PD 211 Principles of Metal Cutting

Dr T Mwinuka

PD 211 Principles of Metal Cutting2hours Lecture + 1hour Tutorials

Objectives To impart to the science and theories underlying the metal cutting processes

Rationale:Engineers need this knowledge to plan, supervise and optimize the process of machining of engineering components.

Course Contents: Introduction to metal cutting: Action of metal cutting, workpiece-tool relationship. Tool geometry: single and multi point tools, abrasives. Theories of chip formation: Shear plane and shear zone theories, merchant theory of chip formation, current theory of chip formation.

Assessment of chip formation: chip compression ratios, force and velocity relationships, types of chips.Cutting forces and cutting power, cutting tool materials. Tool wear and tool lifeMachineability of materialsEconomics of metal cutting: Taylor’s equation, economical tool life equation, economical tool lifeMetal cutting heat: generation and distribution of metal cutting heat

Recommended Text Books1. Masuha, J R (1992): Basic Principles of Metal Cutting, Dar es Salaam University

Press (DUP)2. Ghosh, A and Mallik, A K (1986) Manufacturing Science, Ellis Horwood Limited

Publishers.3. G. Boothroyd, W A Knight (1988) Fundamentals of Machining and Machine

Tools; 2nd Edition, Marcel Dekker, Inc

Assessment Continuous Assessment 2 tests 40%

University Exam 60%

1.0 Introduction to Metal Cutting

1.1 Action of Metal CuttingWhat happens in Metal Cutting? Many people compares the metal cutting action to what happens when an axe or knife

splits wood. This is not a good comparison.

In cutting wood the axe knife splits the wood fibres. In metal cutting the metal is compressed and then plastically deformed as follows:

1. The tool compresses the metal ahead of the tool when the workpiece moves on the face of the cutting tool

2. The compressed metal is then sheared whereby it slips. This process is known as plastic deformation.

3. As the cutting edge moves forward, the metal is strained at the cutting edge by what is known as concentration of stress.

4. The concentration of stress at the cutting edge causes a chip shear (break by force from the workpice)

Factors determining the action of metal cutting(1) The workpiece-its geometry and kind of material e.g. steel, aluminium, cast iron(2) The tool- its geometry and kind of material e.g. tool steel, HSS, carbides(3) The cutting conditions: speed, feed, engagement

1.2 The WorkpieceAs far as metal cutting is concerned, the geometry of the workpiece is defined by three surfaces:

(4) Work Surface to be removed (raw surface)- geometry before cutting(5) Machined surface: surface produced by metal cutting process (geometry after cutting)(6) Transient surface: surface formed on the workpiece and removed during the following

cut(geometry in transition)

The properties of the workpiece material which determines its behaviour during cutting are called cutting constants. They will be discussed latter.

1.3 The Tool

There are 7 elements which characterize the tool, but not all of them must be present in each type of tool. However each tool must have most of the elements:

1. The body that holds the cutting part/blades/inserts2. The shank by which the tool is held3. The tool bore by which the tool can be located and fixed by a spindle or arbour or mandrel4. The cutting part, the chip producing elements (eg cutting edges, face and flank of a turning tool)5. The wedge which is a portion of cutting part between face and flank6. The base for orienting the tool for its manufacture and sharpening7. The Tool Axis, an imaginary straight line with defined geometric relationship to the locating surfaces

used to manufacture and sharpening of the tool

Elements of a Tool

1.4 Cutting ConditionsCutting conditions are described as

kinematic conditionsgeometric conditionsother conditions

1.4.1 Kinematic Conditions1. MotionsThere are basically three, two major motions which result into a third one(1) Primary Motion: causes a relative motion between tool and workpiece(2) Feed Motion: Causes continuous or repeated cutting(3) Resulting cutting motion: resultant of primary and feed motions(4) Other motions: approach motion, positioning motion and adjustment motion

2. Directions of Motions(1) Direction of Primary Motion: direction of instantaneous primary motion of a selected

point on the cutting edge.(2) Direction of Feed Motion: direction of instantaneous feed motion of a selected point on

the cutting edge(3) Resultant Cutting Direction: direction of instantaneous resultant cutting motion of a

selected point on the cutting edge

3. Speeds(4) Cutting speed, v [m/min] the instantaneous velocity of the primary motion of a selected

point on the cutting edge(5) Feed Speed, vf [mm/min](6) Resultant Cutting Speed, ve [m/min]: the instantaneous velocity of the resultant cutting

motion of a selected point on the cutting edge

1.4.2 Geometric ConditionsAre of 2 types

Those which are related to setting of the machine. These are called terms related to setting or simply machine variables

Those which are related to the geometry of the material that is going to be cut. These are called terms which determine the cut.

1. Machine variables (i) Feed, f [mm]: displacement of the tool relative to the workpiece in the direction of feed

motion(ii) Engagement a [mm]: depth of cut i.e. the length of the metal to be cut per unit

revolution/stroke(iii) Cutting Speed, v [m/min]: the instantaneous velocity of the primary motion of a selected

point on the cutting edge

1.4.3 Other ConditionsA complete description of the metal cutting action requires the introduction of other

quantities which give the exact location of the metal

cutting action- angle give the rate of metal cutting action–rate of metal

removedAngles(1) Feed Motion Angle Ψ: The angle between the directions of the simultaneous feed

motion and primary motion(2) Resultant cutting speed angle η : The angle between the directions of the

simultaneous primary motion and the resultant cutting motion.

Rate of Metal Removal

Definition: The volume of material removed per unit time at a particular instant.In turning the metal removal rate is the product of mean speed and the cross

sectional area of the cut A, thus Q=A.vmean

=a.f.vmean

=b.h.vmean

For turning

2

).(... mw ddnfaQ

2

).(... mw ddnhb

Where dw=work surface diameter anddm=machined surface diameter

2 A CutIs a layer of workpiece material that is going to be removed by a single action of a cutting

tool. In turning it is the workpiece material that is going to be removed by the cutting tool when the workpiece rotates once under feed

2.1 Width of Cut, b[mm]Is the distance between two extreme points of the active cutting edge perpendicular to the

primary motion

sin

ab

2.2 Thickness of Cut, h [mm]

Is the thickness of undeformed chip. It is defined as the product of feed and the sine of the cutting edge angle:

Another quantity which is derived from the width and thickness of cut is the Area of cut, A [mm2]: cross sectional area of cut

sinfh

hbfaA ..

