Unit - 1.PrintOUT Docx

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Unit 1

Theory of Metal Cutting

Metal cutting process is basically shearing of work material and th

surface is removed in the form of chip

Metal cutting commonly called machining, produces a desired shape

size and finish on a rough block of work piece material with the help

of a wedge shaped tool that is constrained to move relative to th

work piece in such a way that a layer of metal is removed in th

form of a chip.

Chip formation in conventional machining process


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Elements of metal cutting

Cutting conditions are characterized mainly by such elements as

cutting speed, Feed, Depth of cut, undeformed chip cross-section

Cycle time and machining time.

Cutting speed is the distance traveled by the work surface per uni

time in reference to the cutting edge of the tool.

Cutting speed, v = TDN / 1000 m/min

Where D is the work piece diameter in mm and N is work piec

speed in RPM

Feed (s) is the movement of the tool cutting edge per revolution o

the work: in turning it is expressed in mm/revolution.

Depth of cut (t) is measured in a direction perpendicular to the work

axis and in straight turning in one pass, it is found from the relation

= (D d) / 2 mm, where D is original diameter of the work piece and

d is the diameter of machined work piece in mm.

The time required to machine one work piece is called the cycle tim

(Tp), it includes machining time (Tm), handling time (Th) whic

includes loading, un loading of work piece etc., servicing time (Ts)

which includes time spent on changing blunt tools, chip removal

machine lubrication etc., and (Tf) time for rest and personal needs.

The standard time per piece (Tp) is determined by Tp = Tm + Th + T

+ Tf min. Knowing the standard time per piece, the rate o

production can be calculated.


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Geometry of Single Point Cutting Tool (Tool Signature)

Metal cutting by chip formation occurs when a work piece move

relative to a cutting edge, which is positioned to penetrate its surface

The principle of tool geometry is to provide a sharp cutting edge tha

is strongly supported.The word tool geometry is basically referred to some specific angle

or slope of the salient faces and edges of the tools at their cutting

point.

Rake angle and clearance angle are the most significant for all th

cutting tools.

Rake and Clearance angles of cutting tools

Rake angle () is defined as the angle of inclination of rake surfac

from reference plane and Clearance angle () is defined as the angl

of inclination of clearance or flank surface from the finished surface

Rake angle is provided for ease of chip flow and overall machining

Rake angle may be positive or negative or even zero

Relative advantages of such rake angles are: Positive rake helps reduce cutting force and thus cuttin

power requirement.

Negative rake to increase edge-strength and life of the tool

Zero rake to simplify design and manufacture of the form

tools.


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Clearance angle is essentially provided to avoid rubbing of the too

(flank) with the machined surface which causes loss of energy an

damages of both the tool and the job surface.

Hence, clearance angle is a must and must be positive (3o

~ 15

depending upon tool-work materials and type of the machinin

operations like turning, drilling, boring etc.).

Three possible types of rake: Positive, Zero and Negative rake

angles

Basic Features of Single Point Cutting Tool (Turning)


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Tool signature showing seven elements of single point cutting

tool

Orthogonal and Oblique Cutting

Orthogonal cutting is the simplest, as the tool cutting edge goes instraight line through th

material and the edge of the too

is set perpendicular to th

cutting direction.

In oblique cutting, the cuttin

edge is inclined at an angl

(cutting edge inclination) to line drawn at right angles to th

direction of cutting. The cuttin

edge inclination is measured i

the plane of the new work piec

surface.

Chip Formation


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The chip formed in the metal cutting processes undergoes plasti

deformation, i.e. it becomes shorter and its cross section increase

(Chip contraction). Due to contraction the length of the chip

obtained will be much shorter than the length of travel of the too

along the surface of the work. Depending upon the material beingmachined and the cutting conditions used, four types of chips ar

produced during metal cutting, viz., Continuous chip, Continuou

chip with built up edge, Discontinuous chip / Segmental chips, Non

homogeneous chip.

Types of chips

Continuous chip is produced when the material ahead of the too

continuously deforms without fracture and flows off the tool face i

the form of a ribbon. This type of chip is common when most ductil

materials, such as wrought iron, mild steel, copper and aluminum ar

machined. Basically this operation is one of shearing the work

material to form the chip and the sliding of the chip along the face o


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the cutting tool. The formation of chip takes place in the zon

extending from the tool cutting edge to the junction between th

surfaces of the chip and work piece; this zone is known as th

primary shear zone. This type of chip is associated with low frictio

between the chip and the tool face. Some times chip breakebecome necessary for convenient chip handling.

