tool engg notes by DVSHIRBHATE

8/14/2019 tool engg notes by DVSHIRBHATE

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by a great deal of friction & heat generation is governed by definite laws. Metal cuttingoperation involves three basic requirements. (1) There must be a cutting tool that isharder and wear resistant than the work piece material, (2) there must be interference

between the tool & the work piece as designated by the feed and depth of cut, and (3)There must be relative motion or cutting velocity between the tool & the work piecewith sufficient force and power to overcome the resistance of work piece material. Aslong as above three conditions exist, the portion of the material being machined thatinterferes with free passage of the tool will be displaced to create a chip.

1.2 Classification of production process:The metals are given different usable forms by various processes. These

processes may be classified as under.

Metal Forming

Chip-forming Process Chip-less Process(Metal Cutting)

Continuous-contact Intermittent Continuous Impact or Cutting cutting (Rolling, Spinning Intermittent

Etc.) Contact(Forgoing,Drop-stamping)

Single-edge Double Sizable Ground ChipsCutting edged Swarf (Honing, Grinding,(Turning, cutting (Milling) etc.)Shaping, (Drilling)Boring)

In chip removal processes the desired shape and dimensions are obtained by separatinga layer from the parent work piece in the form of chips. During the process of metalcutting there is a relative motion between the work piece & cutting tool. Such a relativemotion is produced by a combination of rotary and translatory movements either of thework piece or of cutting tool or of both. These relative motions depend upon the type of metal cutting operation. The following table indicates the nature of relative motion for various cutting processes.


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In chipless processes the metal is given the desired shape without removing anymaterial from the parent work piece.

1.3 Basic elements of cutting tools:The cutting tool consists of three basic elements (1) cutting element or

Principle element This is the element, which is actually fed into the material of work piece to cut the chips ex. In drilling lips (or cutting edges) are cutting elements. (2)Sizing element The part, which serves to make up any deficiencies of cutting elementafter sharpening, is sizing element. It imparts final shape to the machined surface andalso provides guidance in tool operation ex. In drill sizing element; (flute portion)immediately follows the lips ). (3) Mounting element It serves for securing the tool inmachine or holding it in hand of worker ex. In the twist drill the shank is mountingelement. The cutting & sizing element taken together is referred as working element of the tool.

1.4 Machining parameters:

1.4.1 Cutting Speed (V) It is the travel of a point on cutting edge relative to surfaceof cut in unit time in process of accomplishing the primary cutting motion. It is denoted

by V. The unit of cutting speed is m/min. In lathe work for turning a blank of diameter D mm, (The diameter of

machined surface is Do mm.) rotating at a speed N (rpm) the cutting speed at periphery (maximum) is given by.

V = D N /1000, m/min .......... 1.41

Fig. 1.1 Elements of cutting process in turning

FEED

SPEED


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Fig. 1.2 Sketches Showing V, f and d

From this formula it is easy to find rotational speed N = 1000 V / D ................... 1.42

From figure 1.1. it is evident that the cutting speed varies along the cutting edgefrom maximum at point m to minimum at point K though the rotational speed issame.

In drilling a work piece with a drill of diameter D mm., rotating at a speed N

(rpm) the cutting speed will vary from zero at center to maximum at periphery given byeqn 1.41.

m/min, 1000

NDV =

Similarly in facing the cutting speed varies from zero at center to maximum at periphery.

1.4.2 Feed (Feed rate) (f, f m)It is the travel of the cutting edge in the direction of feed motion relative to the

machined surface in unit time. The feed may be expressed as distance traveled by thetool in one minute (f m) or distance traveled by the tool in one revolution (f). The termsf and f m are related by

f = f m / N, mm/rev . . . . . . . . . 1.43In lathe work, distinction is made between longitudinal feed, when tool travels

in a direction parallel to work axis, cross feed when tool travels in a direction perpendicular to the work axis, and angular feed when tool travels at an angle to work axis (for example, in turning tapered surface.)

1.4.3 Depth of cut: (d)It is the thickness of the layer of metal removed in one cut or pass; measured in

direction perpendicular to machined surface. The depth of cut is always perpendicular to the direction of feed motion and, in external longitudinal turning; it is half thedifference between the work diameter and the diameter of machined surface obtainedafter one pass.

d = (D Do)/2 mm ............ 1.4.4


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To reduce machining cost machining time should be less i.e. the metal removal rateshould be high. To achieve this following facts should be considered.

1) Proper cutting tool material should be selected.2) Correct tool (angle) geometry should be produced or ground on tool 3) The tool should be rigidly held to avoid vibrations.4) Depending on the rigidity of machine tool system maximum values of speed &

feed should be selected.A process, which removes metal at a faster rate, may not be the most economical

process, since the power consumed & cost factors must be taken into account. Due tothis, to compare two processes, the amount of metal removed per unit of power

consumed in unit time is determined. This is called Specific metal removal rate and is expressed as, mm 3/w/min, if the power is measured in watts.

1.5 Basic shape of cutting tools: Wedge.Almost all cutting tools used in metal cutting operations consist of basic form of

a wedge, which is defined as one form of inclined plane in shape of a triangular prism.Assume that a wedge under the action of force P is penetrating into another body at aconstant speed as shown in Fig.1.3.

N

K

M N N

N P

PL

Fig. 1.2 Force acting on an indenting wedge Fig. 1.4 Force triangle at the wedgecheck

in fig.1.3. The body resists the motion of the wedge. The reaction N.N. appear at the

cheeks of the wedge. The forces N.N. are perpendicular to the cheeks in absence of friction. From the equilibrium of forces (fig.1.4)

2sin2

1

KM2/KL

2

1KLKM

P N

=

==


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, .smaller the angle of wedge, the greater will be the gain in force. In other words, thewedge angle ' ' determines the resisting force of the cutting edge.

The cutting edge must be oriented at certain required angles with the work surface depending on nature of operation to be performed. Fig.1.5 shows that thewedge must be set at right angles to the work surface, so that the driving force "P" is inthe direction of parting. Fig.1.6 shows during chipping the wedge must be set at anangle inclined to work surface so that separation of chip can be done.

Thus for the wedge two geometric parameters can be defined i.e. (1) The wedgeangle ' ' and (2) the axis of symmetry along which 'P' acts. In addition to above, twomore parameters are introduced to confirm conditions of chipping action. These

parameters are set with respect to velocity Vector, 'V' and are defined as (3) cuttingangle ' ' and (4) clearance angle ;, as shown in fig.1.7. The sign convention for describing these angles are set wr.t. left handed cork screw rule with "Z" axis coincidingwith the direction of the velocity vector, V, and the cutting edge lying along 'Y' axis.Hence, ' ' & ' ' are measured positive, when moving from 'Z' to 'X' axis as shown infig.1.7. The parameter ' ' defines the indination of the top face of the wedge (calledRake face) w.r.t. velocity vector V, while the parameter ' ' describes the relief providedfrom the bottom face of the wedge (called flank), often another derived parameter,called (5) Rake angle ' ', is used to describe the indination of the top face of the wedge.This is derived parameter given by

= 900

- .However if > 90 , then ' ' is negative. Thus from this equation it may be seenthat while ' ' is always positive the rake angle can become positive or negativedepending an value of angle ' '.

v

090


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1.6 Types of metal cutting processes:

The metal cutting processes are classified in to two types, on the basis of angular relationship between cutting velocity vector V, & the cutting edge of the tool.

(1) Orthogonal cutting process (two dimensional cutting)(2) Oblique cutting process (three dimensional cutting)

In orthogonal cutting the cutting edge of the tool is perpendicular to cuttingspeed

direction. In oblique cutting, the angle between the cutting edge & cutting velocityvector is different from 90 0. fig 1.9 & fig.1.10

Point Orthogonal Cutting Oblique Cutting1. Definition - The cutting edge of tool

perpendicular to cuttingspeed V;

The Cutting edge of the tool isinclined at an angle other than900 to V.

2. Alternative name Two dimensional cutting Three dimensional cutting3. Volume of metalremoval for a cuttingcondition.

Less metal removal dueto square cuttingcondition.

More metal removal, as greater area of chip is removal for samedepth of cut & other conditions.

4. Tool life - Shorter Longer 5. Friction & -

Chatter More Less, as small amount of heat

developed due to friction at the

Fig1.9 Fig1.10


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,4) The above relative values are affected by changes in cutting, conditions & in

properties of the material to be machined to give chip that range from small lumps tolong continuous ribbons.

These observations indicates that the process of chip formation is one of deformation or plastic flow of the material with the degree of deformation dictating thetype of chip that will be produced.

Fig. 1.11 shows progressive formation of a chip using a wedge shaped (single point) tool. At a tool contacts the work piece material. At b compression of material takes place at point of contact. At c the cutting force overcomes theresistance of penetration of tool is begins to deform by plastic flow. As the cutting

force increase, either a rupture or plastic flow in direction generally perpendicular toface of the tool occurs & the chip is formed as shown at d.

tool tool tool tool

a b c d

Fig. 1.11 Progressive formation of a metal chip.

Fig1.12


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.of slip & the layers are called slip planes. The number of slip planes depends upon thelattice structure of parent workplace material. The distortion of layers tends tostrengthen them (work hardening or strain hardening) & therefore the hardness of chipis much greater than the hardness of the parent material.

