Unit 2 Machinability, Cutting Fluids, Tool Life & Wear, Tool Materials

Machining & Machining Tools
Unit-2
1
Machinability, Tool Wear & Tool Life,
Tool Materials, Economics of
Machining, Cutting Fluids

Syllabus
 Concept of machinability, machinability index, factors
affecting machinability
 Different mechanism of tool wear types of tool wear
(crater, flank etc.), Measurement and control of tool
wear
 Concept of tool life, Taylor's tool life equation (including
modified version)
 Different tool materials and their applications including
effect of tool coating
 Introduction to economics of machining
 Cutting fluids: types, properties, selection and
application methods
2

Machinability
Machinability is the ease with which a given material may be
worked with a cutting tool. The machinability of a material is
usually defined in terms of four factors:
• Surface finish
• Tool life.
• Force and power required.
• The level of difficulty in chip control.
Thus, good machinability indicates good surface finish and surface
integrity, a long tool life, low force and power requirements and
desired chip control in the cutting zone.
4

Machinability Index/Rating
Because of the complex nature of cutting operations, it is difficult to
establish relationships that quantitatively define the machinability of a
particular material. The machinability rating of a material
attempts to quantify the machinability of various materials.
where V60 is the cutting speed for the target material that ensures tool
life of 60 min, V60R is the same for the reference material.
 Reference materials are selected for each group of work materials
(ferrous and non-ferrous) among the most popular and widely used
brands.
 If KM > 1, the machinability of the target material is better that this
of the reference material, and vice versa.
 Note that this system can be misleading because the index is
different for different machining processes. 5

Machinability Rating Example
The reference material for steels, AISI 1112 steel has an index of 1.
Machining of this steel at cutting speed of 0.5 m/s gives tool life of 60 min.
Therefore, V60R = 0.5 m/s.
For the austenitic 302 SS steel
The machinability index is KM = 0.23/0.5 = 0.46 (tool life of 60 min is
reached at 0.23 m/s).
For AISI 1045 steel
The machinability index is KM = 0.36/0.5 = 0.72. (tool life of 60 min is
reached at 0.36 m/s).
So, we can rate these steels in a descending order of machinability:
AISI 1112>AISI 1045>302 SS
Note that Tool life cannot be considered as the only criteria for
judging machinability as it is dependent on many factors other
than cutting velocity. Keeping all such factors and limitations in
view, Machinability can be tentatively defined as “ability of being
machined” and more reasonably as “ease of machining”. 6

Machinability Factors
The machinability is affected by following variables Aspects
(a) Work material Aspects
(b) Cutting tool Aspects
(c) Process parameters Aspects
(d) Machining environments Aspects
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Work Material Aspects
8
Following work material properties govern machinability.
 Nature – Brittle/Ductile
 Microstructure-Coarse/Fine
 Mechanical strength – fracture or yield
 Hardness, hot strength and hot hardness
 Work hardenability
 Thermal conductivity
 Chemical reactivity
 Stickiness / self lubricity.

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Nature-Brittle/Ductile
Brittle materials are easy to machine as the chip separation is due to
brittle fracture requiring lesser energy of chip formation and further
shorter chips causing lesser frictional force and heating at the rake
surface
Ductile materials like mild steel produce better surface finish but BUE, if
formed, may worsen the surface finish. Also cutting forces increase with
the increase in yield shear strength, τs of the work material
Microstructure –Coarse/Fine
The value of shear strength and hence shear force of a given material
depends on its microstructure. Coarse microstructure leads to lesser value
of τs. Therefore, τs can be desirably reduced by either proper heat
treatment like annealing of steels or controlled addition of materials like
Sulphur (S), lead (Pb), Tellurium etc. leading to free cutting of soft ductile
metals and alloys.

10
Hardness, hot strength and hot hardness and work hardening
Harder materials are obviously more difficult to machine for increased cutting
forces and tool damage.
Usually, with the increase in cutting velocity the cutting forces decrease to some
extent making machining easier through reduction in τs and also chip thickness.
τs decreases due to softening of the work material at the shear zone due to
elevated temperature. Such benefits of increased temperature and cutting velocity
are not attained when the work materials are hot strong and hard like Ti and Ni
based superalloys and work hardenable like high manganese steel, Ni- hard,
Hadfield steel etc.
Stickiness/Self Lubricity
Sticking of the materials (like pure copper, aluminium and their alloys) and
formation of BUE at the tool rake surface also hamper machinability by increasing
friction, cutting forces, temperature and surface roughness.
Thermal Conductivity
Lower thermal conductivity of the work material affects their machinability by
raising the cutting zone temperature and thus reducing tool life.

Cutting Tool Aspects
11
Tool Materials:
 In machining a given material, the tool life is
governed mainly by the tool material which also
influences cutting forces and temperature as well as
accuracy and finish of the machined surface.
 The composition, microstructure, strength, hardness,
toughness, wear resistance, chemical stability and
thermal conductivity of the tool material play
significant roles on the machinability characteristics
though in different degree depending upon the
properties of the work material.
 High wear resistance and chemical stability of the
cutting tools like coated carbides, ceramics, cubic
Boron nitride (cBN) etc. also help in providing better
surface integrity of the product by reducing friction,
cutting temperature and BUE formation in high speed
machining of steels.
 Very soft, sticky and chemically reactive material like
pure aluminium attains highest machinability when
machined by diamond tools.
Figure 2.1: Role of cutting tool material
on machinability /Tool life

Cutting Tool Aspects
12
Machinability is affected by tool geometry elements like rake angles ,clearance
angles and nose radius.
Increase in rake angle reduces
cutting forces requirements but
does not affect surface finish
Too high rake angle decrease
tool tip strength and hence
decreases tool life.
Inadequate clearance angle
reduces tool life and surface
finish by tool work rubbing and
again too large clearance
reduces the tool strength and
hence tool life.
Proper tool nose radius
improves machinability
to some extent through
increase in tool life by
increasing mechanical
strength and reducing
temperature at the tool
tip reduction of surface
roughness

Process Parameter Aspects
13
 Proper selection of the cutting velocity, feed and depth of cut provide
better machinability characteristics of a given work – tool pair even
without sacrificing productivity or MRR.
 Amongst the process parameters, depth of cut plays least significant
role and is almost invariable.
 Now increase in cutting velocity in general, reduces tool life but it also
reduces cutting forces or specific energy requirement and improves
surface finish through favorable chip-tool interaction.
 Some cutting tools especially ceramic tools perform better and last
longer at higher V within limits.
 Increase in feed raises cutting forces proportionally but reduces
specific energy requirement to some extent.
 Cutting temperature is also less affected by increase in feed than V.
But increase in feed unlike V raises surface roughness. Therefore,
proper increase in V, even at the expense of feed often can improve
machinability quite significantly.

Work Environment Aspects
14
The basic purpose of employing cutting fluid is to improve
machinability characteristics of any work – tool pair
through :
• improving tool life by cooling and lubrication
• reducing cutting forces and specific energy consumption
• improving surface integrity by cooling, lubricating and
cleaning at the cutting zone
The favorable roles of cutting fluid application depend not
only on its proper selection based on the work and tool
materials and the type of the machining process but also
on its rate of flow, direction and location of application.

Possible ways of Improving Machinability
15
The machinability of the work materials can be more or less improved, without
sacrificing productivity, by the following ways:
 Favorable change in composition, microstructure and mechanical properties
by mixing suitable type and amount of additive(s) in the work material and
appropriate heat treatment
 Proper selection and use of cutting tool material and geometry depending
upon the work material and the significant machinability criteria under
consideration
 Optimum selection of cutting velocity, feed and depth of cut based on the
tool – work materials and the primary objectives.
 Proper selection and appropriate method of application of cutting fluid
depending upon the tool – work materials, desired levels of productivity i.e.,
VC and f and also on the primary objectives of the machining work
undertaken
 Proper selection and application of special techniques like dynamic
machining, hot machining, cryogenic machining etc., if feasible,
economically viable and eco-friendly.

Machinability of Ferrous Metals
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STEELS-Carbon steels
 have a wide range of machinability, depending on their ductility and
hardness.
 If carbon steel is too ductile, chip formation can produce built-up edge, leading to poor
surface finish
 if the steel is too hard, it can cause abrasive wear of the tool because of the presence
of carbides in the steel. Cold-worked carbon steels are desirable from a machinability
standpoint.
 An important group of steels is free-machining steels, containing sulfur and
phosphorus. Sulfur forms manganese-sulfide inclusions (second-phase particles),
which act as stress raisers in the primary shear zone. As a result, the chips produced
break up easily and are small, thus improving machinability. The size, shape,
distribution, and concentration of these inclusions significantly influence machinability.
Elements such as tellurium and selenium, both of which are chemically similar to sulfur,
act as inclusion modifiers in resulfurized steels.
 Phosphorus in steels has two major effects:
 It strengthens the ferrite, causing increased hardness and resulting in better chip
formation and surface finish, and
 It increases hardness and thus causes the formation of short chips instead of
continuous stringy ones, thereby improving machinability.

