17827991 Manufacturing Process2

Manufacturing Process-II

1. Theory of Metal cutting :

In olden days, materials could be shaped by a chipping process.

The early metal cutting work (about 1851) was mainly directed toward measuring the work required to remove a given volume of material in drilling.

In 1881, Mallock suggested correctly that the metal cutting process was basically one of shearing the work material to form the chip and emphasized the importance of the effect of friction on the cutting tool face as the chip was removed.

In 1906 Taylor investigated the effect of tool material and cutting conditions on tool life during roughing operations. One fundamental discovery made by Taylor was that the temperature existing at the tool cutting edge controlled the tool-wear.

In 1941, Ernst & Merchant published their paper dealing with the Mechanics of Metal Cutting.

After World war II, theory of metal cutting process took a tremendous importance in the day to day life.

Differential:-Metal cutting:-

Metal cutting commonly called machining produces a desired shape, size & finish on a rough block of w/p material with the help of a wedge shaped tool that is constrained to move relative to the work piece in such a way that a layer of metal is removed in the form of a chip.In metal cutting process, working motion is imparted to the w/p & cutting tool by the mechanisms of machine tool. So that the work and tool travel relative to each other & cut the w/p material in the form of shavings known as chips.

Difference between single point cutting tool and multipoint cutting tool.

A single point cutting tool has only one cutting edge. (Ex: Tools used din lathe) A multipoint cutting tool has a number of teeth or cutting edges don its periphery.

(Ex: Milling cutter)

Single point cutting tool Nomenclature:-

In order to perform cutting operation satisfactorily, various angles are ground on a tool bit. These angles are known as basic tool angles, & compose what is often termed the tool geometry.

Tool signature is sequence of numbers listing the various angles, in degrees and the size of the nose radius.

A typical tool signature standardized by the American Standards Association is given by.

Tool Signature:- (ASA)

Ex: (Degrees) 1/32 inchBack Rake angle - 10Side rake angle - 20End relief angle - 7Side relief angle - 6End cutting edge angle - 8Side cutting edge angle - 15Nose radius - (0.8mm)

Single – point cutting tool terms:-

(1) Tool bit:-

The term tool bit commonly is applied to small pieces of cutting tool material which are inserted in a tool holder in a manner that permits easy removal for regrinding (or) replacement.

(2) Shank:-The shank is the body portion of the tool.

(3) Face:-The face is the top surface of the tool upon which the chips bear as they are removed from the w/p & slide away.

(4) Nose radius:- The nose radius is the dimension of the round are which forms the nose of

the tool bit. For rough turning, a small nose-radius usually about 0.4mm (1/64”) – is

used.For finish turning, a radius from 0.4-1.6mm (1/64” to 1/61”) is used, depending upon the size of the tool.

A turning tool with a nose radius of 0.8mm (1/32”) will produce a satisfactory finish for general rough or finish turning.

Tool large a nose radius results in chatter.

(5) Base:-The base of the tool is that portion of the tool which bears against the supporting tool holder.

(6) Flank:-The flank of the tool is the surface adjacent to and just b below the cutting edge.

(7) Cutting edge:-The cutting edge is that part of the tool bit that does the actual cutting.

(8) Point:-The point of the tool includes that entire portion of the tool which is shaped to produce the tool face & the cutting edge.

(9) Back rake angle:-

The back rake angle is the slope from the tool point (front) toward the back (i.e., the shank) this is perpendicular to the axis of the work.

Variations in the back rake angle affect the direction of chip flow.

As the back rake angle increases (within limits) with other conditions remaining constant, tool life will increase slightly & cutting force required will decrease.

Generally, small rake angles are used for machining hard materials, while steeper rake angles are used for more ductile materials. Exceptions to this rule

include tools for brass, bronze, certain plastics and non-metals.

Back rake angles may vary from O0 to 35O0 for various applications.

The rake angle described above are called + ve rake angle. When no rake is provided don the tool, it is called zero rake When face slopes upwards, - ve rake angle.

(10) Side rake angle:- Side rake angle is between the tool face and a line which represents

the top of the ungrounded tool as it is viewed from the end. By providing a shearing action for chip removal, this angle enables

the tool to cut more freely. Variations in this angle affect the direction of chip flow. Side rake angles vary from O0 to 220 or more for different

applications.

(11) End Relief Angles:-

Relief angles are also known as clearance angles. The purpose of end relief angle is to prevent the end of the tool from rubbing on the work. If an end relief angle is too small, it will wear down, rubbing with w/p will start, the tool and the work can get too hot, and chatter marks or a smeared surface may show up on the work. An excessive relief angle reduces the strength of the tool. Relief angles are usually fairly small, from 3 to 100

(12) Side relief angle:- The purpose of side relief angle is to prevent the side which is at the cutting edge, from rubbing on the work. The side relief angle allows the tool cutting edge to penetrate into

the metal (w/p) & promotes free cutting by preventing the side flank of the tool from rubbing against the work.

