Pe Full Note
-
Upload
midhun-davis -
Category
Documents
-
view
231 -
download
0
Transcript of Pe Full Note
-
7/28/2019 Pe Full Note
1/167
PRODUCTION ENGINEERING
-
7/28/2019 Pe Full Note
2/167
B.TECH.DEGREECOURSE
SCHEMEANDSYLLABUS
(2002-03 ADMISSION ONWARDS)
MAHATMAGANDHIUNIVERSITY
KOTTAYAM
KERALA
PRODUCTION ENGINEERING
M 801
Module 1
. Theory of Metal cutting: Review of deformation of metals -Tool nomenclature and
geometry - Oblique & orthogonal cutting - Mechanism of chip formation, types of
chips- Mechanism of orthogonal cutting: Merchants cycle diagram shear anglesolutionVelocity relationshipsEffect of rake angle, cutting angle, nose radius etc
on cutting force and surface finishFriction in metal cutting nature of sliding friction,
effect of increasing normal load on apparent to real area of contact.
Module 2
. Machinability of metals: Factors affecting Machinability Evaluation of
MachinabilityMachinability index, significance of tool life factors affecting tool
life
measuremmit of tool lifeTaylors equation economics of Machining - - Tool
wearmodes of tool wearflank & crater weartypes of wear measurements
-
7/28/2019 Pe Full Note
3/167
Tool materials: carbon steel, HSS, coated HSS, ceramics, diamond etc Cutting
fluids: types, selection of liquids, properties and functions.
Module -3
Powdered Metallurgy: Metal powdermethods of preparation of Metal powders
Powder characteristics: properties of fine powder, particle size, size
distribution, shape compressibility, purity etcMixingCompaction techniques
hot
pressing, powder rolling pre sintering and sintering Sintering atmosphere
Finishing
[ operations: heat treatment, surface treatment, impregnation treatment etc - examplesof articles produced and their applications
Module-4
Advanced Materials and ceramics: Advanced Materials: super alloysNickel based
super alloys Titanium and titanium alloys shape memory alloys smart
materials properties and applications. Ceramics: Structure Mechanical,
physical properties and applications common types ;-. of ceramic materials
ceramic fabrication process like slip casting, pressure forming, hot pressing, plastic
forming and ceramic joiningglass ceramics
Module-5
Advanced manufacturing Techniques: Introduction to Rapid prototyping
SteriolithographyNon Traditional machining: EDM, ECM., USM, EBM, LBM, 113M,
AJIv1, waterjet machining, LIGA process principle, types, process parameters,
surface finish, applications etc
-
7/28/2019 Pe Full Note
4/167
References
1. Armarego & Brown, The Machining of Metals, Prentice - Hall2.
Beaman, Barlow & Bourell, Solid Free Foam Fabrication: A new direction inmafg., Kluwer Academic Publishers
3. Brophy, Rose & Wulf, The Structure & Properties of Metals Vol.2, WileyEastern
4. Dixon & Clayton, Powder Metallurgy for Engineers, Machinery publishing co.London
5. HMT, Production Technology, Tata McGraw Hill6. Kalpakjian, Manufacturing Engineering & Technology, Addison Wesley, 4nd
edn.
7. Lal G.K., Introduction to Machining Science, New Age publishers8. Metcut research, Machinablity Data Center Vol.1 & 2, Metcut research
associates, Cincinnati
9. Paul. H. Black, Theory of Metal Cutting, McGraw Hill
-
7/28/2019 Pe Full Note
5/167
MODULE-1
THEORY OF METAL CUTTING
Mechanism of chip formation
Mechanism of chip formation in machining
Machining is a semi-finishing or finishing process essentially done to impart required or
stipulated dimensional and form accuracy and surface finish to enable the product to
fulfill its basic functional requirements
provide better or improved performance
render long service life.
Machining is a process of gradual removal of excess material from the preformed blanks
in the form of chips.
The form of the chips is an important index of machining because it directly or indirectly
indicates :
Nature and behaviour of the work material under machining condition
Specific energy requirement (amount of energy required to remove unit volume
of work material) in machining work
Nature and degree of interaction at the chip-tool interfaces.The form of machined chips depend mainly upon :
Work material
Material and geometry of the cutting tool
Levels of cutting velocity and feed and also to some extent on depth of cut
Machining environment or cutting fluid that affects temperature and friction at
the chip-tool and work-tool interfaces.
Knowledge of basic mechanism(s) of chip formation helps to understand the
characteristics of chips and to attain favourable chip forms.
Mechanism of chip formation in machining ductile materials
During continuous machining the uncut layer of the work material just ahead of the
cutting tool (edge) is subjected to almost all sided compression as indicated in Fig.
-
7/28/2019 Pe Full Note
6/167
The force exerted by the tool on the chip arises out of the normal force, N and frictional
force, F as indicated in Fig. 1.1.
Due to such compression, shear stress develops, within that compressed region, in
Fig. 1.1 Compression of work material (layer) ahead of the tool tip
different magnitude, in different directions and rapidly increases in magnitude. Whenever
and wherever the value of the shear stress reaches or exceeds the shear strength of that
work material in the deformation region, yielding or slip takes place resulting shear
deformation in that region and the plane of maximum shear stress. But the forces causing
the shear stresses in the region of the chip quickly diminishes and finally disappears
while that region moves along the tool rake surface towards and then goes beyond the
point of chip-tool engagement. As a result the slip or shear stops propagating long before
total separation takes place. In the mean time the succeeding portion of the chip starts
undergoing compression followed by yielding and shear. This phenomenon repeats
rapidly resulting in formation and removal of chips in thin layer by layer. This
phenomenon has been explained in a simple way by Piispannen using a card analogy.
Fig. 1.2 Piispanen model of card analogy to explain chip formation in machining ductile
materials
-
7/28/2019 Pe Full Note
7/167
In actual machining chips also, such serrations are visible at their upper surface as
indicated in Fig. 1.2. The pattern and extent of total deformation of the chips due to the
primary and the secondary shear deformations of the chips ahead and along the tool face,
as indicated in Fig. 1.3, depend upon
work material
tool; material and geometry
the machining speed (VC) and feed (s
o)
cutting fluid application
Fig. 1.3 Primary and secondary deformation zones in the chip
The basic two mechanisms involved in chip formation are Yielding generally for ductile materials Brittle fracture generally for brittle materials
During machining, first a small crack develops at the tool tip as shown in Fig. 1.4 due to
wedging action of the cutting edge. At the sharp crack-tip stress concentration takes
place. In case of ductile materials immediately yielding takes place at the crack-tip and
reduces the effect of stress concentration and prevents its propagation as crack. But in
case of brittle materials the initiated crack quickly propagates, under stressing action, and
total separation takes place from the parent workpiece through the minimum resistance
path.
-
7/28/2019 Pe Full Note
8/167
Fig. 1.4 Development and propagation of crack
Machining of brittle material produces discontinuous chips and mostly of irregular size
and shape. The process of forming such chips is schematically shown in Fig. 1.5.
Fig. 1.5 Schematic of chip formation in brittle materials
Chip Thickness ratio
The significant geometrical parameters involved in chip formation are shown in Fig. 1.6
and those parameters are defined (in respect of straight turning) as:
t = depth of cut (mm)perpendicular penetration of the cutting tool tip
in work surface
so= feed (mm/rev)axial travel of the tool per revolution of the job
-
7/28/2019 Pe Full Note
9/167
Fig. 1.6 Geometrical features of continuous chip formation
b1
= width (mm) of chip before cut
b2
= width (mm) of chip after cut
a1
= thickness (mm) of uncut layer (or chip before cut)
a2
= chip thickness (mm)thickness of chip after cut
A1
= cross section (area, mm2
) of chip before cut
Chip thickness ratio is defined as the ratio of uncut chip thickness to the chip
thickness. i.e., r = a1/a2
The degree of thickening of the chip is expressed by
= a2/a1> 1.00 (since a2
> a1)
where, = chip reduction coefficient
-
7/28/2019 Pe Full Note
10/167
Larger value of means more thickening i.e., more effort in terms of forces or energy
required to accomplish the machining work. Therefore it is always desirable to reduce a2
or without sacrificing productivity, i.e. metal removal rate (MRR).
Chip reduction coefficient, is generally assessed and expressed by the ratio of the chip
thickness, after (a2) and before cut (a
1).
But can also be expressed or assessed by the ratio of
* Total length of the chip before (L1) and after cut (L
2)
* Cutting velocity, VC
and chip velocity, Vf
Considering total volume of chip produced in a given time,
a1b
1L
1= a
2b
2L
2
The width of chip, b generally does not change significantly during machining unless
there is side flow for some adverse situation.
