

during the machining operation. Most machine tools are capable of producing or machining

components of different geometry. Production of machined surfaces can be broadly divided

into two major categories: (i) production of round or tapered or formed surfaces that are

symm etrical about their axis (o f rotation) and (ii) production o f flat or plane surfaces (includingslots, key ways, splines, etc.).



6 .1 3.1 P r o d u c t i o n o f R o u n d o r T a p e r e d (C o n i c a l) S u r f a c e s U s i n g a

S i n g l e - p o i n t C u t t i n g T o o l

When a rough (unmachined) cylindrical job revolves around its central axis and the tool

penetr ate s beneath its surf ace and tr avels para llel to the axis o f ro ta tion, a surface o f

revolution is produced and the operation is termed turning [Fig. 6.42(a)]. When a hollowtube is machined on its inside in the similar way (as turning), the operation is termed

boring [Fig. 6.42(b)]. Making an external conical surface of uniformly varying diameter is

called taper turning [Fig. 6.42(c)]. W hen the tool point travels in a path o f varying radius,

a contoured surface is produced [Fig. 6.42(d)]. When a short length contoured surface is

turned using a shaped tool normal to the job , the process is termed contour forming

[Fig. 6.42(e)].

(a) Straight (b) Straight (c) Taper (d) Cont our (e) Contourturning boring turning turning forming

Fl f l . 6.42 Showing how surfaces of revolution are produced.

6 .1 3 .2 P r o d u c t i o n o f F la t o r P l a n e S u r f a c e s U s i n g a S i n g l e -p o i n t C u t t in g T o o l

Flat or plane surfaces can be produced by facing or radial turning [Fig. 6.43(a)] wherein the

tool moves normal to the axis of rotation of the workpiece. In other cases, the workpiece is

held steady on the machine tool table, and the tool is allowed to reciprocate (as in case of

a shaper) in a series of straight line cuts with crosswise feed increment before each cutting

stroke [Fig. 6.43(b)]. In case of a planer, the workpiece mounted on the machine tool table

reciprocates past the tool which is given cross feed after each stroke of reciprocation o f the

workpiece [Fig. 6.43(c)]. Both shaper and planer are also capable of cutting slots and splines

on a job, besides generating flat or plane surfaces.

Workpiece

Tool feed

Workpiece

Workpiece feed per cycle

(b) Shaping(a) Facing

Workpiece

(c) Planing

Fig. 6.43 Generation of plane surfaces.



6 .1 3 .3 P r o d u c t i o n o f R o u n d a n d F l at S u r f a c e s (o r C o n t o u r s ) U s i n g

M u l t i - e d g e d C u t t i n g T o o l s

Th e exam ples of machine tools that employ m ulti-edged cutting tools include drilling machine,

milling machine, broaching machine, etc. A drilling machine employs a drill bit, usually a

twist drill bit which is a twin-edged fluted tool, employed for making holes. When drillingis performed on the drilling machine, it is the drill which rotates, but when drilling is carried

out on a lathe, it is the workpiece which rotates [Fig. 6.44(a)]. In milling operations [Fig.

6.44(b)] to [Fig. 6.44(e)], a rotary cutter w ith a num ber o f cutting edg es engages the w orkpiece,

which is moved slowly and fed to the cutter. Plane or contoured surfaces can be generated

in milling, depending on the geometry of the cutter and the type of feed. Horizontal or

vertical axes of rotation are used for the milling cutters, and the feed of the workpiece may

be in any o f the th re e coord in ate directions.

End mill cutt er.

Milled slot -»

Workpiece

(b) Milli ng a plain slot with end mill cutter(a) Drilling on lathe

Workpiece Workpiece Workpiece

(c) Plane or slab milling (d) Groove milli ng (e) Contour or form milling

Fig. 6.44 Various machining operations performed using multipoint tools.

Some commonly used machine tools have been described in the following.

6 . 1 4 L A T H E

Lathe is the most basic machine tool available in any machine shop. The working principle

of lathe is shown in Fig. 6.45 wherein the job is rotated and the tool is fed to the job to cut

metal. The lathe produces surfaces of revolution by a combination of single-point cutting tool

moving p arallel to the axis of job rotation (as in case of turning show n in Fig. 6.42(a) and

Fig. 6.45) or normal to the rotating job (as in case of facing. Fig. 6.43(a)) or sometimes the

combination of both (as in contour turning, Fig. 6.42(d)). The job is held and rotated on alathe by holding it either between the two centres of lathe (live centre and dead centre,



Spindl e — t To01 P°st Tail stock

Head

stock

Face plate

Fig. 6.46 Components of a lathe.

(a) Principal subassemblies

(b) Important components as detailed below:

1. Live centre, 2. Face plate, 3. Spindle, 4. Bull gear, 5. Cone pulley, 6. Back gear, 7. Spindle

gear. 8. Bearing, 9. Stud gear, 10. Intermediate gear, 11. Lead screw gear, 12. Bed ways,

13. Rack, 14. Lead screw, 15(a) Saddle, 15(b) Apron, 16. Bed, 17. Longitudinal hand feed,

18. Power longitudinal feed, 19. Power cross feed. 20. Split nut lever, 21. Tool post, 22.Tool. 23. Compound rest feed. 24. Tool post support. 25. Hand cross feed. 26. Dead centre.

27. Tail stock sleeve. 28. Clamp nut for tail stock, 29. Sleeve lock lever, 30. Tail stock hand

wheel, 31. Motor, 32. Counter shaft cone pulley, 33. Pulley, 34. Motor pulley, 35. Fins for

air cooling of motor, 36. Belt, 37. Short-centre drive support, 38. Chasing dial.

cone pulley (P). This results in engaging the gear (C) with the bull gear (D). It may be noted

that gear (C) is much sm aller in size than bull gear (D). With this arrangement, po wer at much

reduced rpm will flow from cone pulley (P) to gear (A), then to gear (B), then to gear (C),

and finally to bull gear (D) which rotates the spindle. This arrangement of varying spindle

speed is norm ally available in change-gear type lathes but in geared-head lathes, spindle

speed isvaried by jus t s hifting the lever of speed gea r box to a prope r setting as directed in

the chart normally given with the lathes.



Back gear B

Hollow spindle c

Bull gear P in G (if taken out

disengages bull gear with cone pulley)

Bearing Live centre

E-Spindle gear attached with

spindle

Gear A (connected with

cone pulleys)Cone pulley P

(free on spindle)

Spindle

Bull gear D (fixed with spindle)

Fig. 6.47 Showing the details of power transmission (within the head stock of a lathe) from spindle

cone pulley (P) to the lathe spindle wherein the cone pulley (P) receives power from motor.

2. T a i l s t o c k : The tail stock fits on to the opposite end of the lathe bed and carries the dead

centre, which is used to hold and support the job when rotated between the two lathe centres,

live centre and dead centre. The dead centre does not revolve with the job (as the live centre)

and hence is called dead centre. The sectional view of a tail stock is shown in Fig. 6.48. The

tail stock consists of a sleeve, nut, hand wheel, dead centre and arrangement for holding the

tail stock rigid with lathe bed during machining, besides the tail stock set over system for

taper turning is also there. The tail stock serves several functions, for example, it holds the

jo b th rough its dead centre during the m achin in g opera tions, enables ta per turn ing (by ta il

stock set over method) and supports the drill (with drill holder) for drilling hole in a job held

in chuck (Fig. 6.54(e)).