Thus in simple language the mechanism of chip formation in any machiningoperation is a rapid series of plastic flow & slip movements ahead of the cutting edge.The degree of plastic flow ahead of the cutting tool determines the type of chip that will

be produced. If the w/p material is brittle & has little capacity for deformation beforefracture the chip will separate along the shear plane to form what is known as adiscontinuous segmental chip. Material that are more ductile & have capacity for

plastic flow will deform along the shear plane without rupture. The planes tend to slip& weld to successive shear planes, & the result is a chip that flows in a continuousribbon along the face of tool. This is known as a continuous chip & is usually muchharder than the parent material because of its strain hardened conditions.

1.8. Types of Chips:The tool engineer's handbook lists four different types of chips viz.

1) Segmental chips or Discontinuous chips2) Continuous chips3) Continuous chip with BUE or BUE chips.4) Inhomogeneous chips.

1) Discontinuous Chips: These chips are in the form of small individual segments,which may adhere loosely to each other to form a loose chip. These chips are formed asresult of machining of a brittle material such as gray cast iron or brass castings, etc.These chips are produced by actual rupture or fracture of metal ahead of the tool in

brittle manner. Since the chips break up into small segments and also shorter chipshave no interference with work surface. The friction between chip & tool reducesresulting in better surface finish. These chips are convenient to collect, handle &dispose of during production runs. The conditions favorable for formation of discontinuous chips are:1) Brittle & non ductile metals (like cast iron brass castings Beryllium, titanium etc.)

2) Low cutting speed.3) Small rake angle of the tool.4) Large chip thickness.

2) Continuous Chips: These chips are in the form of long coils having uniformthickness throughout. These chips are formed as result of machining of relatively

.) h d


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.5) Sharp cutting edge.6) Efficient cutting fluid.7) Low friction between chip tool interfaces.

3) BUE Chip (or continuous Chip with BUE): These chips are also produced in theform of long coils like continuous chips, but they are not as smooth as continuous chips.These chips are characterized by formation of built up edge on the nose of the toolowing to welding of chip material on to tool face because of high friction between chiptool interfaces. Presence of this welded material further increases the friction leading to

building up of the edge, layer by layer. As the built-up edge continuous to grow, the

chip flow breaks a portion of it into fragments. Some of them are deposited on the work piece material while the rest are carried away by the chips. The hardness of this BUE istwo to three times higher than the work piece material. This is the reason why thecutting edge remains active even when it is covered with built-up edge. The only pointin favor of BUE is that it protects the cutting edge from wear due to moving chips andthe action of heat. This brings about an increase in tool life. These chips normallyoccur while cutting ductile materials with HSS tools with low cutting speeds. Chipswith BUE are under desirable as they result in higher power consumption poor surfacefinish and higher tool wear. Generally speaking any change in cutting conditions thatwill eliminate or reduce BUE is desirable, since high friction between chip & tool faceis major cause of BUE. Any means of reduction of friction such as use of lubricant &

adhesion preventing agent is often effective to reduce BUE, especially when it isnecessary to operate at low cutting speeds. Tool material with inherent low coefficientof friction or a high polish on tool face can also reduce friction & hence BUE. Theconditions favorable for BUE chip are.1) Ductile material2) Low cutting speed.3) Small rake angle of tool.4) Dull cutting edge.5) Coarse feed.6) Insufficient cutting fluid.7) High friction at chip tool interface.

4) Inhomogeneous Chip: These chips are produced owing to non uniform strain set upin material during chip formation and they are characterized by notches on the free sideof chip, while the side adjoining the tool face is smooth. The shear deformation whichoccurs during chip formation causes temperatures on shear plane to rise which in turnmay decrease the strength of material & cause further strain if the material is poor


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Table 1.1. : Factors responsible for the formation of different types of chips.Types of chipsFactors

Discontinuous Continuous With BUE Inhomogeneous1. Material Brittle Ductile Ductile Which Shows

decreased inYield Strength

with temp. &Thermalconductivitymedium.

2. Cutting speed Low High Low -3. Toolgeometry

Small rake Large rake Small -

4. Friction - Lower Higher -5. Chipthickness

Large Small Small -

6. Cutting fluid - Efficient Poor -

7. Feed - - Coarse -8. Cutting edge - Sharp Blunt -

, , c Where t = undeformed chip thickness (i e before cutting) and


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, ,Where t = undeformed chip thickness (i.e. before cutting) andtc = mean thickness of chip ( i.e., after cutting )Chip reduction coefficient K = 1/r

The following methods can be used to determine cutting ratio1) The cutting ratio "r" can be obtained by direct measurement of "t" & "t c". However since underside of chip is rough the correct value of "t c" is difficult to obtain and hencetc can be calculated by measuring length of chip (1c) and weight of piece of chip "W".

tc = W/ (b c .1c. )Where, b c = length of chip

1c = width of chip

= Density of material assumed to be unchanged during chipformation.

2) Alternatively, the length of chip (1 c) & length of work (l) can be determined.The length of work can be determined by using a work piece with slot, which will break the chip for each revolution of work piece. The length of chip can be measured bystring.

It can be shown that r = 1/1 c as under. When metal is cut there is no change involume of metal cut. Hence volume of chip before cutting is equal to volume of chipafter cutting i.e.

1.b.t. = 1c.b.tcor l.t. = 1c.tc (assuming b = bc)

l/lc = t/tc = r 3) Cutting ratio can also be determined by finding chip velocity (V c) and cuttingspeed (V). The chip velocity (V c) can be accurately determined by determining lengthof chip with a string for a particular cutting time measured with the help of a stopwatch.It can be shown that r = Vc/V, as under. From the continuity equation, we know thatvolume of metal flowing per unit time before cutting is equal to volume of metalflowing per unit time after cutting.i.e. V.b.t. = V c .b.t c or Vc/V = t/t c = r (assuming b = bc)

1.10 Shear Angle:The shear angle is the angle made by shear plane with the direction of tool

travel. In fig 1.7a it is the angle made by the line AB with direction of tool travel. Thevalue of this angle depends on cutting conditions, tool geometry, tool material & work material. If the shear angle is small, the plane of shear is larger, the chip is thicker andtherefore higher fore is required to remove the chip. On the other hand, if the angle is

w ere = ra e ang e


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the derivation of the above equation is as follows. from fig 1.7 a

t1 = AB sin t2 = AB sin cos ( - )

+=

==

sinsinsincoscos

sin)(cos

r 1

tt

c1

2

+= sincoscotr 1c

=

=cosr

sinr 1

cos

sinr 1

cotc

cc

=

sinr 1

cosr tan

c

c

1.11 Velocity relationships in orthogonal cuttingThere are three velocities in orthogonal cutting process, namely

(i) Velocity of chip (V f ) which is defined as the velocity with which the chip movesover the rake face of the cutting tool.(ii) Velocity of shear (V s) is the velocity with which the work piece metal shears alongthe shear plane.


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)(cossin

,VV cf =

Vs = V c,)(cos

cos

where is the rake angle, is the shear angle.From the principle of kinematics, the relative velocity of two bodies (tool and

chip) is equal to the vector difference between their velocities relative to the reference body (here the work piece). The vectors of these three velocities - V c, V s and V f -should form a close velocity diagram (Fig.30.15)andThus V c = V s + V f

Refer Fig. 30.15(b)From right-angled ACE

=== sin.Vsin.AEACor sinAEAC c From right -angled ABC

)(cos,V)(cos,ABACor )(cosABAC

f ===

From Eqs. (a) and (b)= sin.V)(cos.V cf

or )(cos

sin.VV cf

=

Consider ADE

== cos.VDE or cosAEDE

c Consider BDE

)(cosBEDE =

rake angle " " by the following equation:


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=

x)tan(xcotx

xs

+=

= Cot + tan ( - )or =

)cos(sincos

This relation can be obtained from the pack of inclined cards model suggested by Prof. Pushpanen. In this model the formation of chip and its motion along the toolface can be visualized from an idealized model in which a stack of inclined (playing)cards is pushed against the tool (fig.1.16 a). As the tool advances, segments, which had

been part of the work place, become part of the chip. From this figure it can be seenthat card closest to the tool point slips to a finite distance relative to the uncut materialas tool point slips to a finite distance relative to the uncut material as tool advances.When the tool point reaches the next card, the previously lipped card moves up alongthe tool face as a part of the chip.

BA = BE + AEBA = x cot +x cot {90 - ( -)}

= cot ( )+ Tan

( )( )+== 90cotcotCEBA


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R = (P + P + P )1/2 = 222 PPP ++ 1 14 1


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R = (P x + Py + P z ) = 222 ZYX PPP ++ ........ 1.14.1This three-dimensional force system can be reduced to a two-dimensional force

system if in orthogonal plane 0 the forces are considered in such a way that the entireforce system is contained in the considered state, when

R = 2 y x2z PP + ..... . . . 1.14.2

Pxy = 2y2x PP + ..... . . . 1.14.3

This is possible only when P xy is contained in plane 0 which is possible only under conditions of free orthogonal cutting. This corresponds to 'orthogonal system of firstkind' for which conditions are:

i) 0< < 90ii) = 0iii) The chip flow direction lies on the plane 0.

Fig. 4.10 shows the cutting forces for the case of orthogonal system of the first kind.An orthogonal two-dimensional system of second kind can be obtained by choosing

and in such a manner that either P x or P y can be made zero.For the orthogonal system of second kind either

i) "P y" is made zero by having = 0 andii) = 90 when two dimensional force system is

R = 2 x2z PP + ... . . . . 1.14.3 Fig. 4.11 shows the disposition of cutting forces in plane orthogonal turning with = 0and = 90.