Under certain conditions, the friction between the chip and the tool i

so great that the chip material welds itself to the tool face / nose. Th

presence of this welded material further increases the friction an

this friction leads to the building up of layer upon layer of chi

material. The resulting pile of material is referred to as a built-up

edge. The built-up edge continues to grow and then breaks down

when it becomes unstable, the broken pieces being carried out by th

underside of the chip and the new work piece surface. Continuou

chips with built-up edge normally occur while cutting ductil

materials with high speed steel tools at low cutting speeds. Weldin

of chips to the tool forms the built up edge which adversel

influences on tool life, power consumption and surface finish

Therefore chip welding should be prevented by following means

a)Reduce friction by increasing rake angle of the cutting tool anby using a lubricant between the rake face and the chip.

b)Reduce temperature by reducing friction and by reducincutting speedc)Reduce pressure between the chip and the tool by increasing th

rake angle, reducing the feed rate and using oblique instead o

orthogonal cutting

d)Preventing metal to metal contact by use of a high pressurlubricant between chip and tool interface.


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Discontinuous chips are separate, plastically deformed segment

which either loosely adhere to each other or remain completely

unconnected. The work material undergoes severe strain during th

formation of chip, and it is brittle, fracture will occur when the chi

is partly formed. Under these conditions the chip is segmented andthe result is the formation of discontinuous chip. Discontinuou

chips are also produced when machining brittle materials such a

cast iron or cast brass. Such chips may also be produced whe

machining ductile materials at very low speeds and high feeds. Fo

brittle materials discontinuous chip is associated with fair surfac

finish, lower power consumption and reasonable tool life. For ductilmaterials, segmented chips are not desirable as they indicat

excessive tool wear and poor surface finish.

Non-homogeneous chip can be seen in materials in which the yiel

strength decreases with temperature and which have poor therma

conductivity. These chips are formed due to non-uniform-strain in

the material during chip formation. There are notches on the fre

side of the chip, while the side adjoining the tool face is smooth. Th

shear deformation which occurs during chip formation causes th

temperature on the shear plane to rise, which in turn may decreas

the strength of the material and cause further strain if the material i

a poor conductor. Thus a large strain is developed at the point oinitial strain. As the cutting process is continued, a new shear plan

will develop at some distance from the first shear plane and th

deformation shifts to this point thereby giving the characteristi

notch-like appearance of the non-homogeneous chip.


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Forces acting on a single point cutting tool

Determination of the cutting forces are required for:

yEstimation of cutting power consumption, which also enableselection of the power source(s) during design of the machin

tools

y Structural design of the machine fixture tool systemyEvaluation of role of the various machining parameters ( proces

VC, s

o, t, tool material and geometry, environment cutting

fluid) on cutting forces

y Study of behaviour and machinability characterisation of thwork materials

yCondition monitoring of the cutting tools and machine tools.Cutting force components and their significances

The single point cutting tools being used for turning, shaping

planing, slotting, boring etc. are characterized by having only oncutting force during machining. But that force is resolved into two o

three components for ease of analysis and exploitation.

Fig visualizes how the single cutting force in turning is resolved into

three components along the three orthogonal directions; X, Y and Z.


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Pz : (Fc) Cutting Force / Tangential force: acts in a vertical plane

tangent to the cutting surface

Px: (Ff / Ft) Axial force / Feed force / Thrust force: acts in a

horizontal plane parallel to the work axis

Py: (Fr) Radial force: acts in a horizontal plane along a radius of the

work


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Tan = F / N =

Where = Friction angle, F = Frictional force, N = Force normal to

friction force and = Coefficient of friction

Tan = (r cos ) / (1 r sin )

Where = shear angle, = rake angle


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r is the cutting ratio, = t1 / t2

Where t1 is the uncut thickness and t2 is the chip thickness

F = Fc sin + Ft cos

N = Fc cos Ft sin

Fc = Fs cos + FN sin

Ft = FN cos Fs sin

Where, Fs is the Shear force on the shear plane and FN is the force

normal to the Shear force

Characteristics and Applications Of The Primary Cutting Too

Materials

(a) High Speed Steel (HSS)

The basic composition of HSS is 18% W, 4% Cr, 1% V, 0.7% C an

rest Fe.