(13) End cutting edge angle:- The purpose of this angle is to avoid rubbing between the edge of

the tool and the w/p. Excessive end cutting angle reduces tool strength. It may vary from 7o to 30o.

(14) Side cutting Edge angle:- This sis formed by the straight (side) cutting edge and the side of

the tool shank. Increasing side cutting edge angle te4nds to widen the thin chip

and influences the direction and chip flow. Angle may vary from 0o at 30o for machining various materials.

ORTHOGONAL & OBLIQUE CUTTING:-

ORTHOGONAL CUTTING

OBLIQUE CUTTING

Orthogonal cutting oblique cutting.

1. Orthogonal cutting is the simplest, as the tool cutting edge goes din a straight line through the material and the edge of the tool is set perpendicular to the cutting direction ( or to the tool- work motion)

2. Oblique cutting:1. Cutting edge is inclined, at an angle s (known as cutting edge dinclination)

to a line drawn at right angles to the direction of cutting.

With oblique cutting, the chip flows up the tool face in a direction forming an angle r (chip flow angle) with a line drawn on the face at right angles to the cutting.

Orthogonal cutting Oblique cutting3) The chip curls and flows straight up the tool and not side ways.

3) The chip flows side ways.

4) The width of the tool is more. 4) It may or may not.5) Heat developed per unit area due to friction along the tool-w/p interface is considerably more.

5) Heat developed per unit area is less.

Mechanism of chip formation:-

Consider tool to be stationary and the w/p moves to the right.The metal is severely compressed in the area in front of the cutting tool. When the stress in the w/p just ahead of the cutting tool reaches a valve

exceeding the ultimate strength of the metal, the particles will shear to form a chips element which moves up along the face of the work. Because of hardening process a continuous chip is formed having a highly compressed and burnished underside.- The plane along which the element shears is called the shear plane.- The deformation does not occur sharply across the shear plane, but it occurs along

a narrow band.

The structure begins elongating along the line AB, below the shear plane and continues to do to until it is completely deformed along the line CD above the shear plane.

The zone AB to CD is called the shear zone or primary deformation zone.

CHIP-THICKNESS RATIO AND SHEAR ANGLE

The outward flow of the metal causes the chip to thicker after separation from the parent metal. Metal prior to being cut is much longer than the chip which is removed.

Let t1 = Chip thickness before cuttingLet t2 = Chip thickness after cutting

Chip thickness ratio, r = t1/t2

The chip thickness ratio is always less than unity. K = chip reduction co-efficient = 1 / r .When metal is cut there is no change in the volume of the metal cut.

t1b1l1=t2b2l2

where b1 = width of cut l1 = length of chip before cutting b2 = width of chip after cutting l2 = length of chip after cutting

It is observed that, b1= b2

t1l1 = t2l2

Chip thickness ratio can be obtained by measuring l1 and l2

From triangle ABC, we have

BC ----- = Sinφ AB

BC t1

AB = ----- = ----- ---------------- (1) Sinφ Sinφ

From triangle ABD

BD ------ = Sin (90 – φ + α) = cos (φ – α) AB

T2 ----- = cos (φ – α) AB

T2 => AB = --------------- -------------- (2) Cos (φ – α)

From 1 and 2, we get

t 1 t2 ------- = -------- = γ Sin φ cos (φ – α)

Sin φγ = ------------------------------ = 1 Cos φ cos α + sin φ sin α

r cos φ cos α r sin φ sin α ----------------- + ------------------- = 1 sin φ sin φ

r cos α r sin α = 1 --------- + tan φ

r cos α ------------- = 1 - r sinα tanφ

t 1 sin φ ----- = ----------- = γt 2 cos( φ – α )

r cos α tan φ = -------------- 1 – r sinα

Where Φ = shear angle Α = Rake angle

The closer the shear angle approaches 450 , the better the machinability is said to be .

Velocity Relationships:- Where Vc = cutting velocity Vs = shear velocity Vf = velocity of chip-flow up the tool face.

Cos α Vs = Vc -------------- Cos ( φ – α )

Sin φ Vf = Vc ------------ Cos( φ – α )

Vf = Vc . r From principle of kinematics that the relative velocity of two bodies (tool and chip) is equal to the vector difference btw their velocities relative to the reference body Vc = Vs + Vf

Types of chip:-

The form and dimension of a chip in metal machining indicate the nature and quality of a particular machining process, but the type of chip formed in greatly influenced by the properties of the material cut and various cutting conditions. The usage of ductile material are move in machining process rather than brittle material.

There are 4 types of chips:-

(1) The discontinuous or segmental form.(2) The continuous or ribbon type(3) The continuous with built-up edge(BUE)

(4) In homogenous chip

(1) Discontinuous Chip:- The work material undergoes severe strain during the formation of chip, and if it is brittle, fracture will occur under these conditions the chip is segmented and the results in the formation of discontinuous chip. Discontinuous chips are produced when machining brittle material such as cast iron or cast brass. Such chips may also be produced when machining ductile materials at very low speeds and high feeds.

