machining and thermal aspects (mgu s8 me)

Department of Mechanical Engineering, SSET 2014

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Module II

Heat generation in metal cutting

Almost all (90%-100%) of the mechanical energy consumed in a machining operation finally

convert into the thermal energy that in turn raises the temperature in the cutting zone. Heat

has critical influences on machining. To some extent, it can increase tool wear and then

reduce tool life, get rise to thermal deformation and cause to environmental problems, etc.

Sources of heat generation

The main heat sources of heat generations are shear zones.

1. Primary shear zone where the shearing of material takes place due plastic deformation.

(The energy required for shearing is converted into heat 80 to 85%)

2. Secondary deformation zone (chip – tool interface) where secondary plastic

deformation is due to the friction between the heated chip and tool takes place.

3. Tool-work interface -due to rubbing between the tool and finished surfaces (1 to 3%)

Dissipation of heat generated during the machining is as follows

Discarded chip carries away about 60~80% of the total heat (q1)

Workpiece acts as a heat sink drawing away 10~20% heat (q2)

Cutting tool will also draw away ~10% heat (q3).

Effects of heat generation in machining

1. Affects tool life and wear rate

2. It affects strength and hardness of tool

3. Surface roughness (oxidation of machined surface and deformation)

4. Deformation of work and machine

5. High coolant circulation rate

6. Lower dimensional accuracy

7. Forms build up edge

Department of Mechanical Engineering, SSET 2014

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Cutting temperature distribution

Cutting temperature is not constant

through the tool, chip and workpiece. It is

observed, that the maximum temperature is

developed not on the very cutting edge,

but at the tool rake some distance away

from the cutting edge. The temperature

field in the cutting zones is shown in the

figure.

Factors influencing Cutting temperature

1. Work material

a) Energy required (less energy means less temperature)

b) Soft and Ductility-cutting temperature is high

c) High thermal conductivity material reduces temperature

2. Tool geometry

a) Increased Rake angle reduces the cutting temperature

b) Increased cutting edge angle reduces the cutting temperature

c) Increased nose radius reduces the temperature slightly

3. Cutting condition

a. Cutting speed – most significant factor(Increases with speed)

b. Feed has less influence on cutting temperature

c. Depth of cut has least influence

4. Cutting fluid

Cutting fluid reduces the temperature (reduces the friction and carries away the

heat)

From this figure is obvious, that any reduction of the cutting temperature will require

substantial reduction in either the cutting speed or cutting feed, or both (the effect of depth of

cut can be neglected). But cutting time and therefore production rate will decrease. To avoid

the problem, application of cutting fluid (coolants) is the solution.

Department of Mechanical Engineering SSET 2015

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Tool temperature measurement

The power consumed in metal cutting is converted into heat. The magnitude of the

cutting temperature need to be known or evaluated to facilitate

1. Assessment of machinability (judged by cutting forces, temperature and tool life)

2. Design and selection of cutting tools

3. Proper selection and application of cutting fluid

4. Analysis of temperature distribution in the chip, tool and job

5. Evaluate the role of variation of the different machining parameters on cutting

temperature

The temperatures which are of major interests are:

etemperaturcuttingAverage

etemperaturerfacechiptoolAverage

etemperaturzoneshearAverage

avg

i

s

:

int:

:

:

Cutting temperature can be determined by two ways:

1. Analytically – using mathematical models (equations). This method is simple,

quick and inexpensive but less accurate and precise.

2. Experimentally – this method is more accurate, precise and reliable

The feasible experimental methods are:

1. Decolourising method – some paint or tape is pasted on the tool or job near the

cutting point and change in colour with variation of temperature may also often

indicate cutting temperature

2. Tool-work thermocouple – simple and inexpensive but gives only average

3. Photo-cell technique

4. Infra rad detection method

Brief description of few methods (Experimental)

Tool-work thermocouple

In a thermocouple two dissimilar but

electrically conductive metals are

connected at two junctions at different

temperature. The difference in

temperature at the hot and cold junctions

produce a proportional current which is

detected and measured by a milli-

voltmeter (mV). In machining, the tool

and the job constitute the two dissimilar

metals and the cutting zone functions as

the hot junction.


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Photo cell technique

This technique enables accurate measurement of the temperature along the shear zone

and tool flank. The electrical resistance of the cell, like PbS cell, changes when it is

exposed to any heat radiation. Holes are drilled at places where temperature is to be

measured. The cell receiving radiation through the small hole and change in resistance of

photo cell is measured which depends upon the temperature of the heat radiating source.

Infra red pyrometer method

Each body with a temperature above the absolute zero emits an electromagnetic radiation

from its surface, which is proportional to its intrinsic temperature. By knowing the

amount of infrared energy emitted by the object and its emissivity, the object's

temperature can often be determined. The basic design consists of a lens to focus the

infrared (IR) energy on to a detector, which converts to an electrical signal that can be

displayed in units of temperature

Tool material

With the progress of the industrial world it has been needed to continuously develop and

improve the cutting tool materials and geometry

1. To meet the growing demands of

high productivity, quality and

economy of machining

2. To enable effective and efficient

machining of the exotic materials

that are coming up with the rapid

and vast progress of science and

technology

3. For precision and ultra-precision

machining (micro and nano level

machining)

Reasons for development of new tool material


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Properties of Cutting Tool Materials

The cutting tool material should possess the following properties

1. Hot hardness

It is the ability of the tool material to retain its hardness and cutting edge at

elevated temperatures. Hot hardness can be increased by adding cobalt, chromium

molybdenum, tungsten and vanadium.

2. Wear Resistance

It is the ability of the tool material to resist wear when operating at high speeds.

The wear resistance can be increased by adding carbon and alloying elements.

3. Toughness

It is ability to resist shock and vibrations without breaking. This is particularly

important for interrupted cuts.

4. High thermal conductivity and specific heat

To conduct the heat generated at the cutting edge.

