PD 211 Principles of Metal Cutting Dr T

PD 211 Principles of Metal Cutting Dr T

PD 211 Principles of Metal Cutting Dr T Mwinuka PD 211 Principles of Metal Cutting 2hours 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 life Machineability of materials Economics of metal cutting: Taylors equation, economical tool life equation, economical tool life Metal cutting heat: generation and distribution of metal cutting heat Recommended Text Books 1. 2. 3. Masuha, J R (1992): Basic Principles of Metal Cutting, Dar es Salaam University Press (DUP) Ghosh, A and Mallik, A K (1986) Manufacturing Science, Ellis Horwood Limited Publishers. G. Boothroyd, W A Knight (1988) Fundamentals of Machining and Machine Tools; 2nd Edition, Marcel Dekker, Inc

Assessment Continuous Assessment 2 tests University Exam 40% 60% 1.0 Introduction to Metal Cutting 1.1 Action of Metal Cutting What 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 Workpiece

As far as metal cutting is concerned, the geometry of the workpiece is defined by three surfaces: (4) (5) (6) Work Surface to be removed (raw surface)- geometry before cutting Machined surface: surface produced by metal cutting process (geometry after cutting) 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. 2.

3. 4. 5. 6. 7. The body that holds the cutting part/blades/inserts The shank by which the tool is held The tool bore by which the tool can be located and fixed by a spindle or arbour or mandrel The cutting part, the chip producing elements (eg cutting edges, face and flank of a turning tool) The wedge which is a portion of cutting part between face and flank The base for orienting the tool for its manufacture and sharpening 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 Conditions Cutting conditions are described as

kinematic conditions geometric conditions other conditions 1.4.1 Kinematic Conditions 1. Motions There 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 Conditions

Are 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. 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. 1.4.3 Other Conditions A complete description of the metal cutting action requires the introduction of other quantities which

give the exact location of the metal cutting actionangle give the rate of metal cutting actionrate of metal removed Angles (1) Feed Motion Angle : The angle between the directions of the simultaneous feed motion and primary motion (2) Resultant cutting speed angle : 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

Q .a. f .n.(d w d m ) 2 .b.h.n.(d w d m ) 2 Where dw=work surface diameter and dm=machined surface diameter 2 A Cut Is 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 a b sin 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: h f sin Another quantity which is derived from the width and thickness of cut is the Area of cut, A [mm2]: cross sectional area of cut A a. f b.h

3 Tool Geometry 3.0 Introduction The tool must have an appropriate shape to be able to cut. This shape is the tool geometry The tool geometry is provided by grinding the tool 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 Tool The 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 Elements These are 7 elements described in section 1.3

3.1.2 Tool Surfaces Face Minor Flank Major Flank Faces of the cutting tool (Turning Tool) The tool has two major surfaces i. The FACE (A): is the surface of the tool over which the chip flows): is the surface of the tool over which the chip flows ii. The FLANK (AV): is the surface of the tool over which the machined surface flow 3.1.3 Cutting Edges

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

The main view gives 3 angles (1) : The cutting edge angle (2) : Included angle (3) : Minor cutting edge angle + + = 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. : Clearance angle

: wedge angle : Rake angle 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 measured 2. The plane in which they are measured Accordingly we have (i) (ii) (iii) (iv) (v) (vi) (vii)

(viii) (ix) (x) (xi) (xii) Side rake angle ): is the surface of the tool over which the chip flowsf Back rake angle ): is the surface of the tool over which the chip flowsp Orthogonal rake angle ): is the surface of the tool over which the chip flowso Normal rake angle ): is the surface of the tool over which the chip flowsn Orthogonal clearance angle Vo Side clearance angle Vf Back clearance angle Vp Normal clearance angle Vn Orthogonal wedge angle o Side wedge angle f Back wedge angle p Normal wedge angle n

3.3.3 The Cutting Edge Inclination Angle Another 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 . Its main function is to prevent cutting vibrations S Pn Po Pr + - 4.0 Theories of Chip Formation 4.1 Introduction The 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 4.2Definition Chip 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 Formation

4.3.1 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 Study The 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 formation 1. 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 Assumptions 1. Orthogonal cutting: Tool edge is straight, normal to the direction of

cutting; also normal to the direction of feed motion 2. 3. 4. Continuous chip Small ratio of chip thickness hc to chip width bc i.e. (hc:bc) 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 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 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 (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. 2. 3. Plastic deformation zone (in shear zone). About 74% of the chip formation energy is dispersed here Secondary plastic flow (in chip), about 24% of energy 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 chiptool 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 then obtain the value of for which Fc is minimum. Differentiating eq. 4.5 with respect to and equating to zero to find the value of for which the force is minimum gives: 2 ne 2 4.5 Evaluation of Chip Formation Process The effectiveness of the chip formation process can be evaluated in many ways. Following are some of the most common methods:

1. Chip Compression Ratios 2. 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 Ratio The 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 h (ii)

Thickness _ of _ chip hc 1 Thickness _ of _ cut h Chip width compression ratio Width _ of _ chip bc b 1 Width _ of _ cut b (iii) Chip length compression ratio l

length _ of _ chip lc 1 length _ of _ cut l (iv) Chip area compression ratio A Area _ of _ chip Ac 1 Area _ of _ cut A 4.5.2 Shear Angle Relations From the geometry of the figure follows:

h h sin __ or __ ls ls sin h ls . sin hc hc cos( ) __ or ___ ls ls cos( ) hc ls . cos( ) Hence hc ls . cos( ) cos cos sin sin

h h ls . sin sin 1 h . cos sin tan Therefore cos h sin tan And cos tan

n sin 4.5.3 Velocity Relations 3 velocities are of interest: 1. The cutting velocity (normally known as the cutting speed), v 2. The chip velocity vc and 3. The shear velocity, vs chip vs v vc 90o+(-) vc

tool vs workpiece 90o- v Fig. Velocity relations in an orthogonal cutting 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-): is the surface of the tool over which the chip flows)+] = 90] = 90o+(): is the surface of the tool over which the chip flows] = 90) Applying the sine rule to the triangle, we get: vc v sin sin[ 90 ( )] or

v cos( ) vc sin similarly vs cos v cos( ) 4.5.4 Types of Chips The 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) Chip 2. Discontinuous (tear) Chip and 3. Continuous Chip with built up edge Chip W/p TOOL Cutting speed v (a) Continuous Chip (b) Discontinuous (tear) chip Fig. Type of chips in a metal cutting

Continuous Chips Are produced under the following cutting conditions: 1. Ductile material 2. High cutting speeds 3. Large rake angles 4. Small engagements 5. Minimum friction at tool-chip inter face Discontinuous Chips Are produced under the following conditions: 1. Brittle materials 2. Small and negative rake angles 3. Small cutting speeds 4. Big engagements

5. 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 conditions 6. Ductile materials 7. Low cutting speed 8. Small rake angles 9. High feeds 10. Poor cooling 11. High molecular affinity between tool and workpiece materials

5.0 Metal Cutting Forces 5.1Introduction 5.1.1 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 workpiece Tool Fig 5.1 Real area of action of cutting forces 5.1.2 Point of Action of Cutting Force Total tool resistance Force acting at a point on the cutting edge

workpiece Tool 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, F t Chip

Fc Tool Fr Ft Motion of workpiece Fig. 5.3a Resultant 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

Fig. 5.3b Cutting Forces (Turning) The Main Cutting Force Fc is the projection of the resultant cutting force Fr in the direction of primary motion. The feed force F is the projection of the overall resultant cutting force F in the f r direction of the feed motion The passive force F is the projection of resultant cutting force F in the direction p r perpendicular to the generated surface. The three forces are vectors and their vectorial sum (the resultant force) is

F r F c F f F p 2 2 Fr Fc F f Fp and 2 2 2 2 Fr Fc F f Fp

2 or 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. 2. 3. 4. 5. 6. 7. 8.

9. 10. Workpiece material Feed f Engagement a The ratio of cut b:h Rake angle ): is the surface of the tool over which the chip flows Cutting edge angle Cutting Speed v Tool Material Cooling Fluid 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 mechanically 2. 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 Forces There are two approaches to calculate the cutting forces: Using shear stress analysis This approach considers a few of the factors that influence the cutting forces, namely the tool geometry (rake angle ): is the surface of the tool over which the chip flows), workpiece material (shear stress), the shear angle () and the friction angle ().). (a) As . s . cos( ) Fc cos( ) whereby For the shear plane theory (Merchant) F s s

As Or A. s . cos( ) Fc sin cos( ) since As A sin (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. Fc f { f (h); a (b); [b : h; v; ; ; w ; t ; ; ]} Whereby f=feed; h=thickness of cut; a=engagement; b= width of cut;

v=cutting speed;=rake angle;=cutting edge angle;w=workpiece material; t=tool material;=cooling and lubrication and =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

{h=1, b=1; A=1} Log kc Kc1.1 Log h h=1 Relationship between kc and h Kienzle (1952) gave the following relationship: k c1.1 kc z h Wherebykc1.1 is the principal value of the specific cutting force and z is a material constant Substituting kc in the older equation gives: kc1.1 Fc z b.h b.h.h z .kc1.1

h 1 z or Fc b.h .k c1.1 Today Kienzles work has been extended to cover the other two cutting forces i.e. 1 y .k f 1.1 For feed force and 1 m

.k p1.1 For passive force F f b.h Fp b.h 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 1. 2. 3. 4. 5. 6.