3 Tool Geometry3.0 Introductiono The tool must have an appropriate shape to be able to cut.o This shape is the tool geometryo The tool geometry is provided by grinding the toolo The tool geometry of each type of tool differs from another type, but the general

geometry is common to all tools

3.1 General geometry of a ToolThe general geometry of a metal cutting tool is given by the following terms:(a) Tool Elements(b) Tool Surfaces(c) Cutting Edges(d) Tool Angles

3.1.1 Tool ElementsThese are 7 elements described in section 1.3

3.1.2 Tool Surfaces

Faces of the cutting tool (Turning Tool)

The tool has two major surfacesi. The FACE (Aγ): is the surface of the tool over which the chip flowsii. The FLANK (Aα): is the surface of the tool over which the machined surface flow

3.1.3 Cutting Edges

Face

Minor Flank Major Flank

Major cutting edge

Minor Cutting Edge

The Corner

3.2 Reference Systems

Reference planes are required for defining tool angles. There are two major reference systems in which the angle of a cutting tool are defined.

1. Tool in hand reference system: Angles are defined with tool held in hand or put on table or flat surface. The system is used for the manufacture and measurement of tool angles.

Planes in this system are called Tool-In-Hand Planes and angles measured in these planes all except one start with the word Tool

2. The tool in use reference system: Angles are defined when the tool is in application, i.e. when cutting. The system is used for describing the actual metal cutting process

Planes in this system are called Tool-In-Use Planes and angles measured in this system all except one start with the word effective or working and are denoted by ve.

3.2.1 Planes in the Tool-in-Hand Reference System

1. Tool Reference Plane Pr: Plane perpendicular or parallel to the plane or axis of the tool convenient for locating or orienting the tool for manufacture or sharpening.

2. Assumed Working Plane Pf: Plane perpendicular to Pr and so chosen as to be either parallel or perpendicular to a plane or axis of the tool convenient for locating or orienting the tool for its manufacture, sharpening or measurement.

3. Tool Back Plane Pp: Plane perpendicular to Pr and Pf

4. Tool Cutting Edge Plane Ps: Plane tangential to the cutting edge and perpendicular to Pr

5. Cutting Edge Normal Plane Pn: Plane perpendicular to the cutting edge

6. Tool Orthogonal Plane Po: Perpendicular to both Pr and Ps

3.2.2 Planes in the Tool-in-Use Reference System

The important factor here is the relative resultant cutting speed ve, which is the vectorial product of the cutting velocity v and feed velocity f. The planes are:

1. Working Reference Plane Pre: Plane perpendicular or parallel to the plane or axis of the tool convenient for locating or orienting the too

2. Working Plane P fe: Plane perpendicular to Pre and so chosen as to be either parallel or perpendicular to a plane or axis of the tool

3. Working Back Plane Ppe: Plane perpendicular to Pre and Pfe

4. Working Cutting Edge Plane Pse: Plane tangential to the cutting edge and perpendicular to Pre

5. Cutting edge normal plane Pne: plane perpendicular to the cutting edge;6. Working orthogonal Plane Poe: Perpendicular to both Pr and Ps

3.3 Angles of the Cutting Tool

The tool angles are defined in the planes described above (except the cutting edge plane), each plane containing a complete set of tool angles.

The tool reference plane gives the main view of the tool geometry while the other four planes give the cross sectional views.

3.3.1 Tool Angles in the Tool Reference Plane Pr

Pp

Pf

ε

Х’Χ

The main view gives 3 angles(1) χ: The cutting edge angle(2) ε: Included angle(3) Х’: Minor cutting edge angle

Χ+ ε+ Х’ = 180o

3.3.2 Tool Angles in the Tool Working Plane Pf

Tool angles in the four remaining planes give a cross sectional view. Basically there are three angles that can be obtained with different magnitudes depending on the type of plane in which they are taken.

The name, size and the shape of tool angles in the four cross section views depend on:1. The reference system in which they are measured2. The plane in which they are measured

αβ

γα : Clearance angleβ : wedge angleγ : Rake angle

Accordingly we have

(i) Side rake angle γf

(ii) Back rake angle γp

(iii) Orthogonal rake angle γo

(iv) Normal rake angle γn

(v) Orthogonal clearance angle αo

(vi) Side clearance angle αf

(vii) Back clearance angle αp

(viii) Normal clearance angle αn

(ix) Orthogonal wedge angle βo

(x) Side wedge angle βf (xi) Back wedge angle βp

(xii) Normal wedge angle βn

3.3.3 The Cutting Edge Inclination Angle λAnother important angle of the tool is obtained by view S. It shows that the cutting edge is

inclined towards the end (corner). This is called the cutting edge inclination angle λ. Its main function is to prevent cutting vibrations

S

Po

Pn

Pr

+ -

4.0 Theories of Chip Formation4.1 IntroductionThe purpose of Chip Studies is to establish optimum conditions for metal cutting

(machining) through Optimization of Cutting Parameters Improvement of machinability of materials Improvement of cutting tools Optimization of machine tool design and Optimization of tool design

4.2 Historical Development Chip Formation research started in early 19th century Seriously between 1851 and 1900 discovery of steam engine (1769) and industrial revolution made the research

more lively Was devoted to manufacturing of machine tools but latter research also involved

tool materials and machining costs Today is a big part of industry research

4.2DefinitionChip formation is the removal a thin layer of metal called chip or swarf, from a lager body by a wedge shaped tool. It involves plastic deformation of the workpiece material through the tool, under the effect of cutting forces.

4.3Theories of Chip Formation4.3.1 Introduction

Early attempts compared metal cutting to cutting of wood. However the modern view involves plastic deformation of the workpiece material during chip formation

4.3.2 Methods of StudyThe process of chip formation cannot be observed by naked eye nor by

ordinary photography. The following methods have been developed to study the nature of chip formation1. Optical observations: can provide good information about the

chip formation process.2. By suddenly stopping (freezing) the chip formation action: Most

of the important details of chip formation can be retained.3. High speed cine-photography at low magnification, was used to reveal the changing external shape of a chip. Now replaced by high speed digital video camera.

4.3.3 Assumptions1. Orthogonal cutting: Tool edge is straight, normal to the direction of

cutting; also normal to the direction of feed motion

2. Continuous chip3. Small ratio of chip thickness hc to chip width bc i.e. (hc:bc)4. No built up edge

4.3.4 Theories

(1) Shear Plane Theory First theory to be developed by

Russian scientist (1870) and then stated again by French scientist, Tresca in 1873. Other researchers who followed this theory are Svorkin, Piispanen, Schwerd and most recent Ernst and Merchant (1941)

It states “During chip formation the material ahead of the cutting edge is considered to be without stress. As the material advances towards the cutting edge with the relative speed v, stress suddenly builds up in the shear plane and the material is deformed along the shear plane into chip” Fig. The Shear Plane Theory

Model

However, there are some contradictions facing this theory:

1. The moving particle attain infinite acceleration when crossing the shear plane. This is against nature. It always takes time for a particle to accelerate

2. A big stress gradient exists on the shear plane. This is unnatural

3. large sudden strain is obtained at the shear plane. This is also unnatural

Another theory had to be developed which clears these contradictions

(2) The Shear Zone Theory

The theory was founded by Russian scientist, A. A. Bricks and updated by two Japanese Okushima and Hitomi in 1960

The shear zone explains many processes associated with metal cutting. Because of this the modern theory of chip formation is based on the shear zone theory

The theory states: “During chip formation the material ahead of the cutting edge is considered to be without stress. As the material enters a zone, called a shear zone, it moves with a relative speed v, stress slowly builds up and the material is deformed into a chip

(3) Modern Theory of Chip Formation Introduction: In general Chip Formation is regarded as a process in which

the workpiece material is plastically deformed by the tool, whereby the workpiece material is sheared after which it slips along favourable slip lines. The material loses its strength and becomes chip, whereby it slides over the tool face.