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However out of all the above cases shown in fig 4.10 4.11 and 4.12 the cuttingin the first two cases is "non free" or 'restricted" type where the auxiliary cutting edge is

also active in causing deviation of chip flow direction from the orthogonal plane.The contribution of auxiliary cutting edge is to deviate P xy from the orthogonal plane. This deviation is small & neglected if the depth of cut is very large compared tofeed, such process is called "Restricted Orthogonal cutting.

However during cutting of a thin pipe with a cutting edge whose lengthis considered to be very large compared to the width of cut, a "pure" orthogonal cut of first or second kind could be obtained. The principal schemes of metal cutting shall be

based on pure orthogonal cutting from which schemes for oblique or other continuousand intermittent cutting processes like drilling, milling, etc., can be derived by similarly

principles.

FIG1.22 FORCES IN METAL CUTTING

1) The chip behaves as a free body in stable equilibrium under the action of twol i d lli l f i R & R


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equal, opposite and collinear resultant forces viz. R & R.2) The tool edge is sharp.3) The work material suffers deformation across a thin shear plane.4) This is no side spread (or the deformation is two-dimensional).5) There is uniform distribution of normal & shear forces on the shear plane &6) The work material is rigid, perfectly plastic (or behaves like ideal plastic)7) As, (shear plane area). T s (shear stress) & "B" (Friction angle), are constant &are independent of shear angle ' 'Forces on the chip (Merchants Analysis, theory)

From the concept of chip formation and measuring force F t and F f with a cuttingtool dynamometer, Merchant was able to build up a picture of forces acting in theregion of cutting which give rise to plastic deformation and sliding of the chip down thetool rake face.See fig 30.16(a)The forces exerted by the work piece on the chip areFc - Compressive force on the shear plane.Fs - Shear force on the shear plane.The force exerted by the tool on the chips are

N - Normal force at the rake face of tool.F - Frictional force along the rake face of tool.

The forces acting on the tool and measured by dynamometer areFt - tangential or cutting forceFf - feed force

Angle is tool rake angle, is shear plane angle and is the angle of friction


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Fc = AO + OEFc = F f. cos + F t. sin


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c f t

In AGB, GBA = 180 90 - = 90 - Hence ABD = 90 - (90 - ) = 90 - 90 + = -

Now Ft = BD

From ABD )(cosR)(or AB

Ft)(or BD =

Thus F t = R. cos ( - ) 30.24Ft = R. cos ( - ) 30.25

Also from ABE )(cosR Fs +=

Now from Eqs. (30.24) and 30.26)

)cos(R )cos(R

F

F

s

t

+=

or F t = F s.)cos(

)cos(+

.30.27

Assumption MadeThe above derived relations (eqs) are based upon the following assumptions

1. The tool is perfectly sharp and it does not make any flank contact with the job.

2. The cutting velocity remains constant.3. A continuous chip with no built up edge is produced4. The chip does not flow to either side5. Chip shears continuously across the shear plane as the shear stress reaches the

value of shear flow stress.The earlier discussed theoretical analysis of mechanics of metal is of use only if thevalue of the shear angle is known. Machining is an unconstrained process and theshear angle (or chip thickness) has no obvious value as, for example, has the exitthickness in rolling.Apart from the time involved in determining the magnitude of the shear angleexperimentally, the understanding of the machining process is clearly incomplete if one

cannot formulate a satisfactory criterion for the orientation of the shear plane.1. Theory of Ernst and Merchant According to this hypothesis, the shear plane orientates itself so that

(a) the work done in cutting is a minimum, or (b) the maximum shear stress occurs on the shear plane.

1 = . cos - .No from eq tion (30 36) nd (30 37)


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Now from equation (30.36) and (30.37)

)(cos)(cos

sin

AF 1s1 +

= (30.38)

Eq. (30.38) may be differentiated w.r.t. and equated to zero to find the value of shear angle, for which F 1 is a minimum.

).0(zero)(cos.sin

)(sin)(cos.cos).(cosA

dFd

221s1 =

+++=

or cos .cos ( + - ) sin ( + - ) = 0or cos ( + + - ) = 0

cos (2 + + ) = 02 + - =

2

(30.39)

or )(21

4224=+=

Shear angle, )(21

4= (30.40)

-Merchant found that the above theory agreed well with experimental results obtainedwhen cutting synthetic plastics but agreed poorly with experimental results obtained for steel machined with a sintered carbide tool.-It should be noted that in differentiating equation (30.38) with respect to , it wasassumed that A 1, and should be independent of . On reconsidering theseassumptions, Merchant decided to include in a new theory the relationship.

sss k o += (30.41)

1.16 Power and energy Relationship:The power or the total energy per unit time or the rate of energy consumption is the

product of cutting speed "V" and cutting force F c i.e. E = F c x V, K.g. mm/min.

The energy consumed during cutting process is primarily utilized at the shear plane, where plastic deformation takes place and at chip tool interface where frictionresists the flow of chip. The total energy per unit time (E) is approximately equal to thesum of shear energy (E s), Friction energy (E f ) and negligible amount of energy required

Similarly specific shear energy (e s) & specific friction energy (e f ) can be defined by thefollowing relations.


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g

eS = E S/b.t.v. = F S . VS/b.t.v. = F S . cos /b.t.cos ( -), Kg/mm 2 andef = E f /b.t.v. = F S . VS/b.t.v. = F/b.t c ,Kg/mm 2

SOLVED PROBLEMS :

Example :1) In an orthogonal cutting operation, following date have been observed :

Uncut chip thickness, t = 0.125 mm.chip thickness, tc = 0.250 mmWidth of cut, b = 6,500 mm.

V = 100 m/min.Rake angle, = 10 0 Cutting force, F z = 70 Kg.Trust force, Ft = 25 kg.

Determine : Shear angle, the friction angle, shear and normal stress on shear plane,

shear strain, shear strain rate, cutting power, specific shear energy, friction energy,cutting energy.

Solution : (i) Shear angle :

(ii) Friction angle

(iii) Shear and normal stress on shear plane

Thus, shear rate = 103.75/0.026 = 3938.755 -1 (vi) Cutting power


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(vi) Cutting power E = F e V/4500 = 70.000/4500 = 1.55 H.P.

vii) Specific shear energy E' s

viii) Specific cutting energy 'e' = F c x c/b.t.v.(70)/(6x5x0.125) = 86.15 kg/mm 2

ix) Specific friction energy = E - e s = 86.15 - 63.65 = 22.5 kg/mm 2

Example 2 : During machining of a C-30 steel with 0-10-6-7-8-80-0.5 mm (ORS)

shaped tungsten carbide tool, the following observations, have been made, depth of cut,d = 2 mm, feed f = 0.2 mm/rev. speed V = 200 m/min. chip thickness t c 0.40 mm.Calculate shear angle width of chipSolution :d = 2mm, 0 p = 80 0, tc = 0.40, V = 10

Now, thickness of uncut chip t = f.sin 0 P = 0.197 mmChip thickness ratio, r = t/t c = 0.49Shear angle, 0 = tan -1 (r.cos v/(1-r sin v) = 23.82 0 Width of Chip, b = d/sin 0 P = 2/Sin 80 = 2.03 mmExample 3. During machining of C-20, steel with a triple carbide cutting tool 0-8-7-10-70-1mm(ORS) shape the following data was obtained.

Feed = 0.18 mm/rev., Depth of cut = 2.0 mm.Cutting speed = 120 mpm, Chip thickness = 0.4 mm.Determine chip reduction coefficient & shear angle.

Solution : = 8, = 70Uncut chip. thickness = sin

= 0.18 sin 70 = 0.169 mm.chip reduction coefficient k =

= 0.42 Now = tan-1 ( r.cos / ( 1 - r sin ), = 23.98 0 Example 4 : In orthogonal turning process the feed is 0.25 mm/rev. at 50 rpm. The

thickness of chip removed is 0.5 mm.(a) What is the cip thickness ratio ?(b) If the wok diameter is 50 mm before the cut is taken what is the approximate lengthof chip removed in the minute. Assume a continuous chip is produced in process.Solution : Uncut chip thickness, t = f = 0.25 mm & ct c = 0.5 mm

.reduction coefficient) ?9. What are the various methods of estimating cutting ratio ?


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10.What is shear angle ? How it can be measured ?11. Prove that

tan = r cos /(1 - r sin ) Where, = Shear angle.r = Cutting ratio. = Rake angle.

12. Prove thatVc = Vsin /cos ( - ) and V s = Vcos /cos ( - )Where V, V c, V s are cutting, chip & shear velocities respt.

"" is shear angle & is rake angle.13. Prove that shear strain " " in orthogonal cutting is given by = tan ( - ) +cos, where is the shear angle and is the rake angle.

14. How is the thickness & width of undeformed chip estimated in turning operation15. What is metal removal rate ? What is specific metal removal rate ? How can

"MRR" be increased ?16. What is meant by the orthogonal cutting system of first & second kind ?

Illustrate with neat sketchs.17. What are the components of resultant force in an oblique cutting operation ?18. What are the assumptions of Merchant's theory ?19 Prove that = /4 + /2 - /2 where is shear angle, is rake angle & is

friction angle.20. What is the modified Merchant's theory of metal cutting ?21. What is meant by power consumed in metal cutting ? What are it's various

components ?22. What is total specific energy, specific shear energy, and specific friction energy23. How can the resultant force in orthogonal cutting be estimated by graphical

method ?24. What is metal cutting ? What are the basic requirements for metal cutting ?