5. Coefficient of friction

The coefficient of friction between the chip and the tool should be as low as

possible in the operating range of speed and feed.

6. Favorable cost, easy to fabricate and easy to grind and sharpen.

7. It should have low mechanical and chemical affinity for the work material.

Selection of Cutting Tool Materials

The selection of cutting tool material will depend on the following factors:

1. Volume of production demanded

2. Cutting condition

3. Type of machining process

4. Physical and chemical properties of materials

5. Rigidity of the machine.

Classification of Tool Materials

1. High Carbon Steel

2. High Speed Steel

3. Stellite (Cast-Cobalt alloys)

4. Cemented Carbide

5. Coated Carbides

6. Ceramics

7. Cermet

8. CBN

9. Diamond

10. PCD


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Carbon Steel

Carbon steel, also called plain carbon steel, is a metal alloy, a combination of two main

elements, iron and carbon plus small amounts of manganese, phosphorus, sulfur, silicon

to improve the properties. These steels usually with less than 1.5 percent carbon and

generally categorized according to their carbon content. Generally, with an increase in the

carbon content from 0.01 to 1.5% in the alloy, its strength and hardness increases but

causes appreciable reduction in the ductility and malleability of the steel.

Plain carbon steels are further subdivided into groups:

1. Low Carbon Steels/Mild Steels contain up to 0.3% carbon

2. Medium Carbon Steels contain 0.3 – 0.6% carbon

3. High Carbon Steels contain more than 0.6% to 1.5% carbon

High carbon: It is very hard, wear resistant and difficult to weld. Used for cutting tools

Typical Composition carbon steel:

Carbon - 0.8 to 1.3%

Silicon - 0.1 to 0.4 %

Manganese 0.1 to 0.4 %


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General Characteristics/properties/advantages of high carbon steel

1. Suitable for low cutting speeds, limited to about 10 m/min using coolant supply.

2. Low hot hardness (Used where cutting temperature is below 200°C). Hardness

lost at 350°C).

3. Have good hardness and toughness (when heated and tempered).

4. Easy to forge and simple to harden.

5. Low cost

6. Can undergo heat treatment

Limitations of plain carbon steel

1. Plain-carbon steels have poor corrosion resistance

2. Plain-carbon steel oxidises readily at elevated temperatures.

3. There cannot be strengthening beyond a limit without significant loss in

toughness and ductility.

Application

Used to make reamers, hacksaw blades, taps and dies etc

High Speed Steels

The term `high speed steel' was derived from the fact that it is capable of cutting metal at

a much higher rate than carbon tool steel and continues to cut and retain its hardness even

when the point of the tool is heated to a low red temperature. The alloying elements

increase the strength, toughness, wear resistance and cutting ability.

The basic composition of HSS

18% W

4% Cr

1% V

0.7% C and rest is Fe

Properties /features

1. HSS can operate at cutting speeds 2 to 3 times higher that of high carbon steel.

Cutting speed up to 30 m/min

2. Excellent toughness- It takes shocks and vibration during cutting

3. It retains hardness at elevated temperatures in the range of 550°C to 600°C.

4. Higher metal removal rates.

5. Less Costly

Application

1. Used to make lathe tools, drills, reamers, shaper tools, slotting tools, farming

equipments and compression springs etc.

Coated HSS

A very thin coating of 5-7µm thickness of wear resistant material on conventional HSS

tool is provided to improve its performance. Vapor chemical deposition technique is used

to give the thin coatings of titanium or zirconium carbide and nitrides on HSS tools.


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Stellite

Composition

48-53% 30-33% 10-20% 1.5-2.0%

Co Cr W C

Manufacturing Process- Casting

Hot hardness- 760ºC

Cutting Speed -45 m/min

Higher tool life compare to H.S.S.

Lower toughness/brittle

Cemented Carbides

The Cemented Carbides are a range of composite materials, which consist of hard carbide

particles bonded or cemented together by a metallic binder. Pure tungsten carbide powder

and carbon is mixed and then mixed with binder at high temperature and manufactured

by powder metallurgy methods. Its unique combination of strength, hardness and

toughness satisfies the most demanding applications. It is usually used as tool inserts

brazed on to steel holders. Example of tool: Tungsten carbide, titanium carbide etc

Composition of tungsten carbide tool

W C Co

94% 6% cobalt (Binder)

Properties

1. High hardness and wear resistance

2. Maximum limiting cutting temperature -1200 ºC

3. Cutting Speed -100 m/min so High material removal rate

4. High Tool life

5. Gives better surface finish

6. Easy to manufacture with powder metallurgy methods (low cost)

Application

Used in machining tough materials such as carbon steel or stainless steel, as well as in

situations where other tools would wear away, such as high-quantity production runs.

Coated tungsten carbide

Thin film of coating of hard and wear resistive materials like Titanium Nitride (TiN),

Titanium Carbide (TiC) and Aluminium Oxide (Al2O3) etc is deposited on surface to

improve the life and service quality of tool. Chemical Vapor Deposition (CVD) or

Physical Vapor Deposition (PVD) technique is used for coating deposition.

Properties

1. High hot hardness-retains cutting edge sharpness at high temperature

2. High cutting speed -150 to 250 m/min

3. High Tool life (2 to 3 times higher than carbide)

4. Reduction of cutting forces and power consumption

5. Improvement in product quality - Chemically more stable


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Applications:

1. Widely used for high speed machining

2. Machining of very hard materials

Ceramics or Oxide tool

Ceramic tool materials consist primarily of fine grained aluminium oxide, cold-pressed

into insert shapes and sintered under high pressure and temperature.