Modern Theory “Material ahead of the cutting edge is compressed and as a result stresses build up gradually as the material approaches the cutting edge of the tool until the material is sheared when the yield stress is reached. The material then slips along favourable directions of shear and finally slides along the face of the tool chip”

4.3.5 Energy Use in Chip Formation:

1. Plastic deformation zone (in shear zone). About 74% of the chip formation energy is dispersed here

2. Secondary plastic flow (in chip), about 24% of energy3. Sub-surface plastic flow (on machined surface), about 2% of

energy

4.4 Theory of Ernst and Merchant This is based on the assumption that the chip is a rigid body

held in equilibrium by the forces transmitted across the chip-tool interface and across the shear zone.

The whole of the resultant tool force is transmitted across the chip-tool interface

No force acts on tool edge or flank (Orthogonal Cutting)

Basis of Theory: Shear angle takes up such a value as to reduce the work done in cutting to a minimum.

This means it was necessary to express in terms of the shear angle Φ then obtain the value of Φ for which Fc is minimum.

Differentiating eq. 4.5 with respect to Φ and equating to zero to find the value ofΦ for which the force is minimum gives:

22

ne

4.5 Evaluation of Chip Formation ProcessThe effectiveness of the chip formation process can be evaluated in many ways. Following are some of the most common methods:

1. Chip Compression Ratios2. Shear angle relations

3. Velocity relations 4. Types of chips 5. Forms of Chips

These quantities are also used to compare different chip formation processes.

4.5.1 Chip Compression RatioThe chip compression ratio denotes the change in size and form of the cut after it has undergone chip formation. Its numerical value gives the degree of deformation of the cut.

(i) Chip thickness compression ratio

(ii) Chip width compression ratio

(iii) Chip length compression ratio

(iv) Chip area compression ratio

1__

__

h

h

cutofThickness

chipofThickness ch

1__

__

b

b

cutofWidth

chipofWidth cb

1__

__

l

l

cutoflength

chipoflength cl

1__

__

A

A

cutofArea

chipofArea cA

4.5.2 Shear Angle Relations

From the geometry of the figure follows:

sin____sin

hlor

l

hs

s

sin.slh

sin

sinsincoscos

sin.

)cos(.

s

sh

c

l

l

h

h

Hence

)cos(. sc lh

)cos(_____)cos(

c

ss

c hlor

l

h

sincos.tan

1h

tan

cossin h

Therefore

And

sin

costan

n

4.5.3 Velocity Relations3 velocities are of interest:1. The cutting velocity (normally known as the cutting speed), v2. The chip velocity vc and

3. The shear velocity, vs

chip

vcvs

v

workpiece

Φtool

90o+(γ-Φ)

Φ90o- γ

Fig. Velocity relations in an orthogonal cuttingv

vc

vs

γ

The above diagram shows first the velocities as they are during the chip formation process (left), and as they can be combined into a force triangle (right)

The velocity triangle has two known angles and one unknown angle. This can be obtained from the equation:

Xo = 180o-[(90o-γ)+φ] = 90o+(γ-φ)

Applying the sine rule to the triangle, we get:

)](90sin[sin

vvc

or

sin

)cos(

cv

v

similarly

)cos(

cos

v

vs

4.5.4 Types of ChipsThe formation of chips involves a shearing of workpiece material in a shear

plane zone.A very large amount of strain takes place in this region in a very short time

and not all metals and alloys can withstand this strain without fracture.As a result three types of chips are obtained in a chip formation process:1. Continuous (shear) Chip2. Discontinuous (tear) Chip and3. Continuous Chip with built up edge

Chip

W/p

Cutting speed v

TOOL (a) Continuous Chip

(b) Discontinuous (tear) chip

Fig. Type of chips in a metal cutting

Continuous ChipsAre produced under the following cutting conditions:1. Ductile material2. High cutting speeds3. Large rake angles4. Small engagements5. Minimum friction at tool-chip inter face

Discontinuous ChipsAre produced under the following conditions:1. Brittle materials2. Small and negative rake angles3. Small cutting speeds4. Big engagements5. Large friction at tool-chip interface

Continuous Chips with built up edge (BUE)BUE are pseudo unstable cutting edge temporarily formed on the actual cutting edge

during chip formation involving some materials. Its life varies from mili-seconds to several seconds.

These chips are formed under the following cutting conditions6. Ductile materials7. Low cutting speed8. Small rake angles9. High feeds10. Poor cooling11. High molecular affinity between tool and workpiece materials

5.0 Metal Cutting Forces5.1Introduction5.1.1 Nature of the Cutting Forces

Cutting force is the total resistance given by a workpiece material against the process of chip formation effected by the penetrating tool. The cutting force is exerted along the entire length of the cutting edge.

Total tool resitance Force

Tool

workpiece

Fig 5.1 Real area of action of cutting forces

5.1.2 Point of Action of Cutting Force

Tool

workpiece

Total tool resistance Force acting at a point on the cutting edge

Fig. 5.2: Assumed point of action of cutting forces

5.2 Major cutting Forces• The total resistance Fr (also known as resultant cutting force) lies in a plane

perpendicular to the tool cutting edge.

• Fr is usually resolved in directions convenient for its measurement, normally resolved in three convenient coordinates.

• There are two systems in use today:

1. The old (English) system, whereby Fr is resolved in two orthogonal components one of them containing the cutting velocity:one in the direction of cutting velocity: the cutting force, Fc

The other normal to the direction of the cutting-the thrust force, Ft

Motion of workpiece

Tool

ChipFc

FtFr

Fig. 5.3aResultant tool force resolved in a coordinate system containing v

Two cutting forces are obtained with this system• The cutting force Fc in the direction of cutting• The thrust force Ft in the direction normal to the direction of cutting

2. Modern system, whereby Fr is resolved in three direction:(i) in the direction of primary motion - main cutting force(ii) in the direction of feed motion – feed force(iii) in the direction perpendicular to the generated surface

The Main Cutting Force Fc is the projection of the resultant cutting force Fr in the direction of primary motion.

The feed force Ff is the projection of the overall resultant cutting force Fr in the direction of the feed motion

The passive force Fp is the projection of resultant cutting force Fr in the direction perpendicular to the generated surface.