Such HSS tool could machine (turn) mild steel jobs at speed onlupto 20 ~ 30 m/min (which was quite substantial those days)

However, HSS is still used as cutting tool material where;

the tool geometry and mechanics of chip formation are complex

such as helical twist drills, reamers, gear shaping cutters, hobs

form tools, broaches etc.

brittle tools like carbides, ceramics etc. are not suitable unde

shock loading

the small scale industries cannot afford costlier tools

the old or low powered small machine tools cannot accept high

speed and feed.

The tool is to be used number of times by resharpening.


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With time the effectiveness and efficiency of HSS (tools) and thei

application range were gradually enhanced by improving it

properties and surface condition through -

Refinement of microstructure

Addition of large amount of cobalt and Vanadium to increas

hot hardness and wear resistance respectively Manufacture by powder metallurgical process

Surface coating with heat and wear resistive materials like TiC

TiN, etc by Chemical Vapour Deposition (CVD) or Physica

Vapour Deposition (PVD)

Typical composition of HSS tool

Addition of large amount ofCo and V, refinement of microstructur

and coating increased strength and wear resistance and thu

enhanced productivity and life of the HSS tools remarkably.

(b) StelliteThis is a cast alloy ofCo (40 to 50%), Cr (27 to 32%), W (14 to 19%

and C (2%). Stellite is quite tough and more heat and wear resistiv

than the basic HSS (18 4 1)But such stellite as cutting tool material became obsolete for its poo

grindability and specially after the arrival of cemented carbides.

(c) Sintered Tungsten carbidesThe advent of sintered carbides made another breakthrough in th

history of cutting tool materials.


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Straight or single carbide

First the straight or single carbide tools or inserts were powde

metallurgically produced by mixing, compacting and sintering 90 t

95% WC powder with cobalt.

The hot, hard and wear resistant WC grains are held by the binde

Co which provides the necessary strength and toughness.Such tools are suitable for machining grey cast iron, brass, bronz

etc. which produce short discontinuous chips and at cutting

velocities two to three times of that possible for HSS tools.

Composite carbides

The single carbide is not suitable for machining steels because o

rapid growth of wear, particularly crater wear, by diffusion of C

and carbon from the tool to the chip under the high stress and

temperature bulk (plastic) contact between the continuous chip an

the tool surfaces.

For machining steels successfully, another type called composit

carbide have been developed by adding (8 to 20%) a gamma phas

to WC and Co mix. The gamma phase is a mix of TiC, TiN, TaC

NiC etc. which are more diffusion resistant than WC due to theimore stability and less wettability by steel.

Mixed carbides

Titanium carbide (TiC) is not only more stable but also much harde

than WC.

For machining ferritic steels causing intensive diffusion anadhesion wear a large quantity (5 to 25%) of TiC is added with WC

and Co to produce another grade called Mixed carbide.

But increase in TiC content reduces the toughness of the tools

Therefore, for finishing with light cut but high speed, the harde


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grades containing upto 25% TiC are used and for heavy roughin

work at lower speeds lesser amount (5 to 10%) of TiC is suitable.

Up gradation of cemented carbides and their applications

The standards developed by ISO for grouping of carbide tools an

their application ranges are given in Table

K-group is suitable for machining short chip producing ferrous an

non-ferrous metals and also some non metals.

P-group is suitably used for machining long chipping ferrous metal

i.e. plain carbon and low alloy steels

M-group is generally recommended for machining more difficult

to-machine materials like strain hardening austenitic steel an

manganese steel etc.

(d) Plain ceramics

Inherently high compressive strength, chemical stability and ho

hardness of the ceramics led to powder metallurgical production o

indexable ceramic tool inserts since 1950.


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Advantages and limitations of alumina ceramics in contrast t

sintered carbide.

Alumina (Al2O

3) is preferred to silicon nitride (Si

3N

4) for highe

hardness and chemical stability. Si3N

4is tougher but again mor

difficult to process.

The plain ceramic tools are brittle in nature and hence had limite

applications.

Properties of Alumina Ceramics

Basically three types of ceramic tool bits are available in the market

Plain alumina with traces of additives these white or pink

sintered inserts are cold pressed and are used mainly fo

machining cast iron and similar materials at speeds 200 to 25m/min

Alumina; with or without additives hot pressed, black colour

hard and strong used for machining steels and cast iron at V

= 150 to 250 m/min


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Carbide ceramic (Al2O

3+ 30% TiC) cold or hot pressed, blac

colour, quite strong and enough tough used for machining

hard cast irons and plain and alloy steels at 150 to 200 m/min.