Dead

centre

Fig. 6.48 Sectional view of the tail stock of lathe.

A. Hand wheel to move sleeve (C) forward or backward; B. Handle for tightening the sleeve

(C) and thereby holding the dead centre in any fixed position; C. Sleeve; D. Nut; E and F Bolts

to clamp tail stock with lathe bed; G. Bolt when loosened, allows the tail stock set over

(perpendicular to bed) for taper turning; H. Insert to check rotation of sleeve (i.e. allowing

only linear movement of sleeve back and forth).



Refer Fig. 6.48, when the hand wheel (A) is rotated, the nut (D) slides along its axis

taking the sleeve (C) with it. The dead centre is a tapered piece Fitted in the sleeve. Thus, by

rotating the hand wheel (A), the dead centre can be moved forward to hold the job tight

betw een centr es or moved backward to lo osen the jo b held betw een the tw o centres. The

insert (H) does not allow the rotation of the sleeve (C). The tail stock can be clamped with

the lathe bed using bolts (E) and (F). Bolt (G) can be loosened for tail stock set over across

the lathe bed during taper turning.

Note th at the heig ht o f both the live centre and the dead centre is ex actly th e sa me from

the top of the lathe bed, and the axial movement of the dead centre is in perfect alignment

para llel to th e bed w ays and in line with both the centres o f th e la the.

3. Carriage: Devices for controlling the motions of tool are included in this section. Carriage

has two parts: (i) saddle and (ii) apron (Fig. 6.49). The portion which takes on it the cross

slide and compound rest and slides along the bed ways is called saddle. The other one which

covers the controls for hand and power feed of tool and also the thread cutting controls is

called apron.

Fig. 6.49 Carriage is the name given to the assembly formed with the combination of saddle and

apron. It moves along the bed ways. The saddle carries a cross slide with the tool post on

it to hold the tool during machining. The cross slide moves perpendicular to the bed ways.

Apron forms the hanging part of carriage and houses gear system for giving power feed

for the longitudinal movement of carriage and the cross slide.

It is the hanging part in front of carriage (attached to saddle) and houses a number

of gear trains through which power feeds can be given to the saddle (carriage) and the cross

slide. Schema tic details o f apron mech anisms are given in Fig. 6.50. Note that the splined

feed shaft always keeps rotating when lathe is running, and so also the lead screw gear (Q)

and the gear (N) which is a sliding gear. The gear (N) gets attached with gear (P), worm (L)

and worm wheel (M) mounted on the splined shaft (K) and thus all interconnected and

rotating always with gear (N). Longitudinal hand feed to the tool along the bed can be given

by th e hand wheel (C ) an d th ro ugh gear (I). N ote that gears (G ) and (F) are power gears and

always keep rotating when lathe is running as they (gears G and F) get power from gear N.



For power movement of carriage (hence the tool) along the bed, push knob (E) to engage

pow er gear (G ) with gear (H ). G ear (H ) is connecte d with gear (I) w hich, w hile moving alo ng

the rack (fitted with lathe bed), gives longitudinal power movement to the saddle. Similarly,

cross slide hand feed to the tool can be given by rotating the hand wheel (D) but for power

feed of the cross slide, pull knob (E) to engage gears (F) and (R). While cutting threads on

a job held between the centres (or in a chuck), half nut A is engaged with the lead screw with

lever (B), and this moves the saddle (and hence the tool) in a particular relationship based

on the pitch of threads cut on the lead screw and the pitch of threads to be cut on job and

the RPM of both, the lead screw and the job.

Fig. 6.50 Schematic details of apron mechanism with particular reference to power feeds for longitudinaland cross movement of the tool in relation to lathe bed and also the lead screw and half nutfor thread cutting.

It will be seen that all feeds for the tool are in fact controlled by the lead screw which

is the main source of power for movements of saddle or cross slide. The lead screw gets

power from th e spin dle . A lthough the sp indle moves only in one direction, th e lead sc rew can

be made to move in both th e directions, clo ckw ise an d anticlo ckw ise. The m echanism o f

transmitting power from spindle to lead screw and the method of reversing the direction of

lead screw of an engine lathe is illustrated in Fig. 6.51, where spindle gear (E) transmits

pow er to stud gear (D ), th ro ugh th e direction re vers ing gears (A ) an d (B ) which are used torotate the stud gear (D) in the same or opposite direction to that of the spindle gear (E) since

the spindle gear (E) always has a fixed direction of rotation. Gear L is the gear mounted on

the lead screw. The gears between the stud gear D and lead screw are all called intermediate

gears and are used to vary the RPM and direction of lead screw with respect to the shaft of

stud ge ar (D) (and thus the spindle). In a change gear type lathe, the gear train for intermed iate

gears is calculated based on pitch of the threads to be cut on job and the pitch of lead screw,

whereas in geared head lathe, a control chart helps in operating the required lever for this

purp ose. Rem em ber that lead sc rews hav e acm e th re ads with included angle o f 29° fo r easy

engagement and disengagement of half nut.



Fig. 6.51 Showing the mechanism for direction reversing of lead screw.

A and B: direction reversing gears, C. gear on stud gear shaft, 0. stud gear, E. spindlegear, F and K: intermediate gears, L. lead screw gear.

4. Bed: Bed gives support to all the moun tings of lathe, such as tail stock, carriage, head

stock, etc. Bed is made of nickel alloy cast iron and is carefully seasoned, machined and

scrapped because the accuracy of working on a lathe largely depends on the trueness of bed.

The bed carries bed ways or guide ways which are of two types, inverted V-type with included

angle o f V as 90° and flat type (Fig. 6.52). F lat ways give larger bearing surface with

corresponding reduction in wear but need special care for cleaning the bed ways from metal

chips and other foreign matter. The V-type guide ways give better guide to the carriage and

ensure proper alignment. Chips also do not get collected over the V-guide ways. Lathesusually have both the guide ways to take their best advantage. Ribs give strength and rigidity

to the lathe bed structure.

Fig. 6.52 An example of lathe bed with its bed ways, inverted V type and flat type. Ribs provided at

intervals along the bed length provide strength and rigidity to the structure of bed.

6 .1 4 .2 D e f i n i n g t h e L a t h e S i z e

Lathe size is defined in one or more of the following ways (Fig. 6.53).



(i) Swing or m aximum diam eter of the job that can be rotated over the bed ways (E)

or over the carriage (D).

(ii) M aximu m length of job that can be held between the lathe centres, A and/or the

centre height (C).

(iii) Bed length including the head stock.(iv) Sw ing in gap (B). It is applicable only in case o f specially designed gap bed type

lathes.

Lathes are available in sizes ranging from 700 to 3000 mm between centres.