(2) Continuous Chips:- Continuous chips are produced when material ahead of the tool continuous deforms without fracture and flows off the toll face in the form of a ribbon. They are formed while machining most ductile materials at normal cutting speeds. This type of chip is associated with low friction between the chip and the toll face and is the most desirable from of chip.

(3) Continuous chip with built up edge:- This type of chip is similar to the continuous chip except that a built up edge is formed on the nose of the toll. The built-up edge is formed owing to the action of welding of the chip material on the tool face. Presence of this welded material on the friction, leading to the building up of the edge layer by layer. Such chips normally occur while cutting ductile material with high-speed tools at low cutting speeds. Chips with BUE are undesirable as they result in higher power consumption, poor surface finish and higher tool wear.

(4) Non-homogenous chips:- Are produced owing to non-uniform strain in the material during chip formation and are characterized by notices on the free side of the chip, while the side adjoining the tool face is smooth. The shear deformation which occurs during chip formation causes the temperature on the shear plane to rise, which in turn may decrease the strength of the material and causes further strain if the material is a poor conductor. Thus a large strain is developed at the point of initial strain. As the cutting process is continued , a new shear plane will develop at same distance from the first shear plane and the deformation shifts it this point. Non-homogenous chip are produced while machining some steels and titanium alloys at medium cutting speeds.

MERCHANT CIRCLE OF FORCES.

Merchant suggested a compact and easiest way of representing the various forces inside a circle having the vector F as diameter.

Fig shows merchant circle diagram which is convenient to determine the relation between the various forces and angles.

Where Fs = Shear force Fn = Normal force α = Rake angle of tool φ = Shear angle γ = friction angle of the tool face Fh = cutting force Fv = fees force F = Resultant force of FH and FV N = force exerted by the chip, acting normal to the tool face.

From the diagram , we get The shear force on rake angle , P = AQ + QB AQ = FH sinα = AQ + CD CD = FV cosα

P = FH sinα + FV cosα

…………………………(1)

Normal force on rake angle ,

N = CB = QD + PQ – PD

……………………………….(2)

The shear force , FS = AH –HK AH = Fc cosφ = AH – PE PE = Fv sinφ

…………………………..(3)

The normal force,

Fn = CK = CE + EK CE = Fv cosφ = CE + PH PH = FH sinφ …………………….(4)

From triangle APC ,

FH = AC cos ( γ – α ) FH = F cos ( γ – α ) ……………………………………(5)

From triangle APC ,

F2 = FH2 + FV

2 From triangle ACK

FS = F cos(φ + γ – α ) ………………………………..(6)

FH F cos( γ – α )----------- = --------------------------- FS F cos ( φ + γ – α )

FH cos ( γ – α ) ---------- = --------------------------- ……………………..(7) FS cos ( φ + γ – α )

N = FH cosα - FV sinα

FS = FC cosφ – FV sinφ

Fn = FV cos φ + FH sinφ

WKT, as the chip slides over the tool face under pressure, therefore the kinetic co-efficient of friction (μ) may be expressed as

P μ = ----- = tan γ from eqn 1 and 2 we get N

…….(8)

……….(9)

From right angle triangle ABC,we have,

………………………….(10)

OP ----- = tan (PAC) from triangle APC AP

…………………………………….(11)

Stress in the chip :- A chip is supposed to experience both the stress and strain during machining because it is produced as s result of plastic deformation of the method.

FH sinα + FV cosα Tan γ = ------------------------------ FH cosα - FV sinα

FH tanα + FV

μ = tan γ = ------------------------- FH – FV + tanα

P μ = tanγ = ---------- N

FV ------- = tan ( γ - α ) FH

FS

Mean shear stress , fs = -------- …….N/mm2

AS

Fn

Normal stress , fn = -------- ……..N/mm2

As

Where FS = Shear force Fn = Normal force AS = Area of shear plane A1 b1t1

AS = ---------- = -------- Sin φ sin φ

Workdone in cutting :-

The total workdone in cutting is equal to the sum of the work done in shearing the metal and the work done in overcoming the friction .

Power required in cutting :-

= cutting force * cutting speed

Earnst – Merchant theory :-

States that during cutting the metal shear should occur in that direction in which the energy requirement is minimum .

FS sin φ fs = ----------- b1t1

P = FH * V ……..Watt

The other assumptions made are :-1. The behavior of the metal being machined is like that of an ideal plastic.2. At the shear plane the shear stress is maximum,is constant and independent of

shear angle( φ) .