Cutting temperature causes, effects and influencing

parameters

(i) Sources, causes of heat generation and development o

temperature in machining

During machining heat is generated at the cutting point from thre

sources, as indicated in Fig. Those sources and causes o

development of cutting temperature are:


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y Primary shear zone (1) where the major part of the energy iconverted into heat

y Secondary deformation zone (2) at the chip tool interfacwhere further heat is generated due to rubbing and / or shear

y At the worn out flanks (3) due to rubbing between the tool anthe finished surfaces

Sources of heat generation in machining

The heat generated is shared by the chip, cutting tool and the blank

The apportionment of sharing that heat depends upon th

configuration, size and thermal conductivity of the tool wor

material and the cutting condition. Figure visualizes that maximum

amount of heat is carried away by the flowing chip. From 10 to 20%

of the total heat goes into the tool and some heat is absorbed in th

blank. With the increase in cutting velocity, the chip shares hea

increasingly.


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Apportionment of heat amongst chip, tool & job

ii) Effect of high cutting temperature on tool and Job

The effect of cutting temperature, particularly when it is high, i

mostly detrimental to both the tool and the job. The major portion o

the heat is taken away by the chips. But it does not matter becaus

chips are thrown out. So attempts should be made such that the chip

take away more and more amount of heat leaving small amount o

heat to harm the tool and the job. The possible detrimental effects othe high cutting temperature on cutting tool (edge) are:

y rapid tool wear, which reduces tool lifey Plastic deformation of the cutting edges if the tool material i

not enough hot-hard and hot-strong

y Thermal flaking and fracturing of the cutting edges due tthermal shocks

y Built-up-edge formation


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The possible detrimental effects of cutting temperature on the

machined job are:

y dimensional inaccuracy of the job due to thermal distortion andexpansion-contraction during and after machining

y Surface damage by oxidation, rapid corrosion, burning etc.y induction of tensile residual stresses and microcracks at the

surface / subsurface

However, often the high cutting temperature helps in reducing th

magnitude of the cutting forces and cutting power consumption to

some extent by softening or reducing the shear strength, s

of th

work material ahead the cutting edge. To attain or enhance suchbenefit the work material ahead the cutting zone is often additionall

heated externally. This technique is known as Hot Machining and i

beneficially applicable for the work materials which are very har

and hardenable like high manganese steel, Ni-hard, Nimonic etc.

(iii) Role of variation of the various machining parameters oncutting temperature

The magnitude of cutting temperature is more or less governed o

influenced by all the machining parameters like:

Work material : - specific energy requirement, ductility

thermal properties (, cv

)

process parameters : - cutting velocity (VC), feed (s

o), dept

of cut (t)

cutting tool material : - thermal properties, wear resistance

chemical stability


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tool geometry : - rake angle (), cutting edge angl

(), clearance angle (), nose radius (r)

cutting fluid : - thermal and lubricating properties

method of application

Many researchers studied, mainly experimentally, on the effects othe various parameters on cutting temperature. A well establishe

overall empirical equation is given by

where, C= a constant depending mainly on the work-tool materials

The above equation clearly indicates that among the proces

parameters VC

affects imost significantly and the role of t is almos

insignificant. Cutting temperature depends also upon the too

geometry. The equation depicts that i

can be reduced by lowerin

the principal cutting edge angle, and increasing nose radius, r

Besides that the tool rake angle, and hence inclination angle, als

have significant influence on the cutting temperature. Increase in

rake angle will reduce temperature by reducing the cutting forces bu

too much increase in rake will raise the temperature again due to

reduction in the wedge angle of the cutting edge. Proper selectionand application of cutting fluid help reduce cutting temperatur

substantially through cooling as well as lubrication.


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Control of cutting temperature and cutting fluid application

(i) Basic methods of controlling cutting temperature

It is already realized that the cutting temperature, particularly when

is quite high, is very detrimental for both cutting tools and th

machined jobs and hence need to be controlled, i.e., reduced as far apossible without sacrificing productivity and product quality.

The methods generally employed for controlling machinin

temperature and its detrimental effects are:

y Proper selection of cutting tools; material and geometryy Proper selection of cutting velocity and feedy Proper selection and application of cutting fluid

a) Selection of material and geometry of cutting tool for reducin

cutting temperature and its effects

Cutting tool material may play significant role on reduction o

cutting temperature depending upon the work material.