Fig. 6.53 Various ways of defining the lathe size. A. Maximum length of job accommodated between

centres, B. Swing in gap (in gap bed type lathes only), C. Centre height, D. Swing over

carriage, and E. Swing over bed.

6 .1 4 .3 T y p e s o f L a t h e

Lathes can be broadly categorized as follows.

(a) Speed lathe is a power driven simplest lathe often used for wood turning. Tools are

hand-operated.

(b) Centre lathe or engine lathe is the most commonly used general purpose lathe

found in all machine shops. Stepped cone pulley arrangement with motor is used for

varying the speed of lathe spindle. Tool is fed by power.

(c) Gap bed lathe has a section of lathe bed removable to create a gap or cut in the

lathe bed n ear the head stock to acco mm odate job s o f extra diameters (B in Fig. 6.53).

(d) Geared head lathe is a type of centre lathe wherein changes in the spindle speeds

are accomplished by a set of gears (housed in a gear box) operated by a lever.

(e) Bench lathe is a small lathe that can be mounted on a workbench for doing small

jo bs or repair jo bs.

(f) Turret and capstan lathes are production lathes which carry several tools mounted

on the revolving turret or capstan to facilitate performing a number of machining

operations without wasting time in changing the tool (as different tools are needed

for different types of operations).

(g) Tool room lathes are precision lathes suitable for fine tool room work.

(h) Automatic lathes are high speed, heavy duty and semi or fully automatic lathes.

Fully automatic types are designed to perform the complete scheduled operations

without much involvement of the operator.



Fig. 6.55(a) Plain turning between centres.

Chuck

Fig. 6.55(c) Form turning with a form tool. Fig. 6.55(d) Taper turning using combined

longitudinal and cross feed.

2. Facing [Fig. 6.54(b)): It is the operation of making the ends of a job flat when the job

is usually held in a chuck, an d the tool is fed perpen dicular to the axis o f job rotation.

3. Boring [Fig. 6.54(c)): It is the operation of enlarging the hole (or bore) of a workpiece

having its initial bore made either by drilling or by putting a core during casting or the boremade during forging.

4. Threading [Fig. 6.54(d)): Threading on lathe is the operation of making or cutting

threads (of different types and pitches) on a job. Threads may be male threads (external

threads) and female threads (internal threads). The tool used for cutting threads is a single

poin t th re adin g tool.

5. Drilling [Fig. 6.54(e)): It is the operation of making a hole in the end face of the job held

in a chuck. The tool used is a drill bit held in a drill chuck, which itself is mounted in the

tail stock sleeve in much the same way as the dead centre.

6 . Knurling [Fig. 6.56(a)): It is making of roughened surface on a smooth surface ofcylindrical jobs using hardened steel knurles in place of a usual lathe tool. Knurled surface

o f the job helps in holding the job tight by hand. Exam ples include knurled kn obs o f measuring

instruments such as surface gauge or micrometre screw gauge.

7. Grooving or undercutting [Fig. 6.56(b)): It is the operation of reducing the diameter

of a jo b fo r a very short length. The reduced surface produc ed is called groove.

8 . Parting off [Fig. 6.56(c)): It is the operation of separating (or cutting off) usually the

finished (or machined) component from the workpiece blank. It is a very common operation

on lathe.

Lathe tools used for performing above operations are shown in Fig. 6.56(d).

Longitudinal

tool feed Cross feed

of tool



Knurling

tool bit

Fig. 6.56(a) Knurling with a knurling tool bit.

Grooving tool

Fig. 6.56(b) Grooving (or under cutting) operation.

Chuck

rK

t p 1Blank

Finished

job

Parting t ool

Fig. 6.56(c) Parting off operation.

Unturned Turned

surface surface

Turning

tool Side turning

tool

Facing Tapering

tool tool

Necking Knurling

tool tool

(i) Tools used for generating external surfaces

(i i) Tool s used f or generating internal surfaces

Fig. 6.56(d) Different types of lathe tools. The tools used for generating external surfaces are shownat (i) and those used for generating internal surfacing are shown at (ii).



Other lathe operations include reaming (finishing drilled hole with reamer), tapping

(cutting internal threads with taps) and grinding (finishing with a grinder).

6 .1 4 .5 T a p e r a n d T a p e r T u r n i n g

A cylindrical job which decreases gradually in diameter from its one end to the other so asto assume a conical shape is said to be tapered. Taper on jobs is expressed as the ratio of

the difference in end diameters of the tapered job to the axial length of the tapered section.

For example, a taper of 1 mm p er cm m eans that there is a difference of 1 mm in the end

diameters of a tapered job having a tapered length (along axis) of 1 cm. Taper angle is the

included angle between the tapering sides of a job when extended to meet ata poin t (Fig.

6.57). Taper is also given as, say 1 in 20 which m eans that the difference inmajordiameter

( D ) and minor diameter (d ) of the tapered length (/) of 20 mm is 1 mm or ( D - d)ll - 1/20

or tan of half of taper angle = 1/(2 x 20) = 0.025 or 1°26\

Fig. 6.57 Defining taper and taper angle. Taper per uni t length (along job axis) is equal to di f ference

in diameters (0) and (d), divided by the length (/). Tangent of half of taper angle (i.e. tan a) is equal to tan

a =

(D - d)/2/.

Taper turning is a type of turning operation in which the diameter of the job is

gradually reduced as the turning proceeds along the job length. Common methods of taper

turning on lathe arc discussed in the following.

1. Com pound rest method: Com pound rest has a circular base graduated in degrees (Fig. 6.49).

Set the compound rest by swiveling it from the centre line of the lathe centres (or edge of

bed way s) th ro ugh an angle equal to half o f the ta per angle (a°C ) as shown in Fig. 6.58. By

clamping the lathe carriage in place and after adjusting and clamping the tool, take several

cuts for turning the taper. Feeding of tool is done with the compound rest feed handle while

the depth of cut is taken with the help of cross slide. The method is suitable for turning steepand short tapers, both external and internal type.

Centre l ine / of lathe centres 0

Edge of bed ways

a .

Compound

rest

Compound rest

feed handle

a = Hal f o f

taper angle

Swivel

Fig. 6.58 Taper turning by swiveling of compound rest.



The setting o f compound rest is done by swivelling and setting the compound rest at the

half of the taper angle (a). If D is the large diameter, d is the small diameter at taper end,

/ is the tapered length (Fig. 6.59), then half of taper angle (a ) can be calculated as:

a = tan

D - d

21 (6.60)

2. Tail stock set over method: In this method, the tail stock is set over (as shown in

Fig. 6.59) from its centre line equal to half of the taper. Calculating the ‘set over’ of the tail

stock depends on whether the taper is to be given on entire length or part length. The

following example will help in calculating the tail stock set over. The method is used for

mak ing long tape r length on full length o f job.

Ball

centre

a — Half of t ap er ang le

H —Tai l st oc k se t ove r

Fig. 6.59 Taper turning svith tail stock set over method.