How are the metal cutting processes classified.25. What is the effect of setting up of the cutting edge on rake & clearance angle ?26. In an orthogonal cutting operation the following data is obtained.

1) Cutting force = 180 Kg.2) Feed force = 100 Kg.3) Chip thickness ratio = 0.32 Kg.

Find graphically or otherwise shear force on shear plane, normal force on shear plane,Frictional force, Normal force, and resultant Force.

, , ,velocity, shear strain in chip, cutting power and specific cutting power.

0


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29. A tool making an orthogonal cut has a rake angle of - 10 0. The feed is 0.10 mm,the width of cut 6.5 mm. the speed 160 mpm, and a dynamometer measures the cuttingforce to be 180 kg and normal thrust force to be 140 kg. A high speed photographshows a shear angle of 20 0. Estimate,

(a) Chip thickness (b) coefficient of friction. (c) Shear and normal stress on shear plane (d) shearing strain, (e) H.P. to shear the metal (f) H.P. lost in friction.

-000-

.Thus, study of tool wear is important from standpoint of satisfactory performance &economics. However it is very difficult to find out exact cause and nature of tool wear,h h b i l & d d i l k i


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the phenomenon being very complex & dependent on many aspects, viz, tool work pair,environment, temperature of interfaces etc.

2.2. Wear Mechanism or Causes:

Tool wear causes the tool to lose its original shape. So that in time the toolceases to cut efficiently or even fails completely. After a certain degree of wear, thetool has to be resharpened for further use. The following basic causes, which canoperate singly or in various combinations, produce tool wear.

2.2.1 Attrition Wear (Adhesive Wear) :

At low cutting speeds the flow of material past the cutting edge is irregular or less laminar & contact between the two becomes less continuous due to built up edgeformation. Under such condition fragments of tool are torn intermittently from the toolsurface. This phenomenon is called Attrition or Adhesion. This wear progresses,slowly in continuous cutting, but rapidly in interrupted cutting or in cutting wherevibrations are severe due to lack of rigidity. As the cutting speed is increased, the flowof metal becomes uniform & attrition disappears. Such wear is prominent in cuttingwith carbides at low cutting speeds where B.U.E. is likely to form.

2.2.2. Diffusion Wear:

Diffusion wear occurs because of the diffusion of metal and carbon atoms fromtool surface into work material & chips. It is due to high temperature and pressuresdeveloped at the contact surfaces in metal cutting & rapid flow of chip on tool surfaces.The rate of diffusion wear depends upon the metallurgical relationship between the tool& work material. It is one of the major causes of wear and is of special significance inthe case of carbide tools diffusion is a phenomenon strongly dependent upontemperature. For example diffusion rate is approximately doubled for an increment of the order of 20 0 C. in the case of machining steels with HSS tools.

2.2.3. Abrasive Wear:

The abrasive action of the work on tool is basically due to two principal effects.

. process but it accelerates other wear processes, which reduce life of the tool. Thedeformation leads to sudden failure of the tool by fracture or localized heating. The

f l ti d f ti i i it lf i di ti f th t i g f th


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occurrence of plastic deformation is in itself an indication of the overstressing of thetool material.

2.2.5 Fatigue Wear:

When two surfaces slide in contact with each other under pressure, asperities onone surface interlock with those on other. Due to the frictional stresses compressivestress is produced on one side of each interlocking asperity and tensile stress on theother side. After given pair of asperities have moved over or through each other, theabove stresses are relieved. New pair of asperities is, however, soon formed and thestress cycle is repeated. Thus the material of the hard metal near the surface undergoescyclic stress. This phenomenon causes surface cracks, which ultimately combine withone another & lead to crumbing of the hard metal. Further the hard metal may also besubjected to variable thermal stress owing to temperature changes brought about bycutting fluid, chip breakage & variable dimensions of cut, again contributing to fatiguewear.

2.2.6. Electrochemical Effect:

This type of wear may occur when ions are passed between the tool & work piece-causing (an oxidation of the tool surface & consequent) breakdown of the toolmaterial in the region of the chip-tool interface. It has been argued that sincesufficiently high temperatures exist on the chip tool interface, a thermoelectric E.M.F. isset up in the closed circuit due to the formation of a hot junction at chip tool interface

between dissimilar tool & work materials. This current may assist the wear process at

In tension

In compression

In compression

In tensionStress distribution around theinterlocking asperities

Fatigue wear


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2.3. Types (Geometry) of Tool Wear:

The progressive wear of cutting tools can take two forms:i) Wear on the tool flank characterized by the formation of wear land as result of thenewly cut surface rubbing against tool flank; andii) Tool wear on rake face characterized by the formation of a crater or a depression, asa result of chip flowing over the tool rake face.

2.3.1. Flank Wear:

These wear produces wear lands on the side & end flanks of the tool on accountof the rubbing action. In the beginning, the tool is sharp & the wear land on the flank has zero width. However very soon, the wear land develops & grows in size on accountof abrasion adhesion & shear.

A typical case of flank wear development is shown in fig. 2.3.a. This figure can be divided into their definite regions A, B & C. In the region A, the wear grows rapidlywithin a short period of time because during the initial contact of sharp cutting edgewith work piece; the peaks of the micro-unevenness at the cutting edge are rapidly

broken away. In the region C, the wear rate is rapid and may lead to catastrophic failureof the tool. In general it has been found that the most economical wear land at which toremove the tool & re sharpen is just before the start of rapidly increasing portion of the


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The wear land on flank will not be generally uniform along the entire cutting length of cutting edge. Depending on machining conditions, the following types of wear lands of combinations of these are generally observed.

1. Excessive wear at the nose end of the flank (fig. 2.4a) is brought about by plasticdeformation, which reduces relief in the area, thus increasing the rate of wear.This can also be brought about if the crater on the rake face breaks through thenose area.

2. Irregularities in the wear along the whole cutting edge length due to minutechipping or attrition of the cutting edge (Fig. 2.4 b).

3. Excessive wear at the line of depth of cut (Fig.2.4 c). This can be either due tothe work hardened surface by the previous cut or heat treat scales or by abrasivematerials on the work-piece.


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2.3.2 Crater Wear:It occurs on the rake face of the tool in the form of a pit called as crater. The

crater is formed at some distance from the cutting edge. As the cutting speed isincreased the tendency of the cutting tool to fail by cratering is increased. The tool chipinterface temperature increases with cutting speed & at these higher temperatures therate of material removed from the tool increases. The careful measurements haveshown that the location of maximum cratering & maximum chip tool interfacialtemperature coincide with each other. It may there fore be assumed that cratering is atemperature dependent phenomenon caused by diffusion & adhesion etc. fig.2.5 showshow the radius of curvature 'Rc', the depth of crater 'dc', the width of crater and thedistance of start of crater from the tool tip 'a', change with time. The crater significantlyreduces the strength of the tool & may lead to its total failure.

R ca b Rake face

dc

Tool

The time for which a cutting edge or a cutting tool can be usefully employed

without regrinding ( eq HSS) or replacement (eg. Throw away carbides tip) is calledthe tool life. It is not economical to continue to use the tool beyond its useful life. This


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CR

CROSS SECTION OF AA B A B

FLANK WEAR LAND

Dc

ELEVATION

middle portion

hf hf max

flank

A

nose portion

HT

rear portion

yis because increased bluntness of cutting edge causes increase in cutting forces & as aresult tool temperature also increases. Consequently affecting the dimensional accuracy& quality of machined surface, ultimately leading to rejection. Also, the rate of flank wear-after certain critical value increases rapidly. The progress of crater wear is also of similar nature. Continued use of worn out tool would ultimately cause catastrophicfailure or total loss of tool & even damage of the component. If tool is ground or replaced prior to catastrophic failure; the volume of material ground off the tool(therefore the regrinding cost) would not be excessive. Hence certain tool failurecriteria have been devised to specify maximum wear of the tool that can be tolerated

before regrinding or changing it. The tool failure criteria (or tool life criteria) can beclassified as direct & indirect.

2.4.1. Direct criteria:

These depend upon measurement of tool wear or direct visual examination of cutting edge (fig.2.6)

FIG 2.6 WIDTH OF WEAR LAND AND MAXIMUM DEPTH OF CRATER

c . . per revolution. Also Opitz & weber has suggested the ratio dc/hc value between 0.2 &0.4 as tool failure criteria.iii) Limiting extent of chipping & crack formation:


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) g pp gFaulty cutting conditions may lead to appearance of fine cracks near the cutting

edge shortly after the tool is put to operation. This can be detected by visualexamination. The situation may be remedied by correcting the cutting conditions suchas selection of a tougher tool material, a more rigid machine; a stiffer tool material,

proper tool angles, proper machining parameters such as speed, feed & depth of cut etc.

2.4.2 Indirect criteria:

These depends upon the measurement of effects produced by tool wear andchipping etc.

1) Limiting value of surface roughness : The roughness of a machined piece increasesin proportion to the damage suffered by the cutting edge & unevenness of the flank wear. Monitoring of surface roughness can help to keep a control on limiting value of wear land. But surface roughness measurement requires costlier equipments thanmeasurement of width of wear land, which can be measured by a microscope.

2) Limiting value of change in machined dimensions: In this method, the dimensionsof each machined component are measured. When the tool is new, the dimensionalaccuracy is satisfactory & deteriorates as the tool progressively wears out. When thedimensional accuracy falls below a prescribed level, the tool is said to have failed.