1. They are hard, retain their hardness at high temperatures of 1700°C.

2. High Cutting Speed -200 to 400 m/min (Used for high speed machining )

3. Good surface finish and dimensional accuracy in machining steels.

4. They have longer tool life

5. Chemically more stable (less tendency to adhere to metals during machining so

low build up edge)

Limitation:

1. Poor Toughness: They are extremely brittle and can’t take shock and vibrations

(not suitable for interrupted cut)

2. Unreliable (sudden fail) and also high rigidity set up is required

Application

Used for cutting carbon steel and HSS

Cermets

A cermet is a composite material composed of ceramic and metallic materials. As the

name suggests, cermets (derived from ceramics and metals) combine the positive

properties of two material groups. Cermet is a cutting tool material composed mainly of

TiC (Titanium Carbide) and TiN (Titanium Nitride) bonded together with a nickel

(metal) binder.

The objective to combine the properties of ceramics, such as

1. Hardness

2. Resistance to oxidation

3. Heat resistance

1. Toughness properties of metals,

2. Impact strength

in order to create an ideal cutting material, “cermet”

Properties (Properties comes between carbide and ceramics tool)

1. High hardness and more chemically stable, hence more wear resistant

2. High hot hardness greater than carbide tool and less than ceramics tool

3. High thermal shock resistance

4. Excellent surface finish at very high cutting speed

5. High tool life

Application: Used for cutting steel and cast iron at high cutting speed.


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Sialon

Sialons are ceramics based on the elements silicon (Si), aluminium (Al), oxygen (O) and

nitrogen (N. They are formed when silicon nitride (Si3N4), aluminium oxide (Al2O3) and

aluminium nitride (AlN) are reacted together. The materials combine over a wide

compositional range.

Sialon ceramics are a specialist class of high-temperature refractory materials,

1. High strength,

2. Good thermal shock resistance and

3. Exceptional resistance to wetting or corrosion.

4. Oxidation resistance

CBN: cubic born nitride

CBN tools are very effective at machining most steels and cast irons, but they are also

very expensive. It is the second hardest material known (second to diamond). CBN is

used mainly as coating material because it is very brittle. Boron nitride has the formula

(BN)n, (n is a very large number, but the empirical formula is BN). In the cubic form of

boron nitride, alternately linked boron and nitrogen atoms form a tetrahedral bond

network, exactly like carbon atoms do in diamond.

Properties

1. CBN is an excellent abrasive resistance to cut

2. Very hard and high hot hardness -. can efficiently machine steel

3. Long Tool Life: provide uniform hardness and abrasion resistance in all directions.

4. High Material-Removal Rates: 700 -800 m/min

5. Lower cutting cost

Application

1. Used for roughing and finishing operations

2. Machining of hard steel

3. High speed finish machining of hardened steel and super alloys

Diamond

Diamond has most of the properties (hardest) desirable in a cutting tool material.

Diamond also has high strength, good wear resistance and low friction coefficient.

Diamond is composed of pure carbon atoms, arranged in a very special crystal orientation

that gives it its unique physical properties. Single crystal diamond tools have been mainly

replaced by polycrystalline diamond (PCD). This consists of very small synthetic crystals

fused by a high temperature high pressure process to a thickness of between 0.5 and 1mm

and bonded to a carbide substrate.

1. It is the hardest cutting material.

2. It has low coefficient of friction, high compressive strength and extremely wear

resistant.

3. Used for machining very hard material such as glass, plastics, ceramics etc.

http://en.wikipedia.org/wiki/Aluminium


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4. It produces very good surface finish at high speeds with good dimensional

accuracy. Best suited for light cuts and finishing operations.

5. It can resist upto a temperature of 1250°C.

6. High cutting speed (300 to 1000 m/min)

Diamond coated tool

The tool life of cemented carbide cutting tools is greatly improved by diamond coating,

and typically more than 10 times the tool life is obtained. Diamond films are usually

deposited more than 10 pm thick to make tool life longer, since tool life is directly related

to film thickness. Coats tools with polycrystalline diamond, applied with a CVD process.

The diamond coating can withstand the abrasiveness of these materials. High cutting

speed, accuracy of cut and dry cut is possible.

Comparison of diamond and CBN

Diamond and CBN have the same crystal structure, with diamond consisting of pure

carbon, whilst CBN is made up of the elements boron and nitrogen. In contrast to

Diamond, CBN has no carbon atoms, making it suitable for machining steel. CBN is

better suited for machining the following materials:

Hardened steel over approx. 54 HRc hardness and High-speed steel

Stellite and Nickel-based special alloys

Diamond is generally used for machining non ferrous materials. Steel has a high affinity

to carbon. Since Diamond consists of pure carbon, it is not suitable for machining steel.

Indexable inserts

A tipped tool generally refers to any cutting tool where the cutting edge consists of a

separate piece of material, either brazed, welded or clamped on to a separate body.

The advantage of tipped tools is only a small insert of the cutting material is needed to

provide the cutting ability.

Advantages of tipped tool

1. Small size insert makes manufacturing easier

2. Reduces cost because the tool holder can be made of a less-expensive and tougher

material.

3. Easy to handle

The indexable insert technique, meaning a replaceable cutting edge, or so called insert,

was placed in the toolholder. Inserts are removable cutting tips, which means they are not

brazed or welded to the tool body. This saves time in manufacturing by allowing fresh

cutting edges to be presented periodically without the need for tool grinding and setup

changes.


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Tool failure

Cutting tools generally fail by:

1. Gradual wear of the cutting tool at its flanks and rake surface.

2. Mechanical breakage and chipping due to excessive forces and shocks.

3. Plastic deformation of cutting edge due to intensive stresses and temperature.

The last two modes of tool failure are very harmful and need to be prevented by using

suitable tool materials and geometry depending upon the work material and cutting

condition. But failure by gradual wear, which is inevitable, cannot be prevented but can

be slowed down only to enhance the service life of the tool.

It is considered that the tool has failed or about to fail by one or more of the following

conditions:

1. Total breakage of the tool or tool tip(s)

2. Massive fracture at the cutting edge(s)

3. Excessive increase in cutting forces or vibration

4. Average wear (flank or crater) reaches its specified limit(s)

5. Excessive vibration and or abnormal sound (chatter)

6. Dimensional deviation beyond tolerance

7. Rapid worsening of surface finish

Tool wears mechanism / mode of wear

For the purpose of controlling tool wear one must understand the various mechanisms of

wear that the cutting tool undergoes under different conditions.