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STEELS-LEADED steels
 In leaded steels, a high percentage of lead solidifies at the tips of manganese sulfide inclusions. In
nonresulfurized grades of steel, lead takes the form of dispersed fine particles.
 Lead is insoluble in iron, copper, and aluminum and their alloys. Because of its low shear strength,
lead acts as a solid lubricant and is smeared over the tool-chip interface during cutting.
 At high temperatures lead melts directly in front of the tool, acting as a liquid lubricant.
 Lead lowers the shear stress in the primary shear zone, thus reducing cutting forces and power
consumption.
 Lead can be used with every grade of steel and is identified by the letter “L” between the second
and third numerals in steel identification (e.g., 10L45). (Note that in stainless steels, a similar use of
the letter L means “low carbon”
 Lead is a well-known toxin and a pollutant hence efforts being made to eliminate the use of lead in
steels (lead-free steels).
 Bismuth and tin are substitutes for lead in steels, although their performance is not as good.
 Calcium-deoxidized steels contain oxide flakes of calcium silicates (CaSO) that reduce the strength
of the secondary shear zone and decrease tool-chip interface friction and wear. Increases in
temperature are reduced correspondingly. Consequently, these steels produce less crater wear,
especially at high cutting speeds.
 Alloy steels can have a wide variety of compositions and hardness. Consequently, their machinability
cannot be generalized, although they have higher levels of hardness and other mechanical
properties. Alloy steels at hardness levels of 45 to 65 HRC can be machined with polycrystalline
cubic boron-nitride cutting tools, producing good surface finish, integrity, and dimensional accuracy.

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EFFECT OF VARIOUS ELEMENTS IN STEELS
 The presence of aluminum and silicon in steels is always harmful, because these elements combine
with oxygen to form aluminum oxide and silicates, which are hard and abrasive. As a result, tool wear
increases and machinability is reduced.
 Carbon and manganese have various effects on the machinability of steels, depending on their
composition. Plain low-carbon steels (less than 0.15% C) can produce poor surface finish by forming a
built-up edge. Cast steels are more abrasive, although their machinability is similar to that of wrought
steels. Tool and die steels are very difficult to machine and usually require annealing prior to machining.
The machinability of most steels is improved by cold working, which hardens the material and reduces the
tendency for built-up edge formation.
 Other alloying elements (such as nickel, chromium, molybdenum, and vanadium) that improve the
properties of steels generally reduce machinability. The effect of boron is negligible. Gaseous elements
such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel.
 Oxygen has been shown to have a strong effect on the aspect ratio of the manganese-sulfide inclusions:
The higher the oxygen content, the lower the aspect ratio, and the higher the machinability.
 In improving the machinability of steels, however, it is important to consider the possible detrimental
effects of the alloying elements on the properties and strength of the machined part in service. At elevated
temperatures, for example, lead causes embrittlement of steels although at room temperature it has no
effect on mechanical properties.
 Sulfur can reduce the hot workability of steels severely because of the formation of iron sulfide (unless
sufficient manganese is present to prevent such formation).
 At room temperature, the mechanical properties of resulfurized steels depend on the orientation of the
deformed manganese-sulfide inclusions (anisotropy).
 Rephosphorized steels are significantly less ductile and are produced solely to improve machinability.

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STAINLESS STEELS
• Austenitic (300 series) steels generally are difficult to machine. Chatter can be a problem,
necessitating machine tools with high stiffness.
• Ferritic stainless steels (also 300 series) have good machinability.
• Martensitic (400 series) steels are abrasive, tend to form a built-up edge, and require tool
materials with high hot hardness and crater-wear resistance.
• Precipitation-hardening stainless steels are strong and abrasive, thus requiring hard and abrasion-
resistant tool materials.
CAST IRONS
• Gray irons generally are machinable, but they can be abrasive depending on composition,
especially pearlite. Free carbides in castings reduce their machinability and cause tool chipping or
fracture.
• Nodular and malleable irons are machinable with hard tool materials.

Machinability of Non-Ferrous Metals
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Aluminum is generally very easy to machine, although the softer grades tend to form a built-up edge, resulting in
poor surface finish. Thus, high cutting speeds, high rake angles, and high relief angles are recommended. Wrought
aluminum alloys with high silicon content and cast aluminum alloys are generally abrasive; hence, they require
harder tool materials. Dimensional tolerance control may be a problem in machining aluminum, because it has a
high thermal expansion coefficient and a relatively low elastic modulus.
Beryllium generally is machinable, but because the fine particles produced during machining are toxic, it requires
machining in a controlled environment.
Cobalt-based alloys are abrasive and highly work hardening. They require sharp, abrasion-resistant tool materials
and low feeds and speeds.
Copper in the wrought condition can be difficult to machine because of builtup edge formation, although cast
copper alloys are easy to machine. Brasses are easy to machine, especially with the addition of lead (leaded free-
machining brass). Note, however, the toxicity of lead and associated environmental concerns. Bronzes are more
difficult to machine than brass.
Magnesium is very easy to machine, with good surface finish and prolonged tool life. However, care should be
exercised because of its high rate of oxidation (pyrophoric) and the danger of fire.
Molybdenum is ductile and work hardening. It can produce poor surface finish; thus, sharp tools are essential.
Nickel-based alloys and super alloys are work hardening, abrasive, and strong at high temperatures. Their
machinability depends on their condition and improves with annealing.
Tantalum is very work hardening, ductile, and soft. It produces a poor surface finish, and tool wear is high.
Titanium and its alloys have very poor thermal conductivity (the lowest of all metals), causing a significant
temperature rise and built-up edge. They are highly reactive and can be difficult to machine.
Tungsten is brittle, strong, and very abrasive; hence, its machinability is low, although it improves greatly at
elevated temperatures.
Zirconium has good machinability, but it requires a coolant-type cutting fluid because of the danger of explosion
and fire.

Machinability of Miscellaneous Materials
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Thermoplastics
 Generally have low thermal conductivity and a low elastic modulus, and they are thermally softening.
 Consequently, machining them requires sharp tools with positive rake angles (to reduce cutting forces), large
relief angles, small depths of cut and feed, relatively high speeds, and proper support of the workpiece.
 External cooling of the cutting zone may be necessary to keep the chips from becoming gummy and sticking to
the tools. Cooling usually can be achieved with a jet of air, a vapor mist, or water-soluble oils.
Thermosetting plastics
 are brittle and sensitive to thermal gradients during cutting;
 Machining conditions generally are similar to those of thermoplastics.
Polymer-matrix composites
 Very abrasive because of the fibers that are present; hence, they are difficult to machine. Fiber tearing, pulling,
and edge delamination are significant problems and can lead to severe reduction in the load carrying capacity
of the machined component.
 Machining of these materials requires careful handling and removal of debris to avoid contact with and inhaling
of the fibers.
 Metal-matrix and ceramic-matrix composites can be difficult to machine, depending on the properties of the
matrix material and the reinforcing fibers.
Graphite
 Abrasive; it requires sharp, hard, and abrasion-resistant tools.
Ceramics
 Steadily improved machinability, particularly with the development of machinable ceramics and Nano ceramics
and with the selection of appropriate processing parameters.
Wood
 Orthotropic material with properties varying with its grain direction.
 Type of chips and the surfaces produced also vary depending on the type of wood and its condition.
 Two basic requirements are generally sharp tools and high cutting speeds.

Tool Wear
23
DEFINITION: Gradual failure of cutting tools due to regular operations is known as tool wear.
MODES OF CUTTING TOOL FAILURES
FRACTURE FAILURE:
 This mode of failure occurs due to mechanical breakage due to excessive forces and shocks at the tool
point causing it to fail suddenly by brittle fracture.
 Also known as mechanical chipping.
 Such kind of tool failure is random and catastrophic in nature, results in premature loss of tool and
hence is extremely detrimental.
TEMPERATURE FAILURE:
 This failure occurs when the cutting temperature is too high for the tool material, causing the material
at the tool point to soften, which leads to plastic deformation and loss of the sharp edge.
 This type of failure also occurs rapidly, results in premature loss of tool and is quite detrimental and
unwanted.
Note: Both of the above kinds of tool failure need to be prevented by using suitable tool materials and
geometry depending upon the work material and cutting condition.
GRADUAL WEAR:
 Gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency, an
acceleration of wearing as the tool becomes heavily worn, and finally tool failure in a manner similar to
a temperature failure.
 Gradual wear is preferred because it leads to the longest possible use of the tool, option of changing
the tool before the final catastrophic loss of the cutting edge occurs, with the associated economic
advantage of that longer use.
Note: failure by gradual wear cannot be prevented but can be slowed down to enhance the service life of
the tool.