3) Limiting value of increase in cutting forces: With increase in wear the tool forcesincreases (However, cutting forces tend to decrease some - what with increase in crater wear on account of increase in effective rake angle). A tool dynamometer or a power meter can therefore be used to monitor changes in cutting forces or rate of power consumption. When their increase exceeds predetermined amount, the tool life is saidto have been exhausted.

4) Limiting value of Volume of metal removed: If the cutting conditions are keptconstant (eg feed, speed; depth of cut) the progress of tool wear is directly proportionalto actual machining time or volume of metal removed. Thus limiting value of volumeof metal removed can be related to limiting value of width of wear land on flank & can,therefore be used as tool failure criterion.

The tool life equation is an empirical relationship between the tool life and one

or more variables of cutting process, e.g. cutting speed (V), feed (f), and depth of cut (d)etc. The most famous tool life equation is due to F.W. Taylor. On the basis of


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V

T

experimental work, Taylor showed the tool life 'T' and cutting speed 'V' is related toeach other as follows.

V. T n = C . . . . . . (eq n. 2.5.1)

Where the constant 'n' is called the tool life exponent and the parameter 'C' is known asTaylors constant. Making T = 1 in the above equation, we find that C = cutting speedfor 1 min tool life. The constants n & C depends upon the tool and work materials, feedand depth of cut, type of coolant and tool geometry etc. Equation 2.5.1 can be writtenas

LogV + n Log T = log C or log T = (1/n) log C - (1/n) log V . . . . . (eqn 2.5.2)

Cutting speed - Tool life curves can be graphically expressed as shown in fig.2.7.From the graph it can be seen that 'n' is the negative inverse slope of the curve and C isthe intercept velocity at T = 1. The following values may be taken for 'n'.

n = 0.1 to 0.15 for HSS tools= 0.2 to 0.4 for carbide tools= 0.5 to 0.6 for ceramic tools.

The equation 2.5.1 can be generalized or modified to include the effects of feedand depth of cut one such relationship is of the form


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VTnf n1dn2 = C 1 . . .. (Eq n .2.5.3)

Where the exponent n, n1, n2 and constant C1 depend upon tool and work materials,tool geometry and type of coolants etc.Following comments can be made from the tool life equation.

(1) Smaller the values of exponent, n, (as in HSS) the steeper is the slope of log V - log T line and more is the sensitivity of tool life to changes in cutting speed. Thusthe ideal tool material is one, which can be used at any cutting speed without affectingtool life. i.e. when n=1. From this point of view ceramics are superior to carbides andHSS.

(2) The larger the value of 'C' the greater is the tool life and superior is the toolmaterial.

(3) Even under constant cutting conditions values of n and 'C' are found tovary widely.

(4) In some cases, the tool life criterion changes with change in the predominantwear mechanism at different cutting speeds. For example in cutting steel with carbidetools, the predominant mechanism changes from adhesion wear at lower cutting speedsto diffusion wear at higher cutting speeds. Consequently the tool life criterion has to bechanged from limiting width of flank wear land to limiting depth of crater wear. Sincethe values of exponent are different for different wear mechanisms the log T - log V graph has two straight-line segments as shown in fig 2.7. (C).

2.6 VARIABLES (FACTORS) AFFECTING TOOL LIFE:

The various variables, which affect the tool life, are as under -

1. Tool material2. Work material3. Process variables - speed, feed, depth of cut4. Tool geometry5. Cutting fluid6. Vibration behavior of machine tool work system

. better tool life than HSS. Hence tool life is dependant on type of tool material.

2. Work material:


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The properties of the work material that tend to increase the tool life are asfollows, (a) softness (or lack of hardness) to reduce cutting forces, cutting temperature& abrasive wear, (b) absence of abrasive component such as slag inclusions, surfacescale & sand, (c) presence of desirable additives like lead to act as boundary lubricantsand sulphur to reduce cutting forces & temperatures by acting as stress raiser, and (d)lack of work hardening tendency that tend to reduce cutting forces and temperatures andalso abrasive wear and (e) occurrence of favorable microstructure, e.g. presence of spheroidized pearlite instead of lamellar pearlite in high carbon steel improves tool life.Similarly in cast irons, a structure that contains large amount of free graphite & ferriteleads to greater tool life than one, which contains free iron carbide.

3. Process variables: (Speed, feed, depth of cut):

The cumulative effect of speed, feed & depth of cut can be seen from themodified Taylors tool life equation. Increase in any one of the above reduces the toollife, but cutting speed has more impact on tool life followed by feed & depth of cut.Tool life is a direct function of temperature. At higher feed, the cutting force per unitarea of chip tool contact on rake face & work tool contact on flank face is increasedthere by increasing the temperature and hence wear rate. Similarly, at higher depth of cut, the area of chip tool contact is increased roughly in proportion to change in depth of cut (such is not the case with feed change where the chip tool contact area changes bylarger proportion than change in depth of cut), increasing the temperature &consequently the wear rate.

4. Tool geometry:

Rake angles, cutting edge angles, and relief angles & nose radius affect the toollife by varying degree.

I) The cutting forces, tool temperatures & tool wear decrease with increase in rakeangle (fig.2.8 a) consequently tool life improves when rake angles are increased.However larger rake angles make the cutting edge sharper reducing the mechanicalstrength & making the tool liable to chipping. Therefore there is an optimum rake angleassociated with every tool work pair.

T o o l

l i

rake


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Effective Rake Angle

(a)Effective rake angle versus tool life

Positiverake

Negativerake

Relief AngleRelief angle

EFFECT OF RELIEF ANGLE ON TOOL LIFE

smaller relief anglelarger relief angle

f

hf

f

toolhf

tool

T(C)

ii) Large relief angle increases volume of wear required to reach a particular widthof flank wear land as seen from fig. 2.8 (b) and also reduces the tendency of rubbing

between flank & work piece surface, there by increasing the tool life. However, on theother hand, larger the relief angle smaller is the mechanical strength of cutting edge &more liable the tool is to chipping fracture. Thus there is maximum tool life for optimum relief angle as seen for fig.2.8 (c).

,

friction at chip tool interface. Therefore the cutting temperatures are decreased & theuse of cutting fluid in the tool materials with low value of hot hardness (e.g.) showsappreciable increase in tool life. However in carbides & oxides, which have high valueof hot hardness the cutting fluid has negligible effect on tool forces or tool life


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of hot hardness, the cutting fluid has negligible effect on tool forces or tool life.

6. Vibration Behavior of Machine tool Work System: -

If the machine is not properly designed, if the work piece is long and thin or if the tool overhang is excessive, chatter may occur during cutting. It is known thatchatter may cause fatigue failure or calas tropic failure of tool due to mechanical shock.

2.7. Machinability: -

Machinability is the property of material to be machined, which governs thecase or the difficulty with which it can be machined under a given set of conditions. Inspite of the efforts made by the number of investigators, so far, there has been no exactquantitative definition of Machinability. It is due to large number factors involved &their complexity in metal cutting process viz. forces & power, tool life, surface finishetc. These are dependent upon number of variable such as work material, cuttingconditions, M/C tool rigidity tool geometry. Due to this, it is impossible to combinethese factors so as to give a suitable definition for Machinability. It is of a considerableeconomic importance for production engineer to know in advance the Machinability of

work material so that he can its processing in an efficient manner.

2.7.1 Criteria of Machinability: -

The case of machining different materials can be compared in terms of variouscriteria based on tool life values, cutting forces surface finish. Some other criteria, eg.ease of chip disposal & operator safety can also be employed.

1. Tool Life Criterion: -

Tool life is usually the most important of the three main criteria used for

assessing machinability. This is due to the fact that the tool life can be convenientlyexpressed in terms of cutting speed (which being the direct function of cutting speedwhen all other variables are kept constantly. Thus the cutting speed for producing a

predetermined value of tool life, termed as specific cutting speed, could be assessed on basis of comparison of machinability of materials. For example if "Vs" is the cutting

. - -

Mg. alloys - 600-2000

2. Cutting Force Criteria: -


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This criterion is important, where it is necessary to limit values of cutting forcein keeping with rigidity of machine tool & to avoid vibration in machining. If thecutting force is high consequently the power consumption is also high, a larger machinetool may be required, thus increasing the overhead cost and unit production cost. Thespecific cutting energy of a given material, defined as cutting power required for removing a unit volume of material in unit time, is often considered as index for machinability of a given work material. The larger the specific cutting energy, i.e.

higher the cutting forces induced under a set of cutting conditions during the machiningof a material, the lower is its machinability index.

3. Surface Finish Criteria: -

According to this criterion, the work material will have better machinability if under a given cutting conditions, it takes on a better surface finish (Polish or is lessrough) than the other. This criterion is used in situations where surface finish is thecause of rejection of machined parts. In such case the first two criteria may not behelpful, because in spite of particular work material permitting the use of higher cuttingspeeds without an excessive number of tool changes or power consumption, satisfactory

finish may not be achieved.

4. Other Criteria: -

Several other criteria have been put forward to assess machinability of differentwork materials. Prominent among these are (a) Temperature developed at chip toolinterface, (b) depth of hole cut out in a given time by a standard drill that rotates at astandard speed and specified downward thrust (penetration test), and (c) depth of cut

produced by a power hacksaw on a standard bar of given material under standardsawing conditions of speed and downward pressure (sawing test). Even physical

properties like hardness, tensile strength, shearing strain per unit shear stress in plastic

range, and different combination physical, properties ( eg. hardness, specific weight,ductility etc.) have been correlated with tool life in order to use them as machinabilitymeasures.