Experimental determination of Workpiece Material Cutting Constants For constant depth of cut a and cutting edge angle X, feed is varied while the cutting forces are measured using cutting force dynamometers. All the results are tabulated as shown in Table 5.2 Calculate b=a/sin Calculate h=fsin Calculate Fc/b 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 graph tan=1-z and Kc1.1 is the value of the Fc/b where log h=1 Fig. 5.8 Experimental determination of material cutting constants 1-z and Kc1.1

6 Cutting Power Requirement 6.0 Introduction Power is the rate of doing work (Joules per second). It is the scalar Product of Force and Velocity. F . v {Fx ; Fy ; Fz }{v x ; v y ; v z } FR .v F R {Fc ; F f ; Fp }

v {vc; v f ; v p } Pc {Fc ; F f ; Fp }{v; v f ;0} Fc .v F f .v 0 Fc .v F f .v f Vf (Feed force) is normally very small (negligible) compared to v (cutting velocity), therefore, for practical purposes: Pc Fc .v LOSSES: Machine tool Drive (D) Electric Motor (el) Pel

PD Metal cutting PC 1. Electrical losses in the electric motors and circuits (el = 2. Mechanical losses in the machine tool (D = 0.7..0.85) PD PC Pel el el . D 6.1 Power Requirements for Turning, Shaping, Planing and Slotting Major cutting forces are defined as follows Main Cutting Force 1 z

Fc b.h .k c1.1 1 y Feed Force F f b.h .k f 1.1 Passive Force Fp b.h1 m .k p1.1 The cutting velocities are defined as follows: Cutting velocity: v=dn in m/min Feed Velocity vf=fn in mm/min Passive 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. 2. 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=dn/1000dn/1000 m/min; 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 The thickness of cut h is measured in the direction perpendicular

to the lip: h=ft.sin(/2)=ft.sin; =point angle=2 3. 4. 5. The width of cut b is measured along the lip and is equal to the length of the lip: b=d/(2.sin ) in mm) in mm The area of cut A per lip is: f d fd At h.b . sin . 2

2. sin 4 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 Drilling 1. The engagement a in core drilling is d do a 2 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 b d do

2 sin In mm 4. The area of cut A for one lip is: At b.h f d do 4 In mm2 6.2.3 Forces acting on a twist drill There 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 on the axis of the rotation (radius is almost zero) Thrust Force The total thrust force Fth is: Fth=2Ff+Fch+Ffr Ff: Feed force Fch:Force on the chisel edge Ffr: 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%. The Feed Force The 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 F f 1 F f 2 d f . sin 1 y { } .k f 1.1 2 sin 2 (b) Core drilling F f 1 F f 2

d d o f . sin 1 y { } .k f 1.1 2 sin 2 Torque The 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% Calculation of the Torque The 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 full x=0.26d..0.41d for core boring For all practical purposes this distance should be assumed as: x=0.5d With this assumption the drilling torque is then T=0.5dFc1 =0.5dFc2 for drilling in full and for core drilling (boring) T (d o d do d do ) Fc1/ 2 ( ) Fc1/ 2 2 2 6.2.4 The cutting Power Requirement for Drilling and Core Drilling

The drilling power is calculated from the torque as follows: Pc=Cutting torque X angular velocity =T=T.2dn/1000n 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 tools rotational path and remains dormant for the rest Fig. Main Features of the Milling Process

The milling operation can be classified into two major groups: (1) Vertical Milling or Face Milling (2) Horizontal Milling or Plain Milling The latter is also grouped into Up Milling and Down Milling 6.3.1 Cutting Variables and the Definition of the Cut in Vertical Milling 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 B: Cutting Height (Width of the Workpiece)

D: Diameter of Milling Head t: Number of cutting edges (teeth) 4. ] = 901: Cutting angle (feed motion angle) on entry 5. ] = 902: Cutting angle (feed motion angle) on departure. 6. ] = 90c = ] = 902 -] = 901: Cutting arc angle 7. ft : tooth feed 8. fc: Cutting feed 9. e: eccentricity i.e. distance between workpiece and milling head centres Note: For good cutting the ratio between B and D must be: B:D = 3:4 or B = 0.6D 1. 2.