Tool Wear
24
WEAR ZONES & TYPES
OF GRADUAL WEAR
Gradual wear occurs at
three principal locations
on a cutting tool.
Accordingly, three main
types of tool wear can
be distinguished,
 ŒCrater wear (KT)
 •Flank wear (VB)
 ŽCorner wear
Types of wear observed in cutting tools

Tool Wear
25
Crater wear:
 It consists of a cavity or concave section on
the tool face/rake face formed and grows
from the action of the chip sliding against
the surface.
 High stresses and temperatures
characterize the tool–chip contact interface,
contributing to the wearing action.
 The crater can be measured either by its
depth or its area.
 Crater wear affects the mechanics of the
process increasing the actual rake angle of
the cutting tool and consequently, making
cutting easier.
 At the same time, the crater wear weakens
the tool wedge and increases the possibility
for tool breakage.
 This wear predominates at high speed. In
general, crater wear is of a relatively small
concern.
The crater wear is mainly caused due
to
 The presence of friction between the
chip-tool interface,
 The abrasion action of microchips
present at the chip-tool interface.
 The abrasive action of fragments of Built
up Edge (BUE) at the chip-tool interface
and diffusion wear.
 The diffusion wears, due to the atomic
attraction between the tool and work the
atoms of the tool material will get
diffused and deposited over the
workpiece called diffusion wear.

Tool Wear
26
Flank wear
 It occurs on the tool flank as a result of friction between the
machined surface of the workpiece and the tool flank.
 Flank wear appears in the form of so-called wear land and is
measured by the width of this wear land, VB.
 Flank wear affects to the great extend the mechanics of cutting.
 An extreme condition of flank wear often appears on the cutting
edge at the location corresponding to the original surface of the
workpart. This is called notch wear. It occurs because the original
work surface is harder and/or more abrasive than the internal
material, which could be caused by work hardening from cold
drawing or previous machining, sand particles in the surface from
casting, or other reasons. As a consequence of the harder
surface, wear is accelerated at this location.
 Cutting forces increase significantly with flank wear. If the
amount of flank wear exceeds some critical value (VB > 0.5~0.6
mm) then the excessive cutting force may cause tool failure.
 This wear predominates at low speed.
The reasons for flank wear are:
 The presence of friction at the tool work interface.
 The abrasive action of microchips or powdered particles present
at the tool work interface and diffusion wear.
 The diffusion wears, due to the atomic attraction between the
tool and work the atoms of the tool material will get diffused and
deposited over the workpiece called as diffusion wear.

Tool Wear
27
Corner wear/Nose wear
 It occurs on the tool corner.
 It can be considered as a part of the wear land and respectively flank wear since there is no
distinguished boundary between the corner wear and flank wear land.
 We consider corner wear as a separate wear type because of its importance for the precision of
machining.
 Corner wear actually shortens the cutting tool thus increasing gradually the dimension of machined
surface and introducing a significant dimensional error in machining, which can reach values of about
0.03~0.05 mm.
Figure: Top view showing the effect of tool corner wear on the dimensional precision in turning

Tool Wear Mechanisms
28
ABRASION
 This is a mechanical wearing action caused by hard particles in the work material gouging and removing
small portions of the tool.
 This abrasive action occurs in both flank wear and crater wear; it is a significant cause of flank wear.
ADHESION
 When two metals are forced into contact under high pressure and temperature, adhesion or welding occur
between them.
 These conditions are present between the chip and the rake face of the tool.
 As the chip flows across the tool, small particles of the tool are broken away from the surface, resulting in
attrition of the surface.
DIFFUSION
 This is a process in which an exchange of atoms takes place across a close contact boundary between two
materials.
 In the case of tool wear, diffusion occurs at the tool–chip boundary, causing the tool surface to become
depleted of the atoms responsible for its hardness.
 As this process continues, the tool surface becomes more susceptible to abrasion and adhesion.
 Diffusion is believed to be a principal mechanism of crater wear.
CHEMICAL REACTIONS
 The high temperatures and clean surfaces at the tool–chip interface in machining at high speeds can
result in chemical reactions, in particular, oxidation, on the rake face of the tool.
 The oxidized layer, being softer than the parent tool material, is sheared away, exposing new material to
sustain the reaction process.
PLASTIC DEFORMATION
 The cutting forces acting on the cutting edge at high temperature cause the edge to deform plastically,
making it more vulnerable to abrasion of the tool surface.
 Plastic deformation contributes mainly to flank wear.
Most of these tool-wear mechanisms are accelerated at higher cutting speeds and temperatures. Diffusion
and chemical reaction are especially sensitive to elevated temperature.

Tool Wear Curve
29
The general relationship of tool wear versus
cutting time is shown in Figure 23.3. Although
the relationship shown is for flank wear, a
similar relationship occurs for crater wear.
Three regions can usually be identified in the
typical wear growth curve.
Figure: Tool wear as a function of cutting time. Flank
wear (FW) is used here as the measure of tool wear.
Crater wear follows a similar growth curve.
 The first is the break-in period, in which the
sharp cutting edge wears rapidly at the
beginning of its use. This first region occurs
within the first few minutes of cutting.
 The break-in period is followed by wear that
occurs at a fairly uniform rate. This is called
the steady-state wear region. In our figure,
this region is pictured as a linear function of
time, although there are deviations from the
straight line in actual machining.
 Finally, wear reaches a level at which the
wear rate begins to accelerate. This marks
the beginning of the failure region, in which
cutting temperatures are higher, and the
general efficiency of the machining process
is reduced. If allowed to continue, the tool
finally fails by temperature failure..

Tool Life
30
DEFINITION: Tool life is generally defined by the
span of actual uninterrupted machining time
through which the tool or tool-tip renders desired
service and satisfactory performance and after
which that tool needs replacement
TOOL LIFE DETERMINATION PROCESS:
Tool wear is a time dependent process. As cutting
proceeds, the amount of tool wear increases
gradually. Rather than operating the tool until
final catastrophic failure an alternative way in
which a level of tool wear (flank wear at 0.3 to
0.6 mm shown by horizontal line on wear curve)
is set as safe limit. The safe limit is referred to as
allowable wear land (wear criterion), VB. The
cutting time required for the cutting tool to
develop a flank wear land of width VB is called
tool life, T, a fundamental parameter in
machining. When wear curves intersects that line,
the life of the corresponding tool is defined as
ended and the tool is replaced.

Tool Life & Cutting Parameters
31
Wear and hence tool life of any tool for any work
material is governed mainly by the level of the
machining parameters i.e., cutting velocity, feed
and depth of cut. Increased speed, feed, and
depth of cut have a similar effect, with speed
being the most important of the three. Cutting
velocity affects maximum and depth of cut
minimum. Harder work materials cause the wear
rate (slope of the tool wear curve) to increase.
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.3.2.3. The tool life obviously
decreases with the increase in cutting velocity
keeping other conditions unaltered as indicated in
Fig. 3.2.3.
Fig. 3.2.3 Growth of flank wear and assessment of tool
life

Taylor’s Tool Life Equation
32
If the tool lives, T1, T2, T3, T4 etc. are plotted
against the corresponding cutting velocities, V1,
V2, V3, V4 etc. as shown in Fig. 3.2.4, a smooth
curve like a rectangular hyperbola is found to
appear. When F. W. Taylor plotted the same figure
taking both V and T in log-scale, a more distinct
linear relationship appeared as schematically
shown in Fig. 3.2.5.
With the slope, n and intercept, C, Taylor derived
the simple equation as
VTn = C
where, n is called, Taylor’s tool life exponent. The
values of both ‘n’ and ‘C’ depend mainly upon the
tool-work materials and the cutting environment
(cutting fluid application). The value of C depends
also on the limiting value of VB undertaken (i.e.,
0.3 mm, 0.4 mm, 0.6 mm etc.) These constants
are well tabulated and easily available. Note that
the magnitude of C is the cutting speed at T = 1
min.
Figure: Cutting velocity – tool life relationship
Figure: Cutting velocity versus tool life on a log-log scale

Taylor’s Tool Life Equation
33
Problem: If in turning of a steel rod by a given cutting tool (material and geometry) at a given machining
condition (feed and depth of cut) under a given environment (cutting fluid application), the tool life
decreases from 80 min to 20 min. due to increase in cutting velocity, V from 60 m/min to 120 m/min., then
at what cutting velocity the life of that tool under the same condition and environment will be 40 min.?