These assumptions deduced the following relationship

…………………… ( *)

where φ = shear angle , γ = friction angle , α = Rake angle

Derivation :

WKT , FS = fs * AS Bt1 fs FS = -------- ………………… (1) Sinφ

From eqn (7) ..we get

FS cos( γ – α ) FH = --------------------- ……………………(2) Cos ( φ + γ – α )

Sub eqn (1) in eqn (2) .we get

Bt1 fs cos ( γ – α ) 1

FH = -------------------------- [ ----------------------------- ] Sin φ cos ( φ + γ – α )

αAnd power comsumption ,

Bt1 fs cos ( γ – α )P = FH * V = V * --------------------------- ……………………….(3) Sin φ cos ( φ + γ – α )

Л γ αΦ = ----- - ----- + ----- 4 2 2

During a cutting operation , φ takes a value such that the least amount of energy is consumed or P is minimum. v, b,t1 and α are given (const) and if we assume that fs and γ do not change when φ varies and that P is a function of φ and is of the form,

const P(φ) = -------------------------------- ………..(4) Sinφ cos(φ + γ – α )

P(φ) will be minimum when the denominator is maximum, then differentiating the denominator w.r.t φ and equating it to zero ,we get

Cosφ cos( φ+ γ –α ) – sin φ sin ( φ + γ – α ) = 0

cos (2φ + γ – α ) = 0

Л 2φ + γ – α = -----

2

or

..hence proved

(1) In an orthogonal cutting o/p ,the tool nomenclature has been given as.Thickness of undeformed chip is 0.2 mm .The thickness of the chip is formed to be 0.25mm.Calculate the shear angle .

Solution: t1 0.2 Chip thickness ratio , r = ---- = ----- = 0.8

T2 0.25

R cosα Shear angle ,tan φ = -------------- 1- r sinα

0.8 * cos 150

tan φ = ------------------ = 0.97 1- 0.8 sin 150

Л γ αΦ = ----- - ----- + ----- 4 2 2

Φ = 44.260

(2) In an orthogonal cutting o/p , the following data have been observed. Uncut chip thickness, t =0.127mm Width of cut, b= 6.140mm Cutting speed, v = 2.6m/sec Rake angle, α = 200 Cutting force, FC = 589 N Thrust force, Ft = 225 3 Chip thickness, tc = 0.226 mm

Determine (i) Shear angle (ii) Friction angle (iii) Shear Stress along shear plane (iv)Power required for cutting o/p (v)Chip velocity.

Solution : Given :

FH = FC = 589 N Fv =Ft = 225 N T2 = tc = 0.226mm T1=t = 0.127mm b = 6.140mm ; α = 200 ; v = 2.6m/sec

(i) Shear angle : R cosα Tanφ = ------------ 1- r sinα 0.561 * cos 200

tanφ = -------------------- 1 – 0.561 * sin 200

=> φ = tan-1 ( 0.652 ) Shear angle,

FH sin α + FV cos α (ii) tan γ = --------------------------- FH cos α – FV sin α

589 * sin 200 + 225 cos 200 412.88 = ---------------------------------------- = ---------------- 589 cos 200 – 225 sin 20 0 476.52

γ = tan-1 (0.866) = 40.900

friction angle ,

Φ = 33.110

γ = 40.900

(iii) Shear stress along the shear plane,

Fs Fs * sinφ fs = ---- = -------------- A2 A1

Where A1 = b1 *t1 WKT , FS = FH cosφ – Fv sinφ ( FH cosφ – Fv sinφ) sinφ Fs = --------------------------------- A1

FH cosφ sinφ – FV sin2φFs = ---------------------------------- B1t1

589* cos(33.110) * sin( 33.110) – 225 sin2(33.110)Fs = ----------------------------------------------------------------- 6.14 * 0.127

(iv) Power required for cutting o/p :-

P = FH * V = 589 * 2.6 = 1531.4 N.m/sec

(v) Chip velocity (vf):-

v sinφ vf = -------------- = v * r cos( φ – α )

vf = 2.6 * 0.561

(3) In an orthogonal cutting process, the following data were observed, t= depth of cut = 0.25mm FH = horizontal force = 113.5N

Fs = 259.50 N/mm2

P = 1.53 kW

Vf = 1.458 m/s

FV = Force component normal to FH = 110 r = Chip thickness ratio = 0.47 α = Rake angle = 200

b = width of cut = 4mm v = cutting of cut = 30m/min determine , friction angle, Shear plane angle, resultant cutting force,power

Tool failure :-

During the operation, cutting tool may fail due to one more of the following reasons.1. Thermal cracking and softening.2. Mechanical chipping.3. Gradual wear.

1. Thermal cracking and softening. Due to heat generated in the metal cutting process, the tool tip and the area closer to the cutting edge becomes very hot and if this heat crosses the high temp, at which the tool losses its hardness the tool material starts deforming plastically. This is said to have failed due to softening. The factors responsible for this are cutting speed, high feed rate, and excessive depth of cut, smaller nose radius and the choice of a wrong tool material.