In spite of availability of several machinability criteria, a wholly satisfactoryunit of machinability has still not been found. For example, if different tool materials,different cutting conditions or different operations are used to assess the relativemachinability for the same set of work materials, different machinability ratings are


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machinability for the same set of work materials, different machinability ratings areobtained. Therefore the tables of machinability ratings are used only for generalguidance during process planning.

2.7.3. Variables affecting machinability: -

Machinability is influenced by variables pertaining to machine tool, cutting tool,cutting conditions or work material. These variables are listed as under.

A) Machine Variables: -1. Capacity of machine: (Power, torque - accuracy of machine)2. Rigidity of machine & work holding devices.

B) Tool Variables: -1. Tool material (HSS, Carbide, Ceramics etc)2. Tool geometry (Tool angles, radii, type etc)3. Nature of tool engagement with work.

(Continuous of intermittent, entrance & exit conditions)C) Cutting Conditions: -

1. Cutting speed, 2. Feed, 3. Depth of cutD) Work material variables: -

1. Hardness, 2. Tensile Shength, 3. Chemical Composition,4. Microstructure, 5. Degree of cold work, 6. Shape & dimensions of work,7. Rigidity of work piece, 8. Strain hardenability.

2.8. SURFACE FINISH: (ROUGHNESS)

The quality a machined surface is characterized by accuracy of manufacture inaspect to the dimensions specified by the designer. Every machining process leaves itsevidence on the surface that has been machined. This evidence is in the form of finelyspaced micro-irregularities left by the cutting tool. Each kind of tool leaves its own

pattern that can be identified. This pattern in called surface finish or surface roughness.

Whenever two machined surface come in contact with each other, the quality of themating surfaces plays an important role in performance & wear of the mating parts.Some degree of roughness, which may be extremely small, is always present on anysurface.

disappears. This reduces the height of micro irregularities. Further increase in cuttingspeed reduces surface roughness. The absence of built up edge formation in curve 'B'for machining of high alloy steels, non-famous metals & cast iron shows decrease insurface roughness from beginning (i.e. no rise in roughness as in curve 'A' is observed)


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A

( M I C R O N S )

(a)Effect of cutting speed

v3v1 v2m/min v

B

H

g g g ( g )

b) Feed: Increase in feed deteriorates surface, finish. It is observed that a rate of feed(f) in the range 0.12 mm/rev. to 0.15 mm/rev. has a negligible effect on the height of

irregularities but further increase in feed rate increases surface roughness Fig.2.10 (b)shows effect of feed for different coolants on height of surface irregularity.c) Depth of Cut: Increase in depth of cut deteriorates the surface finish.

2) Tool geometry: - The effect tool geometry on surface finish is as follows:a) Nose radius: Increase in nose radius improves surface finish. There is appreciableimprovement in surface finish up to 0.3 mm. nose radius. Higher value of nose radius

produces less improvement in surface finish & on the other hand it causes chatter duringmachining.b) Rake angle: Increase in rake angle of tool improves surface finish.c) Relief angle: Increase in relief angle adversely affects surface finish but the effect, as

a whole is small.d) Side cutting edge angle: An increase in side cutting edge angle improves surfacefinish largely because of the reduced chip thickness but the degree of improvementdepends upon the rigidity of the work & its mounting.

0 . 0 7 50

( M I C

R O N S )

H ( R M S )

1 0 0

0 . 1 4

0 . 1 0

0 . 2 f

activated

kerosene

emulsion

refined kerosenedry

(b) Effect of feed

.

2.8.2. Surface finish Terminology:

a) Roughness: This includes surface irregularities resulting due to the various


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) g g gmanufacturing processes. These irregularities combine to form surface texture.Other definition Roughness (Primary texture): relatively fined-spaced surfaceirregularities. On surfaces produced by machining and abrasive operations, theirregularities produced by cutting action of tool edges and abrasive grains and by thefeed of the machine tool are roughness. Roughness may be considered as beingsuperposed on a wavy surface.b) Roughness Height: It is the height of the irregularities with reference to an average

line: The value of roughness height can be expressed in two waysi) Arithmetic average value (Ra)ii) Root mean Square (rms) value.The arithmetic of average height value is given by

R a = h ta - yi/n . . . . .. eq n 2.8.1Where

Y = Vertical distance of the profile from centerline.n = Total no. of vertical measurements.

hrms =n

y 2i c) Roughness Width: It is the distance parallel to the nominal surface betweensuccessive peaks or ridges, which constitute the predominant pattern of the roughness.d) Roughness width cut off: It is the greatest spacing of the repetitive surfaceirregularities to be included in the measurement of average roughness height. It isalways greater than the roughness width in order to obtain the total roughness heightrating.e) Lay: It is the direction of predominant surface pattern produced & reflects themachining method.Lay: The direction of predominant surface pattern. A typical surface is shown in Fig.14.9. The identification of the macrogeometrical of microgeometrical errors is based on

(l/h) ratio as shown in Fig.14.10. When (l 1/h1) > 1000, the deviation ismacrogeometrical denoting out-of-roundness, taper or barrerl form. When500>(l 2/h2)150 the deviations denote waviness. When (l 3/h3) 50, themicrogeometrical deviations are characteristic of surface roughness.

- ,

of greater spacing than the roughness. On machined surfaces such irregularities mayresult from machine and work deflections, vibrations, etc.h) Flaws: Cracks, scratches & ridges are called flaws. They are not regularly recurring& are imperfections outside the regular pattern of surface texture.


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VALLEYS

ROUGHNESS SPACING

PEAKS

CENTRE LINE

other definition FLAWS: Irregularities, which occur at one place or at relativelyinfrequent intervals in the surface, e.g., a scratch, ridge, hole, crack, etc.

2.8.2 Ideal Surface Roughness & natural surface Roughness:

The surface finish is dependent upon tool geometry, feed rate & other irregularities or contributing factors in the machining process. Such roughness is callednatural surface roughness. If the other contributing factors are entirely eliminated themthe surface roughness produced will depend entirely on tool geometry such surfaceroughness is called ideal surface roughness.

Where "h" is the height of the geometry of the surface roughness. Now from geometry:

f h = Eqn 2 8 4


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ECEA

f H

d

TOOL

SCEA(a)

CE

h/2

f

h = ----------------------------------- . . . . . . . Eqn 2.8.4tan "SCEA" + Cot "ECEA"

from Eqn 2.8.4 & 2.8.3. we haveRa = f/4.(tan "SCEA" + Cot "ECEA")

or Ra = f/4.(tan "Cs" + Cot "Ce") . . . . . . . Eqn. 2.8.5.For rounded corner tool, the ideal value of surface roughness is

hCLA = f 2

/8R and Ra = f 2

/18 3 R . . . . . . . . . Eqn 2.8.5.Where "R" is nose radius. Thus, a change in the rate of feed is more importantthan a change in nose radius and depth out has no effect on the surface geometry.

symbols as shown in the following table.

Table: Surface roughness values.

f


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M

a c h i n i n g c o s t C

grinding

CLA

linear variation

finish turning

1/h

A

hononingD

E

Sr. No. Type of operation Representation as per IS 3073 - 1967Symbol Relative values in microns

1. Rough machining 8 - 252. Fine machining 1.6 - 83. Grinding 0.025 - 1.64. Lapping < 0.025

2.8.5. Cost of surface finishThe cost of machining of any component is greatly dependent on the quality of

surface roughness required on the part. The cost increased, if the specified roughnesson the part decreases. In other words smoother the required surface, the greater is thecost involved. The various reasons for this increase are given below.

1. High feed & depth of cut cannot be used.2. Frequent tool changes are required as the tool failure criteria permits lower

width of wear land.3. The set up requires more of effective clamping methods, special tools or inserts

and more frequent closer inspections.

ELECTRONIC GRINDINGGRINDING

BORING, TURNINGELECTROCHEMICAL

ELECTRON BEAM

LASER


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LAPPINGSUPER FINISHING

HONING

POLISHINGELECTROPOLISH

GRINDING

Fig 2.15 surface finish produced by machining operations

In the figure the inverse of surface roughness (i.e. surface finish) is plottedagainst machining cost. The increase in cost with reduction in roughness is gradual inrough turning (up to points). But for finish turning it increases exponentially. Thesurface roughness produced by various machining operations is given in 2.14 (a)

2.8 Cutting Fluids : -

2.9.1 Functions of cutting Fluid: -

During the metal cutting heat is generated. The heat is generated due to the

plastic deformation of the chips, friction between the rake face & chips & friction between clearance face & work piece. The heat generated increases the temperature of both cutting tool & work piece. The increase in temperature of tool decreases hardness& hence wear resistance & life. The effect is more pronounced in H.S.S. tools than incarbide tools. Fig.2.16 (a) shows zones of heat generation. In shear zone maximumheat in generated (i.e. 60 %). In chip - tool zone, 30% heat is generated, whereas intool-work zone minimum heat (i.e. 10%) is generated. In fig. (2.1% (b) it can be seenthat at higher cutting speeds the distribution of heat in chips, tools & work piece isnearly in the ratio 80:10:10


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Moreover, apart from heat generation the chips also create problems in metalcutting such as bad surface finish due to abrasion of chips to the tool or finished work

piece. On these aspects the functions of cutting fluid can be summarized as follows.1) Lubrication at chip tool interfaces & work tool interface & hence preventing welding

between contact surfaces.2) Cooling action or heat dissipation to avoid loss of hardness & wear resistance &

hence improve tool life.3) Flushing of chip to protect surface finish & improve tool life. Cutting fluids, besidesfulfilling the above functions should have following properties.1) It should be stable & non-foaming.