The common mechanisms of cutting tool wear are:

1) Mechanical wear

a. abrasion,

b. adhesion,

c. chipping

d. fatigue

2) Thermo-chemical wear

a. diffusion wear

3) Chemical wear

a. corrosion

b. oxidation

Abrasive wear: wear due to the abrasive

action between the chip and tool as the

chip passes over the rake surface of tool.


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Adhesive wear: the tool and the chip

weld together at contacts, and wear

occurs by the fracture of these welded

junctions.

Welding is due to

Friction

High temperature

Pressure

Fatigue wear: Due to friction while cutting, compressive force will be produced in one

side and tensile force on other side. This causes the crack formation on the surface of tool

and fails.

Chipping of tool: It refers the breaking away of small chips from the cutting edge of tool

Diffusion wear: When a metal is in

sliding contact with another metal the

temperature at the interface is high. The

high temperature allows the atoms of

hard material to diffuse into softer

material matrix. At high temperature

there is an intimate contact between the

chip and the tool rake face. Atoms of the

softer metal may also diffuse into harder

medium, thus weakening the surface of

harder material. Diffusion phenomenon

is strongly dependent on temperature

Oxidation wear: Oxidation wear is the result of reaction between tool face material and

oxygen present in the vicinity.

Corrosion wear: may be included in wear phenomenon

Gradual or progressive tool wear / (types of wear)

a) Flank wear

b) Crater wear

c) Nose wear

d) Plastic deformation of tools

The main wears in cutting tool are

1) Crater wear –occurs on top rake face due to diffusion and friction between tool

and chip. Crater wear dominates at high speeds.

2) Flank wear –occurs on flank (side of tool) due to abrasive rubbing between the

tool flank and the newly generated work surface. Flank wear dominates at low

speed

3) Nose wear is the rounding of a sharp tool due to mechanical and thermal effects


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The tool wear depends on a number of factors such as:

1. Hardness and type of tool material.

2. Cutting tool geometry.

3. Feed and depth of cut.

4. Cutting fluid.

5. Material and condition of work-piece.

6. Dimensions of the work-piece.

Effects of tool wear

Tool wear causes the tool to lose its original shape and lose its cutting efficiency. Tool

wear is a time dependent process. As cutting proceeds, the amount of tool wear increases

gradually. But tool wear must not be allowed to go beyond a certain limit in order to

avoid tool failure.

1. Increase the cutting force

2. Increase the surface roughness

3. Decreases dimensional accuracy

4. Increase the cutting temperature

5. Creates vibration

6. Reduce the quality of work

7. Increase the cost and lower the cutting efficiency


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Tool wear criteria/tool life criteria

In practical situation, the time at which a tool ceases to produce work-pieces with desired

size, surface quality and acceptable dimensional tolerances, usually determines the end of

tool life. In most cases the tool wears gradually and the roughness of the machined

surface becomes too high, cutting forces rise and cause intolerable deflections or

vibrations etc. As the tool wear rate increases, the dimensional tolerances can not be

maintained. When the wear reaches a certain size, it will accelerate and may lead to a

sudden failure of the edge.

In economical machining under normal cutting condition, wear on the flank is usually the

controlling factor. So flank wear is the criteria (ISO test) normally used to assess the tool

life. Some wear is inevitable. (flank wear is normally the most consistent form of wear

which is easy measurable). It is measured by the width of flank wear (VB/FW shown in

figure) is used to define the tool life. If the amount of flank wear exceeds some critical

value (VB -0.3 to 0.5mm) the excessive cutting force may cause tool failure.

Other failure criteria based on crater wear, notch wear, surface finish and dimensional

variations also used to define the tool life. When the cutting edge of the tool reaches one

of the tool life criterion, the tool should be replaced by a new one or sending it for

regrinding.

Stages of tool wear

Rapid initial wear as the new tool is introduced on to the work. Then wear rate reduces

and takes a uniform rate and finally wear exceeds a limit it gets failed

The width of flank wear land is usually taken as a measure of the amount of wear land

deciding the wear allowable before regrinding or disposal of tool (measured using tool

maker’s microscope).

Region AB- rapid wear rate due the initial broken down of cutting edge

Region BC- uniform wear rate

Region CD- increasing rate


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Tool Life

A tool cannot cut for an unlimited period of time. After using for a certain period of time,

it ceases to cut satisfactorily unless it is re-sharpened. It has a definite life. The tool life

maybe defined as the time elapsed (actual cutting time) between two consecutive

sharpening of the cutting tool. The tool life can be measured in the following ways:

1. Volume of material removed between two successive grindings.

2. Number of pieces machined between two successive grindings.

3. Total time of operation.

4. Equivalent cutting speed.

Factors affecting Tool Life (same factors of machinability)

1. Tool and Work Material

2. Tool Geometry

3. Cutting Speed

4. Feed and Depth of Cut

5. Cutting Fluid

6. Rigidity of tool, work and machine.

7. Area of cut

Tool Life Equation (Taylor’s Equation based on Flank Wear)

Tool life of any tool for any work material is governed mainly by the level of the

machining parameters i.e., cutting velocity (VC), feed (f) and depth of cut (t1). Usually

flank wear (VB) is taken as the criteria for fixing the wear limit for tool change and its

dependence on cutting velocity are schematically shown below:

The tool life obviously decreases with the increase in cutting velocity keeping other

conditions kept constant.

Life of the tools T1, T2, T3, and T4 are plotted against the corresponding cutting

velocities V1, V2, V3, and V4, a smooth curve like a rectangular hyperbola is found.

By plotting VC and T and the log scale the following plot was obtained.


