Metal cutting additional

THEORY OF METAL
CUTTING
Thermal aspects of Machining,
Tool materials, Tool wear
Cutting fluids and Machinability.
Prod Tech Prof. Naman M. Dave

Cutting Temperatures
Of the total energy consumed in machining, nearly all of it is
converted into heat. The heat generated can cause
temperatures to be as high as 6000C at tool chip interface.
Cutting temperature has a controlling influence on the rate of
tool wear and friction between tool and chip.
Elastic deformation- Energy required for the operation is stored in
the material as strain energy and no heat is generated.
Plastic deformation – Most of the energy used is converted as heat.

Cutting Temperatures
Cutting temperatures are important because high
temperatures,
1. Reduce tool life.
2. Produce hot chips that pose safety hazards to the
machine operator.
3. Can cause inaccuracies in work part dimensions
due to thermal expansion of work piece material.

Effect of cutting temperature
The effect of cutting temperature, particularly when it is
high is mostly detrimental to both the tool and the job.
The major portion of the heat is taken away by the chips.
But it does not matter because chips are thrown out.
So attempts should be made such that the chips take away
more and more amount of heat leaving small amount of
heat to harm the tool and the job.

Effect of cutting temperature on tool
The possible detrimental effects of the high cutting
temperature on cutting tool (edge) are
 Rapid tool wear which reduces tool life
 plastic deformation of the cutting edges if the tool
material is not enough hot-hard and hot-strong
 thermal flaking and fracturing of the cutting edges
due to thermal shocks.
 Built up Edge formation.

Effect of cutting temperature on Job
The possible detrimental effects of the high cutting
temperature on machined job are:
 Dimensional inaccuracy of the job due to thermal
distortion and expansion-contraction during and after
machining
 surface damage by oxidation, rapid corrosion,
burning etc.
 induction of tensile residual stresses and micro cracks
at the surface / subsurface.

Effect of Cutting Temperature
However, often the high cutting temperature helps in
reducing the magnitude of the cutting forces and cutting
power consumption to some extent by softening or
reducing the shear strength, τs of the work material ahead
the cutting edge.
To attain or enhance such benefit the work material
ahead the cutting zone is often additionally heated
externally. This technique is known as Hot Machining
and is beneficially applicable for the work materials
which are very hard and hardenable like high manganese
steel, Hadfield steel, Ni-hard, Nimonic etc.

Factors Affecting Temperature

Sources and Causes of heat generation
in Machining
During machining, heat is generated at the cutting point from
three sources, as indicated in Fig.
Those sources and causes of development of cutting
temperature are:
 Primary shear zone (1) where the major part of the energy
is converted into heat.
 Secondary deformation zone (2) at the chip – tool
interface where further heat is generated due to rubbing
and / or shear.
 At the worn out flanks (3) due to rubbing between the tool
and the finished surfaces.

Sources of heat generation in Machining

Thermal Aspects of Machining
The heat generated is shared by the chip, cutting tool and the blank.
The apportionment of sharing the heat depends upon the
configuration, size and thermal conductivity of the tool – work
material and the cutting condition.
The following 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 the blank.
With the increase in cutting velocity, the chip shares heat
increasingly. Prod Tech Prof. Naman M. Dave

Thermal Aspects of Machining

Temperature distribution in Metal
Cutting
Fig. shows temperature distribution in work piece and chip during
orthogonal cutting (obtained from an infrared photograph, for free-
cutting mild steel where cutting speed is 0.38m/s, the width of cut is
6.35mm, the normal rake is 300, and work piece temperature is 6110C)

Temperature distribution in Metal
Cutting

Analytical methods to compute Cutting
Temperatures
Cook’s Method
Where,
δT = Mean temperature rise at tool chip interface, C0
U = Specific Energy in the operation, N-m/mm3
V = Cutting Speed, m/s
t0 = Chip thickness before the cut, m
ρC = Volumetric Specific heat of work material, J/mm3-C0
K = Thermal diffusivity of the work material, m2/s
0.333
04.0




=
K
Vt
C
U
T
ρ
δ

Measurement of tool-chip interface
temperature

Tool work Thermocouple Technique
In a thermocouple two dissimilar but electrically conductive
metals are connected at two junctions.
Whenever one of the junctions is heated, the difference in
temperature at the hot and cold junctions produce a
proportional current which is detected and measured by a
milli-voltmeter.
In machining like turning, the tool and the job constitute the
two dissimilar metals and the cutting zone functions as the hot
junction.
Then the average cutting temperature is evaluated from the mV
after thorough calibration for establishing the exact relation
between mV and the cutting temperature.Prod Tech Prof. Naman M. Dave