As for example,

y PVD orCVD coating of HSS and carbide tools enables reduccutting temperature by reducing friction at the chip-tool an

work-tool interfaces.

y In high speed machining of steels lesser heat and cuttintemperature develop if machined by CBN tools which produclesser cutting forces by retaining its sharp geometry for i

extreme hardness and high chemical stability.

y The cutting tool temperature of ceramic tools decrease further the thermal conductivity of such tools is enhanced (by addin


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thermally conductive materials like metals, carbides, etc in Al2O

or Si3N

4)

Cutting temperature can be sizeably controlled also by prop

selection of the tool geometry in the following ways;

y large positive toolrake helps in reducing heat and temperaturgeneration by reducing the cutting forces, but too much increas

in rake mechanically and thermally weakens the cutting edges

y compound rake, preferably with chipbreaker, also enablereduce heat and temperature through reduction in cutting force

and friction

y even for same amount of heat generation, the cutting temperaturdecreases with the decrease in the principal cutting edge angle, as

y nose radius of single point tools not only improves surface finisbut also helps in reducing cutting temperature to some extent.

b) Selection of cutting velocity and feed

Cutting temperature can also be controlled to some extent, eve

without sacrificing MRR, by proper or optimum selection of th

cutting velocity and feed within their feasible ranges. The rate o

heat generation and hence cutting temperature are governed by th

amount of cutting power consumption, PC

where;

PC

= PZ.V

C= t s

o

sf V

C

So apparently, increase in both so

and VC

raise heat generatio

proportionately. But increase in VC, though further enhances he


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generation by faster rubbing action, substantially reduces cuttin

forces, hence heat generation by reducing sand also the form facto

f. The overall relative effects of variation of VC

and so

on cuttin

temperature will depend upon other machining conditions. Henc

depending upon the situation, the cutting temperature can bcontrolled significantly by optimum combination of V

Cand s

ofor

given MRR.

c) Control of cutting temperature by application of cutting fluid

Cutting fluid, if employed, reduces cutting temperature directly b

taking away the heat from the cutting zone and also indirectly breducing generation of heat by reducing cutting forces

Purposes of application of cutting fluid in machining and grinding.

The basic purposes of cutting fluid application are :

Cooling of the job and the tool to reduce the detrimental effect

of cutting temperature on the job and the tool

Lubrication at the chiptool interface and the tool flanks to

reduce cutting forces and friction and thus the amount of hea

generation.

Cleaning the machining zone by washing away the chip

particles and debris which, if present, spoils the finishesurface and accelerates damage of the cutting edges

Protection of the nascent finished surface a thin layer of th

cutting fluid sticks to the machined surface and thus prevent


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its harmful contamination by the gases like SO2, O

2, H

2S, N

xO

present in the atmosphere.

However, the main aim of application of cutting fluid is to improv

machinability through reduction of cutting forces and temperature

improvement by surface integrity and enhancement of tool life

Essential properties of cutting fluids

To enable the cutting fluid fulfil its functional requirements withou

harming the Machine Fixture Tool Work (M-F-T-W) system

and the operators, the cutting fluid should possess the followin

properties:

For cooling :

high specific heat, thermal conductivity and film coefficient fo

heat transfer

spreading and wetting ability

For lubrication :

high lubricity without gumming and foaming

wetting and spreading

high film boiling point

friction reduction at extreme pressure (EP) and temperature

Chemical stability, non-corrosive\ to the materials of the M-F-T-W

system

less volatile and high flash point


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high resistance to bacterial growth

odourless and also preferably colourless

non toxic in both liquid and gaseous stage

easily available and low cost.

Principles of cutting fluid action

The chip-tool contact zone is usually comprised of two parts; plasti

or bulk contact zone and elastic contact zone as indicated in figure.

The cutting fluid cannot penetrate or reach the plastic contact zon

but enters in the elastic contact zone by capillary effect.

With the increase in cutting velocity, the fraction of plastic contac

zone gradually increases and covers almost the entire chip-too

contact zone as indicated figure

Therefore, at high speed machining, the cutting fluid become

unable to lubricate and cools the tool and the job only by bulk

external cooling


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Cutting fluid action in machining

Apportionment of plastic and elastic contact zone with increase in

cutting velocity.