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Taylor derived the tool life equation,

VTn = C or V1T1

n = V2T2

n =C

Where,

T = tool life in minutes

V = cutting speed in m/min

n = index which depends on tool and work-piece

= 0.1 to 0.5 for HSS tools

= 0.2 to 0.4 for carbide tools

= 0.4 to 0.6 for ceramic tools

C = constant. It is numerically equals to cutting speed that gives a tool life of one

minute. The value of C depends on tool-work material and cutting environment

(feed and depth of cut).

2211 loglogloglog TnVTnV

12

21

loglog

loglog

TT

VVn

Note: If the higher cutting speed is permitted by a tool for the same life, we can say that

the tool is having better cutting properties and it will be more productive.


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Effect of tool life on rake, clearance angle, cutting time and temperature

Tool life increases with increase of rake angle for the given cutting speed. Increase

of rake angle decreases the cutting force and temperature.

As the clearance angle increases, the tool chip contact area increases which reduces

the cutting temperature and wear rate ultimately increases the tool life.

Increases of nose radius also increases the life to a extent

Wear increases with cutting temperature. All types of wear accelerate with increase

of temperature (mechanical, chemical and diffusion wear)

Cutting time- tool wear is a time dependent factor. As cutting proceeds, the amount

of tool wear increases gradually.

There is optimum value of rake and clearance angle which gives maximum tool life


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Machineability

It can be defined as the ease with which a material can be machined by a given tool under

a given set of conditions. Suppose it is states as material A is more machinable than

material B, then it means that:

1. Tool wear rate is less when machining material A

2. Better surface finish is obtained for material A.

3. Less power is consumed during the machining of material A.

Factors affecting machinability

1. Work variables

a) Chemical composition of the workpiece material.

b) Microstructure of the workpiece material.

c) Mechanical properties like ductility, brittleness, toughness etc.

d) Physical properties of the work material.

e) Method of production of the work material.

2. Tool variables

a) Geometry of the tool.

b) Tool material

c) Rigidity of the tool.

d) Nature of engagement of the tool with the work.

3. Machine variables

a) Rigidity of the machine

b) Power and accuracy of the machine tool.

4. Cutting conditions

a) Cutting speed

b) Feed

c) depth of cut

d) cutting fluid

Evaluation or assessment of Machinability (Criteria for evaluating machinability)

1. Tool life for a given cutting speed and tool geometry.

2. Material Removal Rate.

3. Magnitude of cutting forces and power requirement.

4. Quality of the surface finish of the work material.

5. Accuracy of dimensions of the workpiece.

6. Ease of chip disposal

7. Shape and size of chips formed.

8. Heat generated during the cutting process.

9. Power consumption per unit volume of material removed.


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Advantages of high machinability

1. Good surface finish can be obtained.

2. Higher cutting speed can be employed.

3. High Material Removal Rate

4. Less tool wear.

5. Less power consumption.

Machiability Index

It is a quantitative measure of machinability. It is used to compare the machinability of

different metals. The machinability index of free cutting steel is arbitrarily fixed as 100%.

Thus for other metals machinability index is given by:

lifetooluteforsteeldardsofspeedCutting

lifetooluteformaterialofspeedCuttingIIndexityMachinabil

min20tan

min20,

Methods to improve the machinability

The machinability of the work materials can be improved, by the following ways:

Favourable change in composition, microstructure and mechanical properties by

mixing 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 parameters

Proper selection and appropriate method of application of cutting fluid depending

upon the tool – work materials

Proper selection of special techniques like hot, cryogenic machining etc,

By changing the method of work material production, eg: cold worked material

shows higher machinability

Machinability of Titanium, Aluminium and Copper alloys

Materials with good machinability require little power to cut, can be cut quickly, easily

obtain a good finish, and do not wear the tooling much.

Titanium

The machinability of titanium and its alloys is generally considered to be poor owing to

several inherent properties of the materials. The machinability index of titanium is about

30% or less (0.3). Machininability involves careful selection of tool geometry and speed.

Poor machinability may due following properties

Titanium is very chemically reactive

Tendency to weld to the cutting tool during machining, thus leading to chipping

Its low thermal conductivity increases the temperature which affects the tool life

and build up edge


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Aluminum alloys

Compared to other construction materials, aluminium is easy to machine and gives good

tool life. Carbide tool gives 600m/min cutting speed.

In general, the specific cutting force for aluminium is about 30% that of steel.

Most aluminum alloys are easily machined and yields greater tool life and

productivity.

Machinability index of aluminium is about 300 to 1500% (Excellent machinability)

Aluminium is machined using tools made of tool steel, high-speed steel, cemented

carbides and diamond

Although readily machinable, aluminum has a high coefficient of friction and high

thermal coefficient of expansion. This means a special approach is required including the

use of polished tools with different tool geometry and good lubrication to avoid thermal

stress. Chips are of the continuous type. The primary tooling concerns when machining

aluminum are: minimizing the tendency of aluminum to stick (BUE) to the tool cutting

edges and poor surface finish

Copper alloys

All coppers and copper alloys can be machined accurately, cheaply and to a good

standard of tolerances and surface finish. Machining copper alloys is considerably easier

than machining steels of the same strength.

Significantly lower cutting forces

The machinability rating of this alloy is 1.6 to 2 (160 to 200%)

Brass is more easy to machine than bronze

Tools used for machining

Usually tungsten carbide-cobalt, coated HSS, CBN or PCD cutting tools are used.

Effect s of cold working on machinability

Cold working of work materials results in many improvements on its properties resulted

in better machinability. A cold worked material gives following characteristics.

1. Improved surface finish

2. Controlled dimensional tolerance and concentricity

3. Improve machinability

Economics of Metal Cutting

The primary aim is to produce satisfactory parts at the lowest possible cost and highest

possible production rate. As the cutting speed increases the MRR is high giving low

cutting cost, but the tool life will be shorter and consequently high cutting cost per piece.

At some intermediate cutting speed, the tool cost will be minimum. The tool life

corresponding to this speed is economical tool life.