Tool work thermocouple technique

Embedded thermocouple technique
In operations like milling, grinding etc. where the previous methods are
not applicable, embedded thermocouple can serve the purpose. Fig. shows
the principle.
The standard thermocouple monitors the job temperature at a certain
depth, hi from the cutting zone. The temperature recorded in oscilloscope
or strip chart recorder becomes maximum when the thermocouple bead
comes nearest (slightly offset) to the grinding zone.
With the progress of grinding the depth, hi gradually decreases after each
grinding pass and the value of temperature, θm also rises as has been
indicated in Fig.
For getting the temperature exactly at the surface i.e., grinding zone, hi
has to be zero, which is not possible. So the θm vs hi curve has to be
extrapolated up to hi = 0 to get the actual grinding zone temperature. Log
– log plot helps such extrapolation more easily and accurately.Prod Tech Prof. Naman M. Dave

Embedded thermocouple technique

Infra-red photographic technique
This modern and powerful method is based on taking
infra-red photograph of the hot surfaces of the tool, chip,
and/or job and get temperature distribution at those
surfaces.
Proper calibration is to be done before that. This way the
temperature profiles can be recorded as indicated in Fig.
The fringe pattern readily changes with the change in any
machining parameter which affect cutting temperature.

Infra-red photographic technique

Tool wear and failure
The usefulness of tool cutting edge is lost through
 Wear
 Breakage
 Chipping
 Deformation
Tool failure implies that the tool has reached a point
beyond which it will not function satisfactorily until it is
re-sharpened.

Three Modes of Tool Failure
Fracture failure
When the Cutting force at tool point becomes excessive, it
leads to failure by brittle fracture.
Temperature failure
Cutting temperature is too high for the tool material, which
makes the tool point to soften, and leads to plastic
deformation along with a loss of sharp edge.
Gradual wear
Gradual wearing of the cutting edge causes loss of tool
shape, reduction in cutting efficiency and finally tool failure.

Preferred Mode of Tool Failure:
Gradual Wear
 Fracture and temperature failures are premature failures
 Gradual wear is preferred because it leads to the longest
possible use of the tool
 Gradual wear occurs at two locations on a tool:
Crater wear – occurs on top rake face
Flank wear – occurs on flank (side of tool)

Figure: Diagram of worn cutting tool, showing the principal
locations and types of wear that occur

Crater wear
It consists of a concave section on the tool face formed by the
action of the chip sliding on the surface.
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.
In general, crater wear is of a relatively small concern.

Flank wear
 It occurs on the tool flank as a result of friction between the
machined surface of the work piece 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.
 Cutting forces increase significantly with flank wear.
 If the amount of flank wear exceeds some critical value i.e.
(VB > 0.5~0.6 mm), the excessive cutting force may cause
tool failure.

Corner Wear
 It occurs on the tool corner.
 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 :
(a)Crater wear, and
(b)flank wear on a
cemented carbide tool,
as seen through a
toolmaker's microscope

Tool Wear: Mechanisms
Adhesion wear:
Fragments of the work-piece get welded to the tool surface at high
temperatures; eventually, they break off, tearing small parts of the
tool with them.
Abrasion:
Hard particles, microscopic variations on the bottom surface of the
chips rub against the tool surface and break away a fraction of tool
with them.
Diffusion wear:
At high temperatures, atoms from tool diffuse across to the chip; the
rate of diffusion increases exponentially with temperature; this
reduces the fracture strength of the crystals.

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
Tool Wear vs. Time

Factors affecting Tool life
 Tool material
 Hardness
 Work material
 Surface roughness of work piece
 Profile of cutting tool
 Type of machining operation
 Cutting speed, feed and depth of cut
 Cutting temperature

Tool life Criteria
 Actual cutting time to failure.
 Length of work cut to failure.
 Volume of metal removed to failure.
 Cutting speed for a given time to failure.
 Number of components produced.

Figure: Effect of cutting speed on tool flank wear (FW) for three
cutting speeds, using a tool life criterion of 0.50 mm flank wear
Effect of Cutting Speed

Figure: Natural log-log plot of cutting speed vs tool life

Tool Life
 Tool wear is a time dependent process. As cutting
proceeds, the amount of tool wear increases gradually.
 Tool wear must not be allowed to go beyond a certain
limit in order to avoid tool failure.
 Tool life is defined as the time interval for which tool
works satisfactorily between two successive grinding or
re-sharpening of the tool.