3. Definition of the Cut in Vertical (face) Milling The dimension of the cut in vertical (face) milling is shown in Fig. 6.3.3 1. Thickness of cut h The thickness of cut in vertical (face) milling varies with the cutting arc angle ] = 90c or the feed motion angle ] = 90 . It is zero when the cutting edge enters the workpiece, when ] = 90 = 0 and increases to maximum value when ] = 90=90o after which the value decreases to another minimum value on leaving the workpiece, when ] = 90= ] = 90c. It follows therefore, that:

h=f(] = 90) And from triangles in Figures 6.3.2 follows that: h=fc.sin From the diagram on the right follows: fc sin ft Hence f c f t . sin Substituting this value in equation 2 gives: h(] = 90)=ft.sin] = 90.sin 2. The Tooth Main Cutting Force in Vertical (Face) Milling Using the main cutting force equation, the tooth cutting force in milling can be written as Fct ( ) b.( f t. sin . sin )1 z .kc1.1

Using the specific cutting force: Fct()=b.f)=b.ft.sin )=b.f.sin .kc The relationship between Fct()=b.f) and )=b.f 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 The mean cutting force of a tooth is Fcmt 2 1 b. f t . sin . sin .kc .d 2 1 1

The above equation can be written as: Fcmt Which is 2 1 b. f t . sin .k c sin .d c 1 1 Fcmt b. f t . sin (cos 1 cos 2 ).kc c cos 1

B1 2B 1 D2 D 2 90o Fig. Tooth main cutting force as of the feed motion angle cos 2 cos(90 o ) sin B2 2 B2 D/2 D

Equation 5 becomes Fcmt 1 B1 B2 .b. f t . sin .2( ).kc c D 1 2B .b. f t . sin . .kc c D )=b.fc in radians c _ in _ radians

Fcmt Fcmt 2 o 1 . c co 360 57.296 57.296 2B . b . f .

sin . .k c t o c D 114 .6 B o .b. f t . sin .kc . c D Note that only a few teeth are engaged at any given time. So if nt= number of teeth of the milling cutter ta=average no. of teeth engaged at any given time

= angle between two adjacent teeth Then the following relationship has been proved to hold: co .nt co ta 360 3. The mean main cutting Force Fcm Fcmt .t a 4. The Cutting Power in Vertical Milling Pc Fcm .v Fcmt .t a .v 6.3.2 Cutting Variables and Definition of the Cut in Horizontal Milling (Plain Milling) Cutting Variables The following features

distinguish Horizontal Milling from Vertical Milling regardless of whether it is up milling or down milling (i) (ii) (iii) ] = 901=0 ] = 902= ] = 90c; Hence cos ] = 902=cos ] = 90c b=B For better cutting, the centre of the milling head arbour is higher up than the engagement cos 2 cos c D/2 a

2a 1 D/2 D Definition of cut The area of cut A in horizontal milling is equal to the area of the arc extended by an angle ] = 90c at the centre. The area of such an arc is given as: Length of the arc x mean width (thickness) L x hm D L . 2 2

Hence D A a. f t hm . c . 2 hm From which 2.a. f t 114 .6 a o . ft . c .D c D For horizontal milling, therefore, the mean tooth main cutting force is: 114 .6

a Fcmt b.hm .kc B.hm .k o .B. f t . .kc c D the cutting power is Pc Fcm .v Fcmt .t a .v 7 Tool Failure and Tool Life 1. 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. 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 Wear Major factors influencing the type and degree of tool wear are: (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 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 oxychlorides. 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 Life Effective 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 either 1. Catastrophic failure or 2. VB=0.3 mm if the flank is regularly worn in zone B 3.