The three forces are vectors and their vectorial sum (the resultant force) is

andpfcr FFFF

2222pfcr FFFF

222pfcr FFFF

or

Fig. 5.3bCutting Forces (Turning)

5.3 Factors influencing Cutting Forces

There are over 20 factors that influence the major cutting forces. O. Kienzle (1952) classified them into two groups as follows:

Those which are of interest to science are only (15) Those which are of interest to practical metal cutting (machining) are only (10). These are

1. Workpiece material2. Feed f3. Engagement a4. The ratio of cut b:h5. Rake angle γ6. Cutting edge angle χ7. Cutting Speed v8. Tool Material9. Cooling Fluid10. Tool Wear

5.4Measurement of the Cutting Forces

5.4.1 Introduction Forces cannot be measured directly but indirectly through their on object.

A device for measuring any forces is called DYNAMOMETER. Dynamometers convert the effect of a force into a measurable signal e.g. change in electric potential, deflection of an elastic body. The part of a dynamometer that does this work is called TRANSDUCER.

Transducers used in metal cutting are of the following types.1. Mechanical transducers- the deflection of the elastic

body is sensed and measured mechanically2. hydraulic transducers- the deflection of a membrane

is measured hydraulically through change of pressure or otherwise.

3. Electric transducers-capacitive electric transducers-inductive electric transducers-strain gauge transducers-piezoelectric transducers

5.5 Calculation of Cutting ForcesThere are two approaches to calculate the cutting forces:

(a) Using shear stress analysisThis approach considers a few of the factors that influence the cutting forces, namely

the tool geometry (rake angle γ), workpiece material (shear stress), the shear angle (Φ) and the friction angle (ρ).

)cos(

)cos(..

ssc

AF

s

ss A

F

whereby

For the shear plane theory (Merchant)

Or

)cos(sin

)cos(..

s

c

AF since sin

AAs

(b) Using the empirical equations based on experimental data on chip

formation. Kienzle identified 10 practical factors that influence the cutting forces. A

good equation must contain these factors i.e.

]};;;;;;;:[);();({ twc vhbbahffF

Whereby f=feed; h=thickness of cut; a=engagement; b= width of cut;v=cutting speed;γ=rake angle;χ=cutting edge angle;πw=workpiece material; πt=tool material;Ψ=cooling and lubrication and δ=tool wear

The first attempts took into account the direct influence of (a,f) or (b,h) and the rest of the factors were put under the constant kc which was given the name specific cutting force, i.e.

Fc=a.f.kc OR Fc=b.h.kc

Definition:The specific cutting force is the force for a unit area of cut A (b.h). This was though to be constant. It was later (1950) found that kc was not a Constant.

Kienzle (1952) found out experimentally that there exists an exponential relationship between the thickness of cut h and the specific cutting force kc as follows

Log kc

Log hh=1

Kc1.1

{h=1, b=1; A=1}

Relationship between kc and h

Kienzle (1952) gave the following relationship:

zc

c h

kk 1.1

Wherebykc1.1 is the principal value of the specific cutting force andz is a material constant

Substituting kc in the older equation gives:

1.11.1 .... c

zzc

c khhbhbh

kF

1.11 .. cz

c khbF or

Today Kienzle’s work has been extended to cover the other two cutting forces i.e.

1.11 .. fy

f khbF

1.11 .. pm

p khbF

For feed force and

For passive force

In these equations 1-z; 1-y; 1-m; kc1.1; kf1.1; kp1.1 are called workpiece Material cutting constants and are determined experimentally.

5.7 Experimental determination of Workpiece Material Cutting Constants1. For constant depth of cut a and cutting edge angle X, feed is varied

while the cutting forces are measured using cutting force dynamometers.

2. All the results are tabulated as shown in Table 5.23. Calculate b=a/sinχ4. Calculate h=fsinχ5. Calculate Fc/b

6. Plot Fc/b against h on a double logarithmic graph paper as they are (without converting them first into their logarithmic values)Typical graph is shown in Figure 5.8

From the graphtanα=1-z andKc1.1 is the value of the Fc/b where log h=1

Fig. 5.8 Experimental determination of material cutting constants 1-z andKc1.1

6 Cutting Power Requirement6.0 Introduction

Power is the rate of doing work (Joules per second). It is the scalar Product of Force and Velocity.

};;}{;;{. zyxzyx vvvFFFvF

vF R .

};;{ pfcR FFFF

};{ ; pfc vvvv

ffc

fc

fpfcc

vFvF

vFvF

vvFFFP

..

0..

}0;;}{;;{

Vf (Feed force) is normally very small (negligible) compared to v (cutting velocity), therefore, for practical purposes:

vFP cc .

Electric Motor(ηel)

Machine toolDrive (ηD)

Metal cutting

Pel PDPC

LOSSES:

1. Electrical losses in the electric motors and circuits (ηel =0.9….0.98)2. Mechanical losses in the machine tool (ηD = 0.7…..0.85)

Del

C

el

Del

PPP

.

6.1 Power Requirements for Turning, Shaping, Planing and Slotting

Major cutting forces are defined as follows

1.11 .. cz

c khbF

1.11 .. fy

f khbF

1.11 .. pm

p khbF

Main Cutting Force

Feed Force

Passive Force

The cutting velocities are defined as follows:Cutting velocity: v=πdn in m/minFeed Velocity vf=fn in mm/minPassive Force velocity is: vp = 0 The 6 cutting constants are obtained experimentally from laboratory tests. They are found in metal cutting Tables

6.2 Cutting Power for Drilling and Core Drilling

6.2.1 The Cut and other variables

1. The cutting speed v varies for various points on the lips (cutting edges); it is a maximum on the periphery and zero at the drill axis. The maximum cutting speed is taken for power calculation purposes

v=πdn/1000 m/min;2. The feed f is the amount the drill advances axially in one revolution of

the drill. A drill has two main cutting edges (lips) and the feed per lip is therefore:

ft=f/2 mm/rev

4. The width of cut b is measured along the lip and is equal to the length of the lip:

b=d/(2.sin κ) in mm5. The area of cut A per lip is:

4sin.2.sin.

2.

fddfbhAt

3. The thickness of cut h is measured in the direction perpendicular to the lip:

h=ft.sin(σ/2)=ft.sinκ; σ =point angle=2κ

6. Engagement a is the distance from the machined surface to the drill axis a=d/2 in mm

6.2.2 The cut and other variable for Core Drilling1. The engagement a in core drilling is

2odd

a

in mm

2. The thickness of cut is determined in the same way as in drilling in full.3. The width of cut b is

sin2odd

b

4. o

t

ddfhbA

In mm

4. The area of cut A for one lip is:

In mm2

6.2.3 Forces acting on a twist drillThere are 3 cutting edges on a twist drill: the cutting edge (lips), minor Cutting edge (drill margin) and the chisel edge.