Case I For giving taper 1 in 10 on a job 80 mm long, find taper on 80 mm length

= 80/10 = 8 mm. Then, tail stock set over = 8/2 = 4 mm.

Case II For a taper of 12° on a job 80 mm long, here sine of half of taper angle

Set over = — -------; — or set over = 80 sin 6° = 8.36 mm

Ta per length

Case III When the given major dia is D , minor dia is d and tapered length is / and thetotal

length of the job (including tapered portion) is L, all in mm, then.

D - d Ltail stock set over (mm) = — - — * y (6.59)

3. Taper turning with a form tool: Short external tapers can be turned using a form tool

as shown in Fig. 6.60. It should be noted that a form tool when used for turning tapers on

longer lengths, generates vibrations and chattering.

4. Taper turning with taper turning attachment: These attachments are available for

turning tapers on lathe. Longer tapers are easily turned with these. The attachment is also

useful for cutting threads on tapered sections.

The attachment is shown in Fig. 6.60(a). The nut (C) is loosened to disconnect the

motion of cross slide (having tool post on it) from the control of cross feed screw, and thusthe cross slide is made floating by disconnecting it from the saddle so that it can move along

its ways. The link (D) connects cross slide and the block (E) (which can slide in slot (H) through



Block E

Fig. 6.60 Taper turning with a form tool. Angle (a) is equal to half of the taper angle.

Link F Indicator

(graduations in

degrees)

^ _ Lathe bed

-££□—ccn J

Link D Slot (guide) H

Nut C

Cross sl ide

Com pound rest

Fig. 6.60(a) Schematic of a taper turning attachment used on lathe.

an adjustable clamp (A». During normal turning operations, nut (C) is kept tightened to connect

cross slides with cross feed screw and the m otion cross to the bed length m ay be given by simply

loosening the clamp nut (A), so that nut (A) becomes free over the slot (B) provided in the link

(D). The link (F) is hinged at one end and has a guide (H) through which block (E) can travel.

On the other end of the link (F), an indicator is provided which is graduated in degrees.

To turn a taper, hold the job properly between the lathe centres and set the link (F) atdesired angle to give the required taper on the job. Loose nut (C) and tight the clamp (A).

The tool will now be restricted to follow the direction parallel to the centre line of link (F)

and will be guided by the movement of block (E) through the guide (H). This will vary the

depth o f the cut of the tool while mo ving along the jo b length and will render a tap er on the

jo b. T he fe ed to the tool is given by work ing th e handle o f com pound re st. T he com pound

rest is positioned at 90° to the axis o f job. Th e feeding o f the tool is given by com pou nd rest

because th e cro ss sl ide scre w is disconnected.

Advantages of using a taper turning attachment are as follows:

1. The attachment can be quickly and easily set.

2. With the use of this attachment, tapers are turned w ithout disturbing the normal

set-up of the lathe.



3. External and internal tapers can be turned.

4. Tapers are turned with the longitudinal pow er feed and thus the work can be machined

quickly and with better finish.

5. Long tapers are easily given.

6 . Taper turning attachme nt is also used for cutting threads on a tapered surface.

Sometimes taper on a job is turned using combined tool feed both longitudinal and

crossed (Fig. 6.55(d)].

6 .1 4 .6 N u m e r ic a l s o n T a p e r T u r n i n g

E xam p le 6.2 1: Find the angle at which com poun d rest should be swivelled for cutting a

taper on job 150 mm long and having diameter 80 mm. The smallest diameter on the tapered

end should be 60 mm and required length of tapered portion 100 mm.

Solution: Given: D = 80 mm; d - 60 mm; I - 1(X) mmLet a be the angle at which compound rest will be swivelled.

a - tan -l D - d = tan-1

I

0 0 0

1 s

21 J 2 x 100m m

= tan"'[0.1] = 5.71° (Ans.)

E xa m p le 6.22: A mild steel rod has a length of 80 mm and a tapered portion o f length

50 mm. L arge diam eter of taper is 90 mm and small diam eter 80 mm. Find:

(i) Taper in mm/metre and in degrees

(ii) Angle to which compound rest should be set

(iii) Tail stock set over

Solution:

(i) Taper =

Given: L - 80 mm; / = 50 mm; D = 90 mm; d = 80 mm

D - d _ 9 0 - 8 0 _ 10

” 50/ 50

It means that for a length of 50 mm, taper is 10 mm. Then,

10taper in mm /metre = — x 1000 = 200 mm /metre length (Ans.)

50

Also.

= tan" 1 (0.1) = 5.71° (Ans.)

(ii) Compound rest should be set at angle a = 5.71°

(iii) Tail stock set over

D - d L= x —

2 /

^ (90 - 80 ) 80

2 X 50

= 8 mm (Ans.)



6 .1 4 .7 T h r e a d C u t t in g o n L a t h e

External or internal threads may be cut on lathe either with the help of a die or a tap

respectively or by using a thread cutting tool which can cut both external and internal threads.

For cutting threads using a thread cutting tool, a certain relationship is needed between the

speed (revolutions) of the job and the speed (revolutions) of the lead screw to control thelinear movement of the threading tool parallel to the job length when half nut (A) (Fig. 6.50

and Fig. 6.61) is engaged with the lead screw. Many lathes are provided with quick-change

gear box in which different ratios of the speed of spindle (hence job) and lead screw (hence

tool) are readily obtained with shifting of the gear change lever. However, on simple lathes

(change gear type), one has to calculate and arrange change gears (intermediate gear, also

refer Fig. 6.51) to be arranged between the stud gear (driver gear) and the driven gear (the

gear on lead screw) to cut threads of different pitches.

Spindle

gear

Driver or

stud gear

Driven or

lead screw gear

Job

Set o f

reversing

gears

Tool

movement

Tool

Intermediate

gears

Carr iage

Lead screw

Half nut

f lever

Half nut (A)

(engaged)

Fig. 6.61 Set-up for thread cutting on a change gear type lathe.

The general set-up for cutting thread is shown in Fig. 6.61, wherein it should be noted

that only the stud gear (or driver gear) and lead screw gear (driven gear) along with their

intermediate gears are changed for cutting threads of different pitches.

Lead is the axial movem ent o f the screw travelled in its one revolution. In case o f single

start threads, lead is equal to pitch which is the distance from one point on one thread to the

corresponding point on the adjacent thread. In multiple start threads, lead is equal to

Lead = No. of start x Pitch (6.60)

For thread cutting, the ratio of gears between the stud gear (driver) to the lead screw

gear (driven) is found from the following relation.

Driver _ Lead of threads to be cut on job

Driven Lead o f threads on lead screw

and when threads on lead screw are in inch system.

(6.61)

Driver Lead o f threads o f job in mm

Driven 127 Lead o f threads on lead screw in inches (6.62)



The following numerical examples should give clear understanding of these trains.

E xa m ple 6.23: Find the charge gears to cut RH single-start threads of 2 mm pitch on a lathe

having lead screw of 8 mm pitch.