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Fig 2.17 Effects of cutting fluid in metal cutting

2.9.2 Types of Cutting Fluids: -The detailed classification of cutting fluids is shown in 2.16. They are broadly

classified into: (i) Neat oil, (ii) Soluble oil, (iii) Synthetic coolant, & (iv) Gaseous fluid.Besides this a true solution i.e. water mixed with corrosion inhibitors, is also used buthas poor lubricating action.

Fig.2.13: Family of cutting fluids

a) Neat Oil: - is mineral oil, vegetable oil or blends of these oil straight mineral oil i.e.without additives is not suitable for high loading & speed & hence used only in lightmachining of non ferrous metals like Aluminum & magnesium. Compounded oils (i.e.mineral oil blended with fatty acids like lard oil, oleic acid, sperm oil etc) have high

-

Soluble oil is blend of mineral oil, emulsifying agents & couplingagents. Emulsion is formed by mixing soluble oil with water in the ratio 1:10 to 1:40for general machining & up to 1:80 for grinding. Conventional emulsions are milky inappearance. Translucent emulsions are made by reducing oil droplet size of emulsionwith a high ratio of emulsifier to oil. They have higher film strength & better


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with a high ratio of emulsifier to oil. They have higher film strength & better anticorrosive properties than opaque emulsions. They also provide improved toollubrication due to fine dispersion of oil globules. Heavy-duty soluble oils i.e. solubleoils blended with fatty acids & EP additives, can withstand heavy cutting pressure &temperature. Heavy-duty soluble oils are used in rich concentration with ratios rangingfrom 1:5 to 1:15.

c) Synthetic Coolants: - They are usually non petroleum products, though sometimes asmall percentage of mineral oil is added, many chemical agents blended in water formsynthetic coolants. Basically they are coolants, though some are also lubricants. Theyare used in grinding than in other operations & they are mixed in the ratio 1:50 to 1:250

parts of water. The main problem associated with use of synthetic coolant is itscompatibility with the other lubricants, seals metal parts. Because they are chemicallyactive, they react easily with paints, other metal parts, etc. Before deciding on the useof these coolants, this aspect should be thoroughly looked into comparative

performance of these three fluids is given in the Table 2.1.

d) Gaseous Fluid: The high cost restricts use of gaseous fluid. Mist is commonly used

gaseous fluid. Mostly compressed air is used to atomize the coolant & carry it to the point of cutting in the form of mist. Gases like carbon dioxide, Freon, and helium areused for special applications.

Table: Comparative Performance of Various Types of Cutting Fluids

Sr.No.

Soluble oil Neat oil Synthetic coolant

1. Promotes better cooling-hence effective intransporting the heat

Promotes better lubrication-henceeffective in reducing

heat generation.

Poor lubricating propertylike soluble oil, but better cooling property than soluble

oil2. Prone to bacteriacontamination andobnoxious odour-so

Good service life,no unpleasant odour

Life better than soluble oil

. uscep e o r s o o

dermatitis

uscep e o r s

of skin cancer

ome c em ca s may e

toxic8. Tramp oil like hydraulic

and other lubricating oilwill reduce the life

Little effect withtramp oil

Little effect with tramp oil

9 Low initial cost High initial cost Low initial cost


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9. Low initial cost High initial cost Low initial cost

2.9.3. Selection of cutting fluids: -The major factors which govern the selection of cutting fluids are (i) machining

process, (ii) the cutting tool materials, & (iii) the work piece materials. Besides thesefactors, compatibility with the machine, performance requirements, human interaction& economy must also he look into? Specifying a particular fluid as most suitable for a

specific application is almost impossible. When more than one operation is performedon the machine this problem becomes more difficult. Neat oil may be better choice thansoluble oil when the cutting fluid is likely to see into the drive or control system. Thefactors influencing selection of cutting fluid are shown in fig.2.17.

Factors influencing the selection of cutting fluids

5. Reaming 3.4 4.3 4 4 6 6

6. Drilling-deep-hole 5 5 5 5 9 9

7. Honing 1.2 1.2 2 2 6 6


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8. Automats 1.2 1.2 2.1 24 6.1 6.1

9.Drilling,

Boring, turning andMilling

7 7 9.7 9.7 7 8

10. Sawing 7 7 7 7 7 7

11. Grinding 7.8 7.8 8.7 8.7 7 7

(1) Mild sulphurized fatty oils. (2) Mild sulphc chlorinated oils. (3) Medium sulphurized fatty oils. (4) High sulphur fattychlorinated oils. (5) High chlorinated mild sulphurized fatty oils. (6) Fatty mineral oils. (7) Soluble oil (8) Translucent soluble oil(9) Heavy-duty soluble oil.

SOLVED PROBLEMS:Example 1:

Tool life test has been carried at two different cutting speeds. The width of flank wear land has been measured after every 10 minutes at each cutting speed. The widthof flank wears land at V = 150 m/min were noted as 0.10, 0.15, 0.22, 0.25, 0.30, and 0.35, min after 10,20,30,40,50,60,70 minutes of cutting respectively. Similarly at thecutting speed "V" = 200 m/min the values of width of flank wear land were noted as0.20, 0.25, 0.30, 0.40, 0.50, 0.85 mm for cutting time of 10,20,30,40,50,60,70 minutesrespectively. If the criterion of tool failure is selected, as 0.30 mm, width of flank wear land, calculate values of "n" & "c" of the Taylor's tool life equation.

Solution:As the tool failure criterion is 0.3 mm, the value of cutting time corresponding to

this value will be the tool life at that cutting speed. From the above data we have,Tool life T1 = 60 min. for V1 = 150 m/min., and

T2 = 30 min. For V2 = 200 m/min.Therefore putting these values in the Taylor's T.L. equation we have.

(V1) (T1) n = C i.e., (150) (60) n = C . . . . . . . . . . (i)And (V2) (T2) 2 = C i.e., (200) (30) n = C . . . . . . . . . . (ii)Dividing equation (i) by equation (ii), we have.

(150/200) x (60/30) n = 1, or 2n = 1.33n log 2 = log 1.33 i.e.

Let m = No. of components produced between consecutive tool changes,V = Cutting speed, (m/min)T = Tool life (min.)f = Feed rate, (mm/rev)L = Length of cut, (mm)


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D = Work piece diameter (mm)T m = time to produce one work piece = L/f.N.

We express the tool life equation asVTn f n1 = C . . . . . . . . . . (i)

Where n, n1 & C are constants.V = .D.N./1000 . . . . . . . . . (ii)

m = T/Tm = ( T.f.N.)/LTherefore, T = (m.L)/f.N. . . . . . . . . (iii)Substituting (ii) & (iii) in (i), we get

( . D.N./1000) x ( m.L./f.N.) n x f n1 = C N1-n , mn , f n1-n = (1000 C.L. -n/. D.)or N 1-n , mn. f n1-n = C1 . . . . . . . . . . (iv)Substituting values of N,f, & m in (iv) we get.

(250) 1-n x (311) n x (0.100) n1-n = Cl . . . . . . . (v)(250) 1-n x (249) n x (0.125) n1-n = Cl . . . . . . . (vi)

and (300) 1-n x (144) n x (0.125) n1-n = Cl .. . . . . . (vii)Dividing (vii) by (vi) we get, (250/300) 1-n x (249/144) n = l

or (1-n) log (250/300) + n log (249/144) = 0n = 0.25 Similarly by dividing (v) by (vi) we get n1 = 0.500Substituting values of n & n1 in (v) we get C1 = 148.4

Putting N = 350, f = 0.150, n = 0.25 & n1 = 0.500 in (iv) we get m = 75

Example 3 : In a laboratory test on turning operation, the following data have been

recorded,S.No. V, m/min f mm/rev. d, mm T, min.

1 100 0.10 2.0 1202. 130 0.10 2.0 50

3. 100 0.12 3.0 704. 100 0.12 3.0 65

.

(65/70) x (3/2)n2

= 1 or n2 log [3/2] = log 1.66, Therefore n 2 = 0.38Putting values of n in (ii) & (iii) & dividing (iii) by (ii). We get(100/130) x (70/50) n x (0.12/0.10) n1 = 1n1 log (0.12/0.10) = log (1.18), Therefore n1 = 0.885


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Putting these values in eq n (i) we get C = 72.13. Now at V = 120m/minf=0.20 mm/rev, & d=2mm, we get T = 8.77 min.

Example 4: The tool life equation for a turning operation is given as 36.5 = V. T 0.13 , f 0.60 .60 d 0.3 . A 60 min. tool life was obtained using the following cutting conditions; V =

40 m/min, f = 0.25 mm/rev. d=2.0 mm. Calculate the effect on tool life if speed, feed,and depth of cut are together increased by 25% and also if they are increased individually by 25%

Solution: Putting V = m/min, f = 0.25 mm & d = 2 mm we get,40 x (60) 0.13 x (0.25) 0.60 36.50

Now of values of V, f, & d are individually increased by 25% i.e. by Selecting.V = 40 x 1.25 = 50 m/min.f = 0.25 x 1.25 = 0.3125 mm/rev. and

d = 2.0 x 1.25 = 2.5 mm

Tool life T = [36.50/50 x (0.25)0.3

x (2.0)0.3]

1/0.13 = 10.78 min.Similarly, for, V = 40m/min, f=0.3125 mm/rev & d = 2.0 mm, weget T = 21.42 minand for V = 40 m/min, f = 0.25 mm/rev & d = 2.0 mm, we get T = 35.85 min.