Therefore assuming, b1=b
2 , comes up to be
= a2/a1 = L1/L2
Again considering unchanged material flow (volume) ratio, Q
Q = (a1b
1)V
C= (a
2b
2)V
f
Taking b1=b
2, = a2/a1 = Vc/Vf
Shear Angle and Shear Plane
It has been observed that during machining, particularly ductile materials, the chip
sharply changes its direction of flow (relative to the tool) from the direction of the cutting
velocity, VC
to that along the tool rake surface after thickening by shear deformation or
-
7/28/2019 Pe Full Note
11/167
slip or lamellar sliding along a plane. This plane is called shear plane and is schematically
shown in Fig. 1.7.
Shear plane: Shear plane is the plane of separation of work material layer in the form of
chip from the parent body due to shear along that plane.
Shear angle: Angle of inclination of the shear plane from the direction of cutting
velocity
Fig. 1.7 Shear Plane and Shear Angle in chip formation
The value of shear angle, denoted by o(taken in orthogonal plane) depends upon
Chip thickness before and after cut i.e.
Rake angle, o(in orthogonal plane)
From Fig. 1.7,
AC = a2
= OAcos (o-
o)
And AB = a1
= OAsino
Dividing a2by a
1
a2/a1=
-
7/28/2019 Pe Full Note
12/167
Replacing chip reduction coefficient, by cutting ratio, r,
tan o= r cos
o/(1-r sin
o)
This depicts that with the increase in , shear angle decreases and vice-versa. It is also
evident that shear angle increases both directly and indirectly with the increase in tool
rake angle. Increase in shear angle means more favourable machining condition requiring
lesser specific energy.
Cutting Strain
The magnitude of strain, that develops along the shear plane due to machining action, is
called cutting strain (shear). It is given by = cot o+ tan(
o-
o)
Built-up-Edge (BUE) formation
Causes of formation:
In machining ductile metals like steels with long chip-tool contact length, lot of stress and
temperature develops in the secondary deformation zone at the chip-tool interface. Under
such high stress and temperature in between two clean surfaces of metals, strong bonding
may locally take place due to adhesion similar to welding. Such bonding will be
encouraged and accelerated if the chip tool materials have mutual affinity or solubility.
The weldment starts forming as an embryo at the most favourable location and thus
gradually grows as schematically shown in Fig. 1.8
With the growth of the BUE, the force, F (shown in Fig. 1.8) also gradually increases due
to wedging action of the tool tip along with the BUE formed on it. Whenever the force, F
exceeds the bonding force of the BUE, the BUE is broken or sheared off and taken away
by the flowing chip. Then again BUE starts forming and growing. This goes on
repeatedly.
-
7/28/2019 Pe Full Note
13/167
Fig 1.8 Built up edge formation
Characteristics of BUE
Built-up-edges are characterized by its shape, size and bond strength, which depend
upon:
work tool materials stress and temperature, i.e., cutting velocity and feed
cutting fluid application governing cooling and lubrication.
BUE may develop basically in three different shapes as schematically shown in Fig. 1.9.
Fig. 1.9 Different forms of built up edge
In machining too soft and ductile metals by tools like high speed steel or uncoated
carbide the BUE may grow larger and overflow towards the finished surface through the
flank as shown in Fig. 1.10
-
7/28/2019 Pe Full Note
14/167
Fig. 1.10 Overgrowing and overflowing BUE causing surface roughness
While the major part of the detached BUE goes away along the flowing chip, a small part
of the BUE may remain stuck on the machined surface and spoils the surface finish. BUE
formation needs certain level of temperature at the interface depending upon the mutual
affinity of the work-tool materials. With the increase in VC
and so
the cutting temperature
rises and favours BUE formation. But if VC
is raised too high beyond certain limit, BUE
will be squashed out by the flowing chip before the BUE grows. But sometime the BUE
may adhere so strongly that it remains strongly bonded at the tool tip and does not break
or shear off even after reasonably long time of machining. Such detrimental situation
occurs in case of certain tool-work materials and at speed-feed conditions which strongly
favour adhesion and welding.
Effects of BUE formation
Formation of BUE causes several harmful effects, such as:
It unfavourably changes the rake angle at the tool tip causing increase in cutting
forces and power consumption
Repeated formation and dislodgement of the BUE causes fluctuation in cutting
forces and thus induces vibration which is harmful for the tool, job and the
machine tool.
Surface finish gets deteriorated
-
7/28/2019 Pe Full Note
15/167
May reduce tool life by accelerating tool-wear at its rake surface by adhesionand flaking
Occasionally, formation of thin flat type stable BUE may reduce tool wear at the rake
face.
Types of chips and conditions for formation of those chips
Different types of chips of various shape, size, colour etc. are produced by machining
depending upon
type of cut, i.e., continuous (turning, boring etc.) or intermittent cut (milling)
work material (brittle or ductile etc.)
cutting tool geometry (rake, cutting angles etc.)
levels of the cutting velocity and feed (low, medium or high)
cutting fluid (type of fluid and method of application)
The basic major types of chips and the conditions generally under which such types of
chips form are given below:
Discontinuous type
of irregular size and shape : - work materialbrittle like grey cast iron of regular size and shape : - work material ductile but hard and work hardenable -
feedlarge
tool rakenegative cutting fluidabsent or inadequate
Continuous type
Without BUE : work materialductile Cutting velocityhigh Feedlow Rake anglepositive and large Cutting fluidboth cooling and lubricating With BUE : work materialductile cutting velocitymedium
-
7/28/2019 Pe Full Note
16/167
feedmedium or large cutting fluidinadequate or absent.Jointed or segmented type
work materialsemi-ductile cutting velocitylow to medium feedmedium to large tool rakenegative cutting fluidabsent
Often in machining ductile metals at high speed, the chips are deliberately broken into
small segments of regular size and shape by using chip breakers mainly for convenience
and reduction of chip-tool contact length.
Orthogonal and Oblique Cutting
While turning ductile material by a sharp tool, the continuous chip would flow over the
tools rake surface and in the direction apparently perpendicular to the principal cutting
edge, i.e., along orthogonal plane which is normal to the cutting plane containing the
principal cutting edge. But practically, the chip may not flow along the orthogonal plane
for several factors like presence of inclination angle, , etc.
The role of inclination angle, on the direction of chip flow is schematically shown in
Fig. 1.11 which visualises that,
when =0, the chip flows along orthogonal plane, i.e, c= 0
when 0, the chip flow is deviated from o
and c
= where c
is chip flow
deviation (from o) angle
But practically c may be zero even if = 0 and c may not be exactly equal
to even if 0. Because there are some other (than ) factors also which may
cause chip flow deviation.
-
7/28/2019 Pe Full Note
17/167
Fig. 1.11 Orthogonal and Oblique Cutting
Causes and amount of chip flow deviation
The deviation of chip flow in machining like turning by single point tool may deviate
from the orthogonal plane due to the following three factors:
1. Restricted cutting effect (RCE)
2.Tool-nose radius (r)
3.Presence of inclination angle, 0.
Effects of oblique cutting
In contrary to simpler orthogonal cutting, oblique cutting causes the following effects on
chip formation and mechanics of machining:
Chip does not flow along the orthogonal plane;
Positive causes
o Chip flow deviation away from the finished surface, which may result
lesser further damage to the finished surface
-
7/28/2019 Pe Full Note
18/167
but more inconvenience to the operator
o reduction of mechanical strength of the tool tip
o increase in temperature at the tool tip
o more vibration in turning slender rods due to increase in PY
(transverse force)
On the other hand, negative may enhance tool life by increasing mechanical strength
and reducing temperature at the tool tip but may impair the finished surface.
The chip cross-section may change from rectangle (ideal) to skewed trapezium
The ductile metals( materials) will produce more compact helical chips if not broken by
chip breaker
Analysis of cutting forces, chip-tool friction etc. becomes more complex.
Cutting force components and their significances
The single point cutting tools being used for turning, shaping, planing, slotting, boring
etc. are characterised by having only one cutting force during machining. But that force is
resolved into two or three components for ease of analysis and exploitation. Fig. 1.12
visualises how the single cutting force in turning is resolved into three components along
the three orthogonal directions; X, Y and Z.
Merchants Circle Diagram and its use
In orthogonal cutting when the chip flows along the orthogonal plane, O, the cutting
force (resultant) and its components PZ
and PXY
remain in the orthogonal plane. Fig. 1.13
is schematically showing the forces acting on a piece of continuous chip coming out from
the shear zone at a constant speed. That chip is apparently in a state of equilibrium.