Modified Taylor’s Tool Life Equation
34
In Taylor’s tool life equation, only the effect of variation of cutting velocity, V on
tool life has been considered. But practically, the variation in feed (f) and depth
of cut (d) also play role on tool life to some extent. Taking into account the
effects of all those parameters, the Taylor’s tool life equation has been modified
as,
Here-
d is the depth of cut and f is the feed in mm/rev
The exponents x and y must be determined
experimentally for each cutting condition. These
are now also available in machining handbooks.
Here x>y as tool life is affected more by depth
of cut then feed.

Tool Wear Control
35
The rate of tool wear strongly
depends on the cutting
temperature; therefore any
measures which could be
applied to reduce the cutting
temperature would reduce the
tool wear as well. Use of
cutting fluids, lubricants is
another method. Additional
measures to reduce the tool
wear include the application of
advanced cutting tool
materials, such as coated
carbides, ceramics, etc. The
figure shows the process
parameters that influence the
rate of tool wear:

Tool Materials
37
NEEDS OF CUTTING TOOL MATERIALS
With the progress of the industrial world it has
been needed to continuously develop and
improve the cutting tool materials and
geometry;
 to meet the growing demands for high
productivity, quality and economy of
machining
 to enable effective and efficient machining
of the exotic materials that are coming up
with the rapid and vast progress of science
and technology
 for precision and ultra-precision machining
 For micro and even Nano machining
demanded by the day and future.
The relative contribution of the cutting tool
materials on productivity, for instance, can be
roughly assessed from Fig. 2.2
Figure 2.2: Productivity raised by cutting
tool materials

CHRONOLOGICAL DEVELOPMENT OF CUTTING TOOL MATERIALS
38
Figure 2.3:
Chronological
development of
cutting tool
materials

Cutting Tool Characteristics
39
Hardness: The tool material must be harder than the work piece material. Higher the
hardness, easier it is for the tool to penetrate the work material.
Hot hardness: Hot Hardness is the ability of the cutting tool must to maintain its Hardness
and strength at elevated temperatures. This property is more important when the tool is used
at higher cutting speeds, for increased productivity. This property ensures that the tool does
not undergo any plastic deformation and thus retains its shape and sharpness
Toughness: Tool should have enough toughness to withstand the impact loads that come in
the start of the cut to force fluctuations due to imperfections in the work material. Toughness
of cutting tools is needed so that tools don’t chip or fracture, especially during interrupted
cutting operations like milling.
Wear Resistance: The tool-chip and chip-work interface are exposed to severe conditions
that adhesive and abrasion wear is very common. Wear resistance means the attainment of
acceptable tool life before tools need to be replaced.
Low friction: The coefficient of friction between the tool and chip should be low. This would
lower wear rates and allow better chip flow.
Chemical stability and inertness with respect to the material being machined, to avoid or
minimize any adverse reactions, adhesion, and tool-chip diffusion that would contribute to
tool wear.
Thermal characteristics: Since a lot of heat is generated at the cutting zone, the tool
material should have higher thermal conductivity to dissipate the heat in shortest possible
time; otherwise the tool temperature would become high, reducing its life.

Primary Cutting Tool Materials
40
Carbon and Medium alloy steels: These are the oldest of the tool materials
dating back hundreds of years. In simple terms it is a high carbon steel (steel which
contains about 0.9 to 1.3% carbon). Inexpensive, easily shaped, sharpened. No
sufficient hardness and wear resistance. Limited to low cutting speed operation
High Speed Steel (HSS-1905)
 Addition of alloying elements (manganese, chromium, tungsten, vanadium,
molybdenum, cobalt, and niobium) to harden and strengthen the steel and
make it more resistant to heat (hot hardness).
 They are of two types: Tungsten HSS (denoted by T), Molybdenum HSS
(denoted by M).
 The basic composition of HSS (18-4-1) is 18% W (for hot hardness), 4%
Cr (for deformation resistance), 1% V (for wear resistance), 0.7% C
and rest Fe.

41
High Speed Steel (HSS-1905)
 Such HSS tool could machine (turn) mild steel jobs at speed only upto 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 under 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.
With time the effectiveness and efficiency of HSS (tools) and their application
range were gradually enhanced by improving its properties and surface condition
through -
 Refinement of microstructure
 Addition of large amount of cobalt and Vanadium to increase 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 Physical Vapour Deposition (PVD)

42
Figure 2.4: Compositions and types of popular high speed steels
Commonly Used Grades of HSS

PRIMARY CUTTING TOOL MATERIALS
43
STELLITES
 This is a cast alloy of Co (40 to 50%), Cr (27 to 32%), W (14 to 19%)
and C (2%).
 Stellite is quite tough and more heat and wear resistive than the basic HSS
(18 – 4 – 1)
 But such Stellite as cutting tool material became obsolete for its poor
grindability and especially after the arrival of cemented carbides.
CEMENTED/SINTERED TUNGSTEN CARBIDES
 These are basically carbon cemented together by a binder.
 It is a powder metallurgy product and the binder mostly used is cobalt.
 The basic ingredient is tungsten carbide-82%, titanium carbide-10% and
cobalt-8%.
 These materials possess high hardness and wear resistance
 Cutting speed 6 times higher than high speed steel (HSS).

44
SINTERED CARBIDES
Straight or single carbide
 Powder metallurgically produced by mixing, compacting and sintering 90 to 95% WC
powder with cobalt.
 The hot, hard and wear resistant WC grains are held by the binder Co which provides
the necessary strength and toughness.
 Suitable for machining grey CI, brass, bronze etc. which produce short discontinuous
chips and at cutting velocities 2 to 3 times of HSS tools.
Composite carbides
 The single carbide is not suitable for machining steels because of rapid growth of
wear, particularly crater wear by diffusion of Co and carbon from the tool to the chip
under the high stress and temperature bulk (plastic) contact between the continuous
chip and the tool surfaces.
 Used for machining steels
 By adding (8 to 20%) a gamma phase to WC and Co mix. The gamma phase is a mix
of TiC, TiN, TaC, NiC etc. which are more diffusion resistant than WC
Mixed carbides
 Titanium carbide (TiC) is not only more stable but also much harder than WC. So for
machining ferritic steels causing intensive diffusion and adhesion 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.

45
SINTERED CARBIDES
Gradation of cemented carbides and their applications
The standards developed by ISO for grouping of carbide tools and their application
ranges are given in figure below
Figure 2.5: Broad classification of carbide tools

46
PLAIN CERAMICS
 It mainly consists of aluminum oxide (Al2O3) and silicon nitride (Si3N4).
 Ceramic cutting tools are hard with high hot hardness and do not react with the workpiece.
 They can be used at elevated temperature and cutting speed 4 times that of cemented carbide.
 These have low heat conductivity.
 Inherently high compressive strength, chemical stability and hot hardness of the ceramics led to
powder metallurgical production of indexable ceramic tool inserts since 1950.
Table below shows the advantages and limitations of alumina ceramics in contrast to sintered
carbide.
Figure 2.6: Cutting tool properties of alumina ceramics
Alumina (Al2O3) is preferred to silicon nitride (Si3N4) for higher hardness and chemical stability.
Si3N4 is tougher but again more difficult to process. The plain ceramic tools are brittle in nature and
hence had limited applications.
* Cutting tool should resist penetration of heat but should disperse the heat throughout the core.

47
PLAIN 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 for machining cast iron and similar materials at speeds 200 to 250
m/min
 Alumina with or without additives : hot pressed, black color, hard and strong, used for
machining steels and cast iron at speeds = 150 to 250 m/min
 Carbide ceramic (Al2O3 + 30% TiC) cold or hot pressed, black color, quite strong and
enough tough – used for machining hard cast irons and plain and alloy steels at 150 to 200
m/min.
The plain ceramic outperformed the then existing tool materials in some application areas like
high speed machining of softer steels mainly for higher hot hardness as indicated in Fig. 2.7
However, the use of those brittle plain ceramic tools,
until their strength and toughness could be
substantially improved since 1970, gradually decreased
for being restricted to
 uninterrupted machining of soft cast irons and
steels only
 relatively high cutting velocity but only in a narrow
range (200 ~ 300 m/min)
 requiring very rigid machine tools
 Advent of coated carbide capable of machining cast
iron and steels at high velocity made the then
ceramics almost obsolete.
Figure 2.7: Hot hardness of the different
commonly used tool materials.