Taylor Tool Life Equation
This relationship is credited to F. W. Taylor (~1900)
CvT n
=
Where, v = cutting speed;
T = tool life; and
n and C are parameters that depend on feed, depth of cut, work
material, tooling material, and the tool life criterion used
n is the slope of the plot
C is the intercept on the speed axis

Tool Life vs. Cutting Speed
As cutting speed is increased, wear rate increases, so the same wear
criterion is reached in less time, i.e., tool life decreases with cutting speed

Typical Values of n and C in Taylor’s
Tool Life Equation
Tool material n C (m/min) C (ft/min)
High speed steel:
Non-steel work 0.125 120 350
Steel work 0.125 70 200
Cemented carbide
Non-steel work 0.25 900 2700
Steel work 0.25 500 1500
Ceramic
Steel work 0.6 3000 10,000

Tool life
Volume of metal removed per minute Vm
Vm = 𝜋𝜋𝐷𝐷.𝑡𝑡.𝑓𝑓. 𝑁𝑁 𝑚𝑚𝑚𝑚3/ 𝑚𝑚in
D = dia of workpiece, mm
t = depth of cut, mm
f = feed, mm/rev
N = RPM

Tool Life
If T be the time for tool failure in mins, The total volume
removed up to Tool Failure
= 𝝅𝝅𝑫𝑫.𝒕𝒕.𝒇𝒇. 𝑵𝑵. 𝑻𝑻 𝒎𝒎𝒎𝒎𝟑𝟑
Cutting Speed,
V = 𝝅𝝅DN/𝟏𝟏000 m/min
Volume of material removed up to tool failure
= 𝟏𝟏000 V.𝒇𝒇. 𝑻𝑻.𝒕𝒕 𝒎𝒎𝒎𝒎𝟑𝟑

Tool Near End of Life
 Changes in sound emitted from operation.
 Chips become ribbon-like, stringy, and difficult to dispose
off.
 Degradation of surface finish.
 Increased power required to cut.
 Visual inspection of the cutting edge with magnifying
optics can determine if tool should be replaced.

Tool life is measured by:
 Visual inspection of tool edge
 Tool breaks
 Fingernail test
 Changes in cutting sounds
 Chips become ribbony, stringy
 Surface finish degrades
 Computer interface says
- power consumption up
- cumulative cutting time reaches certain level
- cumulative number of pieces reaches certain value
Operator’s Tool life

Wear Control
 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.
 The figure shows the process parameters that influence the
rate of tool wear:
 Additional measures to reduce the tool wear include the
application of advanced cutting tool materials, such as
coated carbides, ceramics, etc..

Wear Control

Cutting Tool Technology
It has two principal aspects:
1. Tool material
Developing materials that can withstand the forces,
temperatures and wearing in machining process.
2. Tool geometry
Optimizing the geometry of the cutting tool for the
tool material and for a given operation.

The cutting tool materials must possess a number of important
properties to avoid excessive wear, fracture failure and high
temperatures in cutting.
The following characteristics are essential for cutting materials to
withstand the heavy conditions of the cutting process and to produce
high quality and economical parts:
Tool failure modes identify the important properties that a tool
material should possess:
 Toughness - to avoid fracture failure.
 Hot hardness - ability to retain hardness at high temperatures.
 Wear resistance - hardness is the most important property to
resist abrasive wear.
CUTTING TOOL MATERIALS

CUTTING TOOL MATERIALS
 hardness at elevated temperatures (so-called hot hardness) so
that hardness and strength of the tool edge are maintained in high
cutting temperatures.
 Toughness: ability of the material to absorb energy without
failing. Cutting is often accompanied by impact forces especially if
cutting is interrupted, and cutting tool may fail very soon if it is
not strong enough.
 wear resistance: although there is a strong correlation between
hot hardness and wear resistance, latter depends on more than
just hot hardness. Other important characteristics include surface
finish on the tool, chemical inertness of the tool material with
respect to the work material, and thermal conductivity of the tool
material, which affects the maximum value of the cutting
temperature at tool-chip interface.Prod Tech Prof. Naman M. Dave

Fig: Typical hot hardness relationships for selected tool materials.
Plain carbon steel shows a rapid loss of hardness as temperature
increases.
High speed steel is substantially better, while cemented carbides
and ceramics are significantly harder at elevated temperatures.