-
7/28/2019 Pe Full Note
19/167
Fig. 1.12 Cutting force R resolved into Px, Py and P
Fig. 1.13Development of Merchants Circle Diagram
-
7/28/2019 Pe Full Note
20/167
From job-side :
PSshear force
Pnforce normal to the shear force where, Ps + Pn=R (resultant)
From tool side
R = R1 (in a state of equilibrium)
where R1 = F + N
N = force normal to the rake face
F = friction force at chip tool interface
The circle(s) drawn taking R or R1
as diameter is called Merchants circle which contains
all the force components concerned as intercepts. The two circles with their forces are
combined into one circle having all the forces contained in that as shown by the diagram
called Merchants Circle Diagram (MCD) in Fig. 1.14.
The significance of the forces in Merchants Circle are:
Ps is the shear force required to produce or separate the chip from parent body
Pninherently exists along with P
S
Ffriction force at the chip tool interface
Nforce acting normal to the rake surface
PZmain force or power component acting in the direction of cutting velocity
-
7/28/2019 Pe Full Note
21/167
Fig. 1.14 Merchant s Circle Diagram with cutting forces.
The magnitude of Ps provides the shear yield strength of the work material under the
cutting condition. The values of F and the ratio of F and N indicate the nature and degree
of interaction like friction at the chip-tool interface. The force components PX, P
Y, P
Zare
generally obtained by direct measurement. Again PZ
helps in determining cutting power
and specific energy requirement. The force components are also required to design the
cutting tool and the machine tool.
Relationship between the forces
Friction force, F, normal force, N and apparent coefficient of friction
F = PZsin
o+ P
XYcos
o
N = PZcos
o- P
XYsin
o
-
7/28/2019 Pe Full Note
22/167
ztano
+ Pxy)/PzPxy o)
Therefore, if PZ
and PXY
are known or determined either analytically or experimentally
the values of F, N and acan be determined using equations only.
Shear force Psand P
n
Ps = Pz cos 0Pxy sin 0
Pn = Pz sin 0 + Pxy cos 0
From Ps, the dynamic yield shear strength of the work material,
scan be determined by
using the relation,
Ps= A
s
s
where, As= shear area = ts0/sin
-
7/28/2019 Pe Full Note
23/167
MODULE-2
THERMAL ASPECTS OF MACHINING
Purposes of application of cutting fluid in machining
The basic purposes of cutting fluid application are : Cooling of the job and the tool to reduce the detrimental effects of cutting
temperature on the job and the tool
Lubrication at the chiptool interface and the tool flanks to reduce cutting
forces and friction and thus the amount of heat generation.
Cleaning the machining zone by washing away the chip particles and debris
which, if present, spoils the finished surface and accelerates damage of the
cutting edges
Protection of the nascent finished surface a thin layer of the cutting fluid
sticks to the machined surface and thus prevents its harmful contamination by the
gases like SO2, O
2, H
2S, N
xO
ypresent in the atmosphere.
However, the main aim of application of cutting fluid is to improve machinability through
reduction of cutting forces and temperature, improvement by surface integrity and
enhancement of tool life.
Essential properties of cutting fluids
To enable the cutting fluid fulfil its functional requirements without harming the Machine
Fixture Tool Work (M-F-T-W) system and the operators, the cutting fluid should
possess the following properties:
For cooling : high specific heat, thermal conductivity and film coefficient for heat transfer spreading and wetting ability For lubrication : high lubricity without gumming and foaming wetting and spreading high film boiling point friction reduction at extreme pressure (EP) and temperature
-
7/28/2019 Pe Full Note
24/167
Chemical stability, non-corrosive to the materials. less volatile and high flash point high resistance to bacterial growth odourless and also preferably colourless non toxic in both liquid and gaseous stage easily available and low cost.
Principles of cutting fluid action
The chip-tool contact zone is usually comprised of two parts; plastic or bulk contact zone
and elastic contact zone as indicated in Fig. 2.1
Fig. 2.1 Cutting fluid action in machining.
The cutting fluid cannot penetrate or reach the plastic contact zone but enters in the elastic
contact zone by capillary effect. With the increase in cutting velocity, the fraction of
plastic contact zone gradually increases and covers almost the entire chip-tool contact
zone as indicated in Fig. 2.2. Therefore, at high speed machining, the cutting fluid
becomes unable to lubricate and cools the tool and the job only by bulk external cooling.
increase in cutting velocity.
-
7/28/2019 Pe Full Note
25/167
Fig. 2.2 Apportionment of plastic and elastic contact zone with
The chemicals like chloride, phosphate or sulphide present in the cutting fluid chemically
reacts with the work material at the chip under surface under high pressure and
temperature and forms a thin layer of the reaction product. The low shear strength of that
reaction layer helps in reducing friction.
To form such solid lubricating layer under high pressure and temperature some extreme
pressure additive (EPA) is deliberately added in reasonable amount in the mineral oil or
soluble oil.
For extreme pressure, chloride, phosphate or sulphide type EPA is used depending upon
the working temperature, i.e. moderate (200o
C ~ 350o
C), high (350o
C ~ 500o
C) and very
high (500o
C ~ 800o
C) respectively.
Types of cutting fluids and their application
Generally, cutting fluids are employed in liquid form but occasionally also employed in
gaseous form. Only for lubricating purpose, often solid lubricants are also employed in
machining and grinding.
The cutting fluids, which are commonly used, are :
-
7/28/2019 Pe Full Note
26/167
Air blast or compressed air only.
Machining of some materials like grey cast iron become inconvenient or difficult if any
cutting fluid is employed in liquid form. In such case only air blast is recommended for
cooling and cleaning.
Water For its good wetting and spreading properties and very high specific heat, water is
considered as the best coolant and hence employed where cooling is most urgent.
Soluble oil
Water acts as the best coolant but does not lubricate. Besides, use of only water may
impair the machine-fixture-tool-work system by rusting
So oil containing some emulsifying agent and additive like EPA, together called cutting
compound, is mixed with water in a suitable ratio ( 1 ~ 2 in 20 ~ 50). This milk like white
emulsion, called soluble oil, is very common and widely used in machining and grinding.
Cutting oils
Cutting oils are generally compounds of mineral oil to which are added desired type and
amount of vegetable, animal or marine oils for improving spreading, wetting and
lubricating properties. As and when required some EP additive is also mixed to reduce
friction, adhesion and BUE formation in heavy cuts.
Chemical fluids
These are occasionally used fluids which are water based where some organic and or
inorganic materials are dissolved in water to enable desired cutting fluid action.
There are two types of such cutting fluid:
- Chemically inactive typehigh cooling, anti-rusting and wetting but less lubricating
- Active (surface) typemoderate cooling and lubricating.
-
7/28/2019 Pe Full Note
27/167
Solid or semi-solid lubricant
Paste, waxes, soaps, graphite, Moly-disulphide (MoS2) may also often be used, either
applied directly to the workpiece or as an impregnant in the tool to reduce friction and thus
cutting forces, temperature and tool wear.
Cryogenic cutting fluid
Extremely cold (cryogenic) fluids (often in the form of gases) like liquid CO2
or N2
are
used in some special cases for effective cooling without creating much environmental
pollution and health hazards.
Selection of Cutting Fluid
The benefits of application of cutting fluid largely depends upon proper selection of the
type of the cutting fluid depending upon the work material, tool material and the
machining condition. As for example, for high speed machining of not-difficult-to-
machine materials greater cooling type fluids are preferred and for low speed machining of
both conventional and difficult-to-machine materials greater lubricating type fluid is
preferred. Selection of cutting fluids for machining some common engineering materials
and operations are presented as follows :
Grey cast iron :
Generally dry for its self lubricating property Air blast for cooling and flushing chips Soluble oil for cooling and flushing chips in high speed machining and grinding
Steels :
If machined by HSS tools, sol. Oil (1: 20 ~30) for low carbon and alloy steels and neatoil with EPA for heavy cuts
If machined by carbide tools thinner sol. Oil for low strength steel, thicker sol. Oil( 1:10 ~ 20) for stronger steels and staright sulphurised oil for heavy and low speed
cuts and EP cutting oil for high alloy steel.
Often steels are machined dry by carbide tools for preventing thermal shocks.
-
7/28/2019 Pe Full Note
28/167
Aluminum and its alloys:
Preferably machined dry Light but oily soluble oil Straight neat oil or kerosene oil for stringent cuts.
Copper and its alloys :
Water based fluids are generally used Oil with or without inactive EPA for tougher grades of Cu-alloy.
Stainless steels and Heat resistant alloys:
High performance soluble oil or neat oil with high concentration with chlorinatedEP additive.