The total cost per piece is based on:

Non productive cost


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Cutting or machining cost

tool changing cost

The machining cost per component is made up of a number of different costs. Non-

productive cost per component includes the cost of loading and unloading component,

and other non-cutting time costs.

Select speed to achieve a balance between high metal removal rate and suitably long tool

life . Mathematical formulas are available to determine optimal speed

Two alternative objectives in these formulas:

1. Maximum production rate

2. Minimum unit cost

Maximizing production rate = minimizing cutting time per unit

Cutting force measurement

The forces generated by a machining process can be monitored to detect a tool failure or

to diagnose and controlling the process parameters, and in evaluating the quality of the

surface produced. There are many force measurement systems, such as strain gages,

inductive and capacitive systems. During machining, the cutting tool exerts a force on the

work piece. The cutting force includes tangential, feed and radial force. These force

components can be measured with a dynamometer.

Quartz crystals experience the piezoelectric effect which produces electric charge when

the material is deformed. The cutting force is measured either directly gauging the

deformation caused or by sensing the deformation with a transducer or converting it.


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Cutting Fluid

During a metal cutting operation heat is generated due to plastic deformation of the metal

and friction of the tool-work interface. The increase in temperature reduces the hardness

of the tool. This leads to tool failure. Cooling the work prevents the excessive thermal

distortion of the work and cooling the tool helps to increase tool life

Cutting fluid is used to carry away the heat produced during the machining and

reduce the friction between the chip and the tool. Cutting fluids in the form of a liquid are

applied to the chip formation zone to improve the cutting conditions.

If the tool has low value of hot-hardness, there is appreciable increase in tool life as result

of cutting fluid supplied to the cutting zone. In the case of carbides and oxides which

have high value of hot-hardness, cutting fluid has negligible effect on the tool life.

Functions of the Cutting Fluid

1. To cool the tool and work during machining (dissipate heat)

2. To lubricate ( to reduce friction)

a. It reduces the power consumption

b. It prevents the formation of BUE

c. Increases tool life

d. Reduces heat generated at the tool chip interface

3. To protect the finished surface from corrosion.

4. To cause the chips break into small pieces.

5. Chips are disposed off easily

6. To improve surface finish

Properties of Cutting Fluid

1. Good lubricating properties to reduce frictional and the power consumption.

2. High heat absorbing capacity (high specific heat)

3. It should high heat conductivity.

4. High flash point.

5. Should be non corrosive to work and tool.

6. It should have low viscosity to permit free flow of liquid.

7. Should not lose its properties when exposed to heat.

8. It should not react with the work piece

9. Should be economical in use.

Types of cutting fluids

a) Water based cutting fluid

b) Straight cutting oils

c) Synthetic cutting fluid (from chemicals)

Water based Cutting Fluids

To improve the cooling and lubricating properties of water, the soft soap and mineral oils

are added to it. Soluble oils are emulsions composed of around 80% of water, remaining

soap and mineral oils. The soap acts as the emulsifying agent which breaks the oil into

particles to minute particles to disperse them throughout water.


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It has got excellent cooling properties at low cost and has got some lubricating effect

between the chip and the tool. By varying the proportion of water cutting fluids with

wide range of cooling and lubricating properties are obtained. Water bases cutting fluids

may be adopted for medium and high speed applications. This is due to the fact that at

low speeds the heat transfer characteristics may be low but lubricity should be high. At

high speeds good heat transfer characteristics are more desirable.

Straight or oil Based cutting fluids

Straight or oil based cutting fluids means undiluted or pure oil based fluids. These oils

have good lubricating properties but poor heat absorption quality. The heat transfer rates

by their application are the lowest among cutting fluids. Thus they are only suitable for

low cutting speeds. Used for heavy cutting process like broaching, threading and gear

etc. Poor cooling and smoke formation are the disadvantages

They can be classified into the following groups:

1. Mineral oils

2. Fatty oils

3. Combination and mineral and fatty oils

Oil with additives

a) The beneficial effect of mineral oils can be improved with the help of additives,

which are generally compounds of sulphur and chlorine.

b) Other additives may also be used to improve stability and anti rust properties.

Selection of Cutting Fluids : The guidelines for selection of cutting fluids are:

a) Type of machining process

b) Based on tool and work materials

c) Based on cutting condition like speed, feed etc

d) Method of application of fluid

e) Temperature involved

Methods of application

a) Flooding – cutting fluid is flooded over the cutting area

b) Jet – cutting fluid under pressure with nozzle

c) Mist – mixed with compressed air

Oil with extreme pressure characteristics may be adopted for low speed cuts.

Example: Machining of aluminium using water based cutting fluid

Alloy steel uses water based and mineral oil etc

Cast iron – dry machining (no cutting fluid is needed as free graphite in cast iron)

************************************************


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Problems

Example 1: A lathe running at a speed of 4000 rev./min is cutting Mild Steel with a

H.S.S. tool. The spindle speed is reduced to 3000 rev./min. calculate the percentage

change in tool life

Given,

V1 = Initial cutting speed. V2 = Reduced cutting speed.

T1 = Tool life at V1. T2 = Tool life at V2.

Take, n = 0.125

VTn = C

VT0.125

= C

So T10.125

= ( C / V1 )

And T20.125

= ( C / V2 )

(T1/T2)0.125

= (C/V2)

(C/V1)

(T2/T1)0.125

= C x V1

V2 x C

(T2/T1)0.125

= V1

V2

(T2/T1) = 9.99

T2 = 9.99xT1

This shows that reducing cutting speed by 25% increases tool life by nearly 10

times (under the cutting conditions of this particular setup).

Problem 2: In the previous example, for instance, if it were to be observed that the initial

tool life (T1) was 4 hours and cutting speed 30m/min., then: -

VTn

= C

30 x (4x60(mins))0.125

C = 59.5


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Problem 3

If the value of n for mild steel workpiece material and carbide tool material is 0.1 then at

the same cutting speed of 30 m/min:

VTn

= C

30 x T0.1

= 59.5

T0.1

= 59.5

30

T = 941 minutes

As opposed to 240 minutes using the H.S.S. tool.