The effect of all forces acting on the drill can be represented by the thrust force Tth and the resisting torque T(M). The action at the chisel edge is not trully a cutting action but rather a pushing into the material like a wedge. But the effect of the chisel edge on the torque is negligible as it is onthe axis of the rotation (radius is almost zero)

6.2.3.1 Thrust ForceThe total thrust force Fth is:

Fth=2Ff+Fch+Ffr Ff: Feed forceFch:Force on the chisel edgeFfr: Friction force on the drill margins which rub on the machined surface of the hole and the friction force due to chip flow.

The horizontal forces Fp (the passive forces), cancel each other. The feed force Ff consumes about 40% of the thrust force and chisel edge force consumes 57% and the friction force 3%.

6.2.3.2 The Feed ForceThe pure feed force is the sum of the two feed forces lips

(cutting edges of a drill):Ff=Ff1+Ff2

Using the cutting variables developed, the feed force acting on one cutting edge is:

(a) Drilling in full

1.11

21 .}2

sin.{

sin2 fy

ff kfd

FF

(b) Core drilling

1.11

21 .}2

sin.{

sin2 fyo

ff kfdd

FF

6.2.3.3 The TorqueThe torque is made up of(1)Torque of the main cutting force (Fc) - T(2)Torque due to scrapping of chisel edge-Tch

(3)Torque of the friction force – Tfr

T=Tc+Tch+Tfr

About 80% of torque is due to the main cutting force, 12% to the chisel edge and 8% to friction.

Calculation of the TorqueThe two main cutting forces form a force couple with a distance x between them.

Theoretically the distance x should be half the twist drill diameter. However, recent research results show that:

x=0.50d……..0.57d for drilling in fullx=0.26d……..0.41d for core boringFor all practical purposes this distance should be assumed as: x=0.5dWith this assumption the drilling torque is then

T=0.5dFc1 =0.5dFc2 for drilling in full and for core drilling (boring)

6.2.4 The cutting Power Requirement for Drilling and Core DrillingThe drilling power is calculated from the torque as follows:Pc=Cutting torque X angular velocity

=Tω=T.2πn

2/12/1 )2

()2

( co

co

o Fdd

Fdd

dT

6.3 Power Requirement in Milling

6.3.0 Introduction• Milling is a metal cutting process in which a multiple cutting edge

tool conducts a rotary motion.• The multiple cutting edges are arranged on the circumferential

surface or on the faces or on both.• Each cutting edge cuts over a small fraction of the tool’s rotational

path and remains dormant for the rest

The milling operation can be classified into two major groups: (1) Vertical Milling or Face Milling (2) Horizontal Milling or Plain MillingThe latter is also grouped into Up Milling and Down Milling

Fig. Main Features of the Milling Process

6.3.1 Cutting Variables and the Definition of the Cut in Vertical Milling

6.3.1.1 Cutting Variables in Vertical Milling (Face Milling)The following variables, which were not considered under turning, are introduced to be able to calculate the milling power

1. B: Cutting Height (Width of the Workpiece)2. D: Diameter of Milling Head3. t: Number of cutting edges (teeth)4. φ1: Cutting angle (feed motion angle) on entry5. φ2: Cutting angle (feed motion angle) on departure.6. φc = φ2 -φ1: Cutting arc angle7. ft : tooth feed8. fc: Cutting feed9. e: eccentricity i.e. distance between workpiece and milling

head centresNote: For good cutting the ratio between B and D must be:B:D = 3:4 or B = 0.6D

6.3.2.2 Definition of the Cut in Vertical (face) MillingThe dimension of the cut in vertical (face) milling is shown in Fig.

6.3.3

1. Thickness of cut hThe thickness of cut in vertical (face) milling varies with the cutting arc angle φc or the feed motion angle φ . It is zero when the cutting edge enters the workpiece, when φ = 0 and increases to maximum value when φ=90o after which the value decreases to another minimum value on leaving the workpiece, when φ= φc.

It follows therefore, that:h=f(φ)

And from triangles in Figures 6.3.2 follows that:

h=fc.sinχFrom the diagram on the right follows:

Substituting this value in equation 2 gives:h(φ)=ft.sinφ.sinχ

2. The Tooth Main Cutting Force in Vertical (Face) MillingUsing the main cutting force equation, the tooth cutting force in milling can be written as

t

c

f

fsin

sin.tc ff

1.11 .)sin.sin..()( cz

tct kfbF

Hence

Using the specific cutting force:Fct(φ)=b.ft.sin φ.sin χ.kc

The relationship between Fct(φ) and φ is shown in the diagram below. The Magnitude of the tooth main cutting force is represented by the area of the sine curve. The same can be represented by a rectangular shaped curve as shown in the diagram.

The mean cutting force of a tooth is

The above equation can be written as:

Which is

2

1

..sin.sin..1

12

dkfbF ctcmt

dkfbF ctc

cmt .sin.sin..1 2

1

ctc

cmt kfbF ).cos(cossin..1

21

D

B

D

B 111

2

2cos

o902

Fig. Tooth main cutting force as of the feed motion angle

D

B

D

Bo 222

2

2/sin)90cos(cos

ctc

ctc

cmt

kD

Bfb

kD

BBfbF

.2.sin...

1

).(2.sin...1 21

oc

occ radiansin

296.57

1.

360

2__

φc in radians

Equation 5 becomes

ctoc

cmt kD

BfbF .

2.sin...

296.57

D

BkfbF cto

ccmt ..sin...

6.114

Note that only a few teeth are engaged at any given time.So ifnt= number of teeth of the milling cutter

ta=average no. of teeth engaged at any given time

τ= angle between two adjacent teethThen the following relationship has been proved to hold:

oct

oc

a

nt

360

.

3. The mean main cutting Force

acmtcm tFF .4. The Cutting Power in Vertical Milling

vtFvFP acmtcmc ...

6.3.2 Cutting Variables and Definition of the Cut in Horizontal Milling (Plain Milling)

Cutting VariablesThe following features distinguish Horizontal Milling from Vertical Milling regardless of whether it is up milling or down milling

(i) φ1=0(ii) φ2= φc; Hence cos φ2=cos φc

(iii) b=BFor better cutting, the centre of the milling head arbour is higher up than the engagement

D

a

D

aDc

21

2/

2/coscos 2

Definition of cutThe area of cut A in horizontal milling is equal to the area of the arc extended by an

angle φc at the centre.

The area of such an arc is given as:Length of the arc x mean width (thickness)L x hm

2.2D

L

2...D

hfaA cmt

D

af

D

fah to

cc

tm ..

6.114

.

..2

Hence

From which

ctoc

mcmcmt kD

afBkhBkhbF ....

6.114....

For horizontal milling, therefore, the mean tooth main cutting force is:

the cutting power is

vtFvFP acmtcmc ...