The brittle ceramics and cermets should be used either under dry condition or light neat oil
in case of fine finishing.
Grinding at high speed needs cooling ( 1: 50 ~ 100) soluble oil. For finish grinding of
metals and alloys low viscosity neat oil is also used.
Methods of application of cutting fluid
The effectiveness and expense of cutting fluid application significantly depend also on
how it is applied in respect of flow rate and direction of application.
In machining, depending upon the requirement and facilities available, cutting fluids are
generally employed in the following ways (flow):
Drop-by-drop under gravity
Flood under gravity
In the form of liquid jet(s)
Mist (atomised oil) with compressed air
Z-Z method centrifugal through the grinding wheels (pores) as indicated in
Fig. 2.3.
-
7/28/2019 Pe Full Note
29/167
Fig. 2.3 Z-Z method of cutting fluid application in grinding.
The direction of application also significantly governs the effectiveness of the cutting fluid
in respect of reaching at or near the chip-tool and work-tool interfaces. Depending upon
the requirement and accessibility the cutting fluid is applied from top or side(s). inoperations like deep hole drilling the pressurised fluid is often sent through the axial or
inner spiral hole(s) of the drill. For effective cooling and lubrication in high speed
machining of ductile metals having wide and plastic chip-tool contact, cutting fluid may
be pushed at high pressure to the chip-tool interface through hole(s) in the cutting tool, as
schematically shown in Fig. 2.4.
Fig. 2.4 Application of cutting fluid at high pressure through
the hole in the tool.
-
7/28/2019 Pe Full Note
30/167
Concept, Definition and Criteria of Judgement of Machinability
It is already known that preformed components are essentially machined to impart
dimensional accuracy and surface finish for desired performance and longer service life
of the product. It is obviously attempted to accomplish machining effectively, efficiently
and economically as far as possible by removing the excess material smoothly and
speedily with lower power consumption, tool wear and surface deterioration. But this
may not be always and equally possible for all the work materials and under all the
conditions. The machining characteristics of the work materials widely vary and also
largely depend on the conditions of machining. A term; Machinability has been
introduced for gradation of work materials w.r.t. machining characteristics.
But truly speaking, there is no unique or clear meaning of the term machinability. People
tried to describe Machinability in several ways such as:
It is generally applied to the machining properties of work material
It refers to material (work) response to machining
It is the ability of the work material to be machined
It indicates how easily and fast a material can be machined.
But it has been agreed, in general, that it is difficult to clearly define and quantify
Machinability. For instance, saying material A is more machinable than material B may
mean that compared to B,
A causes lesser tool wear or longer tool life
A requires lesser cutting forces and power
A provides better surface finish
where, surface finish and tool life are generally considered more important in finish
machining and cutting forces or power in bulk machining.
Machining is so complex and dependant on so many factors that the order of placing the
work material in a group, w.r.t. favourable behaviour in machining, will change if the
consideration is changed from tool life to cutting power or surface quality of the product
and vice versa. For instance, the machining behaviour of work materials are so affected
by the cutting tool; both material and geometry, that often machinability is expressed as
-
7/28/2019 Pe Full Note
31/167
operational characteristics of the work-tool combination. Attempts were made to
measure or quantify machinability and it was done mostly in terms of :
tool life which substantially influences productivity and economy in
machining
magnitude of cutting forces which affects power consumption and
dimensional accuracy
surface finish which plays role onperformance and service life of the product.
Often cutting temperature and chip form are also considered for assessing machinability.
But practically it is not possible to use all those criteria together for expressing
machinability quantitatively. In a group of work materials a particular one may appear
best in respect of, say, tool life but may be much poor in respect of cutting forces and
surface finish and so on. Besides that, the machining responses of any work material in
terms of tool life, cutting forces, surface finish etc. are more or less significantly affected
by the variation; known or unknown, of almost all the parameters or factors associated
with machining process. Machining response of a material may also change with the
processes, i.e. turning, drilling, milling etc. therefore, there cannot be as such any unique
value to express machinability of any material, and machinability, if to be used at all, has
to be done for qualitative assessment.
Machinability Index = (speed of machining the work for 60 min tool life) / speed of
machining the standard metal for 60 min tool life) x 100%
The free cutting steel, AISI1112, when machined (turned) at 100 fpm, provided 60 min
of tool life. If the work material to be tested provides 60 min of tool life at cutting
velocity of 60 fpm (say), as indicated in the table under the same set of machining
condition, then machinability (rating) of that material would be,
60 (based on 100% for the standard material)
or, simply the value of the cutting velocity expressed in fpm at which a work material
provides 60 min tool life was directly considered as the MR of that work material.
-
7/28/2019 Pe Full Note
32/167
But usefulness and reliability of such practice faced several genuine doubts and
questions :
tool life cannot or should not be considered as the only criteria for judging
machinability
under a given condition a material can yield different tool life even at a fixed
speed (cutting velocity); exact composition, microstructure, treatments etc. of
that material may cause significant difference in tool life
the tool life - speed relationship of any material may substantially change with
the variation in
o material and geometry of the cutting tool
o level of process parameters (Vc, s
o, t)
o machining environment (cutting fluid application)
o machine tool condition
Keeping all such factors and limitations in view, Machinability can be tentatively defined
as ability of being machined and more reasonably as ease of machining.
Such ease of machining or machinability characteristics of any tool-work pair is to be
judged by :
magnitude of the cutting forces
tool wear or tool life
surface finish
magnitude of cutting temperature
chip forms
Machinability will be considered desirably high when cutting forces, temperature, surface
roughness and tool wear are less, tool life is long and chips are ideally uniform and short
enabling short chip-tool contact length and less friction.
-
7/28/2019 Pe Full Note
33/167
Factors affecting machinability
The machinability characteristics and their criteria, i.e., the magnitude of cutting forces
and temperature, tool life and surface finish are governed or influenced more or less by
all the variables and factors involved in machining such as,
(a) properties of the work material
(b) cutting tool; material and geometry
(c) levels of the process parameters
(d) machining environments (cutting fluid application etc)
Machinability characteristics of any worktool pair may also be further affected by,
strength, rigidity and stability of the machine
kind of machining operations done in a given machine tool
functional aspects of the special techniques, if employed.
(a) Role of the properties of the work material on machinability.
The work material properties that generally govern machinability in varying extent are:
the basic nature brittleness or ductility etc.
microstructure
mechanical strength fracture or yield
hardness
hot strength and hot hardness
work hardenability
thermal conductivity
chemical reactivity
stickiness / self lubricity.
Machining of brittle and ductile materials
In general, brittle materials are relatively more easily machinable for :
-
7/28/2019 Pe Full Note
34/167
the chip separation is effected by brittle fracture requiring lesser energy ofchip formation
shorter chips causing lesser frictional force and heating at the rake surface
For instance, compared to even mild steel, grey cast iron jobs produce much lesser
cutting forces and temperature. Smooth and continuous chip formation is likely to enable
mild steel produce better surface finish but BUE, if formed, may worsen the surface
finish.
Free Cutting steels
Addition of lead in low carbon steels and also in aluminium, copper and their alloys help
reduce their shear strength. The dispersed lead particles act as discontinuity and solid
lubricants and thus improve machinability by reducing friction, cutting forces and
temperature, tool wear and BUE formation. Addition of sulphur also enhances
machinability of low carbon steels by enabling its free cutting. The added sulphur reacts
with Mn present in the steels and forms MnS inclusions which being very soft act almost
as voids and reduce friction at the tool work interfaces resulting reduction of cutting
forces and temperature and their consequences. The degree of ease of machining of such
free cutting steels depend upon the morphology of the MnS inclusions which can be
made more favourable by addition of trace of Tellurium.
Effects of hardness, hot strength and hot hardness and work hardening of work
materials.
Harder materials are obviously more difficult to machine for increased cutting forces and
tool damage.
Usually, with the increase in cutting velocity the cutting forces decrease to some extent
making machining easier through reduction in sand also chip thickness.
sdecreases due
to softening of the work material at the shear zone due to elevated temperature. Such
benefits of increased temperature and cutting velocity are not attained when the work
-
7/28/2019 Pe Full Note
35/167
materials are hot strong and hard like Ti and Ni based superalloys and work hardenable
like high manganese steel, Ni- hard, Hadfield steel etc.
Sticking of the materials (like pure copper, aluminium and their alloys) and formation of
BUE at the tool rake surface also hamper machinability by increasing friction, cutting
forces, temperature and surface roughness. Lower thermal conductivity of the work
material affects their machinability by raising the cutting zone temperature and thus
reducing tool life.