Problem 4

If in turning of a steel rod by a given cutting tool (material and geometry) at a given

machining condition ( feed f and depth of cut t1) under a given environment (cutting fluid

application), the tool life decreases from 80 min to 20 min. due to increase in cutting

velocity, VC 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.?


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Problem 5

During a tool life-cutting test on HSS tool material, following data were recorded.

Calculate the values of n and C of taylor’s equation


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Module II Question

1. Define machinability.****

2. Define machinability index.****

3. What do you mean by machinability ? Explain the method of determining

machinability of a material and the factors affecting machinability.*****

4. Define machinability. How it is expressed? State the factors affecting the

machinability of material.******

5. Explain the term machining economics and machiuability index.

6. The machinahility index of SS and of Al alloys is 30 and 1500 respectively. What

does it signify

7. Discuss the functions effecting machinability.

8. What is the effect of cold working in machinability?

*****************************************************************

9. What the properties are of required for a tool material? Give the details of some

common tool materials.

10. Present the developments in cutting tool materials in the last decade

11. What are the desirable characteristics of cutting tool materials? Describe how they

are satisfied in High Speed Steel tools. **

12. Discuss the factors affecting tool life and discuss requirements of tool material to get

reasonable tool life.

13. Discuss briefly various cutting tool materials.

14. Explain any two cutting tool material characteristics.

15. What is HSS and what are its advantages '?

16. When do you use the diamond as cutting tool material?

17. What are the factors affecting tool life and how it is improved? ***

18. Define tool life and state the factors- affecting tool life. ***`

19. Define tool life and explain how it is accessed in metal cutting operations.

20. Explain the relationship between tool life and cutting speed.**

21. What is Taylor tool life equation? Explain economic cutting speed.**

22. Define tool life. Derive Taylor's relationship for cutting speed-tool life.**

23. What is the effect of cutting speed on tool life

24. Classify the different types of tool life tests

25. Discuss the significance of too life.

26. Discuss economic tool life.

27. A 25 mm. dia steel bar was turned at 3000 r:p.m. using HSS tool. Tool failure

occurred after 10 minutes. When the speed was reduced to 250 r.p.rn., the tool failed

in 52.5 minutes. Assuming Taylor equation is valid, find the expected tool life at a

speed of 275 rpm.

28. For a HSS tool cutting mild steel at 20 m./min, the life is 4 hours. Calculate its life

when the cutting speed is increased to 24 m./min:

29. What is the tool life of a HSS tool 10-15-6-6-18-14-2 working at 50 m/min?


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30. What are the factors influencing tool wear? Explain how the process parameters can

be controlled to reduce wear and improve life.

31. Explain the mechanism of tool wear. How can it be measured?**

32. State the reasons responsible for tool failure.**

33. Name the different types of tool wear and explain. briefly about use of chip breakers

34. Discuss the various modes of tool wear.

35. Describe different types of tool wear. How is tool wear assessed?

36. What are the classifications of tool wear? Describe them.****

37. Why is flank wear used as the criterion of tool life studies?

38. What are the common mechanisms causing wear on cutting tools? Explain.***

39. Give the composition, properties and applications of any four cutting tool materials.

40. Name the techniques used for measuring tool wear. Described their advantages and

limitations.

41. Explain flank wear.

42. What do you mean by crater wear" and Flank wear? How are they measured?

43. What are "Cermets”? How are they manufactured?

44. Explain the effect of cutting speed, feed and depth of cut on tool life.

45. What determines how wider a wear land can be allowed before the tool should be

resharpened?

*******************************************************************

46. What are the functions of cutting fluids? ****

47. What are the essential properties of a cutting fluid `?***

48. List out the purpose and properties of cutting fluids.**

49. What are the classifications of cutting fluids?**

50. Give an account of different types of cutting fluids and their use.

51. Explain the following types of cutting fluid

a. Water based cutting fluids.

b. Straight or seat oil based cutting fluid

52. Briefly explain the theory of cutting fluid with a diagram.

53. How are cutting fluids selected '?

54. How you will select the cutting fluids for machining? (4 marks)

55. What are the two types of cutting fluids? What are thin major applications?

56. Can cutting fluids have any adverse effects? If so, what are they?

57. Explain the reasons for selecting water based cutting fluids?

************************************************************

58. What are the sources of heat generation in metal cutting?

******************************************************

59. Explain the computation of Cost components" and-"Total production cost" of a

machined

60. Discuss the economics of metal cutting.

61. What are the different cost components in Total production cost”?


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62. Describe the relationships among cutting speed, rate of production and production

cost?

63. Discuss the effect of friction during cutting. How can the friction be reduced?

***************************************

Question before 2003

64. Why is pore size important in the manufacture of self lubricating bearing? How may

pore size be controlled

Ans: Sintering of powder compacts does not generally produce full density products

(density between 0.9 to 0.98 theoretical density). PM bearings are self-lubricating

because their porosity is impregnated with lubricants during the manufacturing process.

In use, heat causes the lubricant to expand out of the pores forming a film between

mating parts. When operation is suspended, the lubricant cools and is drawn back into the

pores for subsequent reuse. These voids take up to10% to 35% of the total volume. In

operation, lubricating oil is stored in these voids and feeds through the interconnected

pores to the bearing surface. Any oil which is forced from the loaded zone of the bearing

is reabsorbed by capillary action. Since these bearings can operate for long periods of

time without additional supply of lubricant, they can be used in inaccessible or

inconvenient places where relubrication would be difficult.

High porosity with a maximum amount of lubricating oil is used for high-speed light-load

applications

Pore size is depend on size and shape of metal powder and compacting pressure, sintering

temperature, sintering time etc

**********************************************************************


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General Extra notes

Modified Taylor’s Tool Life Equation

In Taylor’s Equation only the cutting velocity is considered, but practically, the variation

in feed f, and depth of cut t1, plays an important role in the tool life. The modified Taylor

Equation is given by,

T = C .