2. Mechanical chipping:- The mechanical chipping of the nose and the cutting edge of the tool are commonly observed causes of tool failure. The factors responsible for this are too high cutting pressure. Mechanical impact, excessive wear, too high vibrations and chatter, weak tip and cutting edge etc

3. Gradual wear:- The following two types of wears are generally found to occur in cutting tools.

(b) Crater wear: - This type of wear takes place in a cutting its face, at a small distance from its cutting edge.

This type of wear takes place while machining ductile material like steel alloys, in which continuous chip is produced. The resultant feature of this type of wear is the formation of a crater or a depression at the tool-chip interface. Higher feeds and lack of cutting fluids increases the rate of crater wear.(c) Flank wear: - This type of wear occurs in the flank below the

cutting edge. It occurs due to abrasion btw the tool flank and the w/p excessive heat generated as a result of the same.

The magnitude of this wear mainly depends upon the relative harnesses of the w/p tool material at the time of cutting and also the extent of strain hardness of the chip.

MECHANISMS OF WEAR:-

1. Abrasive wear:- Hard particles on the underside of the sliding chip,which are harder than the tool material plough into the relatively, softer material of the tool face and remove metal particles by mechanical action.The material of the tool face is softened due to the high temperature. The hard particles present on the underside of the chip may be,a) Fragments of hard tool material.b) Broken pieces of built up edge, which are strain hardened.c) Extremely hard constituents, like carbides, oxides, scales, etc .present in the work material.

2. Attrition wear:- At relatively low cutting speeds, the flow of the material past the cutting edge is irregular and fewer streams lined. Sometimes bue may be formed and contact with the tool may not be continous.Under these conditions, Fragments of the tool are torn intermittently from the tool surface. This phenomenon is called attrition. This type of wear progresses slowly in the case of continuous cutting, but with interrupted cutting or where vibrations are severe due to lack of rigidity of the machine tool or uneven work surfaces, it leads to rapid destruction of the cutting edge. As the cutting speed is increased, the flow of metal becomes uniform and attrition disappears.

3. Diffusion wear:- This occurs because of the diffusion of metal and carbon atoms from the tool surface into the work material and the chips. Wear by diffusion is due to the high temperature and pr developed at the contact surfaces in metal cutting and rapid flow of the chip and the work surfaces past the tool. The rate of diffusion wear depends upon the metallurgical relationship btw the tool and the work material. It is one of the major causes of wear and is of special significance in case of carbide tools.

4. Plastic deformation:-

When high compressive stresses act on the tool rake face, the tool may be deformed downwards and this deformation takes place primarily in the nose area of the insert and reduces the relief angle. This is a deformation rather than a wear process, but it accelerates other wear processes which reduce the life of the tool. Deformation leads to the sudden failure of the tool by fracture or localized heating.

TOOL LIFE:- Te length of the period for which a tool can be used is called the tool life.When the wear reaches certain values the tool is not capable of further cutting unless it is resharpened .Tool life is the most important criterion for assessing the performance of a tool material,machinibility of work material and for determining cutting conditions. There are three common ways of expressing tool life.1. As time period in minutes btw two successive grindings.2. In terms of number of components machined btw 2 successive grindings.This

mode is commonly used when the tool operates continuously ,as in case of automatic machine.

3. In terms of the volume of material removed btw two successive grindings.This mode of expression is commonly used when the tool is primarily used for heavy stock removed.

Volume of metal removed per minute = Л D t f N …….mm3/min Where, D = dia of w/p in mm t= depth of cut in mm f = feed rate in mm/ rev N= no of revolutions of work/minIf ‘T’ be the time in minutes to tool failure ,then Total volume of metal removed to tool failure, = Л D t f N T mm3 ………….(1) WKT, cutting speed, V=Л D N/ 1000…m/min Or Л D N = V * 1000…………………..(2)

Sub eqn (2) in eqn (1) we get, Total vol of metal removed to tool failure = V * 1000 * t * f * T mm3

Tool life, TL in terms of the total volume of the metal removed to tool failure is given by.. TL = V * 1000 * t * f * T ….mm3

Factors affecting tool life:-

The life of a cutting tool is affected by the following factors.1. Cutting speed2. feed and depth of cut

3. Tool Geometry4. Tool material5. Work material6. Nature of cutting7. Rigidity of machine and work8. Use of cutting fluids

Out of all the above factors,the maximum effect on tool is of cutting speed. The tool life varies inversely as the cutting speed . i.e the higher the cutting speed the smaller the tool life. According to Taylor ,the relationship btw cutting speed and tool life is,

Speed α 1 / Tool life.

VTn = CWhere V = Cutting speed (m/min) T = Tool life ( minute) n = an exponent ,whose value largely depends on the material of the tool called tool life index. C= A constant called machining constant.

1. During the machining of low carbon steel with HSS cutting tool,the following observations were made..

Cutting speed/min-------40 , 50 Too life, minutes----------40, 10 Using the V-T retionship, find the tool life for a speed of 60m/min. Solution: V1= 40m/min, V2= 50m/min, V3 = 60m/min, T1 = 40min, T2 = 10min; T3 =?