Carbon Steels
 It is the oldest of tool material. It is inexpensive, easily
shaped, sharpened.
 The carbon content is 0.6~1.5% with small quantities of
silicon, chromium, manganese, and vanadium to refine
grain size.
 This material has low wear resistance and low hot hardness.
Maximum hardness is about HRC 62.
 Used for drills taps, broaches, reamers.
 Limited to hand tools and low cutting speed operation. (Red
hardness temp.: 200° C)
 The use of these materials now is very limited.

High Speed Steel (HSS)
 First produced in 1900s. They are highly alloyed with
vanadium, cobalt, molybdenum, tungsten and chromium
added to increase hot hardness and wear resistance.
 Can be hardened to various depths by appropriate heat
treating up to cold hardness in the range of HRC 63-65.
 The cobalt component give the material a hot hardness
value much greater than carbon steels.(Red hardness temp.:
6500 C)
 The high toughness and good wear resistance make HSS
suitable for all type of cutting tools with complex shapes for
relatively low to medium cutting speeds.

High Speed Steel (HSS)
 Highly alloyed tool steel capable of maintaining hardness
at elevated temperatures better than high carbon and low
alloy steels.
 One of the most important cutting tool materials
 Especially suited to applications involving complicated
tool geometries, such as The most widely used tool
material today for taps, drills, reamers, gear tools, end
cutters, slitting, broaches, etc.
Two basic types
1. Tungsten-type, designated T- grades
2. Molybdenum-type, designated M-grades

High Speed Steel Composition
Two basic types of HSS
M-series (6-6-4-2):
 Contains 6% molybdenum, 6% tungsten, 4% chromium,
2% vanadium & cobalt
 Higher, abrasion resistance
 H.S.S. are majorly made of M-series
T-series (18-4-1):
 Contains 18 % tungsten, 4% chromium, 1% vanadium
& cobalt
 undergoes less distortion during heat treating

Cemented Carbides
 Introduced in the 1930s. These are the most important
tool materials today because of their high hot hardness
and wear resistance.
 There may be other carbides in the mixture, such as
titanium carbide (TiC) and/or tantalum carbide (TaC) in
addition to WC.
 The main disadvantage of cemented carbides is their low
toughness.

Cemented Carbides – General
Properties
 High compressive strength, but low to moderate tensile
strength
 High hardness (90 to 95 HRA)
 Good hot hardness
 Good wear resistance
 High thermal conductivity
 High elastic modulus - 600 x 103 MPa (90 x 106 lb/in2)
 Toughness lower than high speed steel

 This hard tool material is produced by a powder
metallurgy technique, sintering grains of tungsten carbide
(WC) in a cobalt (Co) matrix (as the binder, it provides
toughness).
 Particles 1-5 μm in size are pressed & sintered to desired
shape in a H2 atmosphere furnace at 15500 C.
 Amount of cobalt present affects properties of carbide
tools. As cobalt content increases – strength, hardness &
wear resistance increases.
Cemented Carbides

Cemented Carbides

Insert Attachment
In spite of more traditional tool materials, cemented carbides are available as
inserts produced by powder metallurgy process.
Inserts are available in various shapes, and are usually mechanically attached
by means of clamps to the tool holder, or brazed to the tool holder.
The clamping is preferred because after an cutting edge gets worn, the insert is
indexed (rotated in the holder) for another cutting edge.
When all cutting edges are worn, the insert is thrown away. The indexable
carbide inserts are never reground.
If the carbide insert is brazed to the tool holder, indexing is not available, and
after reaching the wear criterion, the carbide insert is re-sharpened on a tool
grinder.

Types of Cemented Carbides
Two basic types:
1. Non-steel cutting grades - only WC-Co
2. Steel cutting grades - TiC & TaC added to WC-Co

Non-Steel Cutting Carbide Grades
• Used for nonferrous metals and gray cast iron
• Properties determined by grain size and cobalt content
– As grain size increases, hardness and hot hardness
decrease, but toughness increases.
– As cobalt content increases, toughness improves at the
expense of hardness and wear resistance.

Steel Cutting Carbide Grades
Used for low carbon, stainless, and other alloy steels
– For these grades, TiC and/or TaC are substituted for
some of the WC.
– This composition increases crater wear resistance for
steel cutting, but adversely affects flank wear resistance
for non-steel cutting applications.