Driver Lead of threads on jobSolution: ur iven Leaa or inreaas on ieaa screw

9 1 v 9 0 9 0

(Fig. 6.62)8 4 x 20 80

Another alternative solution is given below:

D river 1 25 25= — x

Driven 4 25 100

It shows that simple gear train with a 20-tooth gear on stud and 80-tooth gear on lead

screw or 25-tooth gear on stud and 100-tooth gear on lead screw can be the two solutions out

of several others.

E xam p le 6.24: Find charge gears for cutting threads of 1 mm pitch on a job on a lathe

having lead screw of 8 mm pitch.

_ . . D river Lead o f th reads to be cut 1Solution: =— ------------- ;——— = —

Driven Lead o f lead screw 8

= ! 20 _ 20 ’

8 X 20 160

It is not possible to get a gear of 160 teeth and hence compound gear train will berequired as given below:

D river 1 l x l 20 30= - x - — - = — x

Driven 8 4 x 2 80 60

Hence gears with 20 teeth and 30 teeth are the first and the second stud gears, respectively

[Fig. 6.62(b)! and gears with 80 teeth and 60 teeth are respectively the first and the second

driven gears (or lead screw gears).

E xam p le 6.25: Design a suitable gear train for cutting 8 mm pitch, 3 start threads on lathe

having lead screw with 6 mm pitch.

_ . . D ri ver Lead o f th reads to be cu t 3 x 8Solution: — = ------ — -——— = — r— = 4

Driven Lead of lead screw 6



E xa m p le 6.2 6: Square threads of 8 mm pitch, double start are to be cut on a rod having

diameter 60 mm. The lathe has a lead screw with 6 mm pitch. Find:

(a) gear ratio between spindle and lead screw

(b) depth of thread to give 0.1 mm clearance

(c) lead of thread to be cut(d) core diameter

(e) helix angle at core diameter

(f) helix angle of thread

Driver 16 8Solution: (a) Gear ratio = 7 —: = — = —

Driven 6 3

_ 8 x 10 _ 80 (stud gear)

3 x 1 0 3 0 ( le ad s cre w )

^ ^ , pitch 8(b) Depth of square threads = —- — = — = 4 mm

Since a clearance of 0.1 mm is required, hence depth of cut would be: 4 + 0.1

= 4.1 mm (Ans.)

(c) Lead of thread to be cut = Pitch x No. of start

= 8 x 2 = 16 mm (Ans.)

(d) Core diameter = Outside diam eter - 2 x depth of thread

= 60 - 2 x 4.1 = 51.8 mm (Ans.)

lead ^(e) Helix angle at core diameter = ta n '1^

= tan-1

core circumference )

16= 5.59° (Ans.)

(0 Helix angle of thread = tan-1

= tan"1

n x 51.8

lead to be cut

mean circumference o f work

16

]

/ r(60 - 4)

= 5.19° (Ans.)

6 .1 4 .8 C u t t i n g S p e e d , Fe e d a n d D e p t h o f C u t i n T u r n i n g

Cutting speed in lathe means the number of metres measured on the circumference of a

rotating job that passes the cutting edge of the tool in one minute. The length of the chip

removed per minute is its measure.

^ , * D N Cu tting speed = , metres per minute1000

where D - Job diameter, mm

N = rpm o f job



For cutting different metals with a tool made of a particular material, there are recom mended

specific ‘average cutting speed s’ for performing various mach ining ope rations. For example,

with a high speed tool, turning of mild steel is done at a cutting speed of 25-30 m/min and

that o f cast iron at 16-22 m/min and o f brass at 60-8 0 m/min. The cutting speeds will be

different for different operations also such as the cutting speed for drilling will be differentthan that for turning, threading, ream ing, etc. M aintaining correct cu tting speed (i.e. the speed

at which a particular tool and job material combination is most effective) enhances the tool

life greatly.

Feed is the amount of advancement of tool (parallel to the surface being machined) per

revolution of the job. It is usually given in millimetres per revolution of the job. The amount

of feed depends on the finish required, depth of cut and the rigidity of the machine tool. On

lathe, a feed of 0.3 to 1.5 mm per revolution is often used for roughing operations and 0.1

to 0.3 mm per revolution for finishing operations.

Feed ( f ) may be calculated as below:

Feed ( /■ ) = — (6 .66) N T

where L = length of cut, mm

N = rpm of job

T - cutting/machining time, min

/ = feed, mm/rev

D epth of cu t is the advancemen t (or digging) o f tool in the job in a direction perpen dicular

to the surface being machined. It may be expressed as the thickness of the chip of metal

removed by the tool in one cut and is measured in mm. The depth of cut depends on theamount of metal to be removed, tool material and the power and rigidity of machine tool.

d \ ~ d 2Depth of cut ( /) = — - —

where d ] = dia of job before machining

d2 = dia of machined surface

For normal roughing operations, the depth of cut may vary from 2 to 5 mm and for

finishing operations, from 0.5 to 0.1 mm. The depth of cut also depends on the material of

jo b as deeper cuts can be taken in soft metals.

Me ta l r em ova l r a t e (M RR )

M eta l r emoval rate is the volum e o f material removed p er unit time. Volume of metal removed

is a function of speed, feed and depth of cut as the higher the values of these, the higher will

be th e meta l removal rate.

Let Dj = initial dia of workpiece, mm

d = depth of cut, mm

/ = feed, mm/rev

N = rpm of jobV = cutting speed, m/min



Now, metal removed per revo lution = vo lume of chip hav ing leng th jtDt and cross-sectional

area d • f

Hence, volume of metal removed per revolution = n D .d - f mm 3 then,

Metal removal rate (MRR) = i tDdfN, mm 3/min (6.67)

And in terms of cutting speed,

MR R = 1000 • F - </ •/, m m3/min (6 .68)

• xD jN As V = — -— , m /m in

1000

Ma c h i n i n g t ime (o r t u r n i n g t ime )

To calculate machining time, refer Fig. 6.62(c) wherein:

L = total distanc e travelled by tool in feed direction in single cu t, mm

/ = length of surface to be machined, mm/, = tool approach, m m

12 = over travel of tool, mm

d = depth of cut, mm

/ = feed, mm/rev

N = spindle or job rotational speed, rpm

np = Number of cuts taken during machining

D i = initial diameter of work, mm

D f = ma chined o r final d iame ter of work, mm

Dog —

Driving plate

-Hfeh

L-

' i h -

J \

" T l .............................................

- D - D ________________________a a

r

_

w

1 Work piece

/ * /

f

K Feed ^

Mandrel

= ■ - 9 -

Fig. 6.62(c) Calculating machining time in turning.

Distance ( L ) travelled by tool in feed direction in single cut, L = / + / , + l2

In case of single-point tools, /, and /2 are negligible and hence,

Machining time (Tm) = — per cut or per pass o f tool (6.69)



Since cutting speed (V) = , m/min1000

v 1 0 0 0 v where N =-----------

XD,

Substituting for N ,

L k x D . x LM achining time (7*_) per cut = — \ = * m in (6.70)

/1000 V 1 100017*

V 7TD,

Total machining time (T) - Tm x n (6.71)

where np = Number of cuts/passes

( D , - D

f )Machining allowance =------

-

——

Material removed per cut = depth of cut (d)

, v Total machining allowanceAnd number of cuts ( n j = --------------------------------------

' Material remov ed per cut

(6.72)depth o f cut (d)

Powe r r e q u i r e d f o r t u r n i n g

The power required for turning depends on cutting speed (10* depth of cut (d), feed rate (/*)

and hardness and machinability of workpiece metal.