Advanced Cutting Tool Materials
48
1. Coated carbides
2. Cermets
3. Coronite
4. High Performance Ceramics (HPC)
5. Cubic Boron Nitride (cBN)
6. Diamond

49
COATED CARBIDES
The properties and performance of carbide tools could
be substantially improved by
 Refining microstructure
 Manufacturing by casting: expensive and
uncommon
 Surface coating – made remarkable contribution.
Thin but hard coating of single or multilayers of more
stable and heat and wear resistive materials like TiC,
TiCN, TiOCN, TiN, Al2O3 etc. on the tough carbide
inserts (substrate) by processes like chemical Vapour
Deposition (CVD), Physical Vapour Deposition (PVD)
at controlled pressure and temperature enhanced
MRR and overall machining economy remarkably
enabling,
 reduction of cutting forces and power consumption
 increase in tool life (by 200 to 500%) for same V or
increase in V (by 50 to 150%) for same tool life
 improvement in product quality
 effective and efficient machining of wide range of
work materials
 pollution control by less or no use of cutting fluid
through
 reduction of abrasion, adhesion and diffusion wear
 reduction of friction and BUE formation
 heat resistance and reduction of thermal cracking
and plastic deformation
Fig. 2.8 Machining by coated carbide insert.
The cutting velocity range in machining mild steel
could be enhanced from 120 ~ 150 m/min to 300 ~
350 m/min by properly coating the suitable carbide
inserts.
About 50% of the carbide tools being used at
present are coated carbides
The properties and performances of coated inserts
and tools are getting further improved by;
 Refining the microstructure of the coating
 Multilayering (already up to 13 layers within 12 ~
16 μm)
 Direct coating by TiN instead of TiC, if feasible
 Using better coating materials

50
CERMETS
 These sintered hard inserts are made by combining ‘cer’ from ceramics like TiC, TiN
or TiCN and ‘met’ from metal (binder) like Ni, Ni-Co, Fe etc.
 Since around 1980, the modern Cermets providing much better performance are
being made by TiCN which is consistently more wear resistant, less porous and
easier to make.
The characteristic features of such Cermets, in contrast to sintered tungsten carbides,
are:
 The grains are made of TiCN (in place of WC) and Ni or Ni-Co and Fe as binder (in
place of Co)
 Harder, more chemically stable and hence more wear resistant
 More brittle and less thermal shock resistant
 Wt% of binder metal varies from 10 to 20%
 Cutting edge sharpness is retained unlike in coated carbide inserts
 Can machine steels at higher cutting velocity than that used for tungsten carbide,
even coated carbides in case of light cuts.
 TiCN based Cermets with bevelled or slightly rounded cutting edges are suitable
for finishing and semi-finishing of steels at higher speeds, stainless steels
 Not suitable for jerky interrupted machining and machining of aluminium and
similar materials.

51
CORONITE
 It is already mentioned earlier that the properties and performance of HSS tools
could have been improved by refinement of microstructure, powder
metallurgical process of making and surface coating.
 tool material Coronite has been developed for making the tools like small and
medium size drills and milling cutters etc. which were earlier essentially made of
HSS.
 Coronite is made basically by combining HSS for strength and toughness
and tungsten carbides for heat and wear resistance. Microfine TiCN
particles are uniformly dispersed into the matrix.
 Unlike a solid carbide, the coronite based tool is made of three layers;
 the central HSS or spring steel core
 a layer of Coronite of thickness around 15% of the tool diameter
 a thin (2 to 5 μm) PVD coating of TiCN.
 Such tools are not only more productive but also provide better product quality.
The Coronite tools made by hot extrusion followed by PVD-coating of TiN or
TiCN outperformed HSS tools in respect of cutting forces, tool life and surface
finish.

52
HIGH PERFORMANCE CERAMICS (HPC)
Ceramic tools as such are much superior to sintered carbides in respect of hot hardness,
chemical stability and resistance to heat and wear but lack in fracture toughness and
strength.
Through last few years remarkable improvements in strength and toughness and hence overall
performance of ceramic tools could have been possible by several means which include;
 Sinterability, microstructure, strength and toughness of Al2O3 ceramics were improved to
some extent by adding TiO2 and MgO
 Transformation toughening by adding appropriate amount of partially or fully stabilized
zirconia in Al2O3 powder
 Isostatic and hot isostatic pressing (HIP) – these are very effective but expensive route
 Introducing nitride ceramic (Si3N4) with proper sintering technique – this material is very
tough but prone to built-up-edge formation in machining steels
 Developing SIALON – deriving beneficial effects of Al2O3 and Si3N4
 Adding carbide like TiC (5 ~ 15%) in Al2O3 powder – to impart toughness and thermal
conductivity
 Reinforcing oxide or nitride ceramics by SiC whiskers, which enhanced strength,
toughness and life of the tool and thus productivity spectacularly. But manufacture and
use of this unique tool need specially careful handling
 Toughening Al2O3 ceramic by adding suitable metal like silver which also impart thermal
conductivity and self-lubricating property; this novel and inexpensive tool is still in
experimental stage.

53
The enhanced qualities of the unique high performance ceramic tools, specially the whisker
and zirconia based types enabled them machine structural steels at speed even beyond 500
m/min and also intermittent cutting at reasonably high speeds, feeds and depth of cut. Such
tools are also found to machine relatively harder and stronger steels quite effectively and
economically.
The HPC tools can be broadly classified into two groups as:
Fig. 2.9 Classification of HPC Tools

54
NITRIDE BASED CERAMIC TOOLS
Plain nitride ceramics tools
 Compared to plain alumina ceramics, Nitride (Si3N4) ceramic tools exhibit more resistance
to fracturing by mechanical and thermal shocks due to higher bending strength, toughness
and higher conductivity.
 More suitable for rough and interrupted cutting of various material excepting steels, which
cause rapid diffusional wear and BUE formation.
 The fracture toughness and wear resistance can be increased by adding zirconia and coating
the finished tools with high hardness alumina and titanium compound.
 Nitride ceramics cannot be easily compacted and sintered to high density. Sintering with the
aid of ‘reaction bonding’ and ‘hot pressing’ may reduce this problem to some extent.
SIALON tools (Si-Al-O-N)
 By Hot pressing and sintering of an appropriate mix of Al2O3 and Si3N4 powders an excellent
composite ceramic tool called SIALON is made.
 Very hot hard, quite tough and wear resistant.
 Can machine steel and cast irons at high speed (250–300 m/min).
 But machining of steels by such tools at too high speeds reduces the tool life by rapid
diffusion.
SiC reinforced Nitride tools
 The toughness, strength and thermal conductivity and hence the overall performance of
nitride ceramics could be increased remarkably by adding SiC whiskers or fibers in 5 – 25
volume %.
 The SiC whiskers add fracture toughness mainly through crack bridging, crack deflection
and fiber pull-out.
 Such tools are very expensive but extremely suitable for high production machining of
various soft and hard materials even under interrupted cutting.

55
OXIDE BASED CERAMIC TOOLS
Zirconia (or partially stabilized Zirconia-PSZ)
toughened alumina (ZTA) ceramic
 Fine powder of partially stabilized zirconia
(PSZ) is mixed in proportion of ten to
twenty volume percentage with pure
alumina, then either cold pressed and
sintered at 1600–1700oC or hot
isostatically pressed (HIP) under suitable
temperature and pressure.
 The phase transformation of metastable
tetragonal zirconia (t-Z) to monoclinic
zirconia (m-Z) during cooling of the
composite (Al2O3 + ZrO2) inserts after
sintering or HIP and during polishing and
machining imparts the desired strength
and fracture toughness through volume
expansion (3 – 5%) and induced shear
strain (7%).
 Their hardness have been raised further by
proper control of particle size and
sintering process.
 Hot pressing and HIP raise the density,
strength and hot hardness of ZTA tools but
the process becomes expensive and the
tool performance degrades at lower
cutting speeds.
 However such ceramic tools can machine
steel and cast iron at speed range of 150 –
500 m/min.
Alumina ceramic reinforced by SiC whiskers
 Alumina based ceramic tools have been improved by drastic
increase in fracture toughness (2.5 times) and bulk thermal
conductivity without sacrificing hardness and wear resistance.
 This is achieved by mechanically reinforcing the brittle alumina
matrix with extremely strong and stiff silicon carbide whiskers.
 The randomly oriented, strong and thermally conductive
whiskers enhance the strength and toughness mainly by crack
deflection and crack-bridging and also by reducing the
temperature gradient within the tool.
 After optimization of the composition, processing and the tool
geometry, such tools can machine wide range of materials,
over wide speed range (250 – 600 m/min) even under large
chip loads.
 Manufacturing of whiskers need careful handling and precise
control
 Costlier than zirconia toughened ceramic tools.
Silver toughened alumina ceramic
 Alumina-metal composites have been studied primarily using
addition of metals like aluminium, nickel, chromium,
molybdenum, iron and silver.
 Compared to zirconia and carbides, metals were found to
provide more toughness in alumina ceramics.
 Again compared to other metal-toughened ceramics, the
silver-toughened ceramics can be manufactured by simpler
and more economical process routes like pressure less
sintering and without atmosphere control.
 Such HPC tools can suitably machine with large MRR and V
(250 – 400 m/min) and long tool life even under light
interrupted cutting like milling.
 Such tools also can machine steels at speed from quite low to
very high cutting velocities (200 to 500 m/min).