Coated WC
One advance in cutting tool materials involves the application
of a very thin coating (~ 10 μm) to a K-grade substrate, which
is the toughest of all carbide grades.
Coating may consists of one or more
thin layers of wear-resistant
material, such as titanium carbide
(TiC), titanium nitride (TiN),
aluminum oxide (Al2O3), and/or
other, more advanced materials.
Coating allows to increase
significantly the cutting speed for the
same tool life.
Structure of a multi-layer
coated carbide insert

Coated Carbides
Cemented carbide insert coated with one or more thin layers
of wear resistant materials, such as TiC, TiN, and/orAl2O3
Coating is applied by chemical vapor deposition or physical
vapor deposition.
Coating thickness = 2.5 - 13 µm (0.0001 to 0.0005 in)
Applications: cast irons and steels in turning and milling
operations.
Best applied at high speeds where dynamic force and thermal
shock are minimal.

Ceramics
Primarily fine-grained Al2O3, pressed and sintered at high
pressures and temperatures into insert form with no binder.
• Applications: high speed turning of cast iron and steel
• Not recommended for heavy interrupted cuts (e.g. rough
milling) due to low toughness
• There is no occurrence of built-up edge, and coolants
are not required.
• Al2O3 also widely used as an abrasive in grinding.

Ceramics
Two types are available:
 White or cold-pressed ceramics, which consists of only
Al2O3 cold pressed into inserts and sintered at high
temperature.
 Black or hot-pressed ceramics, commonly known as
cermet (from ceramics & metal). This material consists of
70% Al2O3 and 30% TiC.
Both materials have very high wear resistance but low
toughness, therefore they are suitable only for continuous
operations such as finishing turning of cast iron and steel at
very high speeds.

Cermets
Combinations of TiC, TiN, and titanium carbonitride (TiCN),
with nickel and/or molybdenum as binders.
Some chemistries are more complex.
Applications:
high speed finishing and semi-finishing of steels, stainless
steels and cast irons.
– Higher speeds and lower feeds than steel-cutting carbide
grades
– Better finish achieved, often eliminating need for grinding.

Diamond
• Diamond is the hardest substance ever known of all
materials.
• Low friction, high wear resistance.
• Ability to maintain sharp cutting edge.
• Use is limited because it gets converted into graphite at
high temperature (700 °C). Graphite diffuses into iron
and make it unsuitable for machining steels.
• It is used as a coating material in its polycrystalline form,
or as a single- crystal diamond tool for special applications,
such as mirror finishing of non-ferrous materials.

Synthetic Diamonds
Sintered polycrystalline diamond (SPD) - fabricated by
sintering very fine grained diamond crystals under high
temperatures and pressures into desired shape with little or
no binder.
Usually applied as coating (0.5 mm thick) on WC-Co insert
Applications: high speed machining of nonferrous metals
and abrasive nonmetals such as fiberglass, graphite, and
wood.
- Not for steel cutting

Cubic Boron Nitride
 Next to diamond, cubic boron nitride (CBN) is hardest
material known. Retain hardness up to 1000°C.
 By bonding 0.5 mm thick polycrystalline CBN onto a
carbide substrate through sintering under pressure.
 CBN is used mainly as coating material because it is very
brittle.
 In spite of diamond, CBN is suitable for cutting ferrous
materials.
Applications: machining steel and nickel-based alloys.

 SPD and CBN tools are expensive.
 Made by bonding (0.5-1.0 mm) Layer of poly crystalline
cubic boron nitride to a carbide substrate by sintering
under Pressure.
 While carbide provides shock resistance CBN layer
provides high resistance and cutting edge strength.
 Cubic boron nitride tools are made in small sizes without
substrate.
Cubic Boron Nitride

Lubricants – purpose is to reduce friction… usually oil based
Coolants – purpose is to transport heat… usually water based
Both lose their effectiveness at higher cutting speeds!
Cutting Fluids

Dry Machining
• No cutting fluid is used
• Avoids problems of cutting fluid contamination,
disposal, and filtration
• Problems with dry machining:
– Overheating of the tool
– Operating at lower cutting speeds and production rates to
prolong tool life
– Absence of chip removal benefits of cutting fluids in
grinding and milling

Cutting Fluids
• Essential in metal-cutting operations to reduce heat
and friction
• Centuries ago, water used on grindstones
• 100 years ago, tallow used (did not cool)
• Lard oils came later but turned rancid
• Early 20th century saw soap added to water
• Soluble oils came in 1936
• Chemical cutting fluids introduced in 1944