If all these values are increased together i.e. for V = 50m/min, f=0.3125 mm/rev.and d = 2.5 mm, we get T = 1.26 min.

It can be seen that the impact of increase in cutting speed on tool life ismaximum.

Example 5. The tool life for H.S.S. tool is expressed by the relation V.T. 0.14 = C1 and for tungsten carbide is VT 0.2 = C2. It at a speed of 24m/min tool life for both the tools

is 128 minutes; compare the life of tools at a speed of 30 m/min.Solution

For HSS tool, V x T 0.14 = C1 . . . . . . . . . . . . (i)i.e. 24 x (128) 0.2 = C2

, .

tools or the tool life of HSS tool T1 = (29/38) x T2 or 0.7 x T2 at V = 30m/min. Example 6: The cutting speed and tool life relationship for a tool is given by V.T. 0.2 = C. During machining, 18 mm bar on a lathe at a cutting speed of 110 m/min. the tool life is found to be 60 minutes. Calculate spindle speed to give a tool life of 5 hours. If length

f i 50 h i h i i i d h i


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of cut per component is 50 mm, what is the cutting time per piece and how many piecescan be produced between tool changes at a feed of 0.15 mm/rev?

Solution: From tool life relationships.C = (110) x (60) 0.2 = 249.47

Therefore for T = 60 x 5 minutes, we haveV = 249.47/(300) 0.2 = 79.72 m/min.Hence spindle speed N = 1000 C/ x D = 1410 rpm.

Cutting time/piece = L/f. N.= 50 x 60/(0.15 x 1410) = 0.24 min.

Therefore, number of components produced in 5 hours tool life50 x 60 x 6/0.24 = 1249

QUESTIONS1.What are the various causes or mechanisms of tool wear? Explain.

2.Explain what is meant by the terms - (Explain with neat sketches)1.Attrition wears2.Diffusion Wear 3.Abrasive Wear 4.Fatigue Wear 5.Plastic deformation6.Electrochemical wear & chemical wear.

3.Differentiate between "flank wear" and 'Crater wear'4.What is the difference between 'Abrasion wear' & 'Attrition wear?5.Why tool wear is important in metal cutting?

6.What is meant by 'tool life? What is its significance to an engineer who is interestedin productivity?7.What are the various tool life or tool failure criteria?8.State the general from of Taylor's tool life equation and also give the modifiedTaylor's tool life equation.

.20. What are the sources of best generation in metal cutting? How is the heat distributedin chip, tool & work piece?21. What are the basic functions of cutting fluids?22. What are the effects of cutting fluid in metal cutting?23. How are the cutting fluids classified?24 Comp re the perform nce of ne t oil sol ble oil nd s nthetic cool nt


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24. Compare the performance of neat oil, soluble oil and synthetic coolant.25. What are the various factors influencing selection of cutting fluids?26. Which coolants would you suggest for turning of following metals with H.S.S. tool?i) Mild steel, 2) Aluminum, 3) Copper,27. Recommend a cutting fluid for broaching or i) Low Carbon Steel, ii) Stainless steel, iii) Aluminum alloy, iv) Copper.28. State the factors responsible for surface roughness?29. Explain the effect of cutting speed, feed and depth of cut on surface finish.30. What is the effect of tool geometry on surface finish?31. What is surface finish? Describe the elements of surface texture.32. Explain the following terms.i) Roughness, ii) Roughness height, iii) Roughness width, iv) Lay, v) Waviness, vi)Flame,33. What is ideal surface roughness & natural surface roughness?34. What is h CLA or R-value? Derive an expression for the theoretical roughness of asurface machined with single point tool.35. What is the effect of surface roughness on cost of production?

36. In general give a broad range of surface roughness that can be achieved in variousmachining operations.37. How is surface finish represented on drawing?38. The tool life for HSS tool is expressed by relation V.T. 1/7 C1 and for W-C tool isexpressed as V.T.= C. If at a speed of 24 mpm the tool life is 120 minutes, compare thelife of tool at a speed of 30 mpm.39. For a particular machining set up the tool life equation is given as V.T. 0.2 = C.During machining of an 18 mm diameter bar on a lathe at a cutting speed of 110 m/min,the life of tool is found to the 60 minutes. Calculate spindle speed to give tool life of 5hours. If a length of 50 mm per component is machined what is the cutting time per

piece and how many pieces can be produced between tool changes? The feed used is

0.15 mm/revolution 40. During the tool life test for HSS tool used to cut special diesteel the following observations were recorded. Use the above values to calculate theconstants of tool life equation.

CHAPTER-IIICUTTING TOOL MATERIALS AND NOMENCLATURE

3.1 Introduction:Before 1990 machining had been carried out with plain carbon steel tools or air


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Before 1990, machining had been carried out with plain carbon steel tools or air hardening alloy steel known as Mushet steel. The machining speeds were low due tofailure of cutting tool to maintain hardness at high temperature generated due to highcutting speeds. A big break-through has been achieved when F.W. Taylor developed atool material (called high speed steel), which can operate at relatively higher cuttingspeeds. The cast cobalt base alloy tools appeared on the scene in 1915. They havehigher hot hardness; wear resistance & fewer tendencies to form BUE. In 1926,Germans introduced a new material called sintered tungsten carbide tools, manufactured

by the powder metallurgical technique. They can operate at comparatively very highcutting speeds than HSS tools. Addition of carbides of titanium, tantalum & niobium to

basic tungsten carbides enhanced range of application of the carbides. Later on coatingof titanium carbides has been applied on cemented tungsten carbide increasing the

permissible speed by about 50-80% over the conventional carbides. Ceramic tools (or cemented oxide tools) were produced & developed in 1960's. The permissible speedwith cemented oxides is two three times higher than those with cemented carbides.However, the brittleness restricts their use to only on rigid or (more of less vibrationfree) machine tools. In 1971, Union carbide (USA) developed a new material UCON,consisting of columbium, tungsten & titanium, permitting increase in cutting speedcompared to carbides. In 1972, a new tool material called cubic boron nitride,(Borozon) with hardness next to diamond has been developed. Consequently,

permissible machining speed with these tools is five to eight times that of carbides &can he used to cut hardest materials. Polycrystalline diamond bonded to carbide basehas been used as a tool material.


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3.2. Desirable Properties of Cutting tool materials:The three most important desirable properties of tool materials are wear

resistance, hot hardness & Toughness. This is because during machining, the tool tip issubject to relatively high temperature, intense normal pressure, frictional stress at chiptool contact & work tool contact, impact & vibrations.

Apart from these three important properties some other properties such asthermal conductivity for removal of heat from chip tool interface, low coefficient of thermal expansion, weldability, hardenability, dimensional stability, freedom fromdistortion after heat treatment etc are also desired.

a) Wear Resistance: It is also called as abrasive wear resistance (or AWR). Wear resistance, is necessary to enable the cutting tool to retain its shape & cutting efficiency.Wear may be caused due to various mechanisms such as attritition or adhesion,diffusion, abrasion etc. Wear resistance refers to the ability of tool material to retain itssharpness & shape for sufficiently long time while machining a given material at arelatively low cutting speed. Wear resistance & toughness are two independent

properties. The gain of one results in loss of other.

b) Hot hardness: It is also called as high temperature stability (or HTS). It is ameasure of ability of tool material to retain its hardness at elevated temperaturesdeveloped at chip-tool interface. Higher the value of hot hardness higher will he the

d) Other desirable properties.

The following other properties are also desired by tool materials.i) High Thermal conductivity for quick removal of heat from chip tool interface.ii) Low coefficient of friction for reduction of heat generated due to friction at contactsurface.


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surface.iii) Low coefficient of thermal expansion for reduction of effect of thermal stresses &thermal shocks on material.iv) Resistance to distortion after heat treatment.v) Hardenability to achieve hardness at slower cooling rates during hardening.vi) Weldability for ease of wedding of tool with shank vii) Grindability for ease of grinding after tool failure.viii) Dimensional stability etc.

The basic desirable properties namely RBF, HTS & AWR do not always haveconcurrent high attributes in any given tool material. Generally, if a material is mademore refractory to gain "HTS", it becomes more brittle with low "RBF". If it becomesmore abrasion resistant, it also becomes brittle fig.3.31 (a) shows relative "HTS","AWR", & "RBF" for two arbitrary materials in a ternary plot. Such a plot may be used

3.3. Carbon Tool Steels:

AWR

TERNARY PLOT


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These are plain carbon tool steels to which no appreciable amounts of alloyingelements are added. The carbon percentage varies from 0.6 to 1.5% in these steels.Increase in carbon percentage increase toughness and shock resistance whereas decreasein carbon content increases hardness & abrasion resistance. Very small quantities of Silicon, manganese, chromium or vanadium are added for increasing the hardness andgrain refinement.

Carbon tool steel is broadly classified into two categories water hardening steel(W type) & oil hardening steel (0 types), depending on the quench

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tool engg notes by DVSHIRBHATE