Tool life eqn VTn =C

i.e V1T1n= V2T23n = V3T3

n

Consider, V1T1n = V2T2

n

40 (40) n = 50 (10) n

40/50 = (10/40) n

4n = 1.25 n ln = ln (1.25)

n = 0.16

From eqn 1 , V2T2n = V3T3n 50(10)0.16 = 60(T3)0.16

72.27/60 = T3 0.16

1.204 = T3 0.16

ln ( 1.204) = 0.16 ln (T3) T3 = 3.19 min

(2) While rough cutting at a speed of 30 mpm ,a cutting tool had a life of 60min .Calculate the tool life when for finishing vut.Assume ‘n’ as 0.125 for rough cut and 0.11 for the finish cut.

VTn = C

For rough cutting, 30 * (60)0.125 = C C = 50 For finishing cutting , 30(T)0.11 = 50 T0.11 = 1.666 =>

(3) While machining carbon steelby a tungsten-cobalt steel,it was observed that the tool life was observed that the tool life was 60min for a cutting speed of 50m/min.Determine the tool life for a cutting speed of 40m/min.Using taylor’s tool-life equation taking the index ‘3’ as 0.143.

V1T1n = V2T2

n

50 (60)0.143 = 40 ( T2)0.143 89.79 = 40 ( T2)0.143

T2 0.143 = 2.24

0.143 ln(T2) = ln (2.24)

(4) During the machining of law carbon steel with H33 cutting tool, the following observations were made

Cutting speed, m/min = 40 , 50 Tool life , min = 40 ,10

T = 98.87 min

T2 = 281.37 min

Derive the V-T relationship . Solution : Given V1= 40m/min ; V2 = 50m/min T1=40min ; T2 = 10min

V1T1n = V2T2

n = C V1T1

n = V2T2n

40(40)n= 50(10)n

(40/10)n = 50/40 4n = 1.25

n = 0.16 .

V1T1n = C

40 (40)0.16 = C C = 72.17

(5) A 75mm dia MSbar was turned at 220rpm to get tool life of 14min.The rpm was changed to 260 and tool life obtained was 10min .Find out the value of 3 and C.

Solution: D=75mm ; N1=220rpm ;T1=14min; N2=260rpm;T2=10min n=? C=? Л D N1

V1 = ----------- m/min 1000 Л * 75 * 220 V1 = ------------------ = 51.83m/min 1000Similarly

Л * 75 * 260 V2 = -------------------- = 61.25m/min 1000

V1T1n = V2T2

n

51.83 * (14)n = 61.25 (10)3

(14/10)n = (61.25/51.83)

(1.4)n = 1.18

n ln(1.4) = ln(1.18)

n = 0.47

V1T1n = C

51.83(14)0.47 = C

=> C = 180

HEAT GENERATION IN METAL CUTTING:- The temperature generated during the metal cutting controls the rate of tool wear, the practical cutting speed and the metal removed rate.

1. Primary deformation zone:- It is the region in which actual plastic deformation of the metal occurs during machining. Due to this deformation .heat is generated .A portion of heat is carried away by the chip, due to which its temperature is raised .The rest of the heat is retained by the work piece. It is known as primary deformation zone.

2. Secondary deformation zone (tool chip interface) As the chip slides upwards along the face of the tool friction occurs btw their surfaces, due to which heat is generated. A part this heat is carried the chip, which further raises the temp of the chip. This area is known as secondary deformation zone.

3. Tool- work piece interface :- This portion of tool flank which rubs against the work surface is another source of heat generation due to friction. This heat is also shared by the tool ,w/p and the coolant used. Is more pronounced when the tool is not sufficiently sharp.

Measurement of tool tip temperature :- Several methods have used for measuring temperature at a chip –tool interface Tool-work thermocouple is quite commonly used to measure the tool-chip interface temp.

The emf in the thermo-electric circuit depends only on the difference in temperature btw the hot and cold junction. Figure the thermo couple technique to measure the tool chip interface temp for a w/p fitted on a lathe in which the chip and tool junction constitutes the hot junction .A and B the cold junction remains at room temp .The milli voltmeter readings are converted into temperature by using standard tables.

Cutting Tool materials and cutting –fluids Cutting tool materials and their proper selection for a particular application are the most important factors in manufacturing operations. A cutting tool material must have certain characteristics I oeder to produce parts economically and with good quality . These characteristics are:- 1. Hot hardness - The hardness and strength of the tool should be maintained at high

temperatures encountered in metal cutting .2. toughness – The tool should be capable of withstanding the impact forces in the

tool due to interrupted cutting.3. Wear resistance -0 The tool should have an acceptance life before it is

resharpened .The tool should should not chemically react with the work material.4. Co – efficient of friction- The coefficient of friction ‘μ’ for tool material must be

low,so that the tool wear will be minimum and result in a good surface finish.5. Economy – Cost of thetool material selected should be minimum for economical

machining cost.6. It should be capable of withstanding sudden coolong effect of coolant used during

cutting.