Sticking of the materials (like pure copper, aluminium and their alloys) and formation of
BUE at the tool rake surface also hamper machinability by increasing friction, cutting
forces, temperature and surface roughness.
(b) Role of cutting tool material and geometry on machinability of any
work material.
Role of tool materials
In machining a given material, the tool life is governed mainly by the tool material which
also influences cutting forces and temperature as well as accuracy and finish of the
machined surface. The composition, microstructure, strength, hardness, toughness, wear
resistance, chemical stability and thermal conductivity of the tool material play
significant roles on the machinability characteristics though in different degree depending
upon the properties of the work material.
Fig. 2.5 schematically shows how in turning materials like steels, the tool materials affect
tool life at varying cutting velocity.
-
7/28/2019 Pe Full Note
36/167
Fig. 2.5 Role of cutting tool material on machinability (tool life)
High wear resistance and chemical stability of the cutting tools like coated carbides,
ceramics, cubic Boron nitride (CBN) etc also help in providing better surface integrity of
the product by reducing friction, cutting temperature and BUE formation in high speed
machining of steels. Very soft, sticky and chemically reactive material like pure
aluminium attains highest machinability when machined by diamond tools.
Role of the geometry of cutting tools on machinability.
The geometrical parameters of cutting tools (say turning tool) that significantly affect the
machinability of a given work material (say mild steel) under given machining conditions
in terms of specific energy requirement, tool life, surface finish etc. are:
tool rake angles () clearance angle () cutting angles ( and
1)
nose radius (r)The other geometrical (tool) parameters that also influence machinability to some extent
directly and indirectly are:
-
7/28/2019 Pe Full Note
37/167
inclination angle () edge bevelling or rounding (r) depth, width and form of integrated chip breaker
(c) Role of the process parameters on machinability
Proper selection of the levels of the process parameters (VC, s
oand t) can provide better
machinability characteristics of a given work tool pair even without sacrificing
productivity or MRR.
Amongst the process parameters, depth of cut, t plays least significant role and is almost
invariable. Compared to feed (so) variation of cutting velocity (V
C) governs machinability
more predominantly. Increase in VC
, in general, reduces tool life but it also reduces
cutting forces or specific energy requirement and improves surface finish through
favourable chip-tool interaction. Some cutting tools, specially ceramic tools perform
better and last longer at higher VC
within limits. Increase in feed raises cutting forces
proportionally but reduces specific energy requirement to some extent. Cutting
temperature is also lesser susceptible to increase in so
than VC. But increase in s
o, unlike
VC
raises surface roughness. Therefore, proper increase in VC, even at the expense of s
o
often can improve machinability quite significantly.
(d) Effects of machining environment (cutting fluids) on machinability
The basic purpose of employing cutting fluid is to improve machinability characteristics
of any worktool pair through :
improving tool life by cooling and lubrication reducing cutting forces and specific energy consumption improving surface integrity by cooling, lubricating and cleaning at the cutting
zone
The favourable roles of cutting fluid application depend not only on its proper selection
based on the work and tool materials and the type of the machining process but also on its
rate of flow, direction and location of application.
-
7/28/2019 Pe Full Note
38/167
Possible Ways Of Improving Machinability Of Work Materials
The machinability of the work materials can be more or less improved, without
sacrificing productivity, by the following ways :
Favourable change in composition, microstructure and mechanical properties by mixing
suitable type and amount of additive(s) in the work material and appropriate heat
treatment
Proper selection and use of cutting tool material and geometry depending upon the work
material and the significant machinability criteria under consideration
Optimum selection of VC
and sobased on the tool work materials and the primary
objectives.
Proper selection and appropriate method of application of cutting fluid depending upon
the tool work materials, desired levels of productivity i.e., VC
and so
and also on the
primary objectives of the machining work undertaken
Proper selection and application of special techniques like dynamic machining, hot
machining, cryogenic machining etc, if feasible, economically viable and eco-friendly
-
7/28/2019 Pe Full Note
39/167
Failure of cutting tools
Smooth, safe and economic machining necessitate
prevention of premature and catastrophic failure of the cutting tools
reduction of rate of wear of tool to prolong its life
To accomplish the aforesaid objectives one should first know why and how the cutting
tools fail.
Cutting tools generally fail by :
i) Mechanical breakage due to excessive forces and shocks. Such kind of tool
failure is random and catastrophic in nature and hence are extremely
detrimental.
ii) Quick dulling by plastic deformation due to intensive stresses and
temperature. This type of failure also occurs rapidly and are quite detrimental
and unwanted.
iii) Gradual wear of the cutting tool at its flanks and rake surface.
The first two modes of tool failure are very harmful not only for the tool but also for the
job and the machine tool. Hence these kinds of tool failure 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.
The cutting tool is withdrawn immediately after it fails or, if possible, just before it
totally fails. For that one must understand that the tool has failed or is going to fail
shortly.
It is understood or considered that the tool has failed or about to fail by one or more of
the following conditions :
-
7/28/2019 Pe Full Note
40/167
(a) In R&D laboratories
total breakage of the tool or tool tip(s)
massive fracture at the cutting edge(s)
excessive increase in cutting forces and/or vibration
average wear (flank or crater) reaches its specified limit(s)
(b) In machining industries
excessive (beyond limit) current or power consumption
excessive vibration and/or abnormal sound (chatter)
total breakage of the tool
dimensional deviation beyond tolerance
rapid worsening of surface finish
adverse chip formation.
Mechanisms and pattern (geometry) of cutting tool 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 :
i) Mechanical wear
thermally insensitive type; like abrasion, chipping and delamination
thermally sensitive type; like adhesion, fracturing, flaking etc.
ii) Thermochemical wear
macro-diffusion by mass dissolution
micro-diffusion by atomic migration
iii) Chemical wear
iv) Galvanic wear
In diffusion wear the material from the tool at its rubbing surfaces, particularly at the rake
surface gradually diffuses into the flowing chips either in bulk or atom by atom when the
-
7/28/2019 Pe Full Note
41/167
tool material has chemical affinity or solid solubility towards the work material. The rate
of such tool wear increases with the increase in temperature at the cutting zone.
Diffusion wear becomes predominant when the cutting temperature becomes very high
due to high cutting velocity and high strength of the work material.
Chemical wear, leading to damages like grooving wear may occur if the tool material is
not enough chemically stable against the work material and/or the atmospheric gases.
Galvanic wear, based on electrochemical dissolution, seldom occurs when both the work
tool materials are electrically conductive, cutting zone temperature is high and the cutting
fluid acts as an electrolyte.
Essential properties for cutting tool materials
The cutting tools need to be capable to meet the growing demands for higher productivity
and economy as well as to machine the exotic materials which are coming up with the
rapid progress in science and technology.
The cutting tool material of the day and future essentially require the following properties
to resist or retard the phenomena leading to random or early tool failure :
i) high mechanical strength; compressive, tensile, and TRA
ii) fracture toughnesshigh or at least adequate
iii) high hardness for abrasion resistance
iv) high hot hardness to resist plastic deformation and reduce wear rate at
elevated temperature
v) chemical stability or inertness against work material, atmospheric gases
and cutting fluids
vi) resistance to adhesion and diffusion
vii) thermal conductivity low at the surface to resist incoming of heat and
high at the core to quickly dissipate the heat entered
viii) high heat resistance and stiffness
ix) manufacturability, availability and low cost.
-
7/28/2019 Pe Full Note
42/167
Tool Life
Definition
Tool life generally indicates, the amount of satisfactory performance or service rendered
by a fresh tool or a cutting point till it is declared failed.
Tool life is defined in two ways :
(a) In R & D : Actual machining time (period) by which a fresh cutting tool (or point)
satisfactorily works after which it needs replacement or reconditioning. The modern tools
hardly fail prematurely or abruptly by mechanical breakage or rapid plastic deformation.
Those fail mostly by wearing process which systematically grows slowly with machining
time. In that case, tool life means the span of actual machining time by which a fresh
tool can work before attaining the specified limit of tool wear. Mostly tool life is
decided by the machining time till flank wear, VB
reaches 0.3 mm or crater wear, KT
reaches 0.15 mm.
(b) In industries or shop floor : The length of time of satisfactory service or
amount of acceptable output provided by a fresh tool prior to it is required to
replace or recondition.
Assessment of tool life
For R & D purposes, tool life is always assessed or expressed by span of machining time
in minutes, whereas, in industries besides machining time in minutes some other means
are also used to assess tool life, depending upon the situation, such as
no. of pieces of work machined
total volume of material removed
total length of cut.
Measurement of tool wear
The various methods are : i) by loss of tool material in volume or weight, in one life time
this method is crude and is generally applicable for critical tools like grinding wheels.