In fact, the power required depends on the cutting force (Ff) which is estimated as

follows:

Cutting force (F t ) = k • d f

where k is a constant depending on workpiece metal

Power (P) = F( x V = k d f - V (6.73)

Nume r i c a l p r o b l em s

E xa m ple 6.27: Find cutting speed in turning a job having diam eter 100 mm and spindle

speed 100 rpm.

Solution:

r/ t t DN 3 . 14 x1 00 x1 00 . . . . . .V ------- = ------------------------ = 31.4 m/min (Ans.)

1000 1000

E xam p le 6.28: A hollow workpiece of 50 mm diameter and 200 mm long is to be turned

all over in 4 passes. If approach length is 20 mm, over travel 10 mm, feed 0.8 mm/rev andcutting speed 30 m/min, find the machining time.



Solution: Cutting speed ( V) -

D N ,

or m/min318

. . 50 x Ni.e. 30 =------------

318

30x318 l f tAOor N = ------------ = 190.8 rpm

50

Total distance travelled by tool in a single pass ( L ) = 200 + 20 + 10 = 230 mm

Then, length travelled by tool in 4 passes (Z,,) = 230 x 4 = 920 mm

L 920Manufacturing time (T ) = — = ---------------- = 6 min (Ans.)

5 JN 0 . 8x190 .8

E xam p le 6.29: Find the machining tim e to face a job o f 60 mm diameterandrotating at

80 rpm with a cross feed o f 0.3 mm/rev.

o . , , r • , . L D /2 6 0 /2Solution: Time for facing (one pass) = — = -------= ------------

JN JN 0.3 x 80

= 1.25 min (Ans.)

E xa m p le 6.30: A rod 150 mm long and having diam eter 15 mm is reduced to 14 mm

diam eter in one pass of turning. Find the natural removal rate and machining time when

spindle speed is 400 rpm and feed 200 mm/min.

Solution: Given: L - 150 mm; D. = 15 mm; Df - 14 mm; N = 400 rpm; / = 200 mm/min ;

feed rate = 200/400 = 0.5 mm/rev

When d = depth of cut = = 0.5 mm

Then, MRR = n x 15 x 0.5 x 0.5 x 400 = 4170 mm 3/min (Ans.)

Cutting time (T ) =-^—= — — = 0.75 min (Ans.)6 m JN 0 . 5 x 4 0 0

E xam p le 6.31: Ca lculate the time required to mac hine a job 170 mm long, 50 mm diam eter

to 165 mm length and 40 mm diameter when job rotates at 400 rpm, feed is 0.2 mm/rev and

maximum depth of cut 2 mm. Take tool approach and overall travel distance as 8 mm for

turning operation.

Solution: Since both diameter and length of the job will be reduced down by turning and

facing respectively, let us do first turning and then facing.



Time for turning

Total length( L ) of tool travel = 170 + 8 = 178 mm

« • ^ u ^ , Di ~ Df 5 0 - 4 0 .Required depth to be cut, d = ---- - — = — - — = 5 mm

Sincemaxim um depth of cut is 2 mm , then No. of cuts required will be:

n = — = 2.5 or 3 p 2

Total turning time = machining time for one cut x Number of cuts

L_

Time for facing

Diameter of job is to be reduced from 50 m m to 40 mm. But before facing, diameter of jobis already turned down to 40 mm dia. Since in facing, length of tool travel is equal to half

the job diameter, i.e. L = 40/2 = 20 mm.

Time for facing one pass = I — j = ----- — ------= 0.25 min

N 178n_ = -------------- x 3 = 6 . 6 min (Ans .) p 0.2 x 400

N um ber o f pas ses re quired (n p)

J N ) 0 . 2 x 4 0 0

Material to be remov ed ? 5 _ _ = --------------------------------- = ------------------------ = - = 2.5, say 3

Max. depth o f cut Max. depth o f cut 2

Hence total time for facing = 3 x 0.25 = 0.75 min

Total time for machining = 6.6 + 0.75 = 7.35 min (Ans.)

6 .1 4 .9 L a t h e A c c e s s o r i e s a n d A t ta c h m e n t s

Accessories are the devices used for holding or supporting job on a lathe during machining.

These include lathe centres, face plate, dog carrier, chucks, angle plate, mandrel, steady rest,

follower rest, etc.

Attachments are special devices used for special jobs , for example, taper turning attachmentused for turning taper, gear cutting attachment used for cutting gear on lathe or grinding

attachment for performing grinding operations.

Lathe centres [Fig. 6.63(a)! are used for turning jo b b etween the centres. The centre

which is fitted in spindle nose and which revolves with the job is called live centre. The one

fitted with tail stock is called dead centre as it does not revolve with the job. The job is held

betw een tw o la the centres, and can be tighte ned or lo osened by advancin g or re treating th e

dead centre by revolving the tail stock hand wheel. The dead centre is set rigid in a particular

posit io n by clam pin g the sleeve o f th e ta il stock befo re sta rt in g th e work.

Face plate (Fig. 6.63(b)] is screwed on the spindle nose. Slots are provided on face

pla te fo r bolt in g angle plate for holding typical right angled bend jobs for boring, etc.



oCO

Ql

u.

Balancing weight

Face plate

s Job-,

Jnn

'Angle p late

Fig. 6.63(c) Showing the use of face plate with the job mounted on angle plate for carrying out boring

operation on it.

Fig. 6.63(e) Use of dog carrier in turning a job between centres. The dog carrier at its one end is

clamped with the job while its other end is engaged in the open slot of the face plate

screwed on the lathe spindle.

Fig. 6.63(f) A four jaw independent chuck. It has four jaws, each jaw is independently actuated andadjusted (during holding the job) by a key. Almost all types of jobs. e.g. cylindrical, square

and irregular shaped are easily held in this chuck.



Teet h on scroll

plate back

Fig. 6.63(g) A three jaw self-centring chuck. It has three jaws, all of them are advanced or retractedsimultaneously by turning the key placed in any of the three key holes made on the

peripheral edge of the chuck.

Other types of lathe chucks include the following.

Collect chucks, magnetic chucks, hydraulic or pneumatic chucks are also used for

holding jobs on lathe.

Mandrels are hardene d steel pieces of round ba r and are used fo r holding the bo red jobs

(jobs having drilled or bored holes) for the purpose of turning them at outside. These are of

various types, for example, screwed mandrel, taper collar mandrel, expansion mandrel, etc.

A typical tapered collar mandrel is shown in Fig. 6.63(h).

Fig. 6.63(h) Showing the use of a taper collar mandrel in turning the surface of a job having its borealready made. The tapered collars, when fit properly in the bore and tightened, hold the job rigid with the mandrel, which is later revolved between the lathe centres.

