Manufacturing Technology Book

A \

Shear plane

t W orkpiece

-H b H-

Fig. 6 .2 4

G eom etry of chip form ation in orthogonal cutting.

Mean norm al stress ( o) a =^ = A

F= F' s m ‘t‘ A!s\n

(6.22)

Putting the values o f F . = Ff cos

(6.23)

M ean shear stress (t) F F. sin

Fs = ^ ~ - ~ s in ^

(6.24)

It can be show n that ^ cos

sin (p

(F ,c o s 0 -/y s in 0 )s in 0

(6.25)

Shear strain (£) S hear strain is given by the follow ing expressions: e - cot

(6.26)

and also €=

6 .3 .9

cos a

(6.27)

sin cos(
F o rc e s o f a S in g le -p o in t T o o l

During metal cutting, the w orkpiece metal offers resistance to the cutting tool. T his resistance is o v erco m e by the cutting force applied through the tool. T he w ork done by this force in cutting is spent in shearing the chip from the w orkpiece, deform ing the chip and ov ercom ing the friction o f the chip on the tool face. The m agnitu de o f cutting force d ep end s on w orkpiece m aterial, feed, depth o f cut, cutting speed, tool angles and lubricant or coolant used. Forces acting on a single-point turning tool in oblique or conventional cutting are show n in Fig. 6.25 and these are: F a = A xial feed force or thrust fo rce acting in horizontal plane parallel to the axis o f w ork but in the direction opposite to the feed. Fr = Radial force acting in horizontal plane along a radius o f work, i.e. along the axis o f tool. Ft = C utting force or tangential fo r c e acting in vertical plane and tangential to the w ork surface.

Fig. 6 .2 5 S how ing c u ttin g fo rce s in co nven tio nal (o b liq u e ) tu rn in g process, /?— re su lta n t fo rce ; F — axial feed force; F — radial force; F,— cu ttin g force.

In the above three forces, F t is the largest in m agnitude and F r the smallest. For turning operation, F a varies between 0.3 Ft and 0.6 F( and Fr between 0.2 F t and 0.4 Fr The components F t and Fa arc determ ined easily with the help o f suitable force dynamometer. The resultant force (R ) can be com puted as below: R =^ F 2+F2+F2

(6.28)

In orthogonal cutting, only tw o forces (Fa and F f) com e into play and Fr is zero (Fig. 6.26). The resultant force (R ) is as follows w herein Fa an d F t are axial (feed) force and cutting force (or tangential force), respectively. (6.29)

R = sjFa2 + F,:

Fig. 6 .2 6

Forces acting on a cutting to o l in orthogonal cutting.

(a) T orq u e (T) developed on w orkpiece N eglecting F q and Fr being very small, Torque (T) =

^ D (Nm) 2x1000

(6.30)

w here D = dia. o f work, m m , F t = cutting force, N (b) Heat produced (H ) Heat produced = w ork done in metal cutting // =

5 ---T 60x1000

kN m /s o r kW or kJ/s

(6.31)

w here V = cutting speed, m/min, F t = cutting force, N Heat produced is also equal to the follow ing w here F t is in k g f and V is m/min F

V

, kcal/min

(6.32)

427 (c) P ow er required ( F) P =

F .- V 60 x 1000 x i]

, kW

(6.33)

w here F t - cutting force, N V - cutting speed, m/min 11 = efficiency (say 80 to 90% ) (d) M etal rem oval rate (M RR) = V • b • /, c m 3/m in w here V = cutting velocity, cm /min h = w idth o f cut (cm) or feed rate, cm/rev. / = depth o f cut or uncut chip thickness, cm

(6.34)

M axim u m m etal rem oval rate (M R R ) max. Max. pow er available at m achine spindle (kW) , v = -------- — --------------------------------------------- , (cm /min) Power required (kW /cm ' /min)

6 .3 .1 0

(6.35)

P o p u la r T h e o rie s o n M e c h a n ic s o f M e ta l C u ttin g

Various relationships have been derived earlier for shear angle (
E am st-M erch an t Theory M erchant Theory Stabler Theory Lee and Shaffer’s Theory

The follow ing two theories are more popular am ong metal cutting engineers. Earnst-M erchant theory T his theory is based on the principle o f m inim um energy consum ption and implies that during cutting, the metal shear should occur in that direction in w hich energy requirem ent for shearing ism inim um . Further, the behaviour o f metal being machined is like that o f an ideal plastic. Also, the shear stress is m axim um and constant at shear plane and independent o f shear angle
4

ifi-a )

’

(6.36)

Lee and Shaffer’s theory In this theory, the process o f orthogonal cutting has been analyzed by applying theory of plasticity for an ideal rigid plastic material. Follow ing assum ptions are made: (i) (ii) (iii) (iv)

W orkpiece metal ahead o f the cutting tool behaves like an ideal plastic material. D eform ation o f metal takes place on a single shear plane. C hip does not get hardened. C hip separates from the parent metal o f w ork at shear plane.

They derived the follow ing relationship: (p = ^ + a or

= 45° + a - {)

0 + P - a = 45°

(6.37)

This w as further modified as: 0 =—+a +6 - p 4

(6.38)

w here 0 covers the changes in different param eters because o f the formation o f built-up edge.

6 .4

H E A T IN M E T A L C U T T IN G

W hen a metal is deformed plastically as in metal cutting, most o f the energy used is converted into heat. The energy available at the cutting edge in a metal cutting process is converted into heat, mostly in frictional heat as also the heat caused by destruction of molecular or atomic bonds in metal in the shear plane. The main sources o f heat are: (i) the shear zone 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 o f this heat, high temperatures are generated in the region o f tool cutting edge which have a controlling influence on the ra te o f w ear o f tool and on the frictio n between chip and tool; for example, the temperature plays a major role in the formation o f crater on the tool face and leads to failure o f tool by softening and thermal stresses.

Fig. 6 .2 7

Regions of heat generation in metal cu ttin g include: (1) P rim ary shear zone, (2) Secondary shear zone and (3) W ork-to ol interface zone.

The three main regions o f heat generation in metal cutting shown in Fig. 6.27 are discussed in the following: I. T h e shear zone or prim ary deform ation zone No. 1 is the region in w hich plastic deform ation o f metal occurs during machining. D ue to this deform ation, about 80% o f total heat is generated in shear zone. A portion o f this heat (about 75% ) is carried

aw ay by chip and hence the temperature o f chip is increased. The heat produced per minute is equal to the product o f cutting speed and cutting force divided by mechanical equivalent o f heat. The amount o f heat conducted from chip to the tool and workpiece depends on temperature differential betw een these elements, their m asses and time o f contact with each other. Therefore, higher cutting speeds show greater amount and percentage o f heat in the chips because the heat has less tim e to be conducted from chip to tool and workpiece (Fig. 6.28). 1 0 0 % total heat

2. T h e secondary deform ation or chip-tool interface zone No. 2 is the region where secondary plastic deform ation takes place due to friction between heated chip and tool causing a further rise in chip temperature. About 15 to 20% o f total heat generated is produced in this secondary plastic deform ation zone. The frictional heat increases with increasing cutting speed. 3. The work-tool interface zone No. 3 is that portion o f tool flank w hich rubs against the w ork surface and generates heat due to friction. In this region, only 1 to 3% o f heat is generated. Heat generation is higher if the tool is not sufficiently sharp. 6.4.1

H eat G e n e ra te d in Metal C u ttin g

A m ount o f heat generated per unit time is given by the thermal equivalent o f the mechanical w ork done in metal cutting. Work done (W.D.) = cutting force (F t) x cutting speed (V) = F t x V, k g f m/min w here F f is in kg f and V in m/min Then, total heat generated (Q) = ^

l!?1 , kcal/min 427

or

Q=

F x V ' kcal/min 427

(6.39)

6 .4 .2

F a c to rs A ffe c tin g T e m p e ra tu re in M e ta l C u ttin g

All the above discussed three zones o f heat generation in metal cutting lead to temperature rise at tool-chip interface. The tem perature plays a m ajor role in the formation o f crater on the tool face and leads to the failure o f tool by softening and thermal stresses. Factors affecting tem perature in metal cutting are given in the following: (a) Materials o f workpiece and tool: These affect tem perature in metal cutting since m aterials with higher thermal conductivity are responsible for production o f lower tem peratures at cutting edge. (b) Tool geometry: If the rake angle is increased in positive direction, both cutting force and am ount o f heat generated are reduced but too large rake angle weakens the cutting edge and reduces the heat conducting capacity o f tool. (c) Cutting conditions: Cutting speed has great influence on the production o f temperature. Since frictional heat increases with cutting speed, the tool-chip interface temperature increases with cutting speed. The tool-chip interface temperature rises but less rapidly than for a rise in cutting speed. C hanges in depth o f 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 re m e n t o f C h ip -to o l In te rfa c e T e m p e ra tu re

A num ber o f m ethods are available for the m easurem ent o f chip-tool interface tem perature and these include tool w ork therm ocouples, em bedded thermocouples, infrared photographic technique, tem perature sensitive techniques, etc. Tool w ork therm ocouple technique is most widely used.

6 .5

C U T T IN G F L U ID S

C utting fluids (or metal working fluids) are those m aterials w hich are applied to the tool and workpiece to facilitate the cutting operation by rem oving heat and reducing friction. These are also known as coolants when cooling quality o f the coolant is m ore and lubricants when lubricating properties are greater. Cutting fluids are available in the form o f liquid, gas and solids to suit different cutting conditions. T he use o f cutting fluids should be economically justified considering its cost o f pumping, collection and filtering o f the cutting fluid. Prominent metal w orking processes involving the use o f these fluids include machining, grinding, lapping, honing, forging, rolling, extrusion, draw ing, etc.

6.5.1

P u rp o s e o f U s in g C u ttin g F lu id s

Lubrication or cutting fluid action is not fully understood. The hydrodynam ic fluid film lubrication is not possible in metal cutting due to the presence o f high pressure and temperature and higher velocities at cutting point. H owever, som e lubricant fluid may reach the tool point due to surface tension o f the fluid through the capillaries formed by m inute hills and valleys o f the chip and tool surface against the outw ard motion o f the chip. T he m echanism o f

boundary lubrication or extrem e pressure (EP) lubrication has helped in explaining the reduction o f kinetic coefficient o f friction between chip and tool. In the presence o f high tem perature and high pressure, the highly clean reactive surface o f the chip reacts with the special cutting fluids used and forms com pounds having low shear strength and layered structures which are easily sheared by sliding action o f 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, im proved tool life with reduction in cutting forces. C om pounds o f sulphur and chlorine like chlorinated paraffin o r 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 o f fluid flow, its thermal conductivity, etc. The cooling effectiveness does not depend mainly on thermal properties o f the fluid but it also depends on the wetting action and vapour formation for quick removal o f heat. Cutting fluids are used for the follow ing purposes: (i) Cooling o f tool which is necessary to prevent metallurgical dam age and to assist in decreasing friction at tool-chip interface and tool-workpiece interface. Reduced friction results in increased tool life, less power consumption and good surface finish. Cooling action o f the fluid is by direct carrying aw ay o f 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. W ater happens to be a very effective coolant but it may lead to corrosion o f the w orkpiece 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 friction results in m any advantages, for exam ple, pow er consum ption in metal cutting is reduced; abrasion or w ear on the tool is reduced and hence tool life increased; lubrication helps generating less heat at tool tip giving longer tool life; chips are helped out o f the flutes o f drill, tap, dies, etc.; reduction in built-up edge and consequent reduction in friction at tool-w orkpiece area. (iv) Improving surface finish. (v) Protecting machined surface against corrosion. (vi) Breaking chips into small pieces and washing them away from tool.

6 .5 .2

P ro p e rtie s o f C u ttin g F lu id

The good cutting fluid should possess the following properties: • • • • • • •

High heat absorption for quickly absorbing the heat. G ood lubricating property for producing low coefficient o f friction. Neutral so as not to react chemically. Stability against oxidizing in air. High flash point to avoid fire hazards. O dourless, harm less to the skin o f operator and bearings o f machine. Non-corrosive to the w ork and machine.

• • •

Transparent so that cutting action o f tool is observed clearly. Low viscosity perm itting free flow o f cutting fluid. Low price in view o f m inim izing production cost.

6 .5 .3

T y p e s o f L u b ric a n ts

C utting oils, m ainly used as lubricants, are classified into the follow ing main groups: (i) A queous solutions include water, either plain or containing alkali like borax, sodium carbonate or salt or w ater soluble additives. These are cheap and have high specific heat and high heat conductivity. These are used where m ainly cooling and washing away o f chips is required as these are likely to develop rust on m achine tool element. For mainly cooling, the cheapest and best solution is soda or borax in water. (ii) Soluble oils or conventional em ulsions contain up to 80% o f water, fatty acids, mineral oils and soap acting as an em ulsifying agent w hich breaks the oil into m inute particles so that these are dispersed throughout the water. The water provides cooling effect and oil provides lubricating effect and freedom from rusting. These cutting fluids are cheap and used w here cooling is the prim e requirement. M ost o f the cutting and grinding involves the use o f such emulsions.

(iii)

(iv)

(v)

(vi)

(vii) (viii)

C om pound em ulsions arc em ulsions com pounded with some special additives. E m ulsion denotes the solution made by diluting the so lu b le o ili n water. Com pound em ulsions are good lubricants, coolants and wear-resisting fluids. M ineral oils are essentially hydrocarbons such as paraffin and naphthalene. The paraffin hydrocarbons are highly oxidation resistant at elevated temperatures. Mineral 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 com pounded form as discussed in the following. Fatty oils and acids are preferred m ost for boundary lubrication as friction-reducing agents under extrem ely high pressures. Fatty oils (such as lard oil and tallow) and acids are used in various forms, for example, in the form o f greases made by mixing with straight mineral oils and used for lubrication in wire drawing. C om p ou nd mineral oils are actually mineral oils com pounded or m ixed with substances such as fatty oils and acids. As pointed out above that straight mineral oils do not work satisfactorily in boundary lubrication as encountered i n metal w orking processes such as form ing, deep draw ing, extrusion, etc. Mineral oils com pounded with sulphurized fatty oils give a very good lubricant for use under excessively high pressures and excessive friction as in draw ing, grinding, extrusion and forming. Sulphurized mineral oils are used for machining tough low carbon steels. These oils further com pounded with chlorine give chlorinated com pounds, use o f w hich in metal cutting prom otes anti-weld characteristics and hence reduced problem s o f built-up edge. W axes are generally derived from petroleum and used as lubricants in rolling, draw ing and tinning. Various waxes include paraffin wax, natural bees wax, etc. Waxes when com pounded with fatty acids and soap work well under high pressures. G raphite suspensions made by mixing graphite pow der in oil or w ater are used in forging, foundry works, extrusion, wire drawing, etc. Minerals including salt, metals, refractory materials, etc. are used as lubricants or coolants. Salt is a com m on quenching m edia (coolant) in heat-treatment processes.

Various chlorides and hydroxides o f brine and caustic soda are used as quenching m edia for rapid cooling. M etals such as lead and copper coatings on steel wires for draw ing. Graphite, bentonite, lime, etc. arc other lubricants. (ix) C hem ical com pounds consist mainly o f a rust inhibitor, such as sodium nitrate, m ixed with good am ount o f water. These are used in grinding and where corrosion is to be avoided. (x) Solid lubricants include various pow ders, vitreous materials, pastes, greases, dry film, etc. Stick wax and bar soap are som etim es used to lubricate the tool. (xi) Gaseous fluids such as air (still o r com pressed) arc used as blast or suction where fluids cannot be used. Refrigerated com pressed air, nitrogen and carbon dioxide have been used with advantage. Carbon dioxide is effectively used with carbide tools for reducing crater w ear when machining titanium alloys. Liquid carbon dioxide, argon, oil mist, etc. have also been used. In order to reduce adverse chemical effects o f using som e cutting fluids, liquid nitrogen as a coolant in machining and grinding is used by injecting at the tool-w orkpiece interface in cryogenic machining.

6 .5 .4

T y p e s o f C u ttin g F lu id s

Cutting fluids 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 chem ically with work surface to help machining operations. These include highly sulphurized mineral oils, fatty oils, sulphurized and chlorinated fatty oils, etc. A ctive 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 w hich usually get discoloured w ith the use o f 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 ils a n d C o m p o u n d s S u g g e s te d fo r U s e fo r D iffe re n t M e ta ls a n d M a c h in in g O p e ra tio 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 com pounds For broaching, active mineral oil or soluble oil em ulsion For threading, soluble oil For milling, sulphurized mineral fatty oils

(ii) A lloy steel and high carbon steel For turning , mineral oils, or mineral oil and sulphur base oil For milling , mineral or lard oil or soluble oil For tapping, mineral o r lard oil For drilling , soluble oil (iii) C ast iron is generally m achined dry or com pressed air may be used. (iv) Brass is m achined dry. Straight mineral oil may be used. (v) A lum inium m ay be m achined dry. Use o f kerosene oil m ixed with mineral oil gives good results. Soluble oils can also be used. (vi) Plastic is usually m achined dry or using a dilute em ulsion o f soluble oils.

6 .6

T O O L F A IL U R E

Even if a tool is properly designed and ground, it cannot cut the metal satisfactorily for an unlim ited period o f time. It, therefore, has its definite useful life. Tool life is the tim e a tool will cut the metal satisfactorily until it is dulled or becom es blunt and then it causes poor finish on w ork surface, generates chatter vibrations and tool m arks on jo b surface, and increases cutting forces and po w er consum ption and overheating o f tool. U nsatisfactory perform ance o f the tool is indicative o f tool failure w hich implies that the tool has reached a point beyond w hich it will not function satisfactorily until it is reground. Tool failure criteria that could be used for limiting the tool life are given in the following: (a) Based on tool wear : Size o f w ear land; depth/w idth o f crater; fine crack o r chipping developing at cutting edge; w eight o f material worn o ff the tool o r total destruction o f tool. (b) Based on resulting performance o f worn-out tool: Limiting value o f surface finish on workpiece; limiting value o f change in com ponent size; fixed increase in cutting force or pow er required for perform ing the m achining operation.