-
7/28/2019 Pe Full Note
43/167
ii) 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
iii) using optical microscope fitted with micrometervery common and effective method
iv) using scanning electron microscope (SEM)used generally, for detailed study; both
qualitative and quantitative
v) Talysurf, specially for shallow crater wear.
Taylors tool life equation
Wear and hence tool life of any tool for any work material is governed mainly by the
level of the machining parameters i.e., cutting velocity, (VC), feed, (s
o) and depth of cut
(t). Cutting velocity affects maximum and depth of cut minimum.
The usual pattern of growth of cutting tool wear (mainly VB), principle of assessing tool
life and its dependence on cutting velocity are schematically shown in Fig.2.6.
Fig. 2.6 Growth of flank wear and assessment of tool life
-
7/28/2019 Pe Full Note
44/167
The tool life obviously decreases with the increase in cutting velocity keeping other
conditions unaltered.
If the tool lives, T1, T
2, T
3, T
4etc are plotted against the corresponding cutting velocities,
V1, V
2, V
3, V
4etc as shown in Fig. 2.7 a smooth curve like a rectangular hyperbola is
found to appear.
Fig. 2.7 Cutting velocitytool life relationship
When F. W. Taylor plotted the same figure taking both V and T in log-scale, a more
distinct linear relationship appeared as schematically shown in Fig.2.8.
Fig. 2.8 Cutting velocity Vs tool life on a log-log scale
-
7/28/2019 Pe Full Note
45/167
With the slope, n and intercept, c, Taylor derived the simple equation as
VTn
= C
where, n is called, Taylors tool life exponent. The values of both n and C depend
mainly upon the tool-work materials and the cutting environment (cutting fluid
application).
Modified Taylors Tool Life equation
In Taylors tool life equation, only the effect of variation of cutting velocity, VC
on tool
life has been considered. But practically, the variation in feed (so) and depth of cut (t) also
play role on tool life to some extent.
Taking into account the effects of all those parameters, the Taylors tool life equation has
been modified as,
TL = CT/ Vcx.So
y.t
z
where, TL = tool life in min
CT
is a constant depending mainly upon the tool work materials and the limiting
value of VB
x, y and z are exponents so called tool life exponents depending upon the tool
work materials and the machining environment.
Generally, x > y > z as VC
affects tool life maximum and t minimum.
The values of the constants, CT, x, y and z are available in Machining Data Handbooks
or can be evaluated by machining tests.
-
7/28/2019 Pe Full Note
46/167
Needs and Chronological Development of Cutting Tool Materials
With the progress of the industrial world it has been needed to continuously develop and
improve the cutting tool materials and geometry;
to meet the growing demands for high productivity, quality and economy of
machining
to enable effective and efficient machining of the exotic materials that are
coming up with the rapid and vast progress of science and technology
for precision and ultra-precision machining
for micro and even nano machining demanded by the day and future.
It is already stated that the capability and overall performance of the cutting tools depend
upon,
the cutting tool materials
the cutting tool geometry
proper selection and use of those tools
the machining conditions and the environments
Out of which the tool material plays the most vital role.
The relative contribution of the cutting tool materials on productivity, for instance, can be
roughly assessed from Fig. 2.9.
Fig. 2.9 Productivity raised by cutting tool materials.
-
7/28/2019 Pe Full Note
47/167
The chronological development of cutting tool materials is briefly indicated in
Fig. 2.10 Chronological development of cutting tool materials.
-
7/28/2019 Pe Full Note
48/167
Characteristics and Applications of the Primary Cutting Tool Materials
(a) High Speed Steel (HSS)
Advent of HSS in around 1905 made a break through at that time in the history of cutting
tool materials though got later superseded by many other novel tool materials like
cemented carbides and ceramics which could machine much faster than the HSS tools.
The basic composition of HSS is 18% W, 4% Cr, 1% V, 0.7% C and rest Fe. Such HSS
tool could machine (turn) mild steel jobs at speed only upto 20 ~ 30 m/min (which was
quite substantial those days)
However, HSS is still used as cutting tool material where:
the tool geometry and mechanics of chip formation are complex, such as helical
twist drills, reamers, gear shaping cutters, hobs, form tools, broaches etc.
brittle tools like carbides, ceramics etc. are not suitable under shock loading
the small scale industries cannot afford costlier tools
the old or low powered small machine tools cannot accept high speed and feed.
The tool is to be used number of times by resharpening.
With time the effectiveness and efficiency of HSS (tools) and their application range
were gradually enhanced by improving its properties and surface condition through -
Refinement of microstructure
Addition of large amount of cobalt and Vanadium to increase hot hardness and
wear resistance respectively
Manufacture by powder metallurgical process
Surface coating with heat and wear resistive materials like TiC, TiN, etc by
Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD)
The commonly used grades of HSS are given below
-
7/28/2019 Pe Full Note
49/167
Addition of large amount of Co and V, refinement of microstructure and coating
increased strength and wear resistance and thus enhanced productivity and life of the
HSS tools remarkably.
(b) Stellite
This is a cast alloy of Co (40 to 50%), Cr (27 to 32%), W (14 to 19%) and C (2%).
Stellite is quite tough and more heat and wear resistive than the basic HSS (18 41)
But such stellite as cutting tool material became obsolete for its poor grindability and
specially after the arrival of cemented carbides.
(c) Sintered Tungsten carbides
The advent of sintered carbides made another breakthrough in the history of cutting tool
materials.
Straight or single carbide First the straight or single carbide tools or inserts were
powder metallurgically produced by mixing, compacting and sintering 90 to 95% WC
powder with cobalt. The hot, hard and wear resistant WC grains are held by the binder Co
which provides the necessary strength and toughness. Such tools are suitable for
machining grey cast iron, brass, bronze etc. which produce short discontinuous chips and
at cutting velocities two to three times of that possible for HSS tools.
Composite carbides
The single carbide is not suitable for machining steels because of rapid growth of wear,
particularly crater wear, by diffusion of Co and carbon from the tool to the chip under the
high stress and temperature bulk (plastic) contact between the continuous chip and the
tool surfaces.
-
7/28/2019 Pe Full Note
50/167
For machining steels successfully, another type called composite carbide have been
developed by adding (8 to 20%) a gamma phase to WC and Co mix. The gamma phase is
a mix of TiC, TiN, TaC, NiC etc. which are more diffusion resistant than WC due to their
more stability and less wettability by steel.
Mixed carbides
Titanium carbide (TiC) is not only more stable but also much harder than WC. So for
machining ferritic steels causing intensive diffusion and adhesion wear a large quantity (5
to 25%) of TiC is added with WC and Co to produce another grade called Mixed carbide.
But increase in TiC content reduces the toughness of the tools. Therefore, for finishing
with light cut but high speed, the harder grades containing upto 25% TiC are used and for
heavy roughing work at lower speeds lesser amount (5 to 10%) of TiC is suitable.
(d) Plain ceramics
Inherently high compressive strength, chemical stability and hot hardness of the ceramics
led to powder metallurgical production of indexable ceramic tool inserts since 1950.
Table 3.3.4 shows the advantages and limitations of alumina ceramics in contrast to
sintered carbide. Alumina (Al2O
3) is preferred to silicon nitride (Si
3N
4) for higher
hardness and chemical stability. Si3N4 is tougher but again more difficult to process. The
plain ceramic tools are brittle in nature and hence had limited applications.
-
7/28/2019 Pe Full Note
51/167
Cutting tool properties of alumina ceramics.
Basically three types of ceramic tool bits are available in the market;
Plain alumina with traces of additives these white or pink sintered inserts are
cold pressed and are used mainly for machining cast iron and similar materials at speeds
200 to 250 m/min
Alumina; with or without additives hot pressed, black colour, hard and strong
used for machining steels and cast iron at VC
= 150 to 250 m/min
Carbide ceramic (Al2O
3+ 30% TiC) cold or hot pressed, black colour, quite
strong and enough tough used for machining hard cast irons and plain and alloy steels
at 150 to 200 m/min.
The plain ceramic outperformed the then existing tool materials in some application areas
like high speed machining of softer steels mainly for higher hot hardness as indicated in
Fig. 2.11.
Fig. 2.11. Hot hardness of the different commonly used tool materials.
-
7/28/2019 Pe Full Note
52/167
However, the use of those brittle plain ceramic tools, until their strength and toughness
could be substantially improved since 1970, gradually decreased for being restricted to
uninterrupted machining of soft cast irons and steels only
relatively high cutting velocity but only in a narrow range (200 ~ 300 m/min)
requiring very rigid machine tools
Advent of coated carbide capable of machining cast iron and steels at high velocity made
the then ceramics almost obsolete.
e) Coated carbides
The properties and performance of carbide tools could be substantially improved by
Refining microstructure
Manufacturing by casting expensive and uncommon
Surface coating made remarkable contribution.
