Chapter 1

METAL CUTTING

History of Metal Cutting:

The history of metal cutting dates from the later part of the eighteenth century. Before that

time machine tools did not exist, and the following extract from the diary of an English engineer,

Richard Reynolds, dated October 1760, gives some idea of the manufacturing problems that had

to be faced. Richard Reynolds was attempting to produce a cylinder for a fire engine for drawing

water from a coal pit. The cylinder, of cast brass, had a length of 9 ft and a bore 28 in. in

diameter. He wrote:

Having hewed two balks of deal to a suitable shape for the cylinder to lie therein solidly on the

earth in the yard, a plumber was procured to cast a lump of lead of about three hundred

weight, which being cast in the cylinder, with a dike of plank and putty either side, did make it

of a curve to suit the circumference, by which the scouring was much expedited. I then

fashioned two iron bars to go around the lead, whereby ropes must be tied, by which the lead

might be pulled to and fro by six sturdy and nimble men harnessed to each rope, and by

smearing the cylinder with emery and train oil through which the lead was pulled the

circumference of the cylinder on which the lead lay was presently made of a superior

smoothness; after which the cylinder being turned a little and that part made smooth, and so

on, until with exquisite pains and much labor the whole circumference was scoured to such a

degree of roundness as to make the longest way across less than the thickness of my little

finger greater than the shortest way; which was a matter of much pleasure to me, as being

the best that we so far had any knowledge of.

In 1776 James Watt built the first successful steam engine, and one of his greatest difficulties

in developing this machine was the boring of the cylinder casting. His first cylinder was

manufactured from sheet metal, but it could not be made steam tight. Even attempts to fill the gap

between the piston and the cylinder with cloth, leather, or tallow were of no avail. John Wilkinson

eventually solved the problem when he

invented the horizontal-boring machine. This

machine consisted of a cutting tool mounted

on a boring bar that was supported on

bearings outside the cylinder. The boring bar

could be rotated and fed through the

cylinder, thus generating, with the tool

points, a cylindrical surface independent of

Fig.1.1 Generation of a cylindrical surface in the irregularities of the rough casting (Fig.1).

a horizontal boring This boring machine was the first effective

machine tool, and it enabled James Watt to produce a successful steam engine.

This was followed by Henry Maudsley’s engine lathe in 1794. A later machine tool to be

invented was the planer by Roberts in 1817. Maudsley combined a lead screw, cross slide and

change gears in a form, which is very similar to the