7 Tool Failure and Tool Life1. Tool Failure: Inability of the cutting tool to continue cutting or maintain a

required workpiece accuracy in terms of dimension, form or surface finish.

2. Causes of Tool Failure: 1. mechanical breakage 2. Plastic Deformation 3. Thermal Cracking 4. Gradual Tool wear

o Mechanical breakage is mainly caused by shocks and vibrations when a tool strength is exceeded.

o High cutting temperature results into tool to loose its hardness properties and leads to plastic deformation and loses its form.

o Thermal cracking occurs due to repeated thermal expansion usually when a carbide tip is brazed into a tough steel. Different coefficient of friction leads to thermal stresses hence cracks. Also difference in thermal expansion of the various layers of tool material can be caused by intermittent cutting or uneven cooling and lubrication.

o Gradual tool wear is mainly caused by friction between tool and workpiece and tool and chip.

(i) Cutting time(ii) Tool material(iii) Workpiece material(iv) Cutting Speed(v) Tool geometry(vi) Setting variables(vii) cooling and lubricating media(viii) Cutting temperature(ix) Dynamic behaviour of machine tool(x) Environment

6. Mechanisms of Gradual Tool Wear

3. Occurrence Forms of Gradual Tool Wear: 1. Crater Wear 2. Flank Wear 3. Notch Wear 4. Minor Flank Wear 5. Deformation of the tool corner

4. Measure of Tool Wear: 1. Maximum Crater Depth for crater wear 2. width of flank wear land for flak wear

5. Influencing Factors of Tool WearMajor factors influencing the type and degree of tool wear are:

There are 5 major mechanisms of gradual tool wear. These are adhesive wear, abbrasive wear, corrosive wear, surface fatigue wear and diffusion wear.

o Adhesive wear: High pressure on surface peaks of tool and wp leads to plastic deformation, macro-welds which work hardens. Relative motion disrupts these macro welds resulting into material loss.

o Abrasive wear: Penetration of harder particle of one of the contact surfaces or from surrounding into inner boundary layer of the other and hence ploughing out the material.

o Corrosive wear: Chemical reaction occuring in the presence of tool material, workpiece material, air and the cooling media. The products of corrosion are normally oxides, hydroxides, carbonates, chlorides and oxy-chlorides.

o Surface fatigue: caused by repeated surface temperature change. Normally occurs towards the end of tool life.

o Diffusion wear: Atoms/molecules move from high atomic concentation zone to low concentration. Cutting and ceramic tool with cutting temperatures btn 700oC and 2000oC. Experience diffusion wear

7.5 Tool LifeEffective cutting time between two resharpenings or the cutting time

required for a tool to reach a tool life criterion

ISO recommended criteria:HSS and CERAMICS tools are either1. Catastrophic failure or2. VB=0.3 mm if the flank is regularly worn in zone B3. VBBmax=0.6 mm, if the flank is irregularly worn, scratched, chipped, or

badly grooved in zone B

7.5.1 Tool Life Graph and Tool Life EquationTool life can be determined graphically or analytically using tool life equation.

Graphically it can be determined as follows:(i) Measurement of tool wear at suitable cutting time intervals(ii) Setting up a tool life criterion(iii) Determination of Cutting time for the tool life criterion chosen(iv) Plotting of the tool life graph, i.e cutting time against tool life

Sintered CARBIDE tools are either:1. VB=0.3 mm, or2. VBBmax=0.6mm, if the flank is irregularly worn, or3. KT=0.06+0.3f, where f is the feed

Attempts have been made to define tool life graph analytically. The first of such attempts was by F W Taylor in 1906 who, incorporated the cutting speed v as the only influencing factor and obtained the following equation:

vTn=C Whereby v=cutting speed T=tool life c,n= constants

C=intersection of tool life curve with x-axis and

12

21

loglog

loglog

tan

1

TT

vvn

7.5.2 Economical Tool LifeThe economical tool life takes into account both technological and economical factors

of metal cutting process: tool life, tool costs and wages.

The main components of the metal cutting costs are incorporated in the following.1. t1 = minutes to change the tool at the end of its tool life2. t2 = minutes equivalent to sharpen the tool3. t3 = minutes equivalent to depreciation of the tool

The following equations have been proved to hold:

c

ss

xRN

xRtt

22

Whereby ts: tool grinding timeRs: Labour and overhead rate in tool grinding departmentRc: Labour and overhead rate in metal cutting department

N2: Number of tool cutting edges

c

T

xRxNN

Ct

213

Whereby CT: Tool cost

N1: Number of times the tool can be sharpened including once when it is madeN2: Number of cutting edges

To replace a tool that has reached the end of its tool life requires:t=t1+t2+t3

The total chargeable cost includes the time of the tool life, i.etc = t+T

The amount of metal removed during this time isQtc = v.T.f.a

Applying Taylor’s equation gives

afTcQ ntc ... 1

Tt

afTc

t

QR

n

c

tc

... 1

The rate of metal removal is then

For economical machining, the rate of metal removal must be maximum.This means mathematically

tn

T

TtntTtT

Tt

afTc

dT

d

dT

dR

ec

n

n

11

)]([)(

1

...0

2

1

From which we get

For the condition sharpening is done when the cutting process continues, the

economical tool life equation becomes:

11 11

tn

Tec

8 Cutting tool Materials8.0 Introduction

Cutting tool materials are used to remove other materials in a metal cutting process. They are therefore harder than the materials they cut.hardness is the major property of a tool material. Essential is the Hot or Red Hardness i.e. hardness at cutting temperatures.

Hardness Ratios: These are ratios of the hardness of the tool and workpiece materials.

The hardness ratio between the tool and workpiece materials must be considered under elevated temperatures for both materials and increased strain rate of deformation of the workpiece material. This gives the modified ratio

5.135.1mod

ifiedwork

tool

H

H

In contrast to the static hardness ratio:

5.135.1

work

tool

H

H

8.1 Requirements

Requirements, put on cutting tool materials are:1. High hardness – so that it can cut other materials with good

tool life

2. High Toughness- to resist shocks in intermittent cutting processes such as shaping and milling.

3. High Wear Resistance- to resist wear during the cutting process.4. High Impact Strength- to withstand the impacts of metal cutting

especially in processes such as shaping and milling.5. High Bending and Compressional resistance- to overcome

bending and compressional forces.6. High Heat Resistance - to withstand metal cutting heat.7. High Torsional Rigidity8. Low Scaling – the exposure of tool material to the cutting heat

under the cutting conditions lead to developments of scales on the tool surfaces.

9. Good Machinability- so that material can be machined to obtain the required tool geometry.

10. CheapNo single cutting tool material satisfies these requirements. Some of the

requirements are contradictoryExample: An increase in hardness makes the material brittle and reduces its

toughness

A good compromise is always sought for each machining situation. In turning the emphasis is on hardness while in drilling the emphasis is on torsional rigidity.