VCx x f

y x t1

z

Where,

T = tool life in min,

C = constant depending on tool and workpiece

x,y,z = exponents so called tool life exponents depending upon the

tool, work material and cutting conditions.

Crater Wear

The wear that occurs on the tool face in the form of a depression is known as

crater, which begins at a certain distance from the cutting edge. The crater is restricted to

the tool chip contact area. The crater is formed some distance away from the cutting edge

and is maximum when the chip tool interface temperature is maximum. The formation of

crater is due to adhesion, diffusion etc. and is a time dependant phenomenon.

Effect of various tool wear factors

Cutting speed

Higher cutting speed increases tool temperature and softens the tool material. It thereby

aids abrasive, adhesion and diffusion wear. The tool life is given by Taylor’s equation.

Feed

The larger the feed, the greater is the cutting force per unit area of the chip-tool contact

on the rake surface and work-tool contact on the flank surface. The cutting temperature

increases and different types of wear increases.

An increase in cutting force as a result of larger feed also increases the likelihood of

chipping of the cutting edge through material shock. The effect of changes in feed on tool

life is relatively smaller that of proportionate changes in cutting speed.


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Depth of Cut

If the depth of cut is increased, the area of the chip-tool contact increases roughly

in equal proportions to the change in depth of cut. Consequently the rise in temperature is

relatively small. But when comparing with feed the change in temperature is larger. This

is due to the fact that the area of chip-tool contact in increased depth of cut changes by a

small proportion, than that of change in feed rate.

Cutting Fluid

The cutting fluid cools the chip and the workpiece and reduces the frictional stress at the

tool-work interface and the chip-tool interface. Thus the cutting forces are reduced and

thus the cutting temperature. In the case of carbides and oxides which have high value of

hot-hardness, cutting fluid has negligible effect on the tool life.

Rigidity of Tool, work piece and machine

If the machine is not properly designed, if the workpiece is long and thin or if the tool

hang is excessive, chatter may occur during cutting. Chatter may cause fatigue failure or

catastrophic failure of the tool due to mechanical shock.

Measurement of tool wear: The various methods are

a) by loss of tool material in volume or weight, in one life time – this method is

crude and is generally applicable for critical tools like grinding wheels.

b) by grooving and indentation method – in this approximate method wear depth is

measured indirectly by the difference in length of the groove or the indentation

outside and inside the worn area using optical microscope fitted with micrometer

– very common and effective method

c) using scanning electron microscope (SEM) – used generally, for detailed study;

both qualitative and quantitative

The wears were measured by scanning electron microscope and the cutting forces

measured by a dynamometer. There is clear relationship between flank wear and cutting

forces. Lower cutting forces leads to low flank wear and low cutting force provides good

dimensional accuracy of the work material including low surface roughness. Flank wear

formation was mostly caused by abrasion and less by adhesion.

Tool Life Measurement in terms of Metal Removal

The volume of metal removal between tool sharpening for definite depth of cut,

feed and cutting speed can be determined as follows:

Cutting speed V = π D N / 1000 (for turning)

Where,

D = diameter of workpiece (mm)

N = RPM of workpiece

Let,

t1 = depth of cut in mm

f = feed mm/min

TF = time of tool failure in min


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Volume of metal removed per revolution = π D t1 f mm3

Volume of metal removed per minute = π D t1 f N mm3

Volume of metal removed in TF minute = π D t1 f N TF mm3

Tool Life,

L = π D t1 f N TF mm3

= 1000 V t1 f TF mm3

Effects of alloying elements on tool steel properties:

Carbon: Raising carbon content increases hardness slightly and wear resistance

considerably. Dramatic increases to hardness & strength when heat treated.

Manganese: Small amounts of of Manganense reduce brittleness and improve

forgeability. Larger amounts of manganese improve hardenability, permit oil

quenching (less severe queching required - which reduces quenching

deformation).

Silicon: Improves strength, toughness, and shock resistance.

Tungsten: Improves "hot hardness" - used in high-speed tool steel.

Vanadium: Refines carbide structure and improves forgeability, also improving

hardness and wear resistance.

Molybdenum: Improves deep hardening, toughness, and in larger amounts, "hot

hardness". Used in high speed tool steel because it's cheaper than tungsten.

Chromium: Improves hardenability, wear resistance and toughness.

Nickel: Improves toughness and wear resistance to a lesser degree.

Structure of carbon steel

The addition of carbon to the pure iron results in a considerable difference in the

structure.

Austenite This phase is only possible in carbon steel at high temperature. It has

a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon

in solution.

Ferrite: This phase has a Body Centre Cubic structure (B.C.C) which can hold

very little carbon; typically 0.0001% at room temperature.

Cementite: Unlike ferrite and austenite, cementite is a very hard intermetallic

compound consisting of 6.7% carbon and the remainder iron.

Pearlite: A mixture of alternate strips of ferrite and cementite in a single grain. A

fully pearlitic structure occurs at 0.8% Carbon.


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Structure of high speed steel

Aluminium

Next to steel, Aluminum is the most commonly used and commercially available metal.

Its light weight and high strength-to-weight ratio make it a good choice for everything

from aircraft to flashlights. These are light weight having good malleability and

formability, high corrosion resistance and high electrical and thermal conductivity.

However, in the engineering application pure aluminum and its alloys still have some

problems such as relatively low strength, unstable mechanical properties. The

microstructure can be modified and mechanical properties can be improved by alloying,

cold working and heat treatment.

Titanium is one of the fastest growing materials used. Titanium and its alloys are used

extensively in aerospace and other area like petroleum, chemical, marine nuclear fields

because of their excellent combination of

High specific strength (strength-to-weight ratio) at elevated temperature,

High fracture resistant

High exceptional resistance to corrosion etc