Thin but hard coating of single or multilayers of more stable and heat and wear resistive
materials like TiC, TiCN, TiOCN, TiN, Al2O
3etc on the tough carbide inserts (substrate)
(Fig. 3.3.4) by processes like chemical Vapour Deposition (CVD), Physical Vapour
Deposition (PVD) etc at controlled pressure and temperature enhanced MRR and overall
machining economy remarkably enabling,
reduction of cutting forces and power consumption
increase in tool life (by 200 to 500%) for same VC
or increase in VC
(by 50 to
150%) for same tool life
improvement in product quality
effective and efficient machining of wide range of work materials
pollution control by less or no use of cutting fluid through
reduction of abrasion, adhesion and diffusion wear reduction of friction and BUE formation
heat resistance and reduction of thermal cracking and plastic
deformation.
-
7/28/2019 Pe Full Note
53/167
f) Cermets
These sintered hard inserts are made by combining cer from ceramics like TiC, TiN orn
( or )TiCN and met from metal (binder) like Ni, Ni -Co, Fe etc. Since around 1980, the
modern cermets providing much better performance are being made by TiCN which is
consistently more wear resistant, less porous and easier to make. The characteristic
features of such cermets, in contrast to sintered tungsten carbides, are :
The grains are made of TiCN (in place of WC) and Ni or Ni-Co and Fe
as binder (in place of Co)
Harder, more chemically stable and hence more wear resistant
More brittle and less thermal shock resistant
Wt% of binder metal varies from 10 to 20%
Cutting edge sharpness is retained unlike in coated carbide inserts
Can machine steels at higher cutting velocity than that used for tungsten
carbide, even coated carbides in case of light cuts.
Application wise, the modern TiCN based cermets with bevelled or slightly rounded
cutting edges are suitable for finishing and semi-finishing of steels at higher speeds,
stainless steels but are not suitable for jerky interrupted machining and machining of
aluminium and similar materials. Research and development are still going on for further
improvement in the properties and performance of cermets.
g) Cubic Boron Nitride
Next to diamond, cubic boron nitride is the hardest material presently available. Only in
1970 and onward CBN in the form of compacts has been introduced as cutting tools. It is
made by bonding a 0.5 1 mm layer of polycrystalline cubic boron nitride to cobalt
based carbide substrate at very high temperature and pressure. It remains inert and retains
high hardness and fracture toughness at elevated machining speeds. It shows excellent
performance in grinding any material of high hardness and strength. The extremehardness, toughness, chemical and thermal stability and wear resistance led to the
development of CBN cutting tool inserts for high material removal rate (MRR) as well as
precision machining imparting excellent surface integrity of the products. Such unique
tools effectively and beneficially used in machining wide range of work materials
covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like
-
7/28/2019 Pe Full Note
54/167
Ni-hard, Inconel, Nimonic etc and many non-metallic materials which are as such
difficult to machine by conventional tools. It is firmly stable at temperatures upto 1400o
C. The operative speed range for CBN when machining grey cast iron is 300 ~ 400
m/min. Speed ranges for other materials are as follows:
Hard cast iron (> 400 BHN) : 80 300 m/min
Superalloys (> 35 RC) : 80140 m/min
Hardened steels (> 45 RC) : 100300 m/min
In addition to speed, the most important factor that affects performance of cBN inserts is
the preparation of cutting edge. It is best to use cBN tools with a honed or chamfered
edge preparation, especially for interrupted cuts. Like ceramics, CBN tools are also
available only in the form of indexable inserts.
h) Diamond Tools
Single stone, natural or synthetic, diamond crystals are used as tips/edge of cutting tools.
Owing to the extreme hardness and sharp edges, natural single crytal is used for many
applications, particularly where high accuracy and precision are required. Their important
uses are :
Single point cutting tool tips and small drills for high speed machining of
non-ferrous metals, ceramics, plastics, composites, etc. and effective machining
of difficult-to-machine materials
Drill bits for mining, oil exploration, etc.
Tool for cutting and drilling in glasses, stones, ceramics, FRPs etc.
Wire drawing and extrusion dies
Superabrasive wheels for critical grinding.
Limited supply, increasing demand, high cost and easy cleavage of natural diamond
demanded a more reliable source of diamond. It led to the invention and manufacture of
artificial diamond grits by ultra-high temperature and pressure synthesis process, which
enables large scale manufacture of diamond with some control over size, shape and
friability of the diamond grits.
-
7/28/2019 Pe Full Note
55/167
MODULE 3
POWDER METALLURGY
Powder metallurgy is a forming and fabrication technique consisting of three major
processing stages. First, the primary material is physically powdered, divided into many
small individual particles. Next, the powder is injected into a mold or passed through a
die to produce a weakly cohesive structure (via cold welding) very near the dimensions of
the object ultimately to be manufactured. Finally, the end part is formed by applying
pressure, high temperature, long setting times (during which self-welding occurs), or any
combination thereof.
Powder production techniques
Any fusible material can be atomized. Several techniques have been developed which
permit large production rates of powdered particles, often with considerable control over
the size ranges of the final grain population. Powders may be prepared by comminution,
grinding, chemical reactions, or electrolytic deposition. Several of the melting and
mechanical procedures are clearly adaptable to operations in space or on the Moon.
Powders of the elements Ti, V, Th, Nb, Ta, Ca, and U have been produced by high-
temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be
submicrometre powders are obtained by reducing metallic oxalates and formates.
Exceedingly fine particles also have been prepared by directing a stream of molten metal
through a high-temperature plasma jet or flame, simultaneously atomizing and
comminuting the material. On Earth various chemical- and flame-associated powdering
processes are adopted in part to prevent serious degradation of particle surfaces by
atmospheric oxygen.
The common powder production techniques are:
-
7/28/2019 Pe Full Note
56/167
1. Reduction of Oxides: The major world producer of iron powder manufactures powder
by the reduction of iron oxide either in the form of a pure iron ore, or as pure mill-scale
from a large rolling mill. In either case an irregular, spongy powder is produced, with a
particle size of minus 100 mesh, that is the powder will go through a standard sieve of
100 mesh as defined in British Standards.
2. Production from Carbonyl Derivatives: Both iron and nickel are produced in large
quantities by the decomposition of the metal carbonyl. Small, uniform spherical particles
typically 5 microns in diameter are produced.
3. Electrolytic Production: Electro-deposition conditions can be arranged so that the
metal is not plated out as a solid electrode layer, but as a powdery deposit, which does
not adhere to the cathode and can be removed from the electrolyte bath as a fine sludge.
The most common product is pure copper powder.
4. Mechanical Alloying: If elemental powders, produced by the methods described
above, are ball-milled together under the correct conditions the overall composition of
each powder particle becomes that of the average composition of the powders in the ball
mill. This is due to a cycle in which particles of different compositions adhere to each
other, and then break away leaving traces of one particle on the other. If continued for asufficiently long time, the particle compositions become uniform. Again, unusual
compositions can be obtained that are not possible by conventional melting technology,
such as high carbon aluminium alloys, and copper and nickel alloys which contain
oxides.
5. Atomization: Atomization is accomplished by forcing a molten metal stream through
an orifice at moderate pressures. A gas is introduced into the metal stream just before it
leaves the nozzle, serving to create turbulence as the entrained gas expands (due to
heating) and exits into a large collection volume exterior to the orifice. The collection
volume is filled with gas to promote further turbulence of the molten metal jet. On Earth,
air and powder streams are segregated using gravity or cyclonic separation. Most
atomized powders are annealed, which helps reduce the oxide and carbon content. The
-
7/28/2019 Pe Full Note
57/167
water atomized particles are smaller, cleaner, and nonporous and have a greater breadth
of size, which allows better compacting.
Simple atomization techniques are available in which liquid metal is forced through an
orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance
index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the
exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid
jet oscillates, but at higher velocities the stream becomes turbulent and breaks into
droplets. Pumping energy is applied to droplet formation with very low efficiency (on the
order of 1%) and control over the size distribution of the metal particles produced is
rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple
impinging streams, or molten-metal injection into ambient gas are all available toincrease atomization efficiency, produce finer grains, and to narrow the particle size
distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a
few millimeters in diameter, which in practice limits the minimum size of powder grains
to approximately 10 m. Atomization also produces a wide spectrum of particle sizes,
necessitating downstream classification by screening and remelting a significant fraction
of the grain boundary.