Cutting Fluids
Any liquid or gas applied directly to machining operation
to improve cutting performance
Two main problems addressed by cutting fluids:
1. Heat generation at shear zone and friction zone
2. Friction at the tool - chip and tool - work interfaces
Other functions and benefits:
– Wash away chips (e.g., grinding and milling)
– Reduce temperature of work part for easier handling
– Improve dimensional stability of work part

Heat Generated During Machining
• Heat finds its way into one of three places
– Work piece, tool and chips
Too much, work
will expandToo much, cutting edge will
break down rapidly,
reducing tool life
Act as disposable
heat sink

Heat Dissipation
• Ideally most heat taken off in chips
• Indicated by change in chip colour as heat causes
chips to oxidize.
• Cutting fluids assist taking away heat
– Can dissipate at least 50% of heat created during
machining.

Characteristics of a Good
Cutting Fluid

Characteristics of a Good
Cutting Fluid
1. Good cooling capacity
2. Good lubricating qualities
3. Resistance to rancidity
4. Relatively low viscosity
5. Stability (long life)
6. Rust resistance
7. Nontoxic
8. Transparent
9. Non inflammable

Economic Advantages to Using
Cutting Fluids
 Reduction of tool costs.
– Reduce tool wear, tools last longer
 Increased speed of production.
– Reduce heat and friction so higher cutting speeds
 Reduction of labor costs.
– Tools last longer and require less regrinding, less
downtime, reducing cost per part
 Reduction of power costs.
– Friction reduced so less power required by machining

Functions of a Cutting Fluid
Prime functions
– Provide cooling
– Provide lubrication
Other functions
– Prolong cutting-tool life
– Provide rust control
– Resist rancidity

Functions of a Cutting Fluid:
Cooling
• Most effective at high cutting speeds where heat generation
and high temperatures are problems.
• Most effective on tool materials that are most susceptible to
temperature failures. (e.g., HSS)
• Two sources of heat during cutting action
– Plastic deformation of metal
• Occurs immediately ahead of cutting tool
• Accounts for 2/3 to 3/4 of heat
– Friction from chip sliding along cutting-tool face.

Cooling
• Water used as base in coolant - type cutting fluids.
• Water most effective for reducing heat by will promote
oxidation (rust).
• Heat has definite bearing on cutting-tool wear
– Small reduction will greatly extend tool life
• Decrease the temperature at the chip-tool interface by 50
degrees F, and it will increase tool life by up to 5 times.

Lubrication
• Usually oil based fluids and are most effective at lower
cutting speeds.
• Also reduces temperature in the operation.
• Reduces friction between chip and tool face
– Shear plane becomes shorter
– i.e., the area where plastic deformation occurs is smaller
• Extreme-pressure lubricants reduce amount of heat produced
by friction.
• EP chemicals of synthetic fluids combine chemically with
sheared metal of chip to form solid compounds (allows chip
to slide) Prod Tech Prof. Naman M. Dave

Cutting fluid reduces friction and
produces a shorter shear plane.

Cutting-Tool Life
• Heat and friction are prime causes of cutting-tool
breakdown
• Reduce temperature by as little as 500F, life of cutting
tool increases fivefold
• Built-up edge
– Pieces of metal weld themselves to tool face
– Becomes large and flat along tool face, effective rake
angle of cutting tool decreased

Built-up
Edge
Built-up edge keeps breaking off and re-forming and result is
poor surface finish, excessive flank wear, and cratering of
tool face.

Cutting Fluid's Effect on Cutting
Tool Action
1. Lowers heat created by plastic deformation of metal
2. Friction at chip-tool interface decreased
3. Less power is required for machining because of
reduced friction
4. Prevents built-up edge from forming
5. Surface finish of work greatly improved

Rust Control
• Water is the best and most economical coolant
– Causes parts to rust
• Rust is oxidized iron
• Chemical cutting fluids contain rust inhibitors

Rancidity Control
 Rancidity caused by bacteria and other microscopic
organisms, growing and eventually causing bad odours
to form.
 Most cutting fluids contain bactericides that control
growth of bacteria and make fluids more resistant to
rancidity.