6.6.1

R e a s o n s o f T o o l F a ilu re

During operation, a tool may fail because o f one or m ore o f the following reasons: (a) Therm al cracking and softening (b) Excessive stress and m echanical chipping (c) G radual w ear (flank w ear and crater wear)

1. T h erm al cracking and softening is a tool failure that is directly associated with high tem peratures the tool has to stand during cutting. High heat in metal cutting produces high temperatures in the cutting region and a tool is expected to stand those temperatures satisfactorily without losing its hardness. But if that limit is crossed, the cutting edge o f tool starts deform ing losing its cutting ability and the tool is said to have failed due to softening. Factors responsible for creating such a condition o f tool failure include higher cutting speed, higher feed rate and higher depth o f cut, sm aller nose radius and a w rong choice o f tool material. D uring cutting, the tool is subjected to fluctuations in temperature and severe temperature gradients because o f w hich the tool is subjected to local expansion and contraction. This

results in setting up o f thermal stresses due to w hich cracks are developed in the tool known as therm al cracks. U nder high tem peratures, when tool loses strength, som e o f its metal may flow plastically under pressure resulting into edge depression and localized bulging (Fig. 6.29).

Fig. 6 .2 9

Showing edge depression and localized bulging of tool because of high tem peratures involved in metal cutting.

2. Excessive stress and m echanical chipping take place at the nose o r cutting edge of the tool. W hen the tool is acted upon by an excessive force or stress, its cutting edge may fail by crushing or chipping o ff o f the nose m ainly due to lack o f tool strength. Com m on reasons for this type o f failure are too high cutting pressure, too high vibrations or chatter, m echanical impact, excessive w ear and w eak tip or cutting edge. 3. G radual w ear takes place when a tool is used for som e time and its w ear is judged by loss o f weight or material from the tool. Thus, tool wear can be defined as the loss of w eight or mass that accom panies the contact o f sliding surfaces. The following two types o f w ear are found to occur in cutting tools. (i) C rater w ear is the gradual or progressive w ear that develops on the rake surface o f the tool and the region w here w ear 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 w ear gradually takes place while machining ductile materials such as steel in which continuous chips are produced. M ain feature o f this w ear is formation o f a crater or depression at the tool chip interface (or tool face). It is due to the high pressure o f the hot chips sliding up the face o f tool, as a result o f which some metal from the tool face is supposed to be transferred to the sliding (or outgoing) chips (through the m echanism o f diffusion) and the result is the formation o f a crater o r depression on the tool face (the crater wear). C rater m odifies the tool geom etry and increases the cutting forces and softens the tool tip. A continued grow th o f crater results in the cutting edge o f the tool becom ing weak and may finally fail. H igher feeds and lack o f cutting fluid increase the rate o f crater wear. (ii) Flank w ear takes place as a result o f friction o r abrasion between the progressively increasing contact area on the tool flank and the newly m achined w orkpiece surface (Fig. 6.30). Excessive heat is generated as a result o f this. A brasion action is added by the hard m icro-constituents o f the cut metal as also the broken parts o f built-up edge (if it is there). T his type o f w ear is m ore pronounced while machining brittle materials like cast iron or when feed is more than 0.15 mm/rev.

Fig. 6 .3 0

Principal types of to o l wear. The principal region where crater wear takes place is the tool face, at a sm all distance (a) fro m the cutting point of tool. Crater w idth is show n by ( b ) and depth by (d).

Flank w ear is a flat portion w orn behind W L or V B the cutting edge. The w orn region at the flank is called w e a r la n d (Fig. 6.31). The flank w ear occurs on the tool nose and front and side relief faces and its magnitude mainly depends on the relative hardness o f the workpiece and tool material and also the extent o f strain hardening o f the chips. The effect o f flank w ear is expressed in terms o f w idth (or height) o f w ear la n d Flank w e ar land ^ (denoted by V B o r W L ) . It is suggested Hg. 6 -31H ank wear, that the tool be reground before the flank w ear reaches its lim iting value, VB = 0.6 to 2 m m for HSS tool. Increased w ear land means that frictional heat will cause excessive tem perature at the cutting point o f the tool, resulting in rapid loss o f tool hardness w hich may lead to the catastrophic tool failure. The burnishing action o f the tool at its w ear 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 w ear is associated with loss o f w eight or m ass o f the tool. A lthough the w ear m echanism o f cutting tools is a very com plex phenom enon, the com m on m echanism s thought responsible for causing w ear are given in the following. W ear seldom involves a single unique mechanism. The w ear m echanism s associated with progressive tool w ear include the following.

6.7.1

A b ra s io n W e a r

A brasion w ear is a type o f m echanical w ear w hich occurs when hard constituents o f one surface plough through the material o f the other surface. Under this m echanism , hard particles (harder than tool material) on the underside o f sliding chip plough into the relatively softer m aterial o f tool face and rem ove metal particles (from tool) by m echanical action. The material o f the tool face is softened because o f high temperature. Hard particles present on the underside o f the chip may include fragm ents o f hard tool material, broken pieces o f strain hardened built-up edge, extrem ely hard constituents such as carbides, oxide, nitrides, etc. present in the w orkpiece material.

6 .7 .2

A d h e s io n W e a r

Adhesion w ear is due to excessive cutting pressure that results in generation o f a lot o f friction between chip and tool face and consequently extrem ely high localized tem perature w hich causes metallic bond between tool material and chip. Microscopically rough surfaces exist under the chip and on the tool face because o f which in place o f true surface contact, only point contact (Fig. 6.32) takes place between chip material and tool material. Under the effect o f very high temperature at chip-tool interface, a metallic bond takes place (at contact points) betw een the m aterials o f chip and tool in the form o f small spot w elds which keep breaking w hen the chip slides. During the process, a small portion o f the welded tool contact is also carried aw ay by the sliding chips. In this way, small particles o f tool material from the tool face continue to be separated and carried away by the chip by adhesion to its underside. A m ount of material so transferred from tool face to the chip depends on the contact area and relative hardness o f the chip and tool materials.

6 .7 .3

Fig. 6 .3 2 Show ing point contacts and m etallic bonds (w elds) form ed between m ating surfaces of chip and tool.

D iffu s io n W e a r

Diffusion w ear occurs by a solid-state diffusion m echanism w hich consists o f transfer of atom s in a metal crystal lattice. T he transfer o f atom s takes place at elevated temperatures from the area o f high concentration to that o f low concentration. In such a situation, metal atom s are transferred at the points o f contact from tool material to the chip material. It weakens the tool and may ultimately lead to tool failure. The am ount o f diffusion depends on temperature at the contact area between chip and tool face, period o f contact between chip and tool and the bonding affinity between m aterials o f tool and chip. Wear o f carbide tools by diffusion is a w ell-know n phenom enon.

6 .7 .4

C h e m ic a l W e a r

Chem ical w ear occurs when a cutting fluid used during m achining is chem ically active to the material o f tool. Hence, only a suitable type o f cutting fluid should be used to increase the tool life.

6 .8

T O O L L IF E

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 perform ance o f a tool material, assessing machinability o f w orkpiece material and know ing the cutting conditions. Tool life is expressed in the follow ing ways: (i) Time period in minutes between tw o successive grindings o f the tool. (ii) Number o f components machined between tw o successive grindings. (iii) Volume o f metal removed between tw o successive grindings. Volume o f metal rem oved per m inute ( Vm):

Vm = JtD • / f • N, m m 3/m in w here

(6.40)

D - dia o f w orkpiece, mm t = depth o f cut, mm / = feed, mm/rev

N - N um ber o f revolution o f jo b per m inute If T be the tim e for tool failure in minutes, then total volum e o f metal rem oved up to tool failure = xD t f - N ■T, m m 3 (6.41) We know that . xD N . cutting speed — V — , m/min or

xD N = 1000F Substituting this in Eq. (6.41), in term s o f total volum e o f metal rem oved to tool failure, Tool life ( Tl ) = m O V f - T t ( m m 3)

6.8.1

(6.42)

F a c to rs A ffe c tin g T o o l L ife

Various factors which affect tool life are discussed in the following: (i) C utting speed has greatest effect on tool life. H igher the cutting speed, sm aller is the tool life. Reduction in tool life corresponding to an increase in cutting speed is parabolic (Fig. 6.33). Based on w ork o f F.W. Taylor, the relationship betw een cutting speed and tool life can be given as

V•r

= C

(6.43)

Fig. 6 .3 3

Show ing parabolic reduction in to o l life w ith increase in cutting speed.

w here V - cutting speed, m/min T = tool life, m in • 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 cem ented carbide tool = 0.6 to 1.0 for carbide tools C = machining constant w hich is equal to cutting speed (m/min) that will give a tool life o f one minute Use o f proper cutting speed and feed considerably effects the rate o f production, surface finish and production cost. Rate o f feed depends on depth o f cut used. Highest perm issible 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, w orkpiece material, pow er o f machine, finish on the jo b . etc. Average cutting speeds to be used for different tool m aterials and w orkpiece m aterials for various machining operations are given in Table 6.3. TABLE 6 .3

Average cu ttin g speeds in m etres per minute

Operations and tool material Material to he machined

Mild steel Free cutting steel Cast steel Stainless steel Cast iron Aluminium Brass

HSS tools

Cemented carbide tools

Straight turning

Drilling

Reaming & threading

Roughing

50 60 20 20 25 200 120

45 45 15 25 25 150 90

10 15 5 5 5 25 30

140 150 50 40 75 500 300

Ceramic tools

Finishing Roughing 180 200 100 70 140 600 400

400 420 150 120 200

Finishing 500 600 200 200 350

—

—

—

—

Feed is usually kept 0.2 to 0.8 mm/rev. D epth o f cut for rough turning is kept 2 to 5 m m and for finishing, 0.5 to 1 mm. (ii) Feed and depth o f 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 geom etry, particularly tool angles, influences the performance and life o f tool. An increase in rake angle (within limits) increases tool life by reducing cutting force and heat generated. But very large rake angle weakens the cutting edge. The optim um value o f rake angle varies between - 5 and +10°. the m inus sign indicated - iv e rake which gives a stronger tool as cem ented carbide and ceramic tools have - iv e rake and operate effectively at higher cutting speeds. Relief angles or clearance angles are provided on cutting tools to prevent rubbing o f tool flank against the m achined surface o f the work. This way, these angles reduce heat generation and consequently increase tool life. But large relief angles give a w eaker tool. Sim ultaneously an increase in cutting edge angles (both front and end) up to a lim it im proves tool life. N ose radius helps increasing surface finish and a stronger tool. (iv) Tool material: An ideal tool material is the one which will rem ove the largest volume o f material from the jo b 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) W ork material and its microstructure play an important role on the tool performance; for example, the presence o f 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 o f work material, the greater will be the tool wear and shorter will be the tool life. Type o f surface of work material (scaly or smooth) also influences machining operation and the tool life. (vi) Nature o f cutting, w hether continuous cutting o r 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 o f m achine tool and w ork is important for avoiding unw anted vibrations during cutting. (viii) C utting fluids o f proper type and in adequate quantity help increasing the tool life by keeping the cutting edge o f 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 IN G

It is always attem pted to produce an acceptable product at the m inim um possible cost because the m anufacturing cost o f a product plays an important role in the marketing o f the product successfully. The cost o f any new product m ust be com petitive with that o f the similar products already existing in the market. Irrespective o f how well the new product meets the design specifications and quality standards, it has to be the criterion o f the econom y in its cost for being com petitive in the market. It may also be noted that in order to earn m ore profit, it is required that the production or m anufacturing cost should be kept lower.

6.9.1

E le m e n ts o f C o s t

T he term cost represents the expenditure incurred on manufacturing a product. T he three main elem ents o f cost arc given below: (i) M aterial cost may be direct material cost and indirect material cost. Direct material cost is for m aterial purchased from m arket as raw m aterial and con su m ed in m anufacturing the product. Indirect material cost is the cost o f all other materials such as gas, oil, coolants used, etc. in m anufacturing the product but they are never the part o f the product. (ii) Labour cost may be direct labour cost and indirect labour cost. Direct labour cost includes expenditure on workpiece that is directly involved in m anufacturing the product. Indirect labour cost is incurred on all other personnel w ho are indirectly connected with manufacturing the product such as foremen, supervisors, m aintenance staff, staff for support services, stores, time office, etc. (iii) Expenses may also be direct expenses and indirect expenses. All expenditure incurred in m anufacturing a product, other than direct material and direct labour, falls in the category o f expenses. Direct expenses or chargeable expenses are those which can be directly attributed to m anufacturing o f the product, for exam ple, expenditure on patterns, tools, jig s and fixtures, dies, testing, etc. all used in manufacture o f a particular product only. Indirect expenses or overheads or on costs are those expenses which include all the costs which cannot be directly and exclusively attributed to m anufacturing a particular product, for exam ple, factory expenses include indirect material and indirect labour cost, taxes, rent o f buildings, depreciation, electric bills, etc. Administrative expenses include salaries o f office staff, travel bills, insurance, telegram and fax charges, stationery, legal affairs, etc.; selling expenses include salary o f sales personnel, advertisem ent and publicity, com m issions on sale, etc. Overheads should be reduced as far as possible to get better margin for profit. O verheads are charged as percentage o f direct labour cost or percentage o f direct labour m anhours or m achine hours. Cost elements discussed above are com bined to obtained the following cost structure. (i) Prim e cost or direct cost: Prime cost = Direct material + Direct labour + Direct expenses (ii) Factory cost or w orks cost: Factory cost = Prim e cost + Factory expenses (iii) M anufacturing cost or production cost: M anufacturing cost = Factory cost + A dministrative expenses (iv) Total cost: Total cost = M anufacturing cost + Selling and distribution expenses (v) Selling price: Selling price = Total cost + Profit

The following two m ore term s are often used in cost estimation and profitability calculations: (a) Fixed cost (C ^) is that w hich is incurred in setting up the facilities for producing a product and this does not depend on the quality and quantity o f the product or sales thereof, such as staff salaries, rent o f building, machinery, adm inistrative expenses, etc. (b) Variable cost (Cv) varies with the quantity or num ber o f parts produced, for exam ple, cost o f raw materials, direct labour, direct expenses, etc. Hence total cost (C f) for producing a quantity ‘AT will be: Total cost (C f) = Fixed cost (C^) + ^ x variable cost (C v) In order to earn more profit, it is required that the production cost should be less because selling price (total production cost + profit) cannot be increased unreasonably in a com petitive market. Concept o f break-even point analysis can be used effectively for deciding an economical m anufacturing process (out o f a few) to be used effectively for production o f a given quantity o f product. It is illustrated in the following: Suppose there are two manufacturing processes under consideration for the production o f N num ber o f parts. Process (1) has fixed cost (C^,) and variable cost (C vI). Process (2) has fixed cost (C/2 ) and variable cost (C v,2). Total cost for process 1 will be: (6.44)

~ C/ \ + N C v\ Total cost for process 2 will be:

(6.45)

Break-even •.

Fig. 6.34

™

C/4 C . 4 = C/4 +

N '* CC..4 .4

Break-even point analysis fo r com paring tw o m anufacturing processes: process (1) and process (2).

At break-even point, ^ ti ~ Cfi or

*71 + ^bcp Cv, = C/2 +

p c v2

C f l - C f\

or

(6.46)

CvI - C ,v2 The above equation thus helps in determ ining the break-even point quantity ( N ^ ) . Using the break-even analysis, one can decide as to w hich process (1 o r 2) should be used for production o f a given quantity in the m ost econom ical way. As show n 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 com paring econom y aspects o f two or more processes, the break-even point analysis also helps in deciding the size o f batch or quantity o f product to be produced from profitability point o f view as dem onstrated in the following: Let It = total incom e from the sale o f N units with selling price Ps per unit = N ■P s. Total cost o f product (C,) = C y + N C v

(6.47)

The plots o f /, and C, are show n in Fig. 6.35. Break-even point is at the crossing o f two lines o f /, and C r The W. is the quantity produced by the process under consideration when there is no profit at all. Hence to gain profit, the quantity o f product to be produced should be m ore than N,bcp-

Quantily

N

►

Fig. 6 .3 5 Break-even chart fo r a single product fo r assessing the m inim um quantity o f product to be m anufactured.

At break-even point,

, v

or

yV ^

=

c L__ r

cv (6.48)

P. - C„

T he value o f 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 IN IN G

As already mentioned the m anufacturing cost o f a product should be attem pted to keep low to make the product acceptable in the m arket and also to earn m ore profit. In metal machining, attempts have been m ade in different w ays such as optim izing tool life to m inim ize production cost or m axim izing production rate to low er dow n the production cost. But if cutting speed is reduced to increase tool life, metal removal 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 determ ined corresponding to w hich an econom ical tool life may be ensured to result in econom ical production. W.W. Gilbert evaluated tool life for (a) m inim um cost per piece and (b) maximum production rate, as discussed in the following.