56
CUBIC BORON NITRIDE (CBN)
 Next to diamond, cubic boron nitride is the hardest material presently available.
 Only in 1970 and onward cBN in the form of compacts has been introduced as cutting tools.
 It is made by bonding a 0.5 – 1 mm layer of polycrystalline cubic boron nitride to cobalt based
carbide substrate at very high temperature and pressure.
 It remains inert and retains high hardness and fracture toughness at elevated machining speeds.
 It shows excellent performance in grinding any material of high hardness and strength.
 The extreme hardness, toughness, chemical and thermal stability and wear resistance led to the
development of cBN cutting tool inserts for high material removal rate (MRR) as well as precision
machining imparting excellent surface integrity of the products.
 Such unique tools effectively and beneficially used in machining wide range of work materials
covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like Ni-hard,
Inconel, Nimonic etc and many non-metallic materials which are as such difficult to machine by
conventional tools.
 It is firmly stable at temperatures upto 1400o C.
 The operative speed range for cBN when machining grey cast iron is 300 ~ 400 m/min.
 Speed ranges for other materials are as follows :
 Hard cast iron (> 400 BHN) : 80 – 300 m/min
 Superalloys (> 35 RC) : 80 – 140 m/min
 Hardened steels (> 45 RC) : 100 – 300 m/min
 In addition to speed, the most important factor that affects performance of cBN inserts is the
preparation of cutting edge. It is best to use cBN tools with a honed or chamfered edge preparation,
especially for interrupted cuts.
 Like ceramics, cBN tools are also available only in the form of indexable inserts. The only limitation of
it is its high cost.

57
CUBIC BORON NITRIDE (CBN)
 Next to diamond, cubic boron nitride is the hardest material presently available.
 Only in 1970 and onward cBN in the form of compacts has been introduced as cutting tools.
 It is made by bonding a 0.5 – 1 mm layer of polycrystalline cubic boron nitride to cobalt based
carbide substrate at very high temperature and pressure.
 It remains inert and retains high hardness and fracture toughness at elevated machining speeds.
 It shows excellent performance in grinding any material of high hardness and strength.
 The extreme hardness, toughness, chemical and thermal stability and wear resistance led to the
development of cBN cutting tool inserts for high material removal rate (MRR) as well as precision
machining imparting excellent surface integrity of the products.
 Such unique tools effectively and beneficially used in machining wide range of work materials
covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like Ni-hard,
Inconel, Nimonic etc and many non-metallic materials which are as such difficult to machine by
conventional tools.
 It is firmly stable at temperatures upto 1400o C.
 The operative speed range for cBN when machining grey cast iron is 300 ~ 400 m/min.
 Speed ranges for other materials are as follows :
 Hard cast iron (> 400 BHN) : 80 – 300 m/min
 Superalloys (> 35 RC) : 80 – 140 m/min
 Hardened steels (> 45 RC) : 100 – 300 m/min
 In addition to speed, the most important factor that affects performance of cBN inserts is the
preparation of cutting edge. It is best to use cBN tools with a honed or chamfered edge preparation,
especially for interrupted cuts.
 Like ceramics, cBN tools are also available only in the form of indexable inserts. The only limitation of
it is its high cost.

58
DIAMOND TOOLS
SINGLE CRYSTALLINE DIAMOND (SCD)
 Single crystal diamond (natural or
synthetic) are used as tips/edge of
cutting tools.
 Extreme hardness and sharp edges,
natural single crystal is used for many
applications, particularly where high
accuracy and precision are required.
 Their important uses are:
 Single point cutting tool tips and
small drills for high speed
machining of non-ferrous metals,
ceramics, plastics, composites, etc.
and effective machining of
difficult-to-machine materials
 Drill bits for mining, oil
exploration, etc.
 Tool for cutting and drilling in
glasses, stones, ceramics, FRPs
etc.
 Wire drawing and extrusion dies
 Super abrasive wheels for critical
grinding.
 Limited supply, increasing demand,
high cost and easy cleavage of natural
diamond has led to the manufacture
of artificial diamond grits with desired
control on various parameters.
POLYCRYSTALLINE DIAMOND (PCD)
 PCD tools consist of a layer (0.5 to 1.5 mm) of fine grain size,
randomly oriented diamond particles sintered with a suitable
binder (usually cobalt) and then metallurgically bonded to a
suitable substrate like cemented carbide or Si3N4 inserts.
 PCD exhibits excellent wear resistance, hold sharp edge,
generates little friction in the cut, provide high fracture
strength, and had good thermal conductivity. These properties
contribute to PCD tooling’s long life in conventional and high
speed machining of soft, non-ferrous materials (aluminium,
magnesium, copper etc.), advanced composites and metal-
matrix composites, super alloys, and non-metallic materials.
 PCD is particularly well suited for abrasive materials (i.e.
drilling and reaming metal matrix composites) where it
provides 100 times the life of carbides.
 PCD is not usually recommended for ferrous metals because of
high solubility of diamond (carbon) in these materials at
elevated temperature. However, under special conditions can
be used; for example, light cuts are being successfully made in
grey cast iron.
 The main advantage of such PCD tool is the greater toughness
due to finer microstructure with random orientation of the
grains and reduced cleavage. But such unique PCD also suffers
from some limitations like:
 High tool cost
 Presence of binder, cobalt, which reduces wear resistance
and thermal stability
 Complex tool shapes like in-built chip breaker cannot be
made
 Size restriction, particularly in making very small diameter
tools

59
DIAMOND TOOLS
DIAMOND COATED CARBIDE TOOLS
 Since the invention of low pressure synthesis of diamond from gaseous phase, continuous effort
has been made to use thin film diamond in cutting tool field. These are normally used as thin (<50
μm) or thick (> 200 μm) films of diamond synthesized by CVD method for cutting tools, dies, wear
surfaces and even abrasives for Abrasive Jet Machining (AJM) and grinding. Thin film is directly
deposited on the tool surface.
 Thick film ( > 500 μm) is grown on an easy substrate and later brazed to the actual tool substrate
and the primary substrate is removed by dissolving it or by other means. Thick film diamond finds
application in making inserts, drills, reamers, end mills, routers. CVD coating has been more
popular than single diamond crystal and PCD mainly for :
 Free from binder, higher hardness, resistance to heat and wear more than PCD and properties
close to natural diamond
 Highly pure, dense and free from single crystal cleavage
 Permits wider range of size and shape of tools and can be deposited on any shape of the tool
including rotary tools
 Relatively less expensive
 However, achieving improved and reliable performance of thin film CVD diamond coated tools;
(carbide, nitride, ceramic, SiC etc) in terms of longer tool life, dimensional accuracy and surface
finish of jobs essentially need:
 Good bonding of the diamond layer
 Adequate properties of the film, e.g. wear resistance, micro-hardness, edge coverage, edge
sharpness and thickness uniformity
 Ability to provide work surface finish required for specific applications.
 While cBN tools are feasible and viable for high speed machining of hard and strong steels and
similar materials, Diamond tools are extremely useful for machining stones, slates, glass, ceramics,
composites, FRPs and nonferrous metals specially which are sticky and BUE former such as pure
aluminium and its alloys.
 CBN and Diamond tools are also essentially used for ultraprecision as well as micro and nano
machining.

Cutting Tool Materials
60
Figure 2.10: (A) Hardness of various cutting-tool materials as a function of temperature. (B)
Ranges of properties of various groups of materials.