Application of Cutting Fluids
• Cutting-tool life and machining operations influenced
by way cutting fluid applied
• Copious stream under low pressure so work and tool
well covered
– Inside diameter of supply nozzle ¾ width of cutting
tool
– Applied to where chip being formed

Refrigerated Air System
• Another way to cool chip-tool interface
• Effective, inexpensive and readily available
• Used where dry machining is necessary
• Uses compressed air that enters vortex generation
chamber
– Cooled 1000F below incoming air
• Air directed to interface and blow chips away

Types of Cutting Fluids
• Most commonly used cutting fluids
– Either aqueous based solutions or cutting oils
• Fall into three categories
– Straight Cutting oils
– Emulsifiable oils or Water Soluble oils
– Chemical (synthetic) cutting fluids

Straight Cutting Oils
• Derived from petroleum, animal, marine or vegetable
substances and may be used straight or in combination.
• Their main function is lubrication and rust prevention.
• They are chemically stable and lower in cost.
• Usually restricted to light duty machining on metals of high
machinability, such as aluminium, magnesium, brass and
leaded steels.
– Two classifications
» Active
» InactiveProd Tech Prof. Naman M. Dave

Active Cutting Oils
• Those that will darken copper strip immersed for 3 hours
at temperature of 2120F
• Dark or transparent
• Better for heavy-duty jobs
• Three categories
– Sulfurized mineral oils
– Sulfochlorinated mineral oils
– Sulfochlorinated fatty oil blends

Inactive Cutting Oils
• Oils will not darken copper strip immersed in them for 3
hours at 2120F
• Contained sulfur is natural
– Termed inactive because sulfur so firmly attached to oil –
very little released
• Four general categories
– Straight mineral oils, fatty oils, fatty and mineral oil
blends, sulfurized fatty-mineral oil blend

Emulsifiable (Water Soluble) Oils
• About 90% of all metal cutting and grinding operations
make use of emulsions due to their high sp. Heat, high
thermal conductivity and high heat of vapourisation.
• Mineral oils containing soap like material that makes them
soluble in water and causes them to adhere to work piece.
• Emulsifiers break oil into minute particles and keep them
separated in water.
– Water blend is in the ratio of 1 part oil to 15~20 parts water
(for cutting) and 40 to 60 parts of water (for grinding)
• Good cooling and lubricating qualities.
• Used at high cutting speeds, low cutting pressures.

Chemical Cutting Fluids
• Also called synthetic fluids
• Introduced about 1945
• Stable, preformed emulsions
– Contain very little oil and mix easily with water
• Extreme-pressure (EP) lubricants added
– React with freshly machined metal under heat and pressure
of a cut to form solid lubricant
• Reduce heat of friction and heat caused by plastic
deformation of metal

Merits & Demerits of Synthetic Fluids

Advantages of Synthetic Fluids
 Good rust control
 Resistance to rancidity for long periods of time
 Reduction of amount of heat generated during cutting due
to Excellent cooling qualities
 Longer durability than cutting or soluble oils
 Nonflammable – nonsmoking & Nontoxic ??????
 Easy separation from work and chips
 Quick settling of grit and fine chips so they are not re-
circulated in cooling system
 No clogging of machine cooling system due to detergent
action of fluid
 Can leave a residue on parts and tools.

Caution
 Chemical cutting fluids widely accepted
and generally used on ferrous metals.
 They are not recommended for use on alloys of
magnesium, zinc, cadmium, or lead.
 They can mar machine's appearance and dissolve
paint on the surface.

• Ease or difficulty with which metal can be machined
with satisfactory finish at low cost.
• Measured by length of cutting-tool life in minutes or by
rate of stock removal in relation to cutting speed
employed.
Machinability

Machinability
Machinability is defined in terms of:
1. Surface finish and surface integrity of machined part
2. Tool life
3. Force and power required
4. The level of difficulty in chip control
Good machinability indicates
– good surface finish and surface integrity
– a long tool life
– and low force and power requirements
Note,
continuous chips should be avoided for good machinability.

Grain Structure
• Machinability of metal affected by its microstructure.
• Ductility and shear strength modified greatly by
operations such as annealing, normalizing and stress
relieving.
• Certain chemical and physical modifications of steel
improve Machinability.
– Addition of sulfur, lead, or sodium sulfate
– Cold working, which modifies ductility

Machinability Index

Machinability:
Machinability of Nonferrous Metals
• Aluminum
– very easy to machine
– but softer grades: form BUE ⇒ poor surface finish
– ⇒ recommend high cutting speeds, high rake and relief angles
• Beryllium
– requires machining in a controlled environment
– this is due to toxicity of fine particles produced in machining
• Cobalt-based alloys
– abrasive and work hardening
– require sharp, abrasion-resistant tool materials, and low feeds and
speeds
• Copper
– can be difficult to machine because of BUE formationProd Tech Prof. Naman M. Dave