6.10.1

M in im u m C o s t p e r P ie c e

Total production cost o f a product per piece com prises: (a) M achining cost per piece (b) Tool changing cost per piece

Tooling cost per piece (c) Tool grinding cost per piece (d) Idle cost or non-production cost per piece is on account o f tim e lost in replacing and regrinding o f w orn-out tools, loading and unloading the workpiece and for other items during which the m achine remains idle. Let

K . - Direct labour cost + overhead charges (Rs.) K 2 = Cost o f tool per grind (Rs.) L = Length o f machining, mm D = Dia. o f w orkpiece m achined, mm V - Cutting speed, m/min / = Feed rate, mm/rev Ti = Idle time per piece, minutes Tc = Tool changing tim e, minutes (a) Machining cost per piece = (direct labour cost + overheads) x machining time per piece ... . . . length o f machining And m achining tim e/piece = — -------------------- feed rate x rpm

f x N tcDN

In turning operations: V = ------6 1000 Putting the value o f N, Machining tim e/piece =

or

1000F N =----------71D

L f

V X 1000 ttD

ttDL

f

V • 1000

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

M achining cost/piece = K . x — — 1 / V 1000

(6.49)

(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 T aylor’s tool life equation, /n

v r = C

or

7 =

y l/n

Total num ber o f tool failures ( T ) per piece is given by Y _ machining time per piece tool life (T ) or w e can write. nD L

i

T _ IQ O O -/-*' _ x D L { V )" C Un 1000 / C 1'" yl/n

(6.50)

Hence, ‘- i Tool changing cost/piece = K. • * * F 1 1000 / • C

•T

(6.51)

(c) Tool regrinding cost per piece = cost o f tool per grind (K 2) x total num ber o f tool failures per piece (Tx) Hence, i-i ^ 71 D L - ( V ) n Tool re g n n d in g cost per piece = A, • -------------- r—

2

(6.52)

1 0 0 0 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 o f above four costs (a to d) k or

A

DL

JtDLAVy

— A . • -------------------------+ A | -------------------------- T7—

1 1000 f

V

1000 / C 1

Tr

K DL(V)n

+ a ^ ------------------------ — — + A | •T»

(6.54)

1 0 0 0 / C 1'"

The total production cost per piece and its com ponent 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 4A ’ on the total cost per piece curve shows the minimum cost o f production. T he 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 optim um cutting speed ( Vo) will be the most econom ical tool life. The production cost per piece (Km) corresponding to the point 4A ’ is the minimum cost per piece.

Fig. 6 .3 6

6 .1 0 .2

Effect of variations in cu ttin g speed on va rio us costs.

M a x im u m P ro d u c tio n R a te

Relation between production rate (or num b er o f p ie c e s p ro d u c e d p e r unit tim e) and cutting speed is show n in Fig. 6.37. It will be seen that at low er 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 P mx, w here production rate is m axim um . T he corresponding cutting speed ( Vmxp) is the optim um cutting sp e e d at which the rate o f production is highest. A ny increase in cutting speed beyond V will lead to m ore w ear o f tool, frequent changing o f tool, m ore dow n time and hence reduced rate o f production. T he point ‘P ' on the total time curve gives m inim um tim e taken for production o f each piece. T he cutting speed V is the optim um value o f cutting speed at which the total time taken in production o f each com ponent will be m inim um . Total time per piece

Cutting s p e e d

Fig. 6 .3 7

►

V arious tim e curves and m axim um production curve.

The criteria o f minimum cost per piece or maximum production rate when taken individually do not serve the purpose effectively as the ideal criteria should be: producing the com ponents at m axim um rate and at m inim um cost. N ow let us plot both the m inim um cost curve and the m axim um production rate curve in one diagram as shown in Fig. 6.38. The cutting speed Vo at w hich production cost is m inim um is not the same cutting speed at which production rate is m axim um which, in fact, is at cutting speed (F mxp). The area between the tw o values o f cutting speed (V o and Fmxp) is know n 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 econom ical production, the cutting speed should be selected from within this range only.

Fig. 6 .3 8

6 .1 0 .3

High efficiency range of cutting speed.

O p tim u m C u ttin g S p e e d a n d O p tim u m T o o l L ife fo r M in im u m C o s t o f P ro d u c tio n a n d M a x im u m P ro d u c tio n R ate

T h e following relationships can be used for calculating the above quantities. For m inim um cost: In order to find optim um cutting speed and optim um tool life for ‘m inim um cost’, differentiate total production cost (/Q with respect to cutting speed (V) and set it to zero, i.e. d K ld V = 0. By doing so, w e get: O ptim um cutting speed (Vn) C

i- i

(6.55)

K ] x Tc + K 2

n

O ptim um tool life ( 7 ^ ) =

(6.56)

For m axim um production rate: Total production tim e (Tp) = machining tim e + tool changing tim e + non-productive tim e For Finding optim um cutting speed and optim um tool life for m axim um production ra te, differentiate total production time ( Tp ) with respect to V and equate to zero, i.e. ^

dV

= 0

By doing so, w e get, O ptim um cutting speed (F mxp) =

(6.57)

O ptim um tool life (7*

(6.58)

and ) = r- - l | r c

where = Operating cost (direct labour + overheads) (Rs.) K 2 = Tool cost per grind (Rs.) T = Tool changing time (minutes) VQ = Cutting speed for m inim um cost (mpm) Tmc = Tool life for m inim um cost (minutes) Vmxp = Cutting speed for m axim um production (minutes) Tmxp = Tool life for m axim um production (minutes) C = M achining constant n = An exponent depending on cutting conditions

6.11

M A C H IN A B IL IT Y

M a c h in a b ility o f a material refers to the ease with which it can be w orked with a m achine tool. Ease o f metal rem oval (or good machinability) implies: (i) that higher cutting speed and lower power consumption in metal cutting can be expected. (ii) that the forces acting against the cutting tool will be relatively low. (iii) that the chips will be broken easily. (iv) that a good finish will result. (v) that the tool life will increase reducing its frequent re-sharpening or replacement. Ease o f m achining is affected by metal properties such as hardness, tensile strength, chemical com position, m icrostructure, degree o f cold w ork and strain hardening. M achine variables such as cutting speed, feed, depth o f cut, tool material and its form, cutting fluid, etc. also affect machinability.

In view o f the fact that machinability depends on various variable factors, it is not possible to evaluate the sam e directly in term s o f som e numerical value. The follow ing criteria or factors m ay be considered w hile evaluating m achinability. 1. Tool life between grinds:

The longer the tool life, the better the machinability.

2. Quality o f surface finish: T he better the surface finish obtained, the higher the m achinability o f metal, i.e. machinability (or som etim es called fm ishability ) signifies how well a metal takes a good finish. 3. Power consumption: L ow er pow er consum ption per unit volum e o f metal removed show s better machinability.

Form and size o f chips. 5. Cutting forces: Reduced cutting force is indicative o f better machinability o f metal. 6. Shear angle : T he larger the shear angle, the better the machinability.

4.

7. Rate o f metal removal under standard cutting conditions. The m ain factor chosen from above for ju d g in g machinability depends on the type o f operation and requirem ents o f production. The factors often used to predict or evaluate m achinability include the tensile strength, hardness and shear angle.

6.11.1

Im p ro v in g M a c h in a b ility

T he factors responsible for increasing or im proving machinability are as follows: •

C hem ical com position: T he presence o f small am ount o f lead, m anganese, sulphur and phosphorus results in the im provem ent o f machinability. Sulphur in the presence o f m anganese forms m anganese sulphide which in the form o f brittle flakes is spread throughout the metal structure. Phosphorus also helps promoting brittleness in metal and in turn gives ease in machining. T he presence o f these m aterials in the metal enables the chips to break quickly due to brittleness, thereby providing ease in m achining and im proved surface finish. G rey cast iron is much m ore m achinable than white cast iron because the form er has carbon in free form as graphite flakes w hich assist chips to break up easily. Also, graphite lubricates the tool during cutting. T he presence o f carbon content below 0.3% and above 0.6% and the high alloy contents in steel tends to decrease machinability.

•

M ic r o s tr u c tu r e : M etals with uniform m icrostructure having small undistorted grains, lam ellar structure in low and m edium carbon steels and spheroidal structure in high carbon steels ensure higher m achinability rating. N on-uniform structure w ith large and d isto rte d g rain s and p resen ce o f ab rasiv e in c lu sio n s d e c re ase m achinability.

•

T reatm ent given to metal: M edium and high carbon steels are hard and their machinability may be im proved by their hot-working. Machinability o f low carbon steels can be im proved by their cold-w orking. Certain heat treatm ent processes, namely, annealing, normalizing and tempering, also help increasing the machinability.

•

Less hardness, less ductility and less tensile strength.

6 .1 1 .2

M a c h in a b ility In d e x

A s m entioned above that the m achinability o f a m aterial can n o t be quantified directly in the form o f som e absolute value, rather it is given in som e relative form . M achinabilities o f different m etals are com pared (or given) in term s o f th eir m ach inab ility index w hich is defined as below : M achinability index % _ C utting speed o f metal investigated for 20 m in tool life ^ ^ C utting speed o f a standard steel for 20 min tool life A standard steel is free cu ttin g steel w hich is m achined relatively easily and w hose m achinability index is arbitrarily fixed at 100%. T he SA E 1112 steel has been considered to have its m achinability index 100%. T his steel has carbon contents 0.13 (m ax.), m anganese 0 .0 6 .to 1.10 and sulphur 0.08 to 0.03% . R epresentative m achinability index for som e m etals are given below: Low carbon steel 55 to 65% C opper 70% S tainless steel 25% Brass 180% A lum inium alloys 300 to 1500% M agnesium alloys 500 to 2000% It m ay be m entioned in general that (a) m agnesium alloys, alum inium alloys and zinc alloys have excellent machinability ; (b) red brass, gun m etal, grey cast iron, free cutting steels and m alleable cast iron have good machinability ; (c) low carbon steels and low alloy steels have poor machinability ; and (d) stainless steel, sintered carbide, high speed steel and monel m etal have fa ir machinability.

6 .1 1 .3

M e a s u re m e n t o f C u ttin g F o rc e s

1. Need for the m easu rem ent o f cu ttin g forces: im portant role in the follow ing areas:

D eterm ination o f cutting forces plays an

(i) F or analyzing the relationship am ong different forces acting during m etal cutting, determ ining tem perature at the tool-chip interface, assessing tool w ear and that way clearly understanding the process o f m etal cutting. (ii) (iii) (iv) (v)

Investigating m achinability problem s and tool life, wear, pow er requirem ent, etc. F o r helping in the designing o f proper tool to m eet processes requirem ent. H elping in designing the jig s and fixtures o f adequate strength. A nalyzing static and dynam ic behaviour o f m achine tool and designing the proper m achine tool accordingly.

2. C utting forces: E xcept som e typical cases o f m etal cutting such as parting off, facing, etc. involving action o f only tw o com ponent forces (i.e. orthogonal cutting), bulk o f the m achining load involves oblique cutting w herein three com ponent forces act as show n in Fig. 6.25 for the turning operation. T hese forces are:

F a = f e e d fo r c e o r thrust force acting in horizontal plane parallel to axis o f work F t = cu ttin g fo r c e acting in vertical plane and is tangential to w ork surface (also called ta ngential fo rc e ) Fr = ra d ia l fo r c e acting in horizontal plane along the axis o f the tool T he forces w hich are norm ally required to be m easured are cutting force ( F X feed force ( F J and shear force (Fs) as these are generally used in calculating other forces w ith the help o f various equations already discussed. T he device used for m easuring cutting forces is called tool d yn am om eter o r force dynam om eter. T he force in m etal cutting is determ ined by m easuring deflections o r strains in the elem ents supporting the cutting tool. T he design o f the tool dynam om eter should be such as to give strains o r displacem ents large enough to be m easured accurately. 3. Types o f tool dynam om eter: tool can be broadly classified as:

Tool dynam om eters used for m easuring cutting forces o f

(a) M echanical dynam om eter (b) Strain gauge type dynam om eter (c) Pneum atic o r hydraulic dynam om eter (d) E lectrical dynam om eter (e) Piezoelectric dynam om eter O nly the m echanical type and strain gauge type dynam om eters are discussed in the follow ing as these are m ore com m on in use. Mechanical dynamometers They often m ake use o f sensitive dial indicators for direct m easurem ent o f tool forces as the dial indicators are calibrated to show directly the m agnitude o f tool forces corresponding to the deflections caused in the tool holder by these forces; for exam ple, the cutting force (F,) (Fig. 6.39) tends to deflect the tool and tool holder dow nw ards, axial force (Fa) acts along the axis o f w orkpiece (opposite to feed) and radial force (Fr) w ill push the tool aw ay from w orkpiece and m ay cause chatter. A typical tw o-dim ensional dial indicator type m echanical dynam om eter is show n in Fig. 6.39 to m easure force F t and Fa.

or cutting force (F,)

Fig. 6 .3 9

A tw o-dim ensional dial ind icator type m echainical force dynam om eter.

Strain gauge type dynamometers T h ese are co n sid ered su p erio r to m echanical d y n am o m eters and are m ost w idely used. A typical can tilev er ty p e strain g au g e tu rn in g dynam om eter is show n in Fig. 6.40. T h e device w orks in con ju n ctio n w ith a w h eatsto n e flow b rid g e circuit. T h e fo rces b ein g m easu red are F t and Fa. It is w ell know n th at th e electric resistan ce o f a w ire ch an g es if it is stretched. D ifferent p airs o f strain g au g es are cem ented on th e fo u r flat su rfaces (section at A A '). D uring the ex p erim en tatio n , d ifferen t p airs o f strain g au g es a re su bjected to tension or co m pressio n d ep en d in g on th e fo rce ap p lied an d th e su rface on w h ich they are m ounted. It w ill be seen that u n d er the effect o f cu ttin g fo rce (F,), strain gauges T, and T2 are subjected to tension but the bottom strain gauges C, an d C 2 w ill be su b jected to com pression. L ikew ise, fo rce F a puts strain gauges T3 and TA u n d er ten sio n and C 3 and C4 to com pression. A rrangem en t o f o n e set o f strain gauges (7*,, Tv C ,, C 2) in w h eatsto n e brid g e circ u it is show n in Fig. 6.41. T he o th er set com p risin g g au g es Ty 74, C 3, C4 are arranged in another bride circu it. Strain gauges su b jected to tension show an in crease in th eir resistan ce (due to increase in length) and th o se su b jected to com pression show d ecrease in th e ir resistance (due to decrease in length). C hanges in the resistan ce are m easured by w heatstone bridge. T he strain s and stresses co rresp o n d in g to the ch an g es in length (and so the resistan ce) in the strain gauges can be found u sin g standard form ulae and so the co rresp o n d in g forces resp o n sib le for cau sin g th ese changes.

Section at A - A ' (enlarged view)

F ig. 6 .4 0

Fig. 6.41

Using strain gauges fo r m easurem ent o f to o l forces.

The wheatstone bridge and the circu it show ing arrangem ent of strain gauges.

6 .1 1 .4

N u m e ric a ls o n M e c h a n ic s o f M e ta l C u ttin g

Som e typical solved exam ples are given in the follow ing to m ake the reader confident and thorough w ith various m etal cutting principles explained earlier in this chapter. E x a m p le 6.1: T h e follow ing relates to orthogonal turning o f a m ild steel rod o f 50 mm diam eter. Feed 0.8 m m ; chip thickness 1.2 m m ; w ork rotational speed 70 rpm . C alculate chip thickness ratio (r), ch ip reduction ratio (K ) and total length o f chip rem oved per m inute. 0.8 ____ - = — = 0.66 given thickness o f cut chip 1.2 feed rate m m /rev

Solution:

K = - =— = 1.5 r 0.66

then, Now,

(A ns.)

(A ns.)

/ = JlDN, and r =

i

chip length = --------------------/ length o f chip before cutting o r uncut chip length

lc = r I = 0.66 x 71- D ■N = 0.66 x 3.14 x 50 x 70 = 7523.4 m m (A ns.)

or

E x a m p le 6.2: D uring orthogonal m achining, the tool had rake angle 10°, chip thickness m easured as 0.45 m m, uncut metal thickness 0.16 m m. Find (a) shear angle and (b) shear strain. Solution:

G iven a = 10°; / = 0.16 m m ; tc = 0.45 mm .35

r =l =M U /, 0.45

r cos a

Shear angle (0 ) = tan -l

—lan 1

1 - r sin a

—

H ence shear angle (0) = 25/54°

0.45 cos 10 1 - 0 .4 5 sin 10 J

(A ns.)

S hear strain (£) = co t 0 + tan (0 - a ) = cot 25.54° + tan(25.54 - 10) = 2.34 (A ns.) E x a m p le 6.3: A bar o f 85 m m diam eter is turned dow n to 80 m m diam eter. If m ean length o f cut chip is 83 m m, rake angle 12° and cutting is orthogonal, find cutting ratio and shear angle. Solution:

C utting ratio (r) = y

w hen /. = 83 mm

/ = /r-

f 85 + 80 2

or

r =

83

= 0.32

= 259 mm

(A ns.)

259 S hear angle (0) = ta n -1 = 18.52°

r cos a 1 - r sin a \ (A ns.)

tan -l

0.32 cosi 12 ° 1 - 0.32 sin in 12° J

E xa m p le 6.4: From the follow ing data relating to orthogonal cutting, calculate (a) com pression and shear force and (b) coefficient o f friction betw een chip and tool face: feed force = 850 N ; cutting force = 1600 N, chip thickness ratio = 0 .26; tool rake angle = 10° Solution:

G iven, / y = 850 N; F t = 1600 N ; r = 0 .2 6 and a = 10°

Let us first calculate
S hear angle (
1 - r sin a

26 cos 10 ■i[ 0 .2t [ l - O . 26 sin 10

= tan

<(>= 14.89° C om pression force (Fc) - Ff cos

or

Fc = 1320 N

(A ns.)

Shear force (F ?) = F t cos (p - Ff sin

or

(A ns.)

^ /x F/ + F / t a n a 850 + 1 6 0 0 tan 10° C oefficient o f fn ctio n (zi) = —---------------- = -------------------------Ft - F j tan a 1 6 0 0 - 8 5 0 tan 10°

p = 0.780 E x a m p le 6.5:

(A ns.)

T he follow ing observations w ere m ade during an orthogonal cutting operation: t - 49 m m ; b - 4 m m ; / = 160 mm lc - 40 m m ; bc = 4.5 m m ; fi = 0.75 r = 249 N /nm 2; V = 30 m /m in; a - 20°

Find out (a) cutting force, (b) feed force and (c) pow er consum ed. Solution:

l b t = lc - b c - t c

or

I

,

, ...

b

T rcosa 1

160 x 4 .0

Now, shear angle (
Ll-rsinarJ

640

.28 cos 20° T 0.2*

-------

[ 1 - 0 .28 . sin 20°

or shear angle (
co s(fi - a ) cos (

cos { f i - a ) F = - ^ * sin 0 cos (
T herefore,

2 4 0 x 4 x 0.161 sin 16 .119° or

F t = 632.93 N

f

cos(36.87°-20°)

1

J * |_cos( 16.119° +36.87° -2 0 °) J

(A ns.)