Economics of Machining
61
Cutting conditions in a machining operation consist of speed, feed, depth of cut, and
cutting fluid.
Cutting Fluids: Requirements and type are determined by tooling considerations.
Depth of cut: It is often predetermined by workpiece geometry and operation sequence
like roughing operations (high depth of cut) followed by a final finishing operation (low depth
of cut).
The problem then reduces to selection of feed and speed. In general, values of
these parameters should be decided in the order: feed first, speed second.
Feed rate for a given machining operation depends on the following factors:
Tooling: What type of tooling will be used? Harder tool materials (e.g., cemented carbides,
ceramics, etc.) tend to fracture more readily than high-speed steel. These tools are normally
used at lower feed rates. HSS can tolerate higher feeds because of its greater toughness.
Roughing or finishing: Roughing operations involve high feeds, typically 0.5 to 1.25
mm/rev for turning; finishing operations involve low feeds, typically 0.125 to 0.4 mm/rev for
turning.
Constraints on feed in roughing: If the operation is roughing, how high can the feed
rate be set? To maximize metal removal rate, feed should be set as high as possible. Upper
limits on feed are imposed by cutting forces, setup rigidity, and sometimes horsepower.
Surface finish requirements in finishing: If the operation is finishing, what is the
desired surface finish? Feed selected affects the surface finish.
The problem now reduces to selection of speed which is discussed now.

Optimizing Cutting Speed
62
 It means choosing a speed that provides a high metal removal rate
along with long tool life.
 Optimal cutting speed for a machining operation can be determined if
the various time and cost components of the operation are known.
 The original derivation of these machining economics equations was
done by W. Gilbert.
 The formulas allow the optimal cutting speed to be calculated for
either of two objectives:
 (1) Maximum production rate
 (2) Minimum unit cost.
 Both objectives seek to achieve a balance between material removal
rate and tool life.

Maximizing Production Rate Model
63
Objective:
To determine optimum speed that minimizes machining time per workpiece i.e.
maximizes production rate.
Assumptions:
 The formulas are based on Taylor tool life equation.
 Feed, depth of cut, and work material have already been set.
 The derivation is for a turning operation.
Time Elements: Total production cycle time for one part Tc is consists of-
 Part handling time Th: Time for loading & unloading of part on machine.
 Machining time Tm: Actual tool cutting time during the cycle.
 Tool change time Tt: Changing worn tool takes time (transfer for t) which is
apportioned over the number of parts cut during the tool life. Let np the
number of pieces cut in one tool life thus, the tool change time per part =Tt/np

64
The sum of these three time elements gives
the total time per unit product for the
operation cycle
Tc is a function of cutting speed. As cutting
speed is increased, Tm decreases and Tt/np
increases; This unaffected by speed.
The cycle time per part is minimized at a
certain value of cutting speed. This optimal
speed can be identified by recasting total
cycle time equation as a function of speed.
Machining time in a straight turning
operation is given by-
Where Tm = machining time per part, min
D= workpart diameter, mm
L =workpart length, mm
f=feed, mm/rev
v=cutting speed, mm/min

65
The number of pieces per tool np is also a function
of speed. It can be shown that-
Where
T =tool life, min/tool
Tm=machining time per part, min/pc.
Both T and Tm are functions of speed; hence, the
ratio is a function of speed Solving this equation yields the cutting speed for
maximum production rate in the operation
The cycle time per piece is a minimum at the cutting
speed at which the derivative of (1) is zero
The corresponding tool life for maximum production
rate is

Minimizing Cost per Unit Model
66
Objective:
To determine optimum speed that minimizes
production cost per piece for the operation
Assumptions:
 The formulas are based on Taylor tool life
equation.
 Feed, depth of cut, and work material have
already been set.
 The derivation is for a turning operation.
Cost Elements: Total cost of producing one part
during a turning operation is consists of-
 Cost of part handling time: Cost of the time in
loading and unloading the part. Let Co =the cost
for the operator and machine. Thus the cost of
part handling time =CoTh
 Cost of machining time: This is the cost of the
time the tool is engaged in machining. Using Co
again to represent the cost per minute of the
operator and machine tool, the cutting time
cost=CoTm
 Cost of tool change time: The cost of tool change
time = CoTt/np
 Tooling cost: In addition to the tool change time,
the tool itself has a cost that must be added to
the total operation cost. This cost is the cost per
cutting edge Ct divided by the number of pieces
machined with that cutting edge np. Thus, tool
cost per workpiece is given by Ct/np
Note: Variation in Tooling cost
For disposable inserts (e.g.,
cemented carbide inserts),
tool cost is determined as
where
Ct =cost per cutting edge,
Rs/tool life
Pt = price of the insert,
Rs/insert
ne =number of cutting edges
per insert
This depends on the insert
type; for example, triangular
inserts that can be used only
one side (positive rake
tooling) have three
edges/insert; if both sides of
the insert can be used
(negative rake tooling), there
are six edges/insert; and so
forth.
For regrindable tooling (e.g.,
high-speed steel solid shank
tools, brazed carbide tools),
the tool cost includes
purchase price plus cost to
regrind:
Where
Ct=cost per tool life, Rs/tool
life
Pt=purchase price of the solid
shank tool or brazed insert,
Rs/tool
ng=number of tool lives per
tool, which is the number of
times the tool can be ground
before it can no longer be
used (5 to 10 times for
roughing tools and 10 to 20
times for finishing tools);
Tg=time to grind or regrind
the tool, min/tool life
Cg=grinder’s rate, Rs/min.

Minimizing Cost per Unit Model
67
The sum of the four cost components gives the
total cost per unit product Cc for the machining
cycle:
Cc is a function of cutting speed, just as Tc is a
function of v. The relationships for the individual
terms and total cost as a function of cutting speed
are shown in Figure
Eq. (1) can be rewritten in terms of v to yield:
The cutting speed that obtains minimum cost per piece
for the operation can be determined by taking the
derivative of above Eq. with respect to v, setting it to
zero, and solving for vmin
The corresponding tool life is given by

Cutting Fluids: Definition & Purpose
69
DEFINITION: A cutting fluid is any liquid or gas that is applied directly to
the machining operation to improve cutting performance.
PURPOSE:
 Act as lubricant: Reduce friction and wear by acting as a film and
hence also reduce welding tendency.
 Act as coolant: Cooling of cutting zone and hence increasing tool life
& improving dimensional stability, Reducing the temperature of the
workpart for easier handling
 Reduce forces and energy consumption.
 Flush away the chips from the cutting zone to avoid interference in
cutting
 Protect the machined surface from environmental corrosion
(weakening and depletion of surface by deposition of other matter like
rust) & contamination by the gases like SO2, O2, H2S, and NxOy present
in the atmosphere

Essential Properties of Cutting Fluids
70
 For cooling:
 High specific heat (high heat absorbing capacity), thermal conductivity and
film coefficient for heat transfer
 spreading and wetting ability
 For lubrication:
 High lubricity without gumming (accumulation & sticking of dirt & dust)
and foaming
 Wetting and spreading
 High film boiling point
 Friction reduction at extreme pressure and temperature
 Chemical stability, non-corrosive to the materials of the system
 less volatile (tendency of evaporation) and high flash point (temperature at
which fluid vapor ignite by some source)
 high resistance to bacterial growth
 odorless and also preferably colorless
 nontoxic in both liquid and gaseous stage
 Easily available and low cost.
 It should permit clear view of the work operation.

Types of Cutting Fluids
71
 Air blast or compressed air only
 Grey cast iron use no cutting fluid in liquid form as graphite flakes in the cast
iron acts as a lubricant in itself by sliding over each other and producing short
discontinuous chips.
 In such cases only air blast is recommended for cooling and cleaning
 Water
 For its good wetting and spreading properties and very high specific heat,
water is considered as the best coolant and hence employed where cooling is
most urgent.
 Cutting oils or Straight oils
 Compounds of mineral oil with vegetable, animal or marine oils added for
improving spreading, wetting and lubricating properties.
 As and when required some EPA additive including Sulphur, chlorine, and
phosphorus are also mixed to reduce friction, adhesion and BUE formation in
heavy cuts.
 Additives react chemically with the chip and tool surfaces to form solid films
(extreme pressure lubrication) that help to avoid metal-to-metal contact
between the two.
 Advantages:: excellent lubrication, good corrosion protection, easy
maintenance
 Limitation & Disadvantages: poor heat removal, toxic mist, high viscosity,
flammable, expensive, not suitable for high speed machining

72
Emulsified oils or soluble oils
 Oil droplets suspended in water
 Water acts as the best coolant but does not lubricate and also
induce rusting.
 So oil containing some emulsifying agent to improve blending and
stability (An emulsion is a mixture of two or more liquids that are
normally immiscible (unmixable or unblendable)) and extreme
pressure additive (EPA) like Sulphur, chlorine, and phosphorus are
mixed with water in a suitable ratio (1:30). It look likes white milk,
used widely, have less lubrication qualities but have good cooling
ability.
 Advantages: good lubrication, good cooling capability, some
corrosion protection, low cost, nonflammable.
 Disadvantages: anti-bacteria additives and maintenance are
needed, toxic mist, susceptible to hard water (may form insoluble
precipitates).

