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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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