Feed force ( F ^ can be calculated as follows: fjr = * s in t f r - * )

F, or

- a )

R c o s (fl-a )

F j = F t - tan(/3 - a ) = 632.93 x tan(36.87° - 20°)

Hence,

F/ =

179.93 N

(A ns.)

P ow er consum ption (P ) r H ence, pow er (P) = 0.316 kW

F ,x V _ 63293x30 1000 x 60 1000 x 60 (A ns.)

E x a m p le 6.6: A lathe has m axim um spindle pow er o f 5 kW and pow er required for turning a steel rod is found 0.12 kW /cm 3/m in. W ith cutting speed 35 m /m in and feed rate 0.3 mm/rev, calculate (a) m axim um m etal rem oval rate, (b) depth o f cut, (c) cutting force and (d) normal pressure on the chip. Solution:

M ax. m etal rem oval rate (M R R )max M ax. pow er available at spindle Pow er required/cm V m in = 41.66 cm 3/m in

(A ns.)

0.12 D epth o f cu t (/) (M R R )max = V ■t f /= or

here V = 35 x 100 cm /m in

0.3/10 cm /rev

41.66 = 35 x 100 x / x 0.3/10

H ence,

t -

4166 4 1 -66 = m o cm = 3.9 mm = --------0.39 3 5 x 1 0 0 x 0 .0 3 105

C utting force ( F f) F ■V ' kW 1000 x 60

/ a ns.)^ (A

Ft X 35 5 = — --------1000x60

or

or

Ff =

5 x 1 0 0 0 x 60 A —----- = 8571.4 N

x (A ns.)

N orm al p ressure on chip ( P)

P = ----------5 ----------- = 8571 4 chip area (/ x / )

3.9 x 0.3

= 7325 N /m m !

(A ns.)

Exam ple 6.7:

In an orthogonal turning operation o f a m ild steel rod o f 55 m m diameter, cutting speed w as 25 m /m in, rake angle o f tool 30°, feed rate 0 .1 2 mm/rev, tangential force 2900 N , feed force 1200 N, length o f continuous chip in one revolution 95 m m , determ ine coefficient o f friction, shear plane angle, velocity o f chip along tool face and chip thickness. Solution:

Given: D - 55 mm; V - 25 m/min; a - 3 0 ° ;/= 0.12 mm/rev; F t = 1200 N; lc = 95 mm ^ v Ff +F, tan a 1200 + 2900 tan 30° C oefficient o f friction (Li) = —— = — — — ——----- —— Ft - Ff tan a 2900 - 1 2 0 0 tan 30° = 1.30

(A ns.)

Shear plane angle (0) = tan-1 [

r cos a 1 - r sin a

t I 95 But r = — = - f = ---------- , w here I - JtD - n - 55 m m p er rev = 0.55 tc I tux 55 A nd 0 = tan-1

0.55 cos 30

- = 33. 28° 1 - 0.55 sin 30U

(A ns.)

V elocity o f chip along tool face (Vc) We know,

r = — V Vc = r • V = 0.55 x 25 = 13.75 m /m in

(A ns.)

C hip thickn ess (/c) t feed r - — = ------ , since depth o f cu t (l) is feed in m achining operation feed

0.12 = o^5 =

mm

x (

or

tc =

E xam ple 6.8:

T he follow ing observations w ere m ade in an orthogonal cuttin g operation.

D epth o f cu t = 0.3 m m ; ch ip thickness = 0.5 mm; a = 20°; cutting velocity = 100 m /m in; cutting force = 250 N; feed force = 110 N , find shear angle, shear strain, velocity o f chip along the tool face and w ork done in shear.

Solution: G iven: depth o f cut (/) = 0.3 m m ; tc = 0.5 m m ; a = 20°; V F t = 250 N ; Ff = 110 N

100 m /m in;

S h ear an gle (0) r cos a

, w here r - — = — = 0.6 tc 0.5

1 - r sin a = tan-1

0.6 x cos 20

= 35.3°

1 - 0.6 sin 20

(A ns.)

Shear strain (£) e = co t 0 + ta n (0 - a ) = cot 35.3° + tan(35.3 - 20) = 1.68

(A ns.)

W ork d one in shear (W s) w . = F , x V, Fs = F t cos

A nd

= 250 cos 35.3° - 110 sin 35.3° = 140.74 N vs =

V cosa

100 x cos 20

cos (< p -a )

c o s (3 5 .3 - 2 0 )

(A ns.)

= 97.47 m /m in

W ork done in shear (fVs) = F s x Vs - 140.74 x 97.47 = 13,717.92 N M /m in

(A ns.)

V elocity o f chip along th e tool face (V c) y _ y s>n

/A x (A ns.)

E x a m p le 6.9: G iven: chip length = 95 m m ; uncut chip length = 245 m m ; rake angle = 20°; depth o f cut = 0.5 mm H orizontal and vertical com ponents o f cutting force are 2300 N and 250 N , respectively. C onsidering orthogonal cutting, find out (a) shear plane angle, (b) chip thickness, (c) friction angle and (d) resultant cutting force. Solution: G iven: lc = 95 m m ; / = 245 m m ; a = 20°; t = 0.5 m m ; F H (= Ft) = 2300 N and F v (= F ) = 250 N Shear plane angle (^) r _

chip length (/c )

= 95

uncut chip length (/)

r cos a 1 - r sin a

_ Q 3g

245

= tan -i

0.38 cos 20 1 - 0 .3 8 sin 20

= 22.29°

(A ns.)

C hip thickness (/f) depth o f cut (/) chip thickniess (/c ) / = 05 tc = - = r 038

or

- 1*31 m m

(A ns.)

Friction angle (fi) F f + F, tan a u = tan u = —--------------F, - F f tan a 250 + 2300 tan 20

1084.9

2300 - 250 tan 20

2209.1

P = tan' 1 (0.491) = 26.15°

or

= 0.491

(A ns.)

R esu ltant cu ttin g force (/?)

R = yjFt2 + Fa2 = V (2300)2 + (2 5 0 )2 = 2313.54 N

(A ns.)

E x a m p le 6.10: A carbide tipped turning tool w ith designation 0-10-6-6-8-90-1 m m (ORS) is used to turn a m ild steel piece o f 80 m m dia. at a cutting speed o f 280 m /m in and feed 0.25 mm/rev. If the cutting force (F,) is 180 kg and feed force (F^) 100 kg and chip thickness 0.32 m m , Find shear angle, shear force, norm al force acting on shear plane, friction force, coefficient o f friction, friction angle and velocity o f chip flow. o . Solution:

rr 1 feed/rev (i.e. 0.25) A no Here r = — = ---------------------------= 0.78 tc 0.32

From the given tool signature, a = 10° tan 0 = or

r COS ^ = 0.888 1 - r sin a

(p = tan-1 0.888 = 42° S hear force (Fs) = F{ cos - F f sin 0 = 180 cos 42° - 100 sin 42° = 67 kg

(A ns.)

N orm al force acting on shear plane (F f): F c = Fy cos

(A ns.)

Friction force (F ) = Ff cos a + F r sin a = 100 cos 10° + 180 sin 10° = 129.73 kg

(A ns.)

_ . . F f cos a + F. sin a C oefficient o f 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

(A ns.)

A ngle o f friction (/?) = tan-1 p = tan-1 0.67 = 34°

(A ns.)

C hip flow velocity ( Vc): Vc = Vr or

when V = cutting velocity = 280 m /m in

Vc = 280 x 0.78 = 218.4 m /m in

(A ns.)

E x a m p le 6.11: A tool gave a tool life o f one hour betw een regrinding w hile rough cutting at 20 m /m in. W hat w ill be its probable life w hen engaged in perform ing finishing operation, given n = 1/8 for roughing and 1/10 for finishing? Solution:

VT = C

For roughing, 2 0(7r) ,/8 = C = 20(7^.),/10 for finishing w here Tr = tool life for roughing and 7^ = tool life for finishing = (7) ) 1' 10

H ence.

(Tr) m

or

(6 0 ),/8 = (7} ) ,/10

H ence, tool life for finishing (Tj) = 1 6 7 m in (A ns.) E x a m p le 6.12: W hile m achining at a cutting speed o f 30 m /m in the useful life o f an HSS tool w as found one hour. Find out the tool life if the sam e tool operates at a cutting speed o f 40 m /m in. Take n = 0.12 in T aylor’s equation. Solution: A ccording to T aylor’s equation, V T ' = C In the first case, 30 x (60)° 12 = C or C = 49 In the second case, 4 0 ( 7 )° 12 = 49 1/ 0.12

r = II — 49^ T 4o;

or

or

5.5 min

(A ns.)

E x a m p le 6.13: A tool had a life o f 10 m in w hen cutting at 200 m /m in. Find the cutting speed for the sam e tool to have a tool life o f 150 m in. Take n = 0.22 in T aylor’s equation. Solution:

V J f = C = V fl\n T V»

or

V2 =

11

/

\0.22

• V, = 1 0 1 ll5 0 ,

/ , \0.22

1 V •150 = | — J

■150 = 82.67 m /m in

(A ns.)

E x a m p le 6.14: W hile cutting at 20 m /m in, a tool gave a life o f o n e hour betw een regrinding, w hile perform ing a rough turning operation. W hat w ill be its probable life if engaged on light finishing cut? Take n = 1/8 for rough turning and 1/10 for finishing operation in T aylor’s equation. Solution: Given:

cutting

speed (V) = 20 m /m in

Tool life rough turning (Tr) = 1 h = 60 m in, and

Tf - tool life for finishing turning = 1/8 for rough turning = 1/10 for finishing

VT' = C

T aylor’s equation, T hen,

20(T r)]K = C = 20(T f ) mo

or

(Tr) m

= ( T / no

Tf = (6 0 ),o/8 = 167 m in (A ns.)

T hen,

E x a m p le 6.15: W hat should be the percentage change in cutting speed to give 50% reduction in tool life, taking n = 0 .2 ?

V J xn = V2T2n o r

Solution:

i i = fZ L h)

But T2 = (1 - 0 .5 ) ^ = 0 .5 T, -i0.2 T hen,

= [2 ]02 = 1 .1 4

0.57}

T his shows that V2 should be 14% higher than F, for required percentage reduction in tool life. E x a m p le 6.16: A tool had a life o f 10 m in w ith cutting speed 260 m /m in. Find the cutting speed for the sam e tool to have a life o f 170 m in. Take n - 0.2 in T aylor’s tool life equation. Solution:

Given: F, = 260 m /m in; T, = 10 m in; T2 = 170 m in; n = 0.2 V\T\n = V2T2 as V7” = C

or

'

10 \

.

0.2

x 2 6 0 = 147.53 m /m in

(A ns.)

J70j E x a m p le 6.17: Two different tools are used to m achine a m ild steel rod under identical conditions. Tool life equation fo r the tw o tools are given below: Tool A = V T o n = 40 Tool B = k T 04 = 85 Taking V and T in m /s and seconds, respectively, find the cutting speed above w hich tool B w ill give b etter tool life.

Solution:

L et V* = B reak-even speed at w hich both the tools give the sam e tool life. il.032

Then

Tool A ■

m 1/0.4

and

Tool B

also T ■

40

or

m 1/0.4

1/0.32

v x 3.125

'4 0

85

vx

i2.5

Vx

101493

66611.2

vx yX ^3.125 or

101493

71.5

or

= 1.52

66611

( P ) 0-6 = 1.52

or

P = (1 .5 2 ),/0-6 = (1 .5 2 )166 = 2 m /s (A ns.)

E x a m p le 6.18: T aylor’s tool life relation for HSS tool is given as V T W = C, and that for carbide tool VTUS = C2 taking that at a speed o f 25 m /m in, the tool life w as 160 m in in each case, com pare their cutting life at a speed o f 35 m /m in. Solution:

For HSS tool, 25 x (1 6 0 ),/7 = C,

and for carbide tool, 25 x (1 6 0 ),/5 = C2 A t a cutting speed o f 35 m /m in: For HSS tool = 2 5 (1 6 0 ),/7 = 35(THSS) 1/7 THSS =

or

x 160 = 15.17 min

35

25

or

c a r b id e

r carbide

29.74

Thss

15.17

E x a m p le 6.19:

7

2 5 (1 6 0 )1/s = 35(7;) 1/5

F or carbide tool,

then

25

= 1.96

35

x 160 = 29.74 min

(A ns.)

In a turning operation, the follow ing tool life w as given: V T 0,12 x / 07 x cP3 = C

At a cutting speed o f 25 m /m in, feed = 0.3 m m /rev and jo b dia 3 m m, the tool life was one hour. C alculate the tool life if the cutting speed, feed and depth o f cut are increased by 25% individually and all taken together.

Solution:

V T 012 x / ° 7 x
Putting values o f V, f and d, C = 25 x ( 6 0 )° 12 x (0 .3 )° 7 x (3 )03 = 24.36 Now taking increase o f 25% individually in V, / a n d d, taking V = 1.25 x 25 = 31.25 m /s (T)0]2

or

= ------------------------o J = 1 '3 31.25 x (0.3) ° 7 x (3)

T = (1 .3 )1/l2 = 8.89 m in

(A ns.)

N ow feed i f ) = 1.25 x 0.3 = 0.375 m m /rev (T)0A2 = ------------^ 25 x (0.375)

------- — = 1.39 x (3)

T = (1.39) 1/012 = 15.53 m in

(A ns.)

Now diam eter = 1.25 x 3 = 3.75 (T ) ° 12 = ------------ ^ 25 x (0.3) or

-------- o J = x (3.75)

r = (1.52)l/012 = (1.52)8333 = 32.75 m in

1'5 2

(A ns.)

N ow taking increased value o f V, / and d together, V - 31.25 m / s , / = 0.375 m m /rev and d - 3.75 mm ( D 0-12 = ---------------- ^ ----------- o j = 1.04 31.25 x (0.375)° 7 x (3.75) ° 3 or

T = (1 .0 4 ),/0-12 = (1.04)833 = 1.38 min

(A ns.)

E x a m p le 6.20: T he follow ing data w ere obtained during m achining on lathe: M achining constant (C) = 80 Tool changing tim e (7* ) = 5 min Tool regrinding tim e = 3 min Tool depression cost/grind = Rs. 1.20 p er grind O perating cost = 25 paise p er m inute L abour + overhead p er m inute (AT,) = 20 paise W ork loading and unloading tim e = 35 sec Feed = 0.25 m m /rev E xponent (n ) = 0.25 Job length = 450 m m , m achined all over in 4 passes C utting speed = 30 m /m in W ork dia = 55 mm Idle tim e per piece = 4 min Tool grinding cost = Rs. 1.20 p er grind

C alculate: (i) O ptim um cutting speed for m inim um cost (ii) Tool life corresponding to above optim um cutting speed (iii) C utting speed for m axim um production (iv) Tool life for m axim um production (v) M achining cost per piece (vi) Tool changing cost per piece (vii) Tool life (viii) Idle cost (ix) Tool regrinding cost (x) Total cost o f producing 1500 com ponents Solution: (i) O ptim um cutting speed for m inim um cost:

rI - i Kn

( K J C+K2 ) *1

B ut K 2 = total cost per grind = tool regrinding tim e x operating cost + tool depreciation cost + tool changing tim e x operating cost. or

K 2 = 3 x 25 + 1.20 + 5 x 25 = 320 = 3.20 (Rs.) 80

then. i 0.25

(ii) or

-1

V T 025 = C o r T = T = 6.34 m in

= 50.5 m /m in

3.25

^ 0 .2 x 5 + 3.20

(A ns.)

0.20 C

1/0.25

V

r s o i 1/025 [ 50 .5 J

(A ns.)

(iii) C utting speed for m axim um production (F mxp): 80 -1O.25

I H rT [(e or (iv)

Vmxp = 47.6 m /m in

- "

5

(A ns.)

Tool life (NT m xp7) = r i - r T = ' 1 - 1 5 c 1 0.25

= 1 5 min

(A ns.)

(v) For m achining cost per piece, First let us find the m achining tim e per piece.

x. ,. . . . xD L x x 5 5 x 4 5 0 (4 ) M achining tim e per piece = --------------- = ------------------f - V -1000 0.25 x 3 0 x 1 0 0 0 = 41.44 m in M achining cost per piece = Xj x 41.44 = 0.2 x 41.44 = 8.28 (R s.) (vi) Tool life,

T=

( C \ lln (?>o V /0'25 =1^1

(A ns.)

= 1.28 m in

(A ns.)

(vii) Tool changing cost per piece - K x x tool failures p er piece x Tc „ m achining tim e ^ — K. x —— x T tool life = 20 x

x 5 = 3237.5 = 32.37 (R s.)

(A ns.)

1.28 (viii) Idle cost p er piece = Kj x idle tim e/piece = 20 x 4 = 80 = 0.80 (R s.)

(A ns.)

(ix) Tool regrinding cost per piece = C ost p er grind x N um ber o f tool failures p er piece m achine time , 41.44 = 1.20 x --------------------= 1.20 x -------tool life 1.28 = 38.85 (R s.)

(A ns.)

(x) Total cost o f production p er piece (K) = Idle cost p er piece + M achining cost per piece + Tool changing cost p er piece + Tool regrinding cost per piece = 0 .8 0 + 8.28 + 32.37 + 38.85 = 80.3 M anufacturing cost o f 1500 pieces = 1500 x 80.3 = 120,450 (R s.)

6 .1 2

(A ns.)