8.2 Types of Cutting Tool Materials

CUTTING TOOL MATERIALS

1. Carbon Tool Steels2. Alloy Tool Steels3. High SpeedSteels

1. Abrasives

1. Cast Alloys2. Cemented

Carbides3. Ceramics4. Diamond

FERROUS NON-FERROUS ABRASIVES

8.2.1 (Plain) Carbon Tool Steel

Carbon tool steels were the first in history to be used. As early as 1870 they had become the main tool material.Carbon tool steels are characterised by their high carbon contents, which ranges between 0.5 and 1.5%. Their hardness is through:1. Carbides of iron and2. a structure called martensite that is obtained through heat treatmentTools made out of these steels can cut up to only 200-250oC, beyond which they loose their hardness.

Martensite: an unstable steel atomic structure that is obtained through sudden

cooling (quenching). The carbon atoms get no time to go back their positions and get trapped between atoms.

Heat treatment: Heating to cherry red heat level, usually between 780oC and 800oC, then quenched in water of about 20oC followed by tempering (warming up) between 220oC and 320oC before they are allowed to cool down to room temperature

Hardness: Due to carbide and martensitic matrixApplications: milling cutters, twist drills, hand tools, thread cutting tools, turning tools

for easy to cut materials (wood, magnesium, aluminium)

8.2.2 Alloy Tool SteelsThey are steels with the following additions of alloying elementsAlloying Element

Improves/increases Lowers

Chromium (Cr) Hardness, strength, wear resistance, hot strength

Strain

Tungsten (W) Through hardening, hot strength, wear resistance, fine grain, toughness, strength, heat resistance

Over heating sensitivity, strain

Nickel (Ni) Toughness, strength, electrical resistance, heat resistance, through hardening

Over heating sensitivity, strain

Molybdenum (Mo)

Through hardening, hot strength, wear resistance, toughness, fatique resistance

Strain forgeability

Vanadium (V) Through hardening, fatique resistance, hot strength, wear resistance, hardening temperature

Overheating sensitivity

Manganese (Mn)

Through hardening, strength, impact strength, wear resistance, fatique resistance, over heating resistance

Machinability. Hardening temperature

Are known as alloy steels. When properly heat treated, these steels can cut at temperatures up to 250-300oC.

Structure: Similar to carbon steels i.e. MartensiteHeat Treatment: Similar to carbon tool steels.Contents of hard particles: 5-10%Hardness: Due to carbides and martensitic matrix.Cutting Temperature: 250-300oCCutting Speed (at tool life of 60 minutes): 5m/min

Applications: similar to carbon tool steels, but with more cutting strength.

8.2.3 High Speed Steels (HSS)They are High Alloy Tool Steels and as such their structure is similar to that of

tool steels. The only difference are:

(1) They have higher alloy contents of up to 25% (the majority of alloy tool steels have up to 20% alloys)

(2) A special heat treatment process.High Speed Steels were discovered by Taylor and White around 1900.

Chemical Composition:Carbon: 0.6……..2.2%Alloys: ≈ 20(25)% (mostly W & Cr with traces of Cr, V and Co

Heat Treatment: Special process: Heating up to near solidus (the temperature where liquid appears in the structure)- about 1250-1290oC; cooling in a jet of compressed air or bath of molten lead to 620oC, then to room temperature. This is followed by a tempering treatment below 600oC.

Contents of hard particles: 20-25(30)%Hardness: Due to carbides and martensitic matrixCutting temperature: 600oC

Cutting Speed (at tool life of 60 minutes): 30m/min (about 6 times maximum, possible cutting speed at that time- Hence the name High Speed Steels.

HSS, though slightly less than tool steels at room temperatures, show a big improvement in hot hardness properties. They also have higher strength and toughness.

Designation: W-based Mo-basedAmerican: T1-----T15 M1-----M20British: BT1---BT40 BM1---BM42

German: S(plus 3 or more digits) e.g. S18-0-2

Application: In the manufacture of taps, twist drills, milling cutters and in the manufacture of some turning tools.

8.2.4 Non-Ferrous Cast Alloys

8.2.4.1 IntroductionThere was a shortage of HSS during the Second World War, especially for

industries in the US. Among the first cast alloys developed in the US was under the name of Stellite

8.2.4.2 Structure and Chemical CompositionNon-ferrous cast alloys are carbides embodied in a metallic matrix of one of

the elements of the iron group (Fe, Ni, Co), usually cobalt.

The carbides are formed by chemical reaction between the carbon and the carbide forming elements (Cr, Mo, V and W).

The mixture is then melted and cast. The structure of non-ferrous cast alloys is therefore that of a cast. As such it leaves no room for manipulation once it is cast.

Manufacture of non-ferrous cast alloys

Carbon

C

1.5-2.5%

Metallic Matrix

Fe, Ni, or Co

45-50%

Carbide forming Elements

Cr, Mo, V, W

40-50%

Melting and Casting

NON-FERROUSCAST ALLOY

Application: The properties of non-ferrous cast alloys lie between HSS and cemented carbides. Because of this, these tool materials have not penetrated the world market. They are used mostly in the US for machining cast and malleable iron, alloy steels, stainless steels, non-ferrous metals.

8.2.5 Cemented Carbides8.2.5.1 IntroductionCemented Carbide is a compound of grain hard refractory carbides of

Tungsten (W), Tantalum (Ta), or Titanium (Ti) bonded (cemented) in metallic binder (Cobalt (Co) or Molybnenium (Mo)), but usually cobalt

It was discovered by Schroeter in 1920s in the laboratories of Osram in Germany.

8.2.5.1 ManufacturingCemented carbides are manufactured by process of sintering. In this process:

1. The carbides are mixed together in the required ratio according to the type of cemented carbide being manufactured.

2. The mixture is then ground to a fine powder.3. The mixture of a fine powder is compressed in a mould under very high

pressure (around 400 bars)4. The parts are then put in an oven and heated up to 1600oC

8.2.5.3 Chemical CompositionCemented Carbides consists of:

(i) 75-95% hard particles (carbides)(ii) 5-25% Cobalt binding metal

The more the Cobalt the tougher the material.

8.2.5.4 Types of Cemented CarbidesThere are many types of Cemented Carbides. To help the operator choose the

right type of tool, cemented carbide are classified in 3 groups and each group is further classified in grades.

Identificationletter

IdentificationColour

Grade Application

P Blue 01-------50 Cutting of steels and steel castings

K Red 01-------40 Cast iron and non-ferrous metals e.g. brass

M Yellow 10-------40 Exotic metals and alloys which are difficult to cut, such as Titanium

The groups are given international identification letters and colours and the Various grades are given international numbers:

NOTE: (1) the higher the number the tougher the material

(2) the lower the number the harder the material and thus the more wear resistant the material

Because of their brittleness, cemented carbides are either brazed on a toughened shank as carbide tips or are clamped on the toughened shank as carbide inserts.