Steady rest [Fig. 6.63(i)] is used when a long job is machined or drilled at its end by

holding the job in a chuck. The use of steady rest avoids the deflection o f job under its own

weight or cutting forces of the tool. The steady rest is fixed in one position with lathe bed.

Follower rest [Fig. 6.63(j)] is used for turning a long and thin job which may be held

betw een centres and may thus deflect under cutt in g fo rces o f the tool . The fo llow er re st is

connected with the carriage and hence moves with the tool as turning operation proceeds. It

is fitted right opposite to the cutting tool.



6 .1 5 T U R R E T A N D C A P S T A N L A T H E S

It has been seen in earlier discussions that a centre lathe (or engine lathe) is quite capable

of performing various operations to produce a large variety of cylindrical surfaces as also

other flat surfaces. Although it is a useful and versatile machine capable of machining any

or every type o f job s within its limit, it is, however, unsuitable as a m ass production m achine

as it consumes considerable time in setting up different tools on the tool post after each

operation and for each job. A job usually needs several operations to be done on it requiring

a number of tools. On a centre lathe, one has to change the tool every time a new operation

is performe d and thus a comp lete comp onent (requiring several operations) cannot be producedwith a single setting of tools. Moreover, it is often needed to change the set of tools so that

other remaining operations may be done on the job. This replacing and resetting of tools

consumes a lot of time. When a large number of alike pieces are to be produced, every time

posit ioning and changin g o f to ols fo r each jo b not only invo lves a good deal o f tim e but also

results in the workpieces of non-identical dimensions. Thus, a centre lathe is best suited for

‘one o ff’ type job and its use for m ass production work w ould not be effective an d econom ical.

Turret and capstan lathes are the natural developments of a centre lathe and are built to

machine workpieces that are large in number and on repetitive basis (i.e. for mass production

jo bs). Turret and ca pstan la thes bridge up the gap between the slow working centre lathe and

fully automatic lathes specifically designed for mass production of components with very high

production rates. The main distinguishing featu re o f turre t and caps tan la thes is the multip le tool

holders which enables presetting of all the tools for a job. Multiple tooling is provided by

replacing the usual tail stock (o f centre lathe) with a rotating and indexing type hex agon al turret

or capstan head on which six or more tools can be mounted and preset as per the requirement

of various operations to be done on the job. The turret is indexed automatically and each tool

may be brought in line with the lathe axis in a regular sequence. Workpieces are held in collets

or chucks mounted on the head stock of the lathe. The longitudinal and cross feed movements

of the turret saddle and cross slide are regulated by adjustable stops which enable different tools

set at different stations to move a predetermined amount for performing different operations onrepetitive workpieces without measuring length or diameter of machined surface in each case.



These special features of the turret or capstan lathes enable it to perform a series of operations

such as turning, drilling, boring, necking, thread cutting, cutting-off, etc. in a regular sequence

to produce a large number of identical parts in minimum possible time.

6 .1 5.1 D i f f e r e n c e b e t w e e n a T u r r e t L a t h e (o r C a p s t a n L a t h e ) a n d C e n t r e L a t h e

The turret and capstan lathes are the improved version of the centre lathe on account of the

following basic differences in regard to their construction and working.

(i) Th e turret lathe has its head stock s imilar to that o f the centre lathe in construction,

but it possesses a wider range o f speeds, heavie r construction and hig her power

available at the spindle for machining at faster speeds resulting into much higher

pro duction ra tes in com parison to th e centre la the.

(ii) The tu rret has a turret head (or capstan head) in place o f the tail stock of a centre lathe.

The turret head is a six-sided block, each side being capable of carrying one or more

tools. These tools may be indexed in an orderly way to perform different operations.(iii) Th e turret has tool post m ounted on its cross slide which is a four-way tool pos t to

hold four tools capable of being indexed by 90° such that each tool is brought into

operation in regular order. The turret cross slide also carries a rear tool post to hold

another tool (often used for parting off).

(iv) The feed movement of each tool mounted on the turret head is regulated by stops

and feed trips, a feature that enables the same tool to perform operation on each job

to a predetermined amount making duplication work possible without further

measurement.

(v) Combination cuts by several tools simultaneously made possible to machine morethan one surface at a time. It is a unique feature of the turret or capstan lathe.

(vi) The turret and capstan lathes do not usually carry a lead screw to help cutting

threads on the job (as in case of centre lathe). Instead, the external threads are cut

with a die set and internal threads with taps. However, some turret lathes may carry

a short length lead screw, called ‘chasing screw’, to help cutting threads by a chaser.

(vii) Special feature of holding eleven or more tools capab le of being brought into operation

in a prefixed sequence regularly, combined with the use of feed trips and stops for

the tools, makes the turret and capstan lathes a production machine suitable for

pro ducin g a la rge num ber o f id entical com ponents in a m in im um possible time.

(viii) The ce ntre lathe is more suitable for m achining ‘one o ff’ type job and is certainlynot suitable for mass production work. Similarly, turret and capstan lathes arc suitable

for only mass production and not for making one or few jobs because of high initial

tool and job setting time and overall cost of the machining operation.

6 .1 5 .2 P r i n c i p a l P a r t s o f T u r r e t a n d C a p s t a n L a t h e s

The turret lathe (or capstan lathe) is similar to the centre lathe except the turret and some

other mechanisms which have been incorporated in it for making it suitable for mass production

work. The essential features of turret lathe are shown in Fig. 6.64 and that of capstan lathein Fig. 6.65. Principal parts of a turret and capstan lathe are described in the following:



selected before hand and the speed changing lever is placed at the selected position. Just

a push of button runs the lathe at the selected speed.

3. Carriage or chaser saddle: It carries a cross slide over it. Two tool posts, one at the

front and the other at the rear, are mounted on the cross slide (Fig. 6 .66). Both these

tool posts are square tool posts, each capable of holding four tools at a time; tools inthe rear tool post are mounted in an inverted position. Both hand and power feeds are

used for the carriage and the cross slide. Stops and trip dogs are used to disengage the

power feeds (longitudin al feed fo r carr iage or chaser saddle and cro ss feed fo r c ro ss sl ide)

as soon as the required tool travel is completed. The cross slide carriage may be of

(a) bridge (reach over) type (as shown in Fig. 6 .66) capable of carrying a second tool

holder at the rear and (b) side hung type fitted with heavy duty turret lathes and riding

on the top and bottom guide ways on the front of the lathe bed.

c

□ n □ / Square turret

' r rear 1(501P°st| H H W - s ^ Too! (inverted)

S '

□ / □ \Q / Front squareUl I I VTT / turret tool post

h r - T o o lu , a

Cross slideCarriage or chain saddle

Fig. 6.66 Showing cross slide, front square turret tool post and rear square turret tool post. Note the

difference in mounting of tool on the rear turret tool post.