M A C H IN E T O O L S

T he term m achining (or m etal cu ttin g) includes a large num ber o f m etal cutting operations such as saw ing, turning, facing,- boring, taper turning, threading, knurling, m illing, slotting, shaping, grinding, etc. A ll these operations are perform ed on various types o f m achine tools as w ill be discussed in the follow ing. T he cutting tools used for rem oving the excess m etal m ay be operated by hand (as in case o f hacksaw cutting o r filing operation) or by m achine (i.e. pow er) (as in case o f lathe o r shaper). M achine o r pow er actuated m etal cutting tools and system s (or m achines) are called m achine tools. T h e m achine tools are provided w ith facility for holding and rotating (or reciprocating o r moving) the workpiece or jo b as also for supporting,

guiding and feeding the cutting tool into the w orkpiece. They also have facilities for transm itting pow er to th eir various sections to perform various operations. M ore com m only used m achine tools in a m achine shop are as follow s. 1. L athe 3. M illing m achine 5. P laner

2. D rilling m achine 4. Shaper 6. G rinding m achine

A m achine shop is the shop in a w orkshop w here all the above m achine tools (along w ith other m etal cutting m achines) are installed for conducting various m achining operations w ith the purpose o f giving the desired shape to the w orkpiece.

6.12.1

F u n c tio n s o f a M a c h in e T o o l

A m achine tool perform s the follow ing functions. (i) Holding, supporting and rotating (or moving) the workpiece as desired during machining. (ii) H olding, rotating and guiding (and feeding) the cutting tool in relation to the workpiece. (iii) Providing pow er drives to the workpiece and the cutting tool as also to other com ponents o f m achine tools to help perform ing various m etal cutting operations. (iv) R egulating cutting speed, feed, etc. for various m achining operations.

6 .1 2 .2

T y p e s o f M a c h in e T o o ls

M achine tools can be broadly classified as (a) standard m achine tools and (b) special purpose m achine tools. Standard m achine tools are those w hich are capable o f perform ing a num ber o f m achining operations to produce a large variety o f jo b s w ith different shapes and sizes. T hese form part o f any m achine shop. E xam ples include lathe, m illing m achine, shaper, planer, drilling m achine, etc. Special purpose m achine tools are those w hich perform only som e specified m achining operations so as to produce a large num ber o f identical item s, such as au to m atic m ach in es u sed fo r m ass p ro d u ctio n . T ra n sfe r m ach in es and n u m erically controlled (N C ) m achines are also autom atic m achines. T ransfer m achines consist o f a group o f m achine tools arranged in a sequence to work as a single unit which is autom ated. Num erically controlled (N C ) m achines have system s for controlling the relative m ovem ents o f cutting tools and w orkpiece, cutting speeds, feeds, d epth o f cuts, sequencing o f proper tools for a particular operation and all the machining param eters autom atically with the help o f a prearranged program m e fed to the control unit. M uch closer dim ensional tolerances on jo b and higher productivity are ensured with these m achines.

6 .1 3

P R O D U C T IO N O F G E O M E T R IC A L S H A P E S O N M A C H IN E T O O L S

T he geom etrical shape o f the m achined surface depends on the shape o f the tool and its path during the m achining operation. M ost m achine tools are capable o f producing or m achining com ponents o f different geom etry. Production o f m achined surfaces can be broadly divided into tw o m ajor categories: (i) production o f round o r tapered o r form ed surfaces that are sym m etrical about their axis (o f rotation) and (ii) production o f flat o r plane surfaces (including slots, key w ays, splines, etc.).

6.13.1

P ro d u c tio n o f R o u n d o r T a p e re d (C o n ic a l) S u rfa c e s U s in g a S in g le -p o in t C u ttin g T o o l

W hen a rough (u n m ach in ed ) cy lin d rical jo b rev o lv es around its cen tral axis and the tool penetrates b en eath its su rface and trav els p arallel to th e axis o f ro tatio n , a surface o f rev o lu tio n is p ro d u ced an d th e o p eratio n is term ed tu rn in g [Fig. 6 .4 2 (a)]. W hen a hollow tube is m ach in ed on its in sid e in the sim ilar w ay (as tu rn in g ), th e o p eratio n is term ed b orin g [Fig. 6.4 2 (b )]. M ak in g an ex tern al co n ical su rface o f u n ifo rm ly varying d iam eter is called tap er tu rn in g [Fig. 6 .4 2 (c)]. W hen th e tool p o in t trav els in a path o f varying radius, a co n to u red su rface is p ro d u ced [Fig. 6.4 2 (d )]. W hen a short length con tou red su rface is turned u sin g a shaped tool norm al to th e jo b , the process is term ed con tou r form in g [Fig. 6.42(e)]. Workpiece

Tool feed (a) Straight turning

(b) Straight boring

Flfl. 6 .4 2

6 .1 3 .2

(c) Taper turning

(d) Contour turning

(e) Contour forming

Showing how surfaces of revolution are produced.

P ro d u c tio n o f F la t o r P la n e S u rfa c e s U s in g a S in g le -p o in t C u ttin g T o o l

Flat or plane surfaces can be produced by facin g o r radial turnin g [Fig. 6.43(a)] w herein the tool m oves norm al to the axis o f rotation o f the w orkpiece. In oth er cases, the w orkpiece is held steady on the m achine tool table, and the tool is allow ed to reciprocate (as in case o f a shaper) in a series o f straight line cuts w ith crossw ise feed increm ent before each cutting stroke [Fig. 6.43(b)]. In case o f a planer, the w orkpiece m ounted on the m achine tool table reciprocates past the tool w hich is given cross feed after each stroke o f reciprocation o f the w orkpiece [Fig. 6.43(c)]. B oth shaper and p lan er are also cap ab le o f cutting slots and splines on a jo b , besides generating flat o r plane surfaces. Workpiece

Workpiece feed per cycle (a) Facing

Fig. 6 .4 3

(b) Shaping

Generation of plane surfaces.

Workpiece (c) Planing

6 .1 3 .3

P ro d u c tio n o f R o u n d a n d F la t S u rfa c e s (o r C o n to u rs ) U s in g M u lti-e d g e d C u ttin g T o o ls

T he exam ples o f m achine tools that em ploy m ulti-edged cutting tools include drilling m achine, m illing m achine, broaching m achine, etc. A drilling m achine em ploys a drill bit, usually a tw ist drill bit w hich is a tw in-edged fluted tool, em ployed for m aking holes. W hen drilling is perform ed on the drilling m achine, it is the drill w hich rotates, but w hen drilling is carried out on a lathe, it is the w orkpiece w hich rotates [Fig. 6.44(a)]. In m illing operations [Fig. 6.44(b)] to [Fig. 6.44(e)], a rotary cutter w ith a num ber o f cutting edges engages the w orkpiece, w hich is m oved slow ly and fed to the cutter. Plane o r contoured surfaces can be generated in m illing, depending on the geom etry o f the cu tter and the type o f feed. H orizontal or vertical axes o f rotation are used for the m illing cutters, and the feed o f the w orkpiece m ay be in any o f the three coordinate directions.

End mill cutter.

»

Milled slot -

Workpiece

(b) Milling a plain slot with end mill cutter

(a) Drilling on lathe

Workpiece (c) Plane or slab milling

Fig. 6 .4 4

Workpiece (d) Groove milling

Workpiece (e) Contour or form milling

V arious m achining operations perform ed using m u ltip o in t tools.

Som e com m only used m achine tools have been described in the follow ing.

6 .1 4

LATHE

Lathe is the m ost basic m achine tool available in any m achine shop. T he w orking principle o f lathe is show n in Fig. 6.45 w herein the jo b is rotated and the tool is fed to the jo b to cut m etal. T he lathe produces surfaces o f revolution by a com bination o f single-point cutting tool m oving parallel to the axis o f jo b rotation (as in case o f turning show n in Fig. 6.42(a) and Fig. 6.45) or norm al to the rotating jo b (as in case o f facing. Fig. 6.43(a)) or som etim es the com bination o f both (as in co ntour turning, Fig. 6.42(d)). T he jo b is held and rotated on a lathe by holding it eith er betw een the tw o centres o f lathe (live centre and dead centre,

Fig. 6.45) or by holding th e jo b in a chuck as for facing operation (Fig. 6.54(b)). It m ay be noted that in a lathe, pow er for rotating the jo b is transm itted to the lathe spindle w hich projects ou t o f the head stock as spindle (Fig. 6.46(a)) w ith threads on it, on w hich either the face plate [Fig. 6.46(b)] o r the chuck (Fig. 6.54) is screw ed on.

Fig. 6 .4 5

6.14.1

W orking principle of the lathe.

C o m p o n e n ts o f a L a th e

A lathe can be sim ply divided into the follow ing subassem blies [Fig. 6.46(a)). (i) H ead stock (iii) C arriage

(ii) Tail stock (iv) Bed

1. H ead stock: T his section com prises m echanism s for providing pow er to the lathe spindle (Fig. 6.46 and Fig. 6.47), w hich is used to rotate the w orkpiece o r jo b at different speeds. The spindle is the m ain com ponent o f the head stock and is long enough to cover the entire length o f head stock. T he spindle is hollow like a pipe and accom m odates the live centre at its front end (spindle nose). T he face plate [used to rotate the jo b through lathe dog, Fig. 6.64(e)] is screw ed on the spindle nose. L onger stock such as bright bars and oth er round sections used as raw m aterial for m achining are easily fed through the hollow section o f the spindle from its rear end to its front end, w here these m aterials are held in position (for m achining) w ith the help o f three ja w (or four jaw ) chuck screw ed on the spindle nose. W hen a chuck is used to rotate the jo b , the live centre is not used. T he ‘live cen tre’ is called so because during w orking, the centre revolves w ith the jo b , w hereas the ‘dead cen tre’ fitted w ith tail stock norm ally rem ains stationary w hile allow ing the jo b to rotate during m achining. T he m ajor com ponents o f head stock include cone pulleys, speed change gears, back gear, bull gear and spindle. Pow er flows from m otor (31) [(Fig. 6.46(b)] through a counter shaft cone pulley (32) to another cone pulley (5) m ounted on the spindle (3). For further explanation, the cone pulley (5) will henceforth be called cone pulley (P) (Fig. 6.47). The gear (A) form s an integral part o f the cone pulley (P) and hence both the cone pulley (P) and the gear (A) run free on the spindle. Bull gear (D) is rigidly fixed with spindle and it can be engaged with cone pulley (P) with the help o f pin (G). H ence, with pin (G) engaged, the bull gear (D), cone pulley (P), gear (A) and the spindle, all rotate as one unit. D ifferent spindle speeds are obtained by shifting the belt on various steps o f cone pulley (P) and m otor pulley (32) [Fig. 6.46(b)]. S pindle is som etim es required to be rotated at m uch low er speed w hile handling large diam eter jo b s and m anaging various other m achining operations. In that case, back gear (B) (Fig. 6.47) is used. T h e pin (G) is taken out to disengage bull gear (D ) from cone pulley (P). Later, w ith the help o f lever (F), the back gear (B) is engaged w ith gear (A) attached w ith

Spindle — t To01 P°st

Tail stock

Head stock

Face plate

Fig. 6 .4 6 C om ponents o f a lathe. (a) Principal subassem blies (b) Im p o rta n t com ponents 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. Interm ediate gear, 11. Lead screw gear, 12. Bed ways, 13. Rack, 14. Lead screw , 15(a) Saddle, 15(b) A pron, 16. Bed, 17. Longitudinal hand feed, 18. Power longitudinal feed, 19. Power cross feed. 20. S plit nut lever, 21. Tool post, 22. Tool. 23. Com pound rest feed. 24. Tool post support. 25. Hand cross feed. 26. Dead centre. 27. Tail stock sleeve. 28. Clamp nut fo r ta il stock, 29. Sleeve lock lever, 30. Tail stock hand wheel, 31. M otor, 32. C ounter shaft cone pulley, 33. Pulley, 34. M o to r pulley, 35. Fins fo r a ir cooling of m oto r, 36. Belt, 37. S hort-centre drive support, 38. Chasing dial.

cone pulley (P). T his results in engaging the g ear ( C ) w ith the bull gear (D). It m ay be noted that gear ( C ) is m uch sm aller in size than bull gear (D ). W ith this arrangem ent, pow er at much reduced rpm w ill flow from cone pulley (P) to gear (A ), then to gear (B ), then to gear ( C ) , and finally to bull g ear (D) w hich rotates the spindle. T his arrangem ent o f varying spindle speed is norm ally available in change-gear type lathes but in g ea red -head lathes, spindle speed is varied by ju st shifting the lever o f speed g ear box to a proper setting as directed in the chart norm ally given w ith 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

Spindle E-Spindle gear attached with spindle

Gear A (connected with cone pulleys)

Bull gear D (fixed with spindle) Cone pulley P (free on spindle)

Fig. 6 .4 7 S how ing the details of pow er tra nsm ission (w ith in the head stock of a lathe) fro m spindle cone pulley (P) to the lathe spindle w herein the cone pulley (P) receives pow er fro m m otor.

2. T a i l s to c k : T he tail stock fits on to the opposite end o f the lathe bed and carries the dead centre, w hich is used to hold and support the jo b w hen rotated betw een the tw o lathe centres, live centre and dead centre. T he dead cen tre does not revolve w ith the jo b (as the live centre) and hence is called dead centre. T he sectional view o f a tail stock is show n in Fig. 6.48. T he tail stock consists o f a sleeve, nut, hand w heel, dead centre and arrangem ent for holding the tail stock rigid w ith lathe bed during m achining, besides th e tail stock set over system for taper turning is also there. T he tail stock serves several functions, fo r exam ple, it holds the jo b through its dead centre during the m achining operations, enables taper turning (by tail stock set over m ethod) and supports th e drill (w ith drill holder) for drilling hole in a jo b held in chuck (Fig. 6.54(e)).

Dead centre

Fig. 6 .4 8 Sectional view o f the tail sto ck of lathe. A. Hand wheel to move sleeve (C) forw ard o r backward; B. Handle fo r tig h te n in g the sleeve (C) and thereby ho lding the dead centre in any fixed po sitio n; C. Sleeve; D. Nut; E and F Bolts to clam p ta il stock w ith lathe bed; G. Bolt when loosened, allow s the ta il sto ck set over (perpendicular to bed) fo r taper tu rn in g ; H. Insert to check rotation of sleeve (i.e. allow ing on ly linear m ovem ent of sleeve back and fo rth ).

R efer Fig. 6.48, w hen the hand w heel (A) is rotated, the nut (D ) slides along its axis taking the sleeve (C ) w ith it. T he dead centre is a tapered piece Fitted in the sleeve. T hus, by rotating the hand w heel (A ), the dead cen tre can be m oved forw ard to hold the jo b tight betw een centres o r m oved backw ard to loosen the jo b held betw een the tw o centres. The insert (H ) does not allow the rotation o f the sleeve (C). T he tail stock can be clam ped w ith the lathe bed using bolts (E ) and (F). B olt (G ) can be loosened fo r tail stock set over across the lathe bed during taper turning. N ote that the height o f both the live centre and the dead centre is exactly the sam e from the top o f the lathe bed, and the axial m ovem ent o f the dead centre is in perfect alignm ent parallel to the bed w ays and in line w ith both the centres o f the lathe. 3. C arriage: D evices for controlling the m otions o f tool are included in this section. C arriage has tw o parts: (i) saddle and (ii) apron (Fig. 6.49). T he portion w hich takes on it the cross slide and com pound rest and slides along th e bed w ays is called saddle. T he other one w hich covers the controls for hand and pow er feed o f tool and also the thread cutting controls is called apron.

Fig. 6 .4 9 C arriage is the nam e given to the asse m bly fo rm e d w ith the co m b in a tio n o f saddle and apron. It m oves along the bed ways. The saddle carries a cro ss slide w ith the to o l p o st on it to hold the to o l d u rin g m achinin g. The cro ss slide m oves pe rp en dicu lar to the bed ways. A pron fo rm s the hanging part of carriage and houses gear system fo r givin g pow er feed fo r the lo n g itu d in a l m ovem ent of carriage and the cross slide.

It is the hanging part in front o f carriage (attached to saddle) and houses a num ber o f gear trains through w hich pow er feeds can be given to the saddle (carriage) and the cross slide. Schem atic details o f apron m echanism s are given in Fig. 6.50. N ote that the splined feed shaft alw ays keeps rotating when lathe is running, and so also the lead screw gear (Q) and the gear (N) w hich is a sliding gear. T he g ear (N ) gets attached w ith gear (P), w orm (L) and w orm w heel (M ) m ounted on the splined shaft (K) and thus all interconnected and rotating alw ays w ith g ear (N ). L ongitudinal hand feed to the tool along the bed can be given by the hand w heel (C) and through gear (I). N ote that gears (G) and (F) are pow er gears and alw ays keep rotating w hen lathe is running as they (gears G and F) get pow er from gear N.

For pow er m ovem ent o f carriage (hence the tool) along the bed, push knob (E ) to engage pow er gear (G) w ith g ear (H ). G ear (H ) is connected w ith gear (I) w hich, w hile m oving along the rack (fitted w ith lathe bed), gives longitudinal pow er m ovem ent to the saddle. Sim ilarly, cross slide hand feed to the tool can be given by rotating the hand w heel (D ) but for pow er feed o f the cross slide, pull knob (E) to engage gears (F) and (R). W hile cutting threads on a jo b held betw een th e centres (or in a chuck), h alf nut A is engaged w ith the lead screw with lever (B), and this m oves the saddle (and hence the tool) in a particular relationship based on the pitch o f threads cut on the lead screw and the pitch o f threads to be cut on jo b and the RPM o f both, the lead screw and the job.

Fig. 6.50 Schem atic details of apron m echanism w ith particular reference to pow er feeds fo r longitudinal and cross m ovem ent of the to o l in relation to lathe bed and also the lead screw and half nut fo r thread cutting.