TYPES OF CUTTING TOOL MATERIAL: - The following material are commonly used for manufacturing the cutting tools.1. High carbon steels.2. Alloy steels3. High speed steels4. Cemented carbides5. ceramics6. Stellite7. Diamond 8. Abrasives9. Cubic Boron Nitride10. Coated carbides.

1 .High Carbon Steels (Plain carbon steel) :-

These are steels having carbon content ranging from 0.8 to 1.5% . These are used for general applications like in turning, boring,shaping, etc.. for machining of ductile andsoft materials like mild steel. Aluminum ,copper etc.

Advantages –Fabrication is easy and easy to harden.Dis advantages- Suitable only for machining of ductile material at low cutting speeds.They are not able to withstand at high temperatures.

2. Ally steels :-These steels have carbon content upto 1% and alloying elements used are tungsten, vanadium molybdenum, and chromium. These elements impact certain properties like high hot hardness, toughness and resistance to wear and distortion.

Adv – 1. These work for medium cutting speeds.2. Used for harder materials.Dis adv- 1. They can retain hardness upto 300c only 2. Due to inclusion of alloying elements the cost increases.

3. High Speed steels-

The widely use material are high speed steels(HSS).These tools can operate at high cutting speed ,where temperature as high as 900c and can operate 2 to 3 times more speed than plain carbon steels.

There are 3 types HSS :-1. High tungsten steel :- It imparts higher hot hardness.2. High molybdenum Steel :- It retains sharp cutting edge.3. High cobalt steels:- It provides high wear resistance.

18-4-1 HSS :- High tungsten steel contains 18% tungsten , 4% chromium, 1% vanadium . These are efficient high speed steel.

Molybdenum HSS (6-4-2) :- It contains 6% Mo , 4% Cr, 2% V .They posses high toughness and cutting strength.Cobalt HSS :- These are known as super high speed steels due to high hot hardness and wear resistance at higher cutting speeds . It contains 15% cobalt, 10-20% tungsten ,2-4% Cr,2-4%V.

Adv:- high machining performance with longer tool life.Dis adv:- Very costly and difficult to fabricate.

4. Cemented carbides:- Carbides are formed by the mixture of tungsten, titanium with carbon .The carbides in powder form mixed with cobalt which acts as a binder .Then powder metallurgy process is applied for pressing and mixture is sintered at 550C .This mixture is pressed is applied for pressing and mixture is pressed at pressure from 1000kg/cm2 to 4200kg/cm2 into suitable blocks and then heated in hydrogen. The amount of cobalt used will regulate the toughness of the tool. Carbides tool have 82% tungsten carbide, 10% titanium and 8% cobalt.

Adv:- 1. Used for high cutting speeds.2. Very high hardness and wear resistance.Dis –adv-1. They are costly.Low toughness because of brittleness.

5.Ceramics :-

Ceramics tools are made by compacting aluminum oxide powder in a mould at about 280 kg/cm2 .The part is then sintered at 2200C and this method is known as cold pressing .This part is in the form of tool and is ready for use as a cutting tool material.These tool tips are fastened to the shanks.Adv- 1. high compressive strength.2. high hardness 3. high wear resistance and longer tool life.4. withstand stand upto 1200C temp.Dis adv-1. very brittle and not suitable for cutting under impact load.2. low thermal conductivity.

6. Diamond- The diamond is the hardest known material and can be run at cutting speeds about 50 times greater than HSS and at temperatures upto 1650C. This is used when good surface finish and dimensional accuracy are desired.

Adv- 1. It is very hard and has very low co-efficient of friction.2. Low thermal expansion and high heat conductivity.Dis adv-1. It is very costly2. Because of its harness, it is very difficult to produce the required shape(fabricate).Applications:-

Diamonds are used for cutting very hard materials such as glass, ceramics, abrasives and steels .It is also used for dressing the grinding wheel.

7.Abrasive:- These abrasives are mainly used for grinding harder material and where a superior finish is desrired in hardened material.In general two kinds of abrasive are used aluminum oxide and silicon carbide. The aluminium oxide abrasive are used for grinding all high tensile materials,where as silicon carbide abrasives are more suitable for low tensile materials and non-ferrous metals.

8.Stellites :- Stellites is the trade name of a non-ferrous cast alloy composed of cobalt ,chromium and tugsten.The rabge of elements in these alloys is 40-48% cobalt,30-35% chromium,12-19% tungsten,1.8 to 2.5 % carbon.They cannot be forged to shape ,but may be deposited directly on the tool shank in an oxy-acetylene flame.

Adv- 1.they have wear resistance.2.retain its hardness upto 1000C3.It can be used at high cutting speeds.Dis adv- It is brittle and cannot be used under impact machining conditions.