73
Chemical fluids or synthetic fluids
 Blended chemicals with additives, diluted in water, and containing no
oil.
 Synthetic fluids are water based solutions (or emulsions) of synthetic
lubricants (soaps and other wetting agents), corrosion inhibitors, water
softeners, Extreme pressure additives (EPA), anti-bacteria additives
(biocides), glycols and other additives.
 Synthetic fluids are supplied in form of concentrates, which are mixed
with water before use.
 Synthetic fluids are used in a wide variety of metalworking operations
including poorly machinable alloys, heavy duty grinding, high speed
cutting.
 Advantages of synthetic fluids: very good cooling capability, good
lubrication properties, good stability in hard water, quick wetting ability,
low surface tension so good spreading, good corrosion protection, easy
handling, cleaning and maintenance.
 Disadvantages/Limitations of synthetic fluids: some toxicity,
easily contaminated by foreign oils, relatively high cost.

74
Semi-Synthetic or Semi-Chemical Fluids
 These will contain small amounts of oil and other additives to enhance lubrication
while providing maximum cooling.
 Semi-synthetic fluids are water based mixture (solution and emulsion) of synthetic
lubricants, additives, emulsifiers and some amount (2%-30%) of mineral oil.
 Semi-synthetic fluids combine advantages (and disadvantages at some extent) of
mineral emulsions and synthetic fluids
 Advantages: They possess better corrosion protection than synthetic fluids and
better cooling and wetting capabilities, easier handling and maintenance than
mineral emulsions.
 Disadvantages/Limitations: misting, relatively poor stability in hard water,
contaminated by foreign oils, some toxicity, lower lubrication ability, possible skin
irritants, and less corrosion protection.
Solid or Semi-Solid Lubricant
 Paste, waxes, soaps, graphite, Moly-disulphide (MoS2) may also often be used, either
applied directly to the workpiece or as an impregnant in the tool to reduce friction
and thus cutting forces, temperature and tool wear.
Cryogenic cutting fluid
 Extremely cold (cryogenic) fluids (often in the form of gases) like liquid CO2 or N2 are
used in some special cases for effective cooling without creating much environmental
pollution and health hazards.

Selection of Cutting Fluids
75
 For high speed machining of not-difficult-to-machine materials greater cooling type fluids are preferred
 For low speed machining of both conventional and difficult-to-machine materials greater lubricating type fluid is
preferred.
Grey cast iron:
 Generally dry for its self-lubricating property
 Air blast for cooling and flushing chips
Steels:
 Soluble oil for cooling and flushing chips in high speed machining and grinding
 If machined by HSS tools, sol. Oil (1: 20 ~30) for low carbon and alloy steels and neat oil with EPA for heavy
cuts
 If machined by carbide tools thinner sol. Oil for low strength steel, thicker sol. Oil ( 1:10 ~ 20) for stronger
steels and straight sulphurised oil for heavy and low speed cuts and EP cutting oil for high alloy steel.
 Often steels are machined dry by carbide tools for preventing thermal shocks.
Aluminum 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
 Oil with or without inactive EPA for tougher grades of Cu-alloy.
Stainless steels and Heat resistant alloys:
 High performance soluble oil or neat oil with
 High concentration with chlorinated EP additive.
The brittle ceramics and cermets should be used either under dry condition or light neat oil in case of fine finishing.
Grinding at high speed needs cooling (1: 50 ~ 100) soluble oil. For finish grinding of metals and alloys low viscosity
neat oil is also used.

Application Methods of Cutting Fluids
76
 Drop-by-drop under gravity
 In the form of liquid jet(s)
 Flooding or Flood Cooling (under gravity)
 Most common method also called flood as used with coolant-type cutting fluids.
 A flood of cutting fluid is applied at the tool–work or tool–chip interface
 Mist (atomized oil) with compressed air
 Supplies fluid to inaccessible areas in the form of a high-speed mist using
pressurized air stream.
 Provides better visibility of the workpiece being machined
 It is effective with water-based fluids at air pressures ranging from 70 to 600 kPa.
 Limited cooling capacity.
 Requires venting to prevent the inhalation of airborne fluid particles by operator
 Manual Operation:
 Manual application by means of a paint brush
 Used in tapping and other operations in which cutting speeds are low and friction
is a problem.
 Not preferred by most production machine shops because of its variability in
application.

77
 High-pressure systems
These systems use high-pressure refrigerated coolant systems to increase the
rate of heat removal from the cutting zone.
High pressures also are used in delivering the cutting fluid via specially
designed nozzles that aim a powerful jet of fluid to the zone, particularly into
the clearance or relief face of the tool.
The pressures employed, which are usually in the range from 5.5 to 35 MPa,
act as a chip breaker in situations where the chips produced would otherwise
be long and continuous, interfering with the cutting operation.
In order to avoid damage to the workpiece surface by impact from any
particles present in the high-pressure jet, contaminant size in the coolant
should not exceed 20 um.
Proper and continuous filtering of the fluid also is essential to maintain quality.

78
 Through the cutting tool system
Narrow passages can be produced in cutting tools, as well as in tool holders,
through which cutting fluids can be applied under high pressure.
Two applications of this method are
 Gun drilling, with a long, small hole through the body of the drill itself
 Boring bars, where there is a long hole through the shank (tool holder), to
which an insert is clamped.
Delivering cutting fluids through the spindle of the machine tool is also
developed.
 Z-Z method – centrifugal through the grinding wheels (pores)
Figure 2.13 Application of cutting fluid by hole in the tool Figure 2.14: Z-Z Method in grinding

Cutting Fluid Filtration
79
 Cutting fluids become contaminated over time by various substances like tramp
oil (machine oil, hydraulic fluid, etc.), garbage (cigarette butts, food, etc.), small
chips, molds, fungi, and bacteria.
 In addition to causing odors and health hazards, contaminated cutting fluids do
not perform their lubricating function as well.
 Alternative ways of dealing with this problem are to:
replace the cutting fluid at regular and frequent intervals
use a filtration system to periodically clean the fluid
Dry machining; that is, machine without cutting fluids.
 Because of growing concern about environmental pollution and associated
legislation, disposing old fluids is a major concern.
 Filtration systems are being installed which have advantages of prolonged
cutting fluid life between changes, reduced fluid disposal cost, cleaner cutting
fluid for better working environment and reduced health hazards, lower machine
tool maintenance & longer tool life. Several techniques of filtration are settling,
skimming, centrifuging, and filtering.
 Recycling involves treatment of the fluids with various additives, agents,
biocides, and deodorizers, as well as water treatment (for water-based fluids).

Dry Machining
80
 No cutting fluid is used.
 It has advantages of addressing pollution concern, cost reduction & improving
surface quality. Associated problems are overheating the tool, operating at lower
cutting speeds and production rates to prolong tool life, and absence of chip
removal benefits in grinding and milling.
 Application of a fine mist of an air-fluid mixture having very small amount of
cutting fluid.
 The mixture is delivered to the cutting zone through the spindle of the machine
tool, typically through a 1-mm-diameter nozzle and under a pressure of 600
kPa.
 It is used at rates on the order of 1 to 100 cc/hr, which is estimated to be (at
most) one ten-thousandth of that used in flood cooling. Consequently, the
process is also known as minimum-quantity lubrication (MQL).
 Viable alternative in various machining operations (especially turning, milling,
and gear cutting) on steels, steel alloys, and cast irons, but generally not for
aluminum alloys.
 Flushing of chips is achieved by pressurized air, often through the tool shank
which doesn’t serve a lubrication purpose and provides only limited cooling

Cutting Fluid Maintenance
81
 Maintenance and monitoring includes concentration checks
using the appropriate test, including:
 Refractometers which are used to determine the total
amount of soluble in a solution.
 Titration Kits which are used to analyze fluid
concentration in metal cutting fluids contaminated with
tramp oils.
 Tests for PH levels and alkalinity (acid splits) are
also useful.
 The fluid manufacturer’s product data sheets should always
be consulted and rigidly followed.

Unit 2 Machinability, Cutting Fluids, Tool Life & Wear, Tool Materials

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