It w ill be seen that all feeds for the tool are in fact controlled by the lead screw w hich is the m ain source o f pow er for m ovem ents o f saddle o r cross slide. T he lead screw gets pow er from the spindle. A lthough the spindle m oves only in one direction, the lead screw can be m ade to m ove in both the directions, clockw ise and anticlockw ise. T he m echanism o f transm itting pow er from spindle to lead screw and the m ethod o f reversing the direction o f lead screw o f an engine lathe is illustrated in Fig. 6.51, w here spindle gear (E) transm its pow er to stud gear (D ), through the direction reversing gears (A ) and (B) w hich are used to rotate the stud gear (D ) in the sam e o r opposite direction to that o f the spindle gear (E) since the spindle g ear (E ) alw ays has a fixed direction o f rotation. G ear L is the gear m ounted on the lead screw. T he gears betw een the stud gear D and lead screw are all called interm ediate gears and are used to vary the RPM and direction o f lead screw w ith respect to the shaft o f stud gear (D) (and thus the spindle). In a change gear type lathe, the g ear train for interm ediate gears is calculated based on pitch o f the threads to be cu t on jo b and the pitch o f lead screw, w hereas in geared head lathe, a control chart helps in operating the required lever for this purpose. R em em ber that lead screw s have acm e threads w ith included angle o f 29° for easy engagem ent and disengagem ent o f h a lf nut.

F ig. 6.51

Show ing the m echanism fo r direction reversing of lead screw. A and B: direction reversing gears, C. gear on stud gear shaft, 0. stud gear, E. spindle gear, F and K: interm ediate gears, L. lead screw gear.

4. B ed: B ed gives support to all the m ountings o f lathe, such as tail stock, carriage, head stock, etc. B ed is m ade o f nickel alloy cast iron and is carefully seasoned, m achined and scrapped because the accuracy o f w orking on a lathe largely depends on the trueness o f bed. T h e bed carries bed w ays o r guide w ays w hich are o f tw o types, inverted V-type w ith included angle o f V as 90° and flat type (Fig. 6.52). Flat w ays give larger bearing surface with corresponding reduction in w ear but need special care for cleaning the bed w ays from metal chips and other foreign matter. T he V-type guide w ays give b etter guide to the carriage and ensure proper alignm ent. C hips also do not get collected over the V-guide w ays. Lathes usually have both the guide w ays to take their best advantage. R ibs give strength and rigidity to the lathe bed structure.

Fig. 6 .5 2 An exam ple of lathe bed w ith its bed ways, inverted V type and fla t type. Ribs provided at intervals along the bed length provide strength and rig id ity to the structu re of bed.

6 .1 4 .2

D e fin in g th e L a th e S ize

L athe size is defined in one or m ore o f the follow ing w ays (Fig. 6.53).

(i) Sw ing o r m axim um diam eter o f the jo b that can be rotated over the bed w ays (E) o r over the carriage (D). (ii) M axim um length o f jo b that can be held betw een the lathe centres, A and/or the centre height (C). (iii) Bed length including the head stock. (iv) S w ing in gap (B). It is applicable only in case o f specially designed gap bed type lathes. L athes are available in sizes ranging from 7 0 0 to 3000 m m betw een centres.

Fig. 6.53 V arious w ays of defining the lathe size. A. M axim um length of job accom m odated between centres, B. Sw ing in gap (in gap bed type lathes on ly), C. Centre height, D. Swing over carriage, and E. Sw ing over bed.

6 .1 4 .3

T y p e s o f L a th e

L athes can be broadly categorized as follow s. (a) Speed lathe is a pow er driven sim plest lathe often used for w ood turning. Tools are hand-operated. (b) C en tre lathe o r en gin e lathe is the m ost com m only used general purpose lathe found in all m achine shops. Stepped cone pulley arrangem ent w ith m otor is used for varying the speed o f lathe spindle. Tool is fed by power. (c) G ap bed lathe has a section o f lathe bed rem ovable to create a gap or cut in the lathe bed near the head stock to accom m odate jo b s o f extra diam eters (B in Fig. 6.53). (d) G eared head lathe is a type o f cen tre lathe w herein changes in the spindle speeds are accom plished by a set o f gears (housed in a gear box) operated by a lever. (e) Bench lath e is a sm all lathe that can be m ounted on a w orkbench for doing sm all jo b s o r repair jo b s. (f) T u rret and capstan lathes are production lathes w hich carry several tools m ounted on the revolving turret o r capstan to facilitate perform ing a num ber o f m achining operations w ithout w asting tim e in changing the tool (as different tools are needed for different types o f operations). (g) T ool room lathes are precision lathes suitable fo r fine tool room work. (h) A utom atic lathes are high speed, heavy duty and sem i o r fully autom atic lathes. Fully autom atic types are designed to perform the com plete scheduled operations w ithout m uch involvem ent o f the operator.

6 .1 4 .4

O p e ra tio n s P e rfo rm e d o n L a th e

A lthough a very large variety o f m achining operations can be perform ed on a lathe, the m ajor ones are given in the follow ing. 1. T u rn in g (plain turning and step turnin g): It is the operation o f reducing the diam eter o f cylindrical jo b s [Fig. 6.54(a)]. T he jo b m ay be held betw een the lathe centres and driven w ith the help o f face plate (screw ed on lathe spindle) and a dog carrier. Som etim es turning is done by holding the jo b in a chuck (as show n in Fig. 6.54(b) for perform ing the facing operation). T urning m ay be (a) plain turning and (b) step turning. T he plain turnin g results in uniform reduction in the diam eter o f the jo b throughout its turned length [as shown in Fig. 6.55(a)]. A jo b m ay have tw o o r m ore diam eters to be turned on it, each involving shoulders or steps o f different diam eters. Turning o f such steps is called step turning [Fig. 6.55(b)]. Form turnin g involves the use o f a form tool for turning different contours as shown in Fig. 6.55(c). T ap er turnin g [Fig. 6.55(d)] is the operation o f turning conical o r tapered shapes, i.e. cylindrical shapes gradually reducing in d iam eter along their axis. Taper turning w ill be discussed later in m ore detail.

(c ) Boring

(d ) Threading

Tail (e) Drilling Fig. 6 .5 4

Illu stra tin g som e of the im p o rta n t operations carried out on a lathe.

Fig. 6.55(a)

Plain tu rn in g between centres.

Chuck

Longitudinal tool feed

Fig. 6.55(c)

Form tu rn in g w ith a fo rm tool.

Cro ss feed of tool

Fig. 6.55(d) Taper tu rn in g using com bined lon gitudin al and cross feed.

2. F acin g [Fig. 6.54(b)): It is the operation o f m aking the ends o f a jo b flat w hen the jo b is usually held in a chuck, and the tool is fed perpendicular to the axis o f jo b rotation. 3. B oring [Fig. 6.54(c)): It is the operation o f enlarging the hole (or bore) o f a w orkpiece having its initial bore m ade eith er by drilling o r by p utting a co re during casting o r the bore m ade during forging. 4. T h reading [Fig. 6.54(d)): T hreading on lathe is the operation o f m aking or cutting threads (o f different types and pitches) on a job. T hreads m ay be m ale threads (external threads) and fem ale threads (internal threads). T he tool used for cutting threads is a single­ point threading tool. 5. D rillin g [Fig. 6.54(e)): It is the operation o f m aking a hole in the end face o f the jo b held in a chuck. T he tool used is a drill bit held in a drill chuck, w hich itself is m ounted in the tail stock sleeve in m uch th e sam e w ay as the dead centre. 6 . K n urlin g [Fig. 6.56(a)): It is m aking o f roughened surface on a sm ooth surface of cylindrical jo b s using hardened steel knurles in place o f a usual lathe tool. K nurled surface o f the jo b helps in holding the jo b tight by hand. E xam ples include knurled knobs o f m easuring instrum ents such as surface gauge o r m icrom etre screw gauge. 7. G rooving or u nd ercu ttin g [Fig. 6.56(b)): It is the operation o f reducing the diam eter o f a jo b for a very short length. T he reduced surface produced is called groove. 8 . P artin g o ff [Fig. 6.56(c)): It is the operation o f separating (or cutting off) usually the finished (o r m achined) com ponent from the w orkpiece blank. It is a very com m on operation on lathe. L athe tools used for perform ing above operations are show n in Fig. 6.56(d).

Knurling tool bit Grooving tool

Fig. 6.56(a)

Fig. 6.56(b)

K nurling w ith a kn urlin g to o l bit.

Chuck

rK t

p Blank

Fig. 6.56(c)

Unturned surface

Turning tool

1

G rooving (o r under cu ttin g) operation.

Finished job

Parting tool

Parting o ff operation.

Tu rn ed surface

Side turning tool

Facing tool

Tapering tool

Necking tool

Knurling tool

(i) Tools used for generating external surfaces

(ii) Tools used for generating internal surfaces

Fig. 6.56(d) D ifferent types of lathe tools. The to o ls used fo r generating external surfaces are shown at (i) and those used fo r generating internal surfacing are show n at (ii).

O ther lathe operations include ream ing (finishing drilled hole w ith ream er), tapping (cutting internal threads w ith taps) and grin d in g (finishing w ith a grinder).

6 .1 4 .5

T a p e r a n d T a p e r T u rn in g

A cylindrical jo b w hich decreases gradually in d iam eter from its one end to the other so as to assum e a conical shape is said to be tapered. T ap er on jo b s is expressed as the ratio o f the difference in end diam eters o f the tapered jo b to the axial length o f the tapered section. For exam ple, a taper o f 1 m m p er cm m eans th at there is a difference o f 1 mm in the end diam eters o f a tapered jo b having a tapered length (along axis) o f 1 cm . T aper angle is the included angle betw een the tapering sides o f a jo b when extended to m eet at a point (Fig. 6.57). Taper is also given as, say 1 in 20 w hich m eans that the difference in m ajor diam eter (D ) and m inor diam eter (d) o f the tapered length (/) o f 20 m m is 1 mm or (D - d )ll - 1/20 or tan o f h alf o f taper angle = 1/(2 x 20) = 0.025 o r 1 °2 6 \

Fig. 6.57

Defining taper and taper angle. Ta p e r per unit length (along job axis) is equal to difference

in diam eters (0 ) and (d), divided by the length ( /) . Tangent o f half of taper angle (i.e. tan a) is equal to tan a = (D - d)/2/.

T ap er turnin g is a type o f turning operation in w hich the diam eter o f the jo b is gradually reduced as the turning proceeds along the jo b length. C om m on m ethods o f taper turning on lathe arc discussed in the follow ing. 1. C om pound rest m ethod: Com pound rest has a circular base graduated in degrees (Fig. 6.49). Set the com pound rest by sw iveling it from the centre line o f the lathe centres (or edge o f bed w ays) through an angle equal to h alf o f the taper angle ( a ° C ) as show n in Fig. 6.58. By clam ping the lathe carriage in place and after adjusting and clam ping the tool, take several cuts for turning the taper. Feeding o f tool is done w ith the com pound rest feed handle w hile the depth o f cu t is taken w ith the help o f cross slide. T he m ethod is suitable for turning steep and short tapers, both external and internal type. C om pound rest feed handle Centre line / of lathe centres 0 E d ge of bed w ays a . C om pound rest

a = Half of taper angle Swivel

F ig. 6 .5 8

Taper tu rn in g by sw iveling o f com pound rest.

T he se ttin g o f co m p o u n d rest is done by sw ivelling and setting the com pound rest at the h alf o f the taper angle ( a ) . If D is the large diam eter, d is the sm all diam eter at taper end, / is the tapered length (Fig. 6.59), then h a lf o f taper angle (a ) can be calculated as: a = tan

D -d (6.60)

21 2. T ail stock set Fig. 6.59) from its stock depends on follow ing exam ple m aking long tap er

over m ethod: In this m ethod, the tail stock is set over (as show n in centre line equal to h alf o f the taper. C alculating the ‘set over’ o f the tail w hether the taper is to be given on entire length or part length. The w ill help in calculating the tail stock set over. T he m ethod is used for length on full length o f job.

Ball centre

F ig. 6 .5 9

a— H—

Half of taper angle Tail stock set over

Taper tu rn in g svith tail stock set over m ethod.

C ase

I F or giving tap er 1 in 10 on a jo b 80 m m long, find taper on 80 m m length = 80/10 = 8 m m. T hen, tail stock set over = 8/2 = 4 mm. C ase II F or a taper o f 12° on a jo b 80 m m long, here sine o f h a lf o f taper angle Set over = —-------; — or set over = 80 sin 6° = 8.36 mm T ap er length C ase

III

W hen the given m ajor d ia is D , m inor dia is d and tapered length is / and the total length o f the jo b (including tapered portion) is L, all in m m , then. D -d L tail stock set over (m m ) = — - — * y

(6.59)

3. T ap er turnin g w ith a form tool: S hort external tapers can be turned using a form tool as show n in Fig. 6.60. It should be noted that a form tool w hen used fo r turning tapers on longer lengths, generates vibrations and chattering. 4. T ap er turnin g w ith taper turnin g attachm ent: T hese attachm ents are available for turning tapers on lathe. L onger tapers are easily turned w ith these. T h e attachm ent is also useful for cutting threads on tapered sections. T he attachm ent is show n in Fig. 6.60(a). T he nut (C) is loosened to disconnect the m otion o f cross slide (having tool post on it) from the control o f cro ss feed screw, and thus the cross slide is m ade floating by disconnecting it from the saddle so th at it can move along its ways. The link (D) connects cross slide and the block (E) (which can slide in slot (H) through

Fig. 6.60

Taper tu rn in g w ith a fo rm to o l. Angle ( a ) is equal to half of the ta p e r angle. Link F

Indicator Block E (graduations in degrees) ^

_

Lathe bed

-££□—ccn J Link D

Slot (gu id e ) H

Nut C

C ro ss slide C o m po un d rest

Fig. 6.60(a)

Schem atic of a taper tu rn in g attachm ent used on lathe.

an adjustable clam p (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 may be given by simply loosening the clam p nut (A), so that nut (A) becom es free over the slot (B) provided in the link (D). T he link (F) is hinged at one end and has a guide (H) through which block (E) can travel. On the other end o f the link (F), an indicator is provided w hich is graduated in degrees. To turn a taper, hold the jo b properly betw een th e lathe centres and set the link (F) at desired angle to give the required taper on the job. L oose nut ( C ) and tight the clam p (A). T he tool w ill now be restricted to follow the direction parallel to the centre line o f link (F) and w ill be guided by the m ovem ent o f block (E) through the g uide (H ). T h is w ill vary the depth o f the cu t o f the tool w hile m oving along the jo b length and w ill render a taper on the job. T h e feed to the tool is given by w orking the handle o f com pound rest. T h e com pound rest is positioned at 90° to the axis o f job. T he feeding o f the tool is given by com pound rest because the cross slide screw is disconnected. A dvantages o f using a taper turning attachm ent are as follow s: 1. T he attachm ent can be quickly and easily set. 2. W ith the use o f th is attachm ent, tapers are turned w ithout disturbing the norm al set-up o f the lathe.

3. E xternal and internal tapers can be turned. 4. Tapers are turned w ith the longitudinal pow er feed and thus the w ork can be m achined quickly and w ith better finish. 5. Long tapers are easily given. 6. Taper turning attachm ent is also used for cutting threads on a tapered surface. S om etim es taper on a jo b is turned using com bined tool feed both longitudinal and crossed (Fig. 6.55(d)].

6 .1 4 .6

N u m e ric a ls o n T a p e r T u rn in g

E x a m p le 6.21: Find the angle at w hich com pound rest should be sw ivelled for cutting a taper on jo b 150 m m long and having diam eter 80 m m. T he sm allest diam eter on the tapered end should be 60 m m and required length o f tapered portion 100 mm.

a - tan -l

00 0 1 s

I

Solution: G iven: D = 80 m m ; d - 60 m m ; I - 1(X) mm Let a be the angle at w hich com pound rest w ill be sw ivelled. D -d

= tan"'[0.1] = 5.71° (A ns.) 2 x 100 m m E x a m p le 6.22: A m ild steel rod has a length o f 80 mm and a tapered portion o f length 50 m m. Large diam eter o f taper is 90 m m and sm all diam eter 80 mm. Find: 21 J

= tan-1

(i) Taper in m m /m etre and in degrees (ii) A ngle to w hich com pound rest should be set (iii) Tail stock set over Solution:

G iven: L - 80 m m ; / = 50 m m ; D = 90 m m ; d = 80 mm

(i) Taper =

D - d _ 9 0 - 8 0 _ 10

/ 50 ” 50 It m eans that for a length o f 50 m m , taper is 10 m m. Then, 10 taper in m m /m etre = — x 1000 = 200 m m /m etre length 50 A lso.

= tan" 1 (0.1) = 5.71° (ii) C om pound rest should be set at angle a = 5.71° (iii) Tail stock set over L x — 2 / ^ (90 - 80) 80 =

D -d

2 = 8 mm

X 50 (A ns.)

(A ns.)

(A ns.)

6 .1 4 .7

T h re a d C u ttin g o n L a th e

E xternal o r internal threads m ay be cut on lathe eith er w ith the help o f a die or a tap respectively o r by using a thread cutting tool w hich can cut both external and internal threads. For cutting threads using a thread cutting tool, a certain relationship is needed betw een the speed (revolutions) o f the jo b and the speed (revolutions) o f the lead screw to control the linear m ovem ent o f the threading tool parallel to the jo b length w hen h alf nut (A ) (Fig. 6.50 and Fig. 6.61) is engaged w ith the lead screw. M any lathes are provided w ith quick-change gear box in w hich different ratios o f the speed o f spindle (hence jo b ) and lead screw (hence tool) are readily obtained w ith shifting o f the g ear change lever. H ow ever, on sim ple lathes (change gear type), one has to calculate and arrange change gears (interm ediate gear, also refer Fig. 6.51) to be arranged betw een the stud gear (driver gear) and the driven gear (the gear on lead screw ) to cut threads o f different pitches. ■Job

Spindle gear

S e t of reversing gears

Driver or stud gear

Driven or lead screw gear

Tool m ovem ent

Intermediate gears

Tool

f

Half nut lever

Carriage

Lead screw

Fig. 6.61

Half nut (A ) (engaged)

Set-up fo r thread cu ttin g on a change gear type lathe.