9. Cubic Boron Nitride(CBN) :- Next to diamond, CBN is the hardest material currently available. It consists of atoms of nitrogen and boron, produced by high pressure and high temperature processing. As cutting tool materials CBN is used in the polycrystalline form. It has much higher tensile strength as compared to diamond.CBN being chemically inert is used as a substitute for diamond for machining steel.

Adv- 1. CBN has high hardness and high thermal conductivity.2. The life of a CBN tool is 4 to 5 times higher than that of a diamond tool.Application-As a grinding wheel on HSS tools for machining high temperature alloys, titanium, nimonic stainless steel, stellites and chilled CI.

10.Coated Carbides- Coated carbides have a thin coating of TiC on all faces of the tip. The coating thickness is of the order of a few micros(0.0025 to 0.005).These tools resist the diffusion wear on the crater and give a tough shock resistance tool. Oxide coating- The diffusion of atoms btw the tool and chip material can be retorted by coating the tool surface of the carbide tools with oxides of aluminium and

zirconium.This considerably increases the tool life.Coated carbides are used for machining super alloys.

Cutting fluids;-

Cutting fluids ,sometimes referred to as lubricants coolants are liquids and gases applied to the tool and w/p tot assist on the cutting operations.

Purpose of cutting fluid (Functions)1. To cool the tool:-

Cooling the tool is necessary to prevent metallurgical damage and to assist in decreasing friction at the tool-chip interface and at the tool workpiece interface.

2. To Cool the w/p:-The role of the cutting fluid in cooling the w/p is to prevent its excessive thermal distortion.

3. To wash the chip away from the tool.4. To improve surface finish and tool life.5. Reduces force and energy consumptions.6. To cause chips break up into small part.7. To protect the finished surface from corrosion.Properties of cutting fluids:-

A cutting fluid should have the following properties

1. Good lubricating qualities to produce low-co-efficient of friction.2. High flash point so as to eliminate the hazard of fire.3. High heat absorption for readily absorbing the heat developed.4. Harmless to the skin of the operators.5. Non-corrosive to the work or the m/c.6. Low priced to minimize production cost.7. Low viscosity to permit free flow of the liquid.8. Harmless to the bearings.9. Should be chemically natural inert.10. Transparency so that the cutting action of the tool may be observed.

Types of cutting fluids:-

The types of cutting fluid to be used depends upon the work material and the characteristic of the machining process.

1. Water:-It is principally a coolant and not a lubricant water with alkali, salt or water soluble additive and little soap are sometimes used as a coolant. But water alone is objectionable for its corrosiveness.

2. Soluble Oils:-

These are emulsions composed of around 80% or more of water, soap and mineral oil. The soap acts as an emulsifying agent which break the oil into minute particles to disperse them throughout water. The water increases the cooling effect, and the oil provides the best lubricating properties and ensures freedom from rust.

3. Straight Oils:-These sare straight mineral oils (petroleum), Kerosine, low viscosity petroleum factions. Straightr fixed or fatty oils consisting of animal, vegitable, lard oil etc. They have both cooling and lubricating properties and are used in light machining operations.

4. Mixed oils:-This is a combination of straight mineral ands straight fatty oil. This makes an excellent lubricant and coolant for automatic screw m/c work and where accuracy and good surface finish are of prime importance .

(a) Chemical additive oil :- Straight oil or mixed oil when mixed with sulphur or chlorine

Is known as chemical additive oil.Sulphur and chlorine are used to increase boththe lubricating and cooling qualities of the various oils with which they are combined.

(b) Chemical compounds:-These consist mainly of sodium nitrate mixed with a high percentage of water. These compounds act as coolants particularly in grinding.

(c) Solid lubricants:-Stick waxes and bar soaps are sometimes used as a convenient means of applying lubrication to the cutting tool.

Selection of cutting fluid:-

Cutting fluid should be carefully chosen. The selection of particular type of cutting fluid depends on factors listed below.

1. Cutting speed 2. Feed rate 3. Depth of cut4. Cutting tool material5. W/p material6. Velocity of cutting fluid7. Expected tool life.8. Cost of the cutting fluid.9. Life of cutting fluid and loss of cutting fluid during operation.

Machinability:-The case with which a given material may be worked with a cutting tool is

machinability. Machinability depends on:

1. Chemical composition of w/p material.2. Micro structure3. Mechanical properties.4. Physical properties.

5. Cutting conditions.

Factors affecting machinability:-

Common machine variables affecting ease of cutting are.1. Cutting speed2. Dimensitions of the cut.3. Tool material 4. Tool form (angles, radii, etc.,)5. Cutting fluid6. Nature of engagement of tool with the work.

Common work material variables affecting ease to cutting:-

1. Hardness2. Tensile properties3. Chemical composition4. Microstructure5. Degree of cold work6. Strain hare ability7. Shape and dimensions of work.8. Rigidity of work piece.