Powder pressing (Compacting)
Although many products such as pills and tablets for medical use are cold-pressed
directly from powdered materials, normally the resulting compact is only strong enough
to allow subsequent heating and sintering. Release of the compact from its mold is
usually accompanied by small volume increase called "spring-back."
In the typical powder pressing process a powder compaction press is employed with tools
and dies. Normally, a die cavity that is closed on one end (vertical die, bottom end closed
by a punch tool) is filled with powder. The powder is then compacted into a shape and
then ejected from the die cavity. Various components can be formed with the powder
compaction process. Some examples of these parts are bearings, bushings, gears, pistons,
levers, and brackets. When pressing these shapes, very good dimensional and weight
-
7/28/2019 Pe Full Note
58/167
control are maintained. In a number of these applications the parts may require very little
additional work for their intended use; making for very cost efficient manufacturing.
In some pressing operations (such as hot isostatic pressing) compact formation and
sintering occur simultaneously. This procedure, together with explosion-driven
compressive techniques, is used extensively in the production of high-temperature and
high-strength parts such as turbine blades for jet engines. In most applications of powder
metallurgy the compact is hot-pressed, heated to a temperature above which the materials
cannot remain work-hardened. Hot pressing lowers the pressures required to reduce
porosity and speeds welding and grain deformation processes. Also it permits better
dimensional control of the product, lessened sensitivity to physical characteristics of
starting materials, and allows powder to be driven to higher densities than with coldpressing, resulting in higher strength. Negative aspects of hot pressing include shorter die
life, slower throughput because of powder heating, and the frequent necessity for
protective atmospheres during forming and cooling stages.
Hot isostatic pressing (HIP) is a manufacturing process used to reduce the porosity of
metals and influence the density of many ceramic materials. This improves the
mechanical properties, workability and ceramic density.
The HIP process subjects a component to both elevated temperature and isostatic gas
pressure in a high pressure containment vessel. The pressurizing gas most widely used is
argon. An inert gas is used, so that the material does not chemically react. The chamber is
heated, causing the pressure inside the vessel to increase. Many systems use associated
gas pumping to achieve necessary pressure level. Pressure is applied to the material from
all directions (hence the term "isostatic").
For processing castings, the argon is applied between 15,000 p.s.i. (103 MPa) and 45,000
p.s.i. (310 MPa). 15,000 is the most common. Process soak temperatures range from
900F (480C) for aluminum castings to 3632F (2,000C) for nickel base superalloys.
When castings are treated with HIP, the simultaneous application of heat and pressure
-
7/28/2019 Pe Full Note
59/167
eliminates internal voids and microporosity through a combination of plastic
deformation, creep, and diffusion bonding. Primary applications are the reduction of
microshrinkage, the consolidation of powder metals, ceramic composites and metal
cladding. Hot isostatic pressing is also used as part of a sintering (powder metallurgy)
process and for fabrication of Metal Matrix Composites.
Sintering
Sintering is a method for making objects from powder, by heating the material (below its
melting point - solid state sintering) until its particles adhere to each other. Sintering is
traditionally used for manufacturing ceramic objects, and has also found uses in such
fields as powder metallurgy. A special form of sintering still considered part of powder
metallurgy, is liquid state sintering. In liquid state sintering, at least one but not all
elements are existing in a liquid state. Liquid state sintering is required for making
cemented carbides or tungsten carbide. The thermal treatment of a powder or compact at
a temperature below the melting point of the main constituent, for the purpose of
increasing its strength by bonding together of the particles is called sintering.
The word "sinter" comes from the Middle High German Sinter, a cognate of English
"cinder".
Sintered bronze in particular is frequently used as a material for bearings, since its
porosity allows lubricants to flow through it or remain captured within it. In the case of
materials with high melting points such as Teflon and tungsten, sintering is used when
there is no alternative manufacturing technique. In these cases very low porosity is
desirable and can often be achieved.
Sintered bronze and stainless steel are used as filter materials in applications requiring
high temperature resistance while retaining the ability to regenerate the filter element. For
example, sintered stainless steel elements are used for filtering steam in food and
pharmaceutical applications.
-
7/28/2019 Pe Full Note
60/167
Static sintering is when a metal powder under certain external conditions may exhibit
coalescence yet revert to its normal behavior when such conditions are absent. In most
cases the density of a collection of grains increases as material flows into voids, causing a
decrease in overall volume. Mass movements that occur during sintering consist of the
reduction of total porosity by repacking, followed by material transport due to
evaporation and condensation from diffusion. In the final stages, metal atoms move along
crystal boundaries to the walls of internal pores, redistributing mass from the internal
bulk of the object and smoothing pore walls. Surface tension is the driving force for this
movement.
Metallurgists can sinter most, if not all, metals. This applies especially to pure metals
produced in vacuum which suffer no surface contamination. Many nonmetallicsubstances also sinter, such as glass, alumina, zirconia, silica, magnesia, lime, ice,
beryllium oxide, ferric oxide, and various organic polymers. Sintering, with subsequent
reworking, can produce a great range of material properties. Changes in density, alloying,
or heat treatments can alter the physical characteristics of various products. For instance,
the tensile strength En of sintered iron powders remains insensitive to sintering time,
alloying, or particle size in the original powder, but depends upon the density of the final
product according to:
En/E = (D/d)3.4
where D is the density, E is Young's modulus and d is the maximum density of iron.
Atomic diffusion takes place and the welded areas formed during compaction grow until
eventually they may be lost completely.
Recrystallisation and grain growth may follow, and the pores tend to become rounded
and the total porosity, as a percentage of the whole volume tends to decrease. The
operation is almost invariably carried out under a protective atmosphere, because of the
large surface areas involved, and at temperatures between 60 and 90% of the melting-
point of the particular metal or alloys.
-
7/28/2019 Pe Full Note
61/167
Control over heating rate, time, temperature and atmosphere is required for reproducible
results.
The type of furnace most generally favoured is an electrically heated one through which
the compacts are passed on a woven wire mesh belt.
The belt and the heating elements are of a modified 80/20 nickel/chromium alloy and
give a useful life at temperatures up to 1150C.
For higher temperatures walking beam furnaces are preferred, and these are increasingly
being used as the demand for higher strength in sintered parts increases.
Silicon carbide heating elements are used and can be operated up to 1350C.
For special purposes at still higher temperature molybdenum heating elements can be
used, but special problems are involved, notably the readiness with which molybdenum
forms a volatile oxide.
Molybdenum furnaces must operate in a pure hydrogen atmosphere.
SHAPE FACTOR AND ASPECT RATIO
Particle shape has a major influence on processing characteristics. The shape is
usually described in terms of aspect ratio or shape factor.
Aspect ratio is the ratio of the largest dimension to the smallest dimension of the
particle. This ratio changes from unity (for a spherical particle) to about 10 for flake like
or needle like particles.
Shape factor or shape index, is a measure of the ratio of the surface area of the
particle to its volume, normalized by reference to a spherical particle of equivalent
volume. Thus for eg., the shape factor for a flake is higher than that for a sphere.
ADVANTAGES OF POWDER METALLURGY
The powder metallurgy process has certain basic advantages over conventional
melting and casting method of producing metals, alloys and finished articles. These
-
7/28/2019 Pe Full Note
62/167
advantages include,
1. The dimensional accuracy and surface finish ontainable are such that for
many applications all machining can be eliminated.
2. Cleaner and quieter operation and long life of the components.
3. High production rates.
4. No material is wasted as scrap the process makes use of 100% raw material
unlike casting, press forming etc.
5. Economy, greater accuracy (i.e.; close dimension at tolerance in the
finished part) and smooth surfaces.
6. Lack of voids, gas pockets, porosity or blow holes, stringering of segregatedparticles and various inclusions common in castings.
7. control of grain size and relatively much uniform structure.
8. Excellent reproduce ability.
9. Improved physical properties.
10. Quite complex shapes can be produced.
11. No requirement of highly qualified or skilled personnel.
12. Greater freedom of design in the case of production of machined part.
LIMITATION OF P/M
1. Powder metallurgy parts possess comparatively poor plastic properties
(impact strength, plasticity, elongation etc) which limit their use in many
applications.
2. Powder metallurgy is not economical for small scale production.
3. It may be difficult, sometimes, to obtain particular alloy powders.
4. Parts pressed from the top tend to be less dense at the bottom
5. Extreme care is required in handling pyrophoric powders.
6. Relatively high and die cost is associated with the process.
-
7/28/2019 Pe Full Note
63/167
7. Parts made by powder metallurgy, in most case, do not have good physical
properties as w