Department of Mechanical Engineering, SSET 2014 Page1 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
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Department of Mechanical Engineering, SSET 2014
Pag
e1
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
Pag
e2
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
Pag
e3
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.
Department of Mechanical Engineering SSET 2015
Pag
e4
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
Department of Mechanical Engineering SSET 2015
Pag
e5
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
Department of Mechanical Engineering SSET 2015
Pag
e6
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 %
Department of Mechanical Engineering SSET 2015
Pag
e7
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.
Department of Mechanical Engineering SSET 2015
Pag
e8
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
Department of Mechanical Engineering SSET 2015
Pag
e9
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.
Department of Mechanical Engineering SSET 2015
Pag
e10
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.