The chemicals like chloride, phosphate or sulphide present in th

cutting fluid chemically reacts with the work material at the chip


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under surface under high pressure and temperature and forms a thin

layer of the reaction product.

The low shear strength of that reaction layer helps in reducin

friction.

To form such solid lubricating layer under high pressure an

temperature some extreme pressure additive (EPA) is deliberatel

added in reasonable amount in the mineral oil or soluble oil.

For extreme pressure, chloride, phosphate or sulphide type EPA i

used depending upon the working temperature, i.e. moderate (200oC

~ 350oC), high (350

oC ~ 500

oC) and very high (500

oC ~ 800

oC

respectively

Types of cutting fluids and their application

Generally, cutting fluids are employed in liquid form bu

occasionally also employed in gaseous form.

Only for lubricating purpose, often solid lubricants are als

employed in machining and grinding.

The cutting fluids, which are commonly used, are :

Air blast or compressed air only.

Machining of some materials like grey cast iron becom

inconvenient or difficult if any cutting fluid is employed in liqui

form. In such case only air blast is recommended for cooling and

cleaning


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Water

For its good wetting and spreading properties and very high specifi

heat, water is considered as the best coolant and hence employed

where cooling is most urgent.

Soluble oil

Water acts as the best coolant but does not lubricate. Besides, use o

only water may impair the machine-fixture-tool-work system b

rusting

So oil containing some emulsifying agent and additive like EPA

together called cutting compound, is mixed with water in a suitabl

ratio ( 1 ~ 2 in 20 ~ 50). This milk like white emulsion, called

soluble oil, is very common and widely used in machining an

grinding.

Cutting oils

Cutting oils are generally compounds of mineral oil to which ar

added desired type and amount of vegetable, animal or marine oil

for improving spreading, wetting and lubricating properties.

As and when required some EP additive is also mixed to reduc

friction, adhesion and BUE formation in heavy cuts.

Chemical fluids

These are occasionally used fluids which are water based wher

some organic and or inorganic materials are dissolved in water to

enable desired cutting fluid action.


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There are two types of such cutting fluid;

Chemically inactive type high cooling, anti-rusting and wetting bu

less lubricating

Active (surface) type moderate cooling and lubricating.

Solid or semi-solid lubricant

Paste, waxes, soaps, graphite, Moly-disulphide (MoS2) may als

often be used, either applied directly to the workpiece or as a

impregnant in the tool to reduce friction and thus cutting forcestemperature and tool wear.

Cryogenic cutting fluid

Extremely cold (cryogenic) fluids (often in the form of gases) lik

liquid CO2or N

2are used in some special cases for effective coolin

without creating much environmental pollution and health hazards.

Selection of Cutting Fluid

The benefit of application of cutting fluid largely depends upon

proper selection of the type of the cutting fluid depending upon th

work material, tool material and the machining condition.

As for example, for high speed machining of not-difficult-to

machine materials greater cooling type fluids are preferred and fo

low speed machining of both conventional and difficult-to-machin

materials greater lubricating type fluid is preferred.


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Selection of cutting fluids for machining some common engineerin

materials and operations are presented as follows :

Grey cast iron:

Generally dry for its self lubricating property

Air blast for cooling and flushing chips

Soluble oil for cooling and flushing chips in high speed machining

and grinding

Steels:

If machined by HSS tools, sol. Oil (1: 20 ~30) for low carbon an

alloy steels and neat oil with EPA for heavy cuts

If machined by carbide tools thinner sol. Oil for low strength stee

thicker sol. Oil ( 1:10 ~ 20) for stronger steels and starigh

sulphurised oil for heavy and low speed cuts and EP cutting oil fo

high alloy steel.

Often steels are machined dry by carbide tools for preventin

thermal shocks.

Aluminium and its alloys:

Preferably machined dry

Light but oily soluble oil

Straight neat oil or kerosene oil for stringent cuts.

Copper and its alloys :

Water based fluids are generally used


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Oil with or without inactive EPA for tougher grades ofCu-alloy.

Stainless steels and Heat resistant alloys:

High performance soluble oil or neat oil with high concentratio

with chlorinated EP additive.

The brittle ceramics and cermets should be used either under dr

condition or light neat oil in case of fine finishing.

Grinding at high speed needs cooling ( 1: 50 ~ 100) soluble oil. Fo

finish grinding of metals and alloys low viscosity neat oil is als

used.