4. Turret saddle and auxiliary slide: The turret saddle in a capstan lathe bridges the

gap between bed ways and its top face provides base for an auxiliary slide (often called

ram or short slide). The turret saddle is adjusted (according to the length of workpiece)

and clamped on the bed ways at required position while the hexagonal turret or capstan

is mounted on the auxiliary slide (Fig. 6.67) which slides longitudinally on the turret

saddle. This arrangement permits quick movement of the turret. Trip stops are there to

stop the feeding motion of the turret at any predetermined position. This type of lathe

is used for bar stock of smaller diameter and light-duty chucking work.

In case o f a turret lathe, the turret is directly mounted on the top of the turret saddle and

any movement of the turret is effected by the movement of the saddle, by hand or by power

(Fig. 6 .68). The turret is a hexagonal-shaped tool holder for holding six or more tools.

Different types of turret heads are shown in Fig. 6.60. Through centre of each face of the

turret, bored holes are provided for accommodating shanks of different tool holders; the

centre line of each hole coincides with the axis of the lathe when aligned with the spindle

i head stock). Besides these central holes, there are four tapped holes on each face o f turret

for securing different tool holding attachments. In addition, six stop bars are mounted on the

saddle which restrict movement of each tool mounted on each face of the turret, a feature

that helps duplicating work. After one operation is over and the turret is brought back away

from the job. the turret indexes automatically so that the tool mounted on the next face of

the turret is aligned with the work. The turret lathe is heavier in construction (in comparisonto the capstan lathe) and is particularly adapted for larger diameter bar work and chucking

work and the machine can also accommodate longer workpieces.



6 .1 5 .8 M e t h o d s o f H o l d i n g T o o l s

It has been mentioned earlier that on turret and capstan lathes, a number of tools can be

mounted on the turret as also on the tool posts (front and back) mounted on the carriage (or

chaser saddle). The methods for mounting tools on tool posts are similar to that used on

centre lathe, but the methods of mounting tools on the turret or capstan head are different. Note the genera l shape an d pro vis io ns o f a tu rret head shown in Fig . 6.72. The common

methods for mounting tools on the turret are given in the following:

Star hand wheel

Tr ip control

Fig. 6.72 View of turret lathe saddle and turret head from the point of view of mounting tools. Note

the hole in the centre of each face of turret for accommodating tool shank and small holes

for bolting different attachments or tool holders.

Method (A): There are holes in the centre o f each face of the turret head. Split adapter

bushes [Fig. 6.73(a )] may be in troduced th ro ugh th ese hole s to hold sm all boring bars , bar

stops, drills and reamers. This type of tool mounting is often used on the capstan lathe. Use

of the split adapter bush for holding a threading die is shown in Fig. 6.73(b).

Fig. 6.73(b) Showing the use of split adapter bush for

holding a threading die.



Method (B): Tools or their attachments may be bolted directly on to the face of the turret

head. Tool (a boring bar) mounting by directly bolting the tool attachment on the face of the

turret is shown in Fig. 6.74.

Fig. 6.74 Showing mounting of a boring bar tool by directly bolting with turret face.

Method (C ): Some tool attachm ents bolted to the face of the turret head have a hole in their

back flange in line with the bore o f the tu rret hea d. In that case, to ols like dri lls o r reamerscan be inserted through the hole and can be used to work over and above to the main tools

fixed on the attachments.

Apart from the above methods, there may be numerous types of tools for which people

may design their own specific devices for mounting on the turret head.

6 .1 5 .9 C o m m o n T o o l s a n d A t ta c h m e n t s

A great variety of tools, tool holding devices or attachments are used on turret and capstan

lathes. These attachments are mounted on the turret faces or tool posts on cross slide. Some

more common types of these devices have been listed in the following with reference toFig. 6.75. A pilot bar [Fig. 6.75(h)] is used on turret lathes for performing heavy-duty

machining. One end of the pilot bar is attached to the multiple turning and boring attachment

and the other end to the head stock of the lathe such that pilot bar gives support to the tool

head for taking heavy cuts.

(a) Straight cutter holder

(b) Multiple cutter holder

(c) Slide tool cutter

(d) Combined bar stop and centring tool

(e) Knurling tool holder ( f ) R o l l e r b o x t u r n i n g a t t a c h m e n t

(g) Adjustable knee-tool holder

(h) Pilot bar and multiple turning and boring head

6 .1 5 .1 0 B a r F e e d i n g M e c h a n i s m

Out of several methods employed for feeding bar stock forward (for machining) after each

finished product is cut off (or parted off) from the bar stock, the simplest method is to feed

the bar by m eans of a wire rope and weight. The m ethod is, however, limited for use on small

machines such as capstan lathes.



T oo l s

r < h

S h a n k

r i

. * * ° i O l.. r \ . ' . _ / _ . - . 1 . - .V> 7 , 1 - \

; □ / ° A t f t

Jo b

(b ) M u l t i p l e cu t t e r ho l de r

Rol l e r

s u p p o r t s

C h i p

(f ) R o l l e r b o x t u r n i n g a t t a c h m e n t

Fig. 6.75 (Contd.)



Second Edition

MANUFACTURING PROCESSES

J.P. KAUSHISHThe revised and updated second edition of this book gives an in-depth presentation of the basic principles and operational

procedures of general manufacturing processes. It aims at assisting the students in developing an understanding of the important

and often complex interrelationship among various technical and economical factors involved in manufacturing.

The book begins with a discussion on material properties while laying emphasis on the influence of materials and processing

parameters in understanding manufacturing processes and operations. This is followed by a detailed description of various

manufacturing processes commonly used in the industry. With several revisions and the addition of four new chapters, the new

edition also includes a detailed discussion on mechanics of metal cutting, features and working of machine tools, design of molds

and gating systems for proper filling and cooling of castings. Besides, the new edition provides the basics of solid-state welding

processes, weldability. heat in welding, residual stresses and testing of weldments and also of non-conventional machiningmethods, automation and transfer machining, machining centres, robotics, manufacturing of gears, threads and jigs and fixtures.

The book is intended for undergraduate students of mechanical engineering, production engineering and industrial engineering. The

diploma students and those preparing for AMIE, Indian Engineering Services and other competitive examinations will also find the

book highly useful.

NEW TO THIS EDITION

• Includes four new chapters Non-conventional Machining Methods; Automation: Transfer Machining, Machining Centres and Robotics; Manufacturing Gears and Threads: and Jigs and Fixtures to meet the course requirements.

• Offers a good number of worked-out examples to help the students in mastering the concepts of the various manufacturing

processes.• Provides objective-type questions drawn from various competitive examinations such as Indian Engineering Services and GATE.

THE AUTHOR

J.P. KAUSHISH, former Deputy Director, Central Building Research Institute (CBRI), Roorkee, and former faculty, University of

Roorkee (now IIT Roorkee), has worked in CBRI in different capacities and was the Head of Building Plants and Processes Division

for over two decades. He has received three times the coveted National Award, constituted by the National Research Development

Corporation for meritorious innovations. Mr. Kaushish has to his credit seven Indian Patents on his innovative development of

machines. He has authored over half-a-dozen books on various aspects of production engineering and has also published fifty

manufacturing technology book

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