Machinability:
Machinability of Nonferrous Metals
• Magnesium
– very easy to machine, good surface finish, prolonged tool life
– Caution: high rate of oxidation and fire danger
• Titanium and its alloys
– have very poor thermal conductivity
– ⇒ high temp. rise and BUE ⇒ difficult to machine
• Tungsten
– brittle, strong, and very abrasive
– ⇒ machinability is low
• Zirconium
– Good machinability
– Requires cooling cutting fluid (danger of explosion, fire)

Aluminum
• Pure aluminum generally more difficult to machine
than aluminum alloys
– Produces long stringy chips and harder on cutting tool
• Aluminum alloys
– Cut at high speeds, yield good surface finish
– Hardened and tempered alloys easier to machine
– Silicon in alloy makes it difficult to machine
• Chips tear from work (poor surface)

Copper
• Heavy, soft, reddish-colored metal refined from copper
ore (copper sulfide)
– High electrical and thermal conductivity
– Good corrosion resistance and strength
– Easily welded, brazed or soldered
– Very ductile
• Does not machine well: long chips clog flutes of cutting
tool
– Coolant should be used to minimize heat

Copper/Beryllium
• Heavy, hard, reddish-colored copper metal with Beryllium
added
– High electrical and thermal conductivity.
– Good corrosion resistance and strength.
– Can be welded.
– Somewhat ductile.
– Withstands high temperature.
• Machines well
– Highly abrasive to HSS Tooling.
– Coolant should be used to lubricate and minimize tool wear.

Copper-Based Alloys: Brass
• Alloy of copper and zinc with good corrosion
resistance, easily formed, machines, and cast.
• Several forms of brass.
– Alpha brasses: up to 36% zinc, suitable for cold working.
– Alpha 1 beta brasses: Contain 54%-62% copper and used
in hot working.
• Small amounts of tin or antimony added to minimize
pitting effect of salt water.
• Used for water and gas line fittings, tubings, tanks,
radiator cores, and rivets.

Copper-Based Alloys: Bronze
• Alloys of copper and tin which contain up to 12% of
principal alloying element
– Exception: copper-zinc alloys
• Phosphor-bronze
– 90% copper, 10% tin, and very small amount of phosphorus
– High strength, toughness, corrosion resistance
– Used for lock washers, cotter pins, springs and clutch discs

• Silicon-bronze (copper-silicon alloy)
– Contains less than 5% silicon
– Strongest of work-hardenable copper alloys
– Mechanical properties of machine steel and corrosion
resistance of copper
– Used for tanks, pressure vessels, and hydraulic pressure
lines

• Aluminum-bronze (copper-aluminum alloy)
– Contains between 4% and 11% aluminum
– Other elements added
 Iron and nickel (both up to 5%) increases strength
 Silicon (up to 2%) improves machinability
 Manganese promotes soundness in casting
– Good corrosion resistance and strength
– Used for condenser tubes, pressure vessels, nuts and bolts

Effects of
Temperature and Friction
• Heat created
– Plastic deformation occurring in metal during process of
forming chip
– Friction created by chips sliding along cutting-tool face
• Cutting temperature varies with each metal and
increases with cutting speed and rate of metal
removal

Effects of
Temperature and Friction
• Temperature of metal immediately ahead of cutting tool
comes close to melting temperature of metal being cut.
• Greatest heat generated when ductile material of high
tensile strength is cut.
• Lowest heat generated when soft material of low tensile
strength is cut.
• Maximum temperature attained during cutting action.
– affects cutting-tool life, quality of surface finish, rate of
production and accuracy of work piece.

Friction
• Kept low as possible for efficient cutting action
• Increasing coefficient of friction gives greater
possibility of built-up edge forming
– Larger built-up edge, more friction
– Results in breakdown of cutting edge and poor surface finish
• Can reduce friction at chip-tool interface and help
maintain efficient cutting temperatures if use good
supply of cutting fluid.

Factors Affecting Surface Finish
• Feed rate
• Nose radius of tool
• Cutting speed
• Rigidity of machining operation
• Temperature generated during machining process

Surface Finish
• Direct relationship between temperature of work piece
and quality of surface finish
– High temperature yields rough surface finish
– Metal particles tend to adhere to cutting tool and form built-
up edge
• Cooling work material reduces temperature of
cutting-tool edge
– Result in better surface finish

Metal cutting additional

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