T he general set-up for cutting thread is show n in Fig. 6.61, w herein it should be noted that only the stud gear (or driver gear) and lead screw gear (driven gear) along w ith their interm ediate gears are changed for cutting threads o f different pitches. Lead is the axial m ovem ent o f the screw travelled in its one revolution. In case o f single start threads, lead is equal to pitch w hich is the distance from one point on one thread to the corresponding point on the adjacent thread. In m ultiple start threads, lead is equal to Lead = No. o f start x Pitch

(6.60)

For thread cutting, the ratio o f gears betw een the stud gear (driver) to the lead screw gear (driven) is found from the follow ing relation. D river _ Lead o f threads to be cut on jo b Driven

(6.61)

Lead o f threads on lead screw

and w hen threads on lead screw are in inch system . Driver Driven

Lead o f threads o f jo b in mm 127

Lead o f threads on lead screw in inches

(6.62)

W hen B ritish S tandard pitch threads are cu t on a lathe w ith lead screw having m etric pitch, then use the follow ing relation D riv er D riv en

=

127 5

L ead o f th read s (in ch e s) to be cut x --------------------------L ead o f th read s on lead screw (m m )

,, ... (6.65)

A fter arriving at the gearing ratio betw een the stud (or driver gear) and the lead screw gear (or driven gear), the next step is to find out the gears or gear trains to be fitted betw een the stud and the lead screw end to fill up the space betw een the stud and the lead screw. The two types o f gear trains are (a) sim ple gear train and (b) com pound g ear train. Sim ple gear train is show n in Fig. 6.62(a). It consists o f a driver o r stud gear m ounted on stud and a driven g ear m ounted on lead screw and an interm ediate gear o r idle gear w hich has no effect on the speed ratio but an interm ediate gear is used to obtain the desired direction o f rotation o f lead screw. N ote that lead screw has single-start threads only w hereas singleor m ulti-start threads are easily cut on lathe. Driver (Stud ge a r)— 20 teeth

Intermediate gear

Driven (G e a r on lead screw )— 80 teeth

Fig. 6.62(a)

Sim ple gear train.

C om pound gear train is shown in Fig. 6.62(b). It consists o f tw o stud gears (driver gears) and tw o driven gears. T his train is used w hen the desired gear ratio is such that it is not possible to achieve that w ith a sim ple gear train. First stud (20 T )

Fig. 6.62(b)

Com pound gear train.

T he follow ing num erical exam ples should give clear understanding o f these trains. E x a m p le 6.23: Find the charge gears to cut RH single-start threads o f 2 m m pitch on a lathe having lead screw o f 8 m m pitch. Solution:

D riv er u r iv e n

L ead o f th read s on jo b L ea a o r in re a a s on ie aa screw 9

1 v 90

90

8

4 x 20

80

(Fig. 6.62)

A nother alternative solution is given below: D riv er

1 25 = —x D riv en 4 25

25 100

It show s that sim ple gear train w ith a 20-tooth g ear on stud and 80-tooth gear on lead screw or 25-tooth gear on stud and 100-tooth g ear on lead screw can be the tw o solutions out o f several others. E x a m p le 6.24: Find charge gears for cutting threads o f 1 m m pitch on a jo b on a lathe having lead screw o f 8 m m pitch. _ . . Solution :

D river D riven

L ead o f th read s to be cu t 1 = — ------------- ;— —— = — L ead o f lead screw 8 = ! 2 0 _ 20 ’ 8 X 20 160

It is not possible to get a g ear o f 160 teeth and hence com pound gear train w ill be required as given below: D riv e r 1 lx l 20 30 = - x -— - = — x D riv en 8 4 x2 80 60 H ence gears w ith 20 teeth and 30 teeth are the first and the second stud gears, respectively [Fig. 6.62(b)! and gears w ith 80 teeth and 60 teeth are respectively the first and the second driven gears (or lead screw gears). E x a m p le 6.25: D esign a suitable gear train for cutting 8 mm pitch, 3 start threads on lathe having lead screw w ith 6 m m pitch. _ . . Solution :

D riv er L ead o f th read s to be cu t 3x8 — = ------— -——— = — r— = 4 D riven L ead o f lead screw 6

E x a m p le 6.26: S quare threads o f 8 m m pitch, double start are to be cu t on a rod having diam eter 60 m m . T he lathe has a lead screw w ith 6 m m pitch. Find: (a) (b) (c) (d) (e) (f)

gear ratio betw een spindle and lead screw depth o f thread to give 0.1 m m clearance lead o f thread to be cut core diam eter helix angle at core diam eter helix angle o f thread

Solution:

D riv er 16 8 (a) G ear ratio = 7—: = — = — D riven 6 3 _ 8 x 10 _ 80 (stu d gear) 3x10 30 (lead screw )

^ ^ , pitch 8 (b) D epth o f square threads = —- — = — = 4 mm Since a clearance o f 0.1 m m is required, hence depth o f cut w ould be: 4 + 0.1 = 4.1 mm

(A ns.)

(c) Lead o f thread to be cut = Pitch x No. o f start = 8 x 2 = 16 m m

(A ns.)

(d) C ore diam eter = O utside diam eter - 2 x depth o f thread = 60 - 2 x 4.1 = 51.8 m m (e) H elix angle at core d iam eter = t a n '1^

(0

H elix angle o f thread = tan-1

= 5.59°

(A ns.)

lead to be cut m ean circu m feren ce o f w ork

= tan" 1

6 .1 4 .8

lead ^ co re circu m feren ce ) 16 n x 51.8

= tan-1

(A ns.)

16 /r(60 - 4)

= 5.19°

]

(Ans.)

C u ttin g S p e e d , F e e d a n d D e p th o f C u t in T u rn in g

C u ttin g speed in lathe m eans the num ber o f m etres m easured on the circum ference o f a rotating jo b that passes the cutting edge o f the tool in one m inute. T he length o f the chip rem oved per minute is its m easure. ^C utting speed, = * D N , m etres p er m inute 1000

w here

D - Job diam eter, mm N = rpm o f jo b

For cutting different metals w ith a tool made o f a particular material, there are recom m ended specific ‘average cutting sp eed s’ for perform ing various m achining operations. F or exam ple, w ith a high speed tool, turning o f m ild steel is done at a cutting speed o f 2 5 -3 0 m /m in and th at o f cast iron at 1 6 -2 2 m /m in an d o f brass at 6 0 -8 0 m /m in. T he cuttin g speeds w ill be different for different operations also such as the cutting speed for drilling w ill be different than that for turning, threading, ream ing, etc. M aintaining correct cutting speed (i.e. the speed at w hich a particular tool and jo b m aterial com bination is m ost effective) enhances the tool life greatly. Feed is the am ount o f advancem ent o f tool (parallel to the surface being m achined) per revolution o f the job. It is usually given in m illim etres p er revolution o f the job. T he am ount o f feed depends on the finish required, depth o f cut and the rigidity o f the m achine tool. On lathe, a feed o f 0.3 to 1.5 m m p er revolution is often used for roughing operations and 0.1 to 0.3 m m per revolution for finishing operations. Feed ( f ) m ay be calculated as below: Feed ( / ■ ) = — NT w here

L = N = T / =

(6 .66)

length o f cut, mm rpm o f jo b cutting/m achining tim e, min feed, m m /rev

D ep th o f c u t is the advancem ent (or digging) o f tool in the jo b in a direction perpendicular to the surface being m achined. It m ay be expressed as the thickness o f the ch ip o f m etal rem oved by the tool in one cut and is m easured in m m. T he depth o f cut depends on the am ount o f m etal to be rem oved, tool m aterial and the pow er and rigidity o f m achine tool. d \~ d 2 D epth o f cut (/) = — - — w here

d ] = dia o f jo b before m achining d 2 = dia o f m achined surface

For norm al roughing operations, the depth o f cut m ay vary from 2 to 5 m m and for finishing operations, from 0.5 to 0.1 m m. T he depth o f cu t also depends on the m aterial o f jo b as deeper cuts can be taken in soft m etals. Metal rem oval rate (M R R ) M eta l rem oval ra te is the volum e o f m aterial rem oved per unit tim e. Volum e o f m etal rem oved is a function o f speed, feed and d epth o f cut as the higher the values o f these, the higher will be the m etal rem oval rate. L et D j = initial dia o f w orkpiece, mm d = depth o f cut, mm / = feed, mm/rev N = rpm o f jo b V = cutting speed, m /m in

Now, metal rem oved per revolution = volume o f chip having length jtD t and cross-sectional area d •f H ence, volum e o f m etal rem oved per revolution = n D .d - f m m 3 then, M etal rem oval rate (M R R ) = itD d fN , m m 3/m in

(6.67)

A nd in term s o f cutting speed, M RR = 1000 • F -
(6 .68)

• x D jN A s V = — -— , m /m in 1000 M achining time (o r turning time) To calculate m achining tim e, refer Fig. 6.62(c) w herein: L = total distance travelled by tool in feed direction in single cut, mm / = length o f surface to be m achined, m m /, = tool approach, mm 12 = over travel o f tool, mm d = depth o f cut, mm / = feed, m m /rev N = spindle o r jo b rotational speed, rpm np = N um ber o f cuts taken during m achining D i = initial diam eter o f w ork, mm D f = m achined o r final diam eter o f w ork, mm D og —

LDriving plate

'i h -

-Hfeh

Mandrel

J \ "

■

-

_

w

T

D a

l ............................................. -D a ________________________

1

W orkpiece

r

= ■ -9 -

*/

/ f

K

Fig. 6.6 2(c)

Feed

^

Calculating m achining tim e in turning .

D istance (L ) travelled by tool in feed direction in single cut, L = / + / , + l2 In case o f single-point tools, /, and /2 are negligible and hence, M achining tim e (Tm) = —

p er cut o r per pass o f tool

(6.69)

Since cutting speed (V) =

, m /m in 1000

w here

N =----------XD , v

1

0

0

0

v

S ubstituting for N , M achining tim e (7*_) per cut = — /

L

\ = 1000 V 1

k x

D. x L

100017*

* m in

(6.70)

V 7TD,

Total m achining tim e (T) - Tm x n

(6.71)

w here np = N um ber o f cuts/passes (D , - D f ) M achining allow ance = ------ -—— M aterial rem oved per cu t = depth o f cut (d) , v Total m achining allow ance A nd num ber o f cuts ( n j = -------------------------------------' M aterial rem oved per cut

(6.72)

depth o f cut (d)

P ow er required for turning T he pow er required for turning depends on cutting speed (10* d epth o f cut (d), feed rate (/*) and hardness and m achinability o f w orkpiece metal. In fact, the pow er required depends on the cutting force ( Ff) w hich is estim ated as follow s: C utting force (F t) = k • d f w here k is a constant depending on w orkpiece m etal Pow er (P) = F ( x V = k d f - V

(6.73)

Num erical problem s E x a m p le 6.27: speed 100 rpm.

Find cutting speed in turning a jo b having diam eter 100 m m and spindle

Solution: r/ V

ttD N 3 .1 4 x 1 0 0 x 1 0 0 . . . ------- = ------------------------ = 31.4 m /m in 1000 1000

.. . (A ns.)

E x a m p le 6.28: A hollow w orkpiece o f 50 mm diam eter and 200 mm long is to be turned all over in 4 passes. If approach length is 20 m m, over travel 10 m m, feed 0.8 m m /rev and cutting speed 30 m /m in, find the m achining tim e.

Solution :

C utting speed ( V) DN

or

318

, m/min

50 x N 30 =-----------318 .

i.e.

.

30x318 lftAO N = ------------ = 190.8 rpm 50

or

Total distance travelled by tool in a single pass (L ) = 200 + 20 + 10 = 230

mm

T hen, length travelled by tool in 4 passes (Z,,) = 230 x 4 = 920 mm L 920 M anufacturing tim e (T) = — = ---------------- = 6 min 5 JN 0 .8 x 1 9 0 .8

(A ns.)

E x a m p le 6.29: Find th e m achining tim e to face a jo b o f 60 m m diam eter and rotating at 80 rpm w ith a cross feed o f 0.3 mm/rev. o . , S olu tion :

, r • , . L D /2 6 0 /2 Tim e for facing (one pass) = — = -------= -----------JN JN 0.3 x 80 = 1.25 m in

(A ns.)

E x a m p le 6.30: A rod 150 m m long and having d iam eter 15 m m is reduced to 14 m m d iam eter in one pass o f turning. Find the natural rem oval rate and m achining tim e w hen spindle speed is 400 rpm and feed 200 m m /m in. S o lu tio n : G iven: L - 150 m m ; D. = 15 m m ; Df - 14 m m ; N = 400 rpm ; / = 200 m m /m in; feed rate = 200/400 = 0.5 m m /rev W hen d = depth o f cu t =

= 0.5 mm

T hen, M RR = n x 15 x 0.5 x 0.5 x 400 = 4170 m m 3/m in C utting tim e ( T ) = -^—= — — = 0.75 m in 6 m JN 0 .5 x 4 0 0

(A ns.)

(A ns.)

E x a m p le 6.31: C alculate the tim e required to m achine a jo b 170 m m long, 50 m m diam eter to 165 m m length and 40 m m d iam eter w hen jo b rotates at 400 rpm , feed is 0.2 m m /rev and m axim um depth o f cut 2 m m. Take tool approach and overall travel distance as 8 m m for turning operation. Solution: Since both diam eter and length o f the jo b w ill be reduced dow n by turning and facing respectively, let us do first turning and then facing.

T im e for turning Total length (L ) o f tool travel =

170 + 8 = 178 mm

« • ^ u ^, Di ~ Df R equired depth to be cut, d = ---- -— Since m axim um

5 0 -4 0 . = — - — = 5 mm

depth o f cut is 2 m m , then No. o f cuts required w ill be: n = — = 2.5 o r 3 p 2 Total turning tim e = m achining tim e for one cut x N um ber o f cuts 178 L_ N n_ = -------------- x 3 = 6 . 6 m in p 0.2 x 400

(A ns.)

T im e for facing D iam eter o f jo b is to be reduced from 50 m m to 40 mm. But before facing, diam eter o f jo b is already turned dow n to 40 m m dia. Since in facing, length o f tool travel is equal to h alf the jo b diam eter, i.e. L = 40/2 = 20 mm. Tim e for facing one pass = I — j = -----—------= 0.25 min JN) 0 .2 x 4 0 0 N um ber o f passes required (np)

M aterial to be rem oved ? 5 _ _ = --------------------------------- = ------------ ------------ = - = 2.5, say 3 Max. depth o f cut M ax. depth o f cut 2 H ence total tim e for facing = 3 x 0.25 = 0.75 min Total tim e for m achining = 6.6 + 0.75 = 7.35 m in

6 .1 4 .9

(A ns.)

L a th e A c c e s s o rie s a n d A tta c h m e n ts

A ccessories are the devices used for holding or supporting jo b on a lathe during m achining. T hese include lathe centres, face plate, dog carrier, chucks, angle plate, m andrel, steady rest, follow er rest, etc. Attachm ents are special devices used for special jobs, for example, taper turning attachm ent used for turning taper, g ear cutting attachm ent used fo r cutting gear on lathe or grinding attachm ent for perform ing grinding operations. L athe centres [Fig. 6.63(a)! are used for turning jo b betw een the centres. The centre w hich is fitted in spindle nose and w hich revolves w ith the jo b is called live centre. T he one fitted w ith tail stock is called dead cen tre as it do es not revolve w ith the job. T he jo b is held betw een tw o lathe centres, and can be tightened o r loosened by advancing or retreating the dead centre by revolving the tail stock hand w heel. T he dead centre is set rigid in a particular position by clam ping the sleeve o f the tail stock before starting the work. Face plate (Fig. 6.63(b)] is screw ed on the spindle nose. S lots are provided on face plate for bolting angle p late for holding typical right angled bend jo b s for boring, etc.

[Fig. 6.63(c)] or the jo b itse lf bolted on face plate. An open slot in the face plate is provided for the dog carrier [Fig. 6.63(d)] w hich at one end grips the jo b with its bolt and at the other end is hooked w ithin the open slot o f the face plate [Fig. 6.63(e)]. C hu ck s are screw ed on to the nose o f lathe spindle. T hese arc four independent jaw type o r three ja w type (or self-centring chuck). T he four ja w chuck [Fig. 6 .6 3 (0 ] can even hold irregular shaped jobs as all its four jaw s have independent movements to help accomm odating and holding irregular shaped jo b s. A three jaw chuck [Fig. 6.63(g)] can, however, hold only the cylindrical jo b s as all its three jaw s m ove forw ard o r backw ard sim ultaneously. By rotating the key [Fig. 6 .6 3 (0 ]. the jaw s o f both four ja w chuck and three ja w chuck can be moved forw ard o r backw ard parallel to the face o f the chuck.

13 Ordinary centre

Ball centre Ball bearing

Revolving centre

Fig. 6 .6 3 (a )

Lathe centres of d ifferent types. Ordinary centres are used fo r general purpose turning w ith included angle 60° fo r lig h t w o rk and 90° fo r heavy w ork. Ball centres are em ployed fo r taper tu rn in g w ith ta il stock set over m ethod to m inim ize wear and strain on centres. The revolving centre is used in tail stock fo r supporting a heavy job revolving at high speed and w herein th is centre (unlike a dead centre) revolves w ith the job. Slots for clamping jobs

G a p for lathe dog

)

\T h re a d s

-i

\

^

J

\ \

n

N. 0

rI

-

i p i\ \ ^ y ^ F a c e plate* i

Fig. 6 .6 3 (b ) A face plate. It carries a num ber of slots fo r bolting job s directly on the face plate or through the angle plate on w hich the job is m ounted. The open slo t (o r gap) is fo r engaging the ta il of the lathe dog carrier.

Balancing weight Face plate

o Ql CO

s

Jo b -, nn

u.