New Developments: Cemented carbide inserts are coated with a thin layer of ceramics, which increases the tool life considerably

Cemented Carbides

Co + WC________________________

Type K

Co + TaC + (TiC+WC)________________________

Types P and M

8.2.6 Ceramics

8.2.6.1 IntroductionCeramic is a compound of up to 99.7% Aluminium Oxide (Al2O3) as hard substance plus

traces of other oxides and additives.Though they are highly temperature and wear resistant, they are very susceptible to

shocks. Therefore, they are used for very high cutting speeds and shock and vibration free operations.

8.2.6.2 ManufacturingCeramic cutting tool materials are manufactured through SINTERING as elaborated

above.

8.2.6.3 Types of CeramicsFour major types:

1. Pure Aluminium Oxide2. Aluminium oxide plus other metallic oxides (Si, Mg, Cr, Ti, V, Ni, Fe)3. Aluminium oxide plus binding metal (Co, Mo, Cr, Fe, Ni)4. Aluminium oxide plus metallic carbides (W, Ti, Mo)

8.2.6.3 Properties (i) High abrasion resistance (ii) high red hardness, and show no sign of deformation even at cutting

temperature above 1000oC(iii) Remain hard at temperatures which would affect cemented carbides.

NOTE: use of cutting fluids is not recommended because of the danger of thermal cracking-pure aluminium oxide will be destroyed by a sudden temperature change of more than 200oC

8.2.7 Cubic Boron Nitride (CBN)

8.2.7.1 IntroductionDoes not occur naturally. It is a synthetic material.Second hardest material, second to Diamond. It has exceptionally high abrasion

resistance and cutting tool life in severe cutting conditions.

8.2.7.2ManufacturingThe synthesis of CBN involves structural transformation of Boron Nitride from

HEXAGONAL to CUBIC form at high pressure, high temperature plus a catalyst.

8.2.7.3Properties(i) Very hard- diamond and CBN are known as super hard materials;(ii) Less reactive to ferrous alloys(iii) It does not react with other materials or oxidize at temperatures below 1000oC

8.2.7.4 ApplicationsA layer of CBN approximately 0.5 mm thick is bonded to a cemented carbide

tip approximately 5 mm thick. The cemented carbide provides a shock resistance base. This tool material will

machine effectively workpiece materials difficult to cut.

8.2.8 Diamond

Diamond is the hardest known material. It occurs in nature in many types. Not all types are suitable for jewelry. Those types, which are unsuitable for jewelry, are used in industry for various tasks including:(i) dressing for grinding wheels

(ii) as cutting tool materials – for non ferrous and non-metallic workpiece materials

9 Machinability of Materials9.1 Introduction

Machinability is understood as the degree of ease with which a workpiece material can be cut.

It does not denote a single property but it encompasses a large number of uncoherent cutting properties and variables.

Therefore it is not possible to quantify machinabiity through a single variable or number nor it can be expressed by one equation.

ASME’s numerical relative machinability rating, has rated steel SAE Number 1112 at 100 and all other materials are relatively rated. However ASME and SAE ratings are based on cutting speeds which give certain specified tool wear or tool life.

It is internationally acceptable nowadays to assess machinability of workpiece material using four major criteria. These are(i) Wear of the cutting tool or tool life(ii) Cutting forces or cutting power(iii) Chip formation (chip forms)(iv) Surface finish

9.2 Pre-requisite for Determination of Machinability

Machinability can only be determined qualitatively by means of comparison. However the conditions under which the parameters for comparison are carried out are same. These variables are

(i) Cutting speed (ii) Cutting feed (iii) the cut ie the width of cut and thickness of cut (iv) tool geometry (v) tool size

(vi) the cantilever length of the tool (vii) state of wear of cutting edge (viii) tool material (ix) cutting edge angle

(x) type of mounting and clamping (xi) cutting fluid(xii) Geometry of workpiece (xiii) type of measuring equipments and

machines (xiv) experience of testing personnel(xv) Method for manufacture of workpiece material.

9.3 Criteria for Machinability

9.3.1 Wear of cutting tool as criterion There are several methods of assessing machinabilty using tool wear and tool

life a criteria. These are

(i) The tool wear-cutting time method(ii) The tool wear-cutting speed method(iii) The cutting temperature method(iv) The tool life graph method

9.3.1.1 Tool wear-cutting time methodTool wear of different workpiece materials are measured after a fixed time

interval. Provides good bases for comparing the machinability of the materials.

9.3.1.2 Tool wear-cutting speed methodMost American machinability index numbers were obtained using this

method.Machinability is assessed by comparing tool wear from cutting two or more

materials with a fixed cutting speeds.There are two ways:1 Comparison done on the cutting speed which will produce a pre-

selected amount of tool wear e.g VBB=0.25mm or a total tool wear failure within a given cutting time, or length.

2 Direct application of tool wear-cutting speed curve. Position of the curve determines machinability.

9.3.1.3 Cutting temperature methodLess applied method.Cutting temperature can be used as a measure of amount of tool wear in a

fixed period of cutting time. If all other variables are kept constant, the amount of too wear will depend on the machinability of the wp material.

9.3.1.4 Tool life graph method

Tool wear-cutting time and tool temperature-cutting time graphs plotted for different workpiece materials can be used.

If graph of material A lies above that for material B then A is less machinable than B.

9.3.2 Cutting forces as criterion for machinabilityForces required to cut a workpiece material denotes resistance of that

material against penetrating tool. This is dependent on properties of workpiece material. This is in fact the machinability of the material.

The magnitude of the principal value of specific cutting force, kc1.1, is used as a measure for the machinability.

9.3.3 Chip formation as criterion for machinabilityIt is the form of chips that is used to assess the machinability of workpiece

material.According to ISO, 8 major forms are identified:

i. Ribbon chips (long, short and snarled)ii. Tubular chips (long, short and snarled)iii. Spiral chips (flat and conical)iv. Washer type helical chips (long, short and snarled)v. Conical helical chips (long, short and snarled)vi. Arc chips (connected and loose)vii. Elemental chipsviii. Needle chips

These forms of chips can be used assess machinability of workpiece materials (good, satisfactory and unstatisfactory) and their usefulness in machining

9.3.4 Surface finish as criterion for machinabilityA relatively weak criterion for machinability, even weaker than chip form

criterion.Good machinability indicates good surface finish and integrity

9.4 Assessment of Machinability

A quantitative assessment of machinability of workpiece materials is not possible.

Only a relative evaluation (comparison), feasible. i.e. assessing a material as being more or less machinable than another material.

A systematic approach:Each of the 4 criteria has to be considered separately.For each case the materials have to be arranged in order of precedence giving

score 1 to least machinable materialMaterial with the lowest score for all four criteria is the least machinable and

material with the highest score is the most machinable