VBBmax=0.6 mm, if the flank is irregularly worn, scratched, chipped, or badly grooved in zone B Sintered CARBIDE tools are either: 1. VB=0.3 mm, or 2. VBBmax=0.6mm, if the flank is irregularly worn, or 3. KT=0.06+0.3f, where f is the feed 7.5.1 Tool Life Graph and Tool Life Equation Tool 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

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 1 log v1 log v2 n tan log T2 log T1 7.5.2 Economical Tool Life The 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 life 2. t2 = minutes equivalent to sharpen the tool 3. t3 = minutes equivalent to depreciation of the tool The following equations have been proved to hold: t s xRs t2 N 2 xRc Whereby ts: tool grinding time Rs: Labour and overhead rate in tool grinding department Rc: Labour and overhead rate in metal cutting department N2: Number of tool cutting edges CT t3

N1 xN 2 xRc Whereby CT: Tool cost N1: Number of times the tool can be sharpened including once when it is made N2: 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.e tc = t+T The amount of metal removed during this time is Qtc = v.T.f.a Applying Taylors equation gives Qtc c.T 1 n . f .a The rate of metal removal is then Qtc c.T 1 n . f .a R

tc t T For economical machining, the rate of metal removal must be maximum. This means mathematically dR d c.T 1 n . f .a 0 dT dT t T 1 n [t n(t T )] 2 T (t T ) From which we get

1 Tec 1 t n For the condition sharpening is done when the cutting process continues, the economical tool life equation becomes: 1 Tec1 1 t1 n 8 Cutting tool Materials 8.0 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 H tool 1.35 1.5 H work mod ified In contrast to the static hardness ratio: H tool 1.35 1.5 H work

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 Rigidity 8. 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. Cheap No single cutting tool material satisfies these requirements. Some of the requirements are contradictory Example: 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 FERROUS 1. Carbon Tool Steels

2. Alloy Tool Steels 3. High Speed Steels NON-FERROUS ABRASIVES 1. Cast Alloys 2. Cemented Carbides 3. Ceramics 4. Diamond 1. 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 and 2. a structure called martensite that is obtained through heat treatment Tools made out of these steels can cut up to only 200-250 oC, 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 matrix

Applications: milling cutters, twist drills, hand tools, thread cutting tools, turning tools for easy to cut materials (wood, magnesium, aluminium) 8.2.2 Alloy Tool Steels They are steels with the following additions of alloying elements Alloying 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. Martensite Heat Treatment: Similar to carbon tool steels. Contents of hard particles: 5-10% Hardness: Due to carbides and martensitic matrix. Cutting Temperature: 250-300oC Cutting 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:

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. (1) 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 1250-1290 oC; cooling in a jet of compressed air or bath of molten lead to 620 oC, then to room temperature. This is followed by a tempering treatment below 600 oC. Contents of hard particles: 20-25(30)% Hardness: Due to carbides and martensitic matrix Cutting 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: American: British: W-based T1-----T15 BT1---BT40 Mo-based M1-----M20 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 Introduction There 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 Structure and Chemical Composition Non-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 Carbide forming Elements Metallic Matrix C Cr, Mo, V, W Fe, Ni, or Co 1.5-2.5%

40-50% 45-50% Melting and Casting NON-FERROUS CAST 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, nonferrous metals. 8.2.5 Cemented Carbides Introduction Cemented 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. Manufacturing Cemented 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 1600 oC Chemical Composition Cemented Carbides consists of: (i) 75-95% hard particles (carbides) (ii) 5-25% Cobalt binding metal The more the Cobalt the tougher the material. Types of Cemented Carbides There 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. The groups are given international identification letters and colours and the Various grades are given international numbers: Identification letter Identification Colour Grade P Blue 01-------50

Cutting of steels and steel castings K Red 01-------40 Cast iron and nonferrous metals e.g. brass M Yellow 10-------40 Exotic metals and alloys

which are difficult to cut, such as Titanium NOTE: Application (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 8.2.6 Ceramics Introduction Ceramic 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. Manufacturing Ceramic cutting tool materials are manufactured through SINTERING as elaborated above. Types of Ceramics Four major types: 1. Pure Aluminium Oxide 2. 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) 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) Introduction Does 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. Manufacturing The synthesis of CBN involves structural transformation of Boron Nitride from HEXAGONAL to CUBIC form at high pressure, high temperature plus a catalyst. Properties (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 1000 oC Applications A 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 Materials 9.1 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. ASMEs 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 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. (i) 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 Tool wear-cutting time method Tool wear of different workpiece materials are measured after a fixed time interval. Provides good bases for comparing the machinability of the materials. Tool wear-cutting speed method Most 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 preselected 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. Cutting temperature method Less 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. 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 machinability Forces 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, k c1.1, is used as a measure for the machinability. 9.3.3 Chip formation as criterion for machinability It 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 chips viii. 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 machinability A 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 material Material with the lowest score for all four criteria is the least machinable and material with the highest score is the most machinable

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