J

'A n g le plate

F ig. 6 .6 3 (c )

S how ing the use of face plate w ith the job m ounted on angle plate fo r ca rryin g out boring operation on it.

F ig. 6 .6 3 (e )

Use of dog ca rrier in tu rn in g a jo b between centres. The dog ca rrier at its one end is clam ped w ith the jo b w hile its other end is engaged in the open slo t of the face plate screwed on the lathe spindle.

F ig. 6 .6 3 (f)

A fo u r jaw independent chuck. It has fo u r jaw s, each ja w is independently actuated and adjusted (during ho lding the jo b ) by a key. A lm ost all types o f jobs. e.g. cylin d rica l, square and irreg ular shaped are easily held in th is chuck.

Teeth on scroll plate back

Fig. 6 .6 3 (g )

A three jaw se lf-centring chuck. It has three jaws, all o f them are advanced o r retracted sim ultaneously by tu rn in g the key placed in any of the three key holes made on the peripheral edge of the chuck.

O ther types o f lathe chucks include the follow ing. C ollect chucks, m agnetic chucks, h ydraulic o r pneum atic chucks are also used for holding jo b s on lathe. M andrels are hardened steel pieces o f round b ar and are used for holding the bored jobs (jobs having drilled o r bored holes) for the purpose o f turning them at outside. These are o f various types, for exam ple, screw ed m andrel, taper co llar m andrel, expansion m andrel, etc. A typical tapered collar m andrel is show n in Fig. 6.63(h).

F ig. 6 .6 3 (h )

Show ing the use of a taper collar m andrel in tu rn in g the surface of a job having its bore already made. The tapered collars, when fit properly in the bore and tightened, hold the job rigid w ith the m andrel, w hich is later revolved between the lathe centres.

Steady rest [Fig. 6.63(i)] is used when a long jo b is m achined o r drilled at its end by holding the jo b in a chuck. T he use o f steady rest avoids the deflection o f jo b under its own w eight or cutting forces o f the tool. The steady rest is fixed in one position w ith lathe bed. Follow er rest [Fig. 6.63(j)] is used for turning a long and thin jo b w hich m ay be held betw een centres and m ay thus deflect under cutting forces o f the tool. T he follow er rest is connected w ith the carriage and hence m oves w ith the tool as turning operation proceeds. It is fitted right opposite to the cutting tool.

6 .1 5

TU R R E T A N D C APSTA N LATHES

It has been seen in earlier discussions that a centre lathe (or engine lathe) is quite capable o f perform ing various operations to produce a large variety o f cylindrical surfaces as also other flat surfaces. A lthough it is a useful and versatile m achine capable o f m achining any or every type o f jo b s w ithin its lim it, it is, however, unsuitable as a m ass production m achine as it consum es considerable tim e in setting up different tools on the tool post after each operation and for each job. A jo b usually needs several operations to be done on it requiring a num ber o f tools. O n a centre lathe, one has to change the tool every tim e a new operation is perform ed and thus a com plete com ponent (requiring several operations) cannot be produced with a single setting o f tools. M oreover, it is often needed to change the set o f tools so that other rem aining operations m ay be done on the job. T his replacing and resetting o f tools consum es a lot o f tim e. W hen a large num ber o f alike pieces are to be produced, every time positioning and changing o f tools for each jo b not only involves a good deal o f tim e but also results in the w orkpieces o f non-identical dim ensions. T hus, a centre lathe is best suited for ‘one o ff’ type jo b and its use for m ass production w ork w ould not be effective and econom ical. Turret and capstan lathes are the natural developm ents o f a centre lathe and are built to m achine w orkpieces that are large in num ber and on repetitive basis (i.e. for mass production jobs). Turret and capstan lathes bridge up the gap betw een the slow w orking centre lathe and fully autom atic lathes specifically designed for mass production o f com ponents w ith very high production rates. T he main distinguishing feature o f turret and capstan lathes is the m ultiple tool holders w hich enables presetting o f all the tools for a job. M ultiple tooling is provided by replacing the usual tail stock (of centre lathe) with a rotating and indexing type hexagonal turret or capstan head on which six o r m ore tools can be m ounted and preset as per the requirem ent o f various operations to be done on the job. T he turret is indexed autom atically and each tool m ay be brought in line with the lathe axis in a regular sequence. W orkpieces are held in collets or chucks m ounted on the head stock o f the lathe. T he longitudinal and cross feed m ovem ents o f the turret saddle and cross slide are regulated by adjustable stops w hich enable different tools set at different stations to move a predeterm ined am ount for perform ing different operations on repetitive w orkpieces without m easuring length or diam eter o f m achined surface in each case.

These special features o f the turret o r capstan lathes enable it to perform a series o f operations such as turning, drilling, boring, necking, thread cutting, cutting-off, etc. in a regular sequence to produce a large num ber o f identical parts in m inim um possible time.

6.15.1

D iffe re n c e b e tw e e n a T u rre t L ath e (o r C a p s ta n L a th e ) a n d C e n tre L a th e

T he turret and capstan lathes are the im proved version o f the centre lathe on account o f the follow ing basic differences in regard to their construction and w orking. (i) T he turret lathe has its head stock sim ilar to that o f the centre lathe in construction, but it possesses a w ider range o f speeds, heavier construction and higher pow er available at the spindle for m achining at faster speeds resulting into m uch higher production rates in com parison to the centre lathe. (ii) The turret 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 o f carrying one or more tools. These tools may be indexed in an orderly way to perform different operations. (iii) T he turret has tool post m ounted on its cross slide w hich is a four-w ay tool post to hold four tools capable o f being indexed by 90° such that each tool is brought into operation in regular order. T he turret cross slide also carries a rear tool post to hold another tool (often used for parting off). (iv) T he feed m ovem ent o f each tool m ounted on the turret head is regulated by stops and feed trips, a feature that enables the sam e tool to perform operation on each jo b to a p red eterm in ed am ount m aking d u p licatio n w o rk po ssib le w ithout fu rther m easurem ent. (v) C om bination cuts by several tools sim ultaneously m ade possible to m achine m ore than one surface at a tim e. It is a unique feature o f the turret o r capstan lathe. (vi) T he turret and capstan lathes do not usually carry a lead screw to help cutting threads on the jo b (as in case o f centre lathe). Instead, the external threads are cut w ith a die set and internal threads w ith taps. However, som e turret lathes m ay carry a short length lead screw, called ‘chasing screw ’, to help cutting threads by a chaser. (vii) Special feature o f holding eleven o r m ore tools capable o f being brought into operation in a prefixed sequence regularly, com bined w ith the use o f feed trips and stops for the tools, m akes the turret and capstan lathes a production m achine suitable for producing a large num ber o f identical com ponents in a m inim um possible tim e. (viii) T he centre lathe is m ore suitable for m achining ‘one o ff’ type jo b and is certainly not suitable for m ass production work. Sim ilarly, turret and capstan lathes arc suitable for only m ass production and not for m aking one or few jo b s because o f high initial tool and jo b setting tim e and overall cost o f th e m achining operation.

6 .1 5 .2

P rin c ip a l P a rts o f T u rre t a n d C a p s ta n L a th e s

T he turret lathe (o r capstan lathe) is sim ilar to the centre lathe except the turret and som e other m echanism s w hich have been incorporated in it for m aking it suitable for mass production work. T he essential features o f turret lathe are shown in Fig. 6.64 and that o f capstan lathe in Fig. 6.65. Principal parts o f a turret and capstan lathe are described in the follow ing:

F ig. 6 .6 4 Turret lathe parts. Rear to o l post on cross slide not show n. A— hand wheel fo r longitudinal feed of saddle, B— hand wheel fo r cross-slide feed, C— sta r hand w heel fo r longitudinal feed of 2 tu rre t saddle. C ro ss slide front H ead stock

Spindle

tooJ P081 Hexagonal turret

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F ig. 6 .6 5

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Sa dd le for auxiliary slide

Saddle for cross slide

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Capstan lathe parts. Rear to o l post on cross slide not show n.

1. Bed: It is a long box-shaped grey iron casting w ith stiffening ribs and carries accurate guide w ays upon w hich are m ounted turret saddle and carriage (w ith cross slide).

2. H ead stock:

The head stock o f a turret and capstan lathe com prises a speed gearbox sim ilar to that o f a centre lathe but differing in a few m ajor aspects, for exam ple, m ethods o f pow er drive and speed change system and controls. O ne o f the m ain features o f turret head stock is the provision for rapid stopping, starting and speed changing w hich helps in taking m axim um advantage o f useful cutting speed for any w ork and at •the sam e tim e to m inim ize the loss o f tim e in speed changing, stopping and starting. T here are step cone pulley driven head stock s w hich are o f very sim ple design and w herein starting, stopping and reversing o f the lathe spindle are effected by pressing a foot pedal. In electric m otor d riven head stocks, the spindle o f lathe and the arm ature shaft o f m otor are the one and the sam e. W ith three o r four speeds, the lathe is suitable for sm all diam eter w orks rotated at high speeds during w orking. L arger lathes carry all geared head stocks equipped w ith speed gears and m echanism for speed changing through levers and w ithout stopping the m achine. P re-selective head stock is an all geared headstock w ith arrangem ents for rapid stopping, starting and speed changing sim ply by pushing a button o r pulling a lever. R equired speed for next operation is

selected before hand and the speed changing lever is placed at the selected position. Just a push o f button runs the lathe at the selected speed. 3. C arriage or ch aser saddle: It carries a cross slide over it. Two tool posts, one at the front and the oth er at the rear, are m ounted on the cross slide (Fig. 6 .66). Both these tool posts are square tool posts, each capable o f holding four tools at a tim e; tools in the rear tool post are m ounted in an inverted position. Both hand and pow er feeds are used for the carriage and the cross slide. Stops and trip dogs are used to disengage the pow er feeds (longitudinal feed for carriage o r chaser saddle and cross feed for cross slide) as soon as the required tool travel is com pleted. T he cross slide carriage m ay be o f (a) bridge (reach over) type (as show n in Fig. 6 .66) capable o f carrying a second tool holder at the rear and (b) sid e hung type fitted w ith heavy duty turret lathes and riding on the top and bottom guide w ays on the front o f the lathe bed. □

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Fig. 6.66 Show ing cross slide, fro n t square tu rre t to o l post and rear square tu rre t to o l post. Note the difference in m ounting of to o l on the rear tu rre t to o l post.

4. T u rret saddle and auxiliary slide: T he turret saddle in a capstan lathe bridges the gap betw een bed w ays and its top face provides base for an auxiliary slide (often called ram o r short slide). T he turret saddle is adjusted (according to the length o f w orkpiece) and clam ped on the bed w ays at required position w hile the hexagonal turret or capstan is m ounted on the auxiliary slide (Fig. 6.67) w hich slides longitudinally on the turret saddle. T his arrangem ent perm its quick m ovem ent o f the turret. Trip stops are there to stop the feeding m otion o f the turret at any predeterm ined position. T his type o f lathe is used for bar stock o f sm aller diam eter and light-duty chucking work. In case o f a turret lathe, the turret is directly m ounted on the top o f the turret saddle and any movement o f the turret is effected by the movement o f 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 o f turret heads are shown in Fig. 6.60. Through centre o f each face of the turret, bored holes are provided for accom m odating shanks o f different tool holders; the centre line o f each hole coincides with the axis o f 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 w hich restrict movement o f each tool mounted on each face o f the turret, a feature that helps duplicating work. A fter 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 comparison to the capstan lathe) and is particularly adapted for larger diam eter bar work and chucking work and the machine can also accomm odate longer workpieces.

6 .1 5 .7

W o rk H o ld in g D e v ic e s

As already m entioned the turret and capstan lathes do not have a tail stock (like the one used on a centre lathe) and hence w ork cannot be held betw een centres. T he work is, therefore, supported at the spindle end w ith the help o f chucks and fixtures. T he follow ing m ethods are used for hold in g w ork on turret and capstan lathes. (i) Jaw chucks include self-centring jaw chuck, independent four ja w chuck, com bination chuck, air-operated chuck, soft ja w chucks, etc. C hucks can hold irregular shaped jobs. A part from those used on centre lathes, there are few m ore special chucks used for holding the stock on turret o r capstan lathe. T hese are p o w e r chucks o r tw o-jaw box chucks used for holding jo b s w ith parallel flat sides. Pow er chucks are op erated by air pressure, hydraulic pressure o r electricity. A ir chucks are com m on due to th eir ease o f operation and sensibility to follow up any looseness in the grip. (ii) C ollet chucks used on turret lathes are sim ilar in w orking principle to the collet chucks used on centre lathe for holding the blank o r bar. A typical spring collet chuck is show n in Fig. 6.71. W hen the split collet (D) is advanced to the right by a lever, the bar (B) is released due to the opening (or spring action) o f the collet jaw s (D ). A feed finger (A) is also a split bush and holds the b ar (B) tight. As soon as the b ar (B) is released, due to the w eight system for b ar feeding, the b ar (B) is fed autom atically forw ard against a bar stop held at the turret bead. By sw inging back the collet lever, collet jaw (D) is pulled back to hold the bar tight during m achining.

Fig. 6.71 A typical spring collet chuck. A— Feed finger, B— Bar stock, C— Spindle of machine and D— Split collar. B esides the spring collet chucks, other types o f collet chucks include draw -in type collet, push-out type collet and dead-length type collet chuck. T he draw -in type collet exerts grip over the bar w hen it is draw n in. T he p ush -out type collet exerts grip over the bar stock w hen it is pushed forw ard. T he dead-length type collet ensures positive locking against the m ovem ent o f the bar. M ost o f the b ar w ork is done on round o r hexagonal bars. T he gripping m outh o f the collet is m ade to have the sam e shape as the cross-section o f the b ar it has to hold. (iii) A rb ors m ay be som etim es used to hold relatively sm all jo b s w hich already have hole drilled in them . D ifferent types o f arbors, for exam ple, threaded type and expanding type are in com m on use. (iv) F ixtures are special type o f chucks built for holding and locating typical jo b s during m achining.

6 .1 5 .8

M e th o d s o f H o ld in g T o o ls

It has been m entioned earlier that on turret and capstan lathes, a num ber o f tools can be m ounted on the turret as also on the tool p o sts (front an d back) m ounted on the carriage (or chaser saddle). T he m ethods for m ounting tools on tool posts are sim ilar to that used on centre lathe, but the m ethods o f m ounting tools on the turret o r capstan head are different. N ote the general shape and provisions o f a turret head show n in Fig. 6.72. The com m on m ethods for m ounting tools on the turret are given in the follow ing:

Star hand wheel

Trip control

F ig. 6 .7 2 View of tu rre t lathe saddle and tu rre t head fro m the po int o f vie w o f m ounting tools. Note the hole in the centre of each face of tu rre t fo r accom m odating to o l shank and sm all holes fo r bolting d ifferent attachm ents o r to o l holders.

M ethod (A ): T here are holes in the centre o f each face o f the turret head. Split adapter bushes [Fig. 6.73(a)] m ay be introduced through these holes to hold sm all boring bars, bar stops, drills and ream ers. T his type o f tool m ounting is often used on the capstan lathe. U se o f the split adapter bush fo r holding a threading die is show n in Fig. 6.73(b).

Fig. 6.73(b)

S how ing the use of s p lit adapter bush fo r ho lding a threading die.

M ethod (B): Tools o r their attachm ents m ay be bolted directly on to the face o f the turret head. Tool (a boring bar) m ounting by directly bolting the tool attachm ent on the face o f the turret is show n in Fig. 6.74.

Fig. 6.74

Show ing m ounting of a boring bar to o l by directly bolting w ith tu rre t face.

M ethod (C ): Som e tool attachm ents bolted to the face o f the turret head have a hole in their back flange in line w ith the bore o f the turret head. In that case, tools like drills o r ream ers can be inserted through the hole and can be used to w ork over and above to the m ain tools fixed on the attachm ents. A part from the above m ethods, there m ay be num erous types o f tools for w hich people m ay design their ow n specific devices for m ounting on the turret head.

6 .1 5 .9

C o m m o n T o o ls a n d A tta c h m e n ts

A great variety o f tools, tool holding devices or attachm ents are used on turret and capstan lathes. T hese attachm ents are m ounted on the turret faces o r tool posts on cross slide. Som e m ore com m on types o f these devices have been listed in the follow ing w ith reference to Fig. 6.75. A pilot b ar [Fig. 6.75(h)] is used on turret lathes for perform ing heavy-duty m achining. One end o f the pilot bar is attached to the m ultiple turning and boring attachm ent and the other end to the head stock o f the lathe such that pilot bar gives support to the tool head for taking heavy cuts. (a) Straight cu tter holder (b) M ultiple cutter holder (c) Slide tool cutter (d) C om bined bar stop and centring tool (e) K nurling 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) A djustable knee-tool holder (h) Pilot bar and m ultiple turning and boring head

6 .1 5 .1 0

B a r F e e d in g M e c h a n is m

O ut o f several m ethods em ployed for feeding bar stock forw ard (for m achining) after each finished product is cu t o ff (or parted off) from the b ar stock, the sim plest m ethod is to feed the bar by m eans o f a w ire rope and w eight. T he m ethod is, however, lim ited for use on sm all m achines such as capstan lathes.

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Fig. 6 .7 5

(Contd.)

S e c o n d Edition

MANUFACTURING PROCESSES J.P. KAUSHISH The 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 w ith 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 machining methods, 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 w ill 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 tw o 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 research papers in reputed journals.

O u r o t h e r u s e fu l b o o k s Fundam entals o f Heat a n d M ass Transfer, B .K . Venkanna Heat Treatm ent: Principles a n d Techniques, 2nd ed., T V. Rajan, C.P. Sharm a and Ashok Sharm a Introduction to H ydraulics a n d Pneum atics, Revised Edition, S. Ilango and V. Soundararajan D e sign o f M achine Elem ents, C .S . Sharm a and Kam lesh Purohit P rinciples o f Environ m e n ta l Science a n d Engin e e rin g, P. Venugopala Rao

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