Methods of application of cutting fluid

The effectiveness and expense of cutting fluid application

significantly depend also on how it is applied in respect of flow rat

and direction of application.

In machining, depending upon the requirement and facilitieavailable, cutting fluids are generally employed in the followin

ways (flow) :

Drop-by-drop under gravity

Flood under gravity

In the form of liquid jet(s)

Mist (atomised oil) with compressed air

Z-Z method centrifugal through the grinding wheels (pores

as indicated in figure.


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The direction of application also significantly governs th

effectiveness of the cutting fluid in respect of reaching at or near th

chip-tool and work-tool interfaces.

Depending upon the requirement and accessibility the cutting fluid i

applied from top or side(s). in operations like deep hole drilling thpressurised fluid is often sent through the axial or inner spiral hole(s

of the drill.

For effective cooling and lubrication in high speed machining o

ductile metals having wide and plastic chip-tool contact, cutting flui

may be pushed at high pressure to the chip-tool interface throughhole(s) in the cutting tool, as schematically shown.

Z-Z method of cutting fluid application in grinding


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Application of cutting fluid at high pressure through the hole in the

tool.

Failure of cutting tools

Cutting tools generally fail by :

i) Mechanical breakage due to excessive forces and shocks. Such

kind of tool failure is random and catastrophic in nature an

hence are extremely detrimental.

ii) Quick dulling by plastic deformation due to intensive stresse

and temperature. This type of failure also occurs rapidly an

are quite detrimental and unwanted.

iii) Gradual wear of the cutting tool at its flanks and rake surface

The first two modes of tool failure are very harmful not only for th

tool but also for the job and the machine tool.


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Hence these kinds of tool failure need to be prevented by usin

suitable tool materials and geometry depending upon the work

material and cutting condition.

Mechanisms and pattern (geometry) of cutting tool wear

For the purpose of controlling tool wear one must understand th

various mechanisms of wear that the cutting tool undergoes unde

different conditions.

The common mechanisms of cutting tool wear are :

i) Mechanical wear

Thermally insensitive type; like abrasion, chipping and

delamination

Thermally sensitive type; like adhesion, fracturing, flakin

etc.

ii) Thermochemical wear

Macro-diffusion by mass dissolution

Micro-diffusion by atomic migration

iii) Chemical wear

iv) Galvanic wear

In diffusion wear the material from the tool at its rubbing surfaces

particularly at the rake surface gradually diffuses into the flowin

chips either in bulk or atom by atom when the tool material ha

chemical affinity or solid solubility towards the work material.


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The rate of such tool wear increases with the increase in temperatur

at the cutting zone.

Diffusion wear becomes predominant when the cutting temperatur

becomes very high due to high cutting velocity and high strength o

the work material.

Chemical wear, leading to damages like grooving wear may occur i

the tool material is not enough chemically stable against the wor

material and/or the atmospheric gases.

Galvanic wear, based on electrochemical dissolution, seldom occur

when both the work tool materials are electrically conductivecutting zone temperature is high and the cutting fluid acts as an

electrolyte.

The usual pattern or geometry of wear of turning and face milling

inserts are typically shown in figure


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Geometry and major features of wear of turning tools

Photographic view of the wear pattern of a turning tool insert

Schematic (a) and actual view (b) of wear pattern of face milling

insert


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In addition to ultimate failure of the tool, the following effects ar

also caused by the growing tool-wear :

increase in cutting forces and power consumption mainly du

to the principal flank wear

increase in dimensional deviation and surface roughnes

mainly due to wear of the tool-tips and auxiliary flank wea

(Vs)

odd sound and vibration

worsening surface integrity

mechanically weakening of the tool tip

Tool Life

Tool life generally indicates, the amount of satisfactory performanc

or service rendered by a fresh tool or a cutting point till it is declare

failed.

Tool life is defined in two ways :

(a) In R & D : Actual machining time (period) by which a fresh

cutting tool (or point) satisfactorily works after which it need

replacement or reconditioning.

The modern tools hardly fail prematurely or abruptly by mechanical

breakage or rapid plastic deformation.

Those fail mostly by wearing process which systematically grows

slowly with machining time.


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In that case, tool life means the span of actual machining time by

which a fresh tool can work before

attaining the specified limit of tool wear.

Mostly tool life is decided by the machining time till flank wear, VB

reaches 0.3 mm or crater wear, KT

reaches 0.15 mm.

(b) In industries or shop floor : The length of time of satisfactor

service or amount of acceptable output provided by a fresh too

prior to it is required to replace or recondition.

Measurement of tool wear

The various methods are :

i) by loss of tool material in volume or weight, in one life time this method is crude and is generally applicable for critica

tools like grinding wheels.

ii) by grooving and indentation method in this approximat

method wear depth is measured indirectly by the difference in

length of the groove or the indentation outside and inside th

worn area

iii) using optical microscope fitted with micrometer ver

common and effective method

iv) using scanning electron microscope (SEM) used generally

for detailed study; both qualitative and quantitative


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v) Talysurf, specially for shallow crater wear.

Taylors tool life equation.

Wear and hence tool life of any tool for any work material i

governed mainly by the level of the machining parameters i.e

cutting velocity, (VC), feed, (s

o) and depth of cut (t). Cutting velocit

affects maximum and depth of cut minimum.

The usual pattern of growth of cutting tool wear (mainly VB)

principle of assessing tool life and its dependence on cutting velocity

are schematically shown in Fig

Growth of flank wear and assessment of tool life


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The tool life obviously decreases with the increase in cuttin

velocity keeping other conditions unaltered as indicated

If the tool lives, T1

, T2

, T3

, T4

etc are plotted against th

corresponding cutting velocities, V1, V

2, V

3, V

4etc as shown i

figure, a smooth curve like a rectangular hyperbola is found t

appear.

Cutting velocity tool life relationship

When F. W. Taylor plotted the same figure taking both V and T in

log-scale, a more distinct linear relationship appeared a

schematically shown

With the slope, n and intercept, c, Taylor derived the simple equationas

VTn= C

where, n is called, Taylors tool life exponent.


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The values of both n and c depend mainly upon the tool-wor

materials and the cutting environment (cutting fluid application).

The value ofC depends also on the limiting value of VB

undertaken

i.e., 0.3 mm, 0.4 mm, 0.6 mm etc.)

Cutting velocity vs tool life on a log-log scale

Modified Taylors Tool Life equation

In Taylors tool life equation, only the effect of variation of cutting

velocity, VC

on tool life has been considered. But practically, th

variation in feed (so) and depth of cut (t) also play role on tool life to

some extent.

Taking into account the effects of all those parameters, the Taylor

tool life equation has been modified as,

where, TL = tool life in min


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CT

a constant depending mainly upon the tool work materials an

the limiting value of VB

x, y and z exponents so called tool life exponents depending upo

the tool work materials and the machining environment.

Generally, x > y > z as VC

affects tool life maximum and t minimum

The values of the constants, CT, x, y and z are available in

Machining Data Handbooks or can be evaluated by machining tests.

Machinability

Machinability is expressed as operational characteristics of thwork-tool combination. Attempts were made to measure or quantify

machinability and it was done mostly in terms of :

tool life which substantially influences productivity an

economy in machining

magnitude of cutting forces which affects power consumption

and dimensional accuracy

surface finish which plays role on performance and service lif

of the product.

Often cutting temperature and chip form are also considered foassessing machinability.

But practically it is not possible to use all those criteria together fo

expressing machinability quantitatively.


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In a group of work materials, one may appear best in respect of, say

tool life but may be much poor in respect of cutting forces an

surface finish and so on. Besides that, the machining responses o

any work material in terms of tool life, cutting forces, surface finish

etc. are more or less significantly affected by the variation; known o

unknown, of almost all the parameters or factors associated witmachining process. Machining response of a material may als

change with the processes, i.e. turning, drilling, milling etc

therefore, there cannot be as such any unique value to expres

machinability of any material However, earlier, the relativ

machining response of the work materials compared to that of

standard metal was tried to be evaluated quantitatively only based on

tool life (VB* = 0.33 mm) by an index,

Following graph shows such scheme of evaluating Machinabilit

rating (MR) of any work material. The free cutting steel, AISI

1112, when machined (turned) at 100 fpm, provided 60 min of too

life. If the work material to be tested provides 60 min of tool life a

cutting velocity of 60 fpm (say), as indicated in graph, under th

same set of machining condition, then machinability (rating) of tha

material would be, or, simply the value of the cutting velocit

expressed in fpm at which a work material provides 60 min tool lif

was directly considered as the MR of that work material.


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In this way the MR of some materials, for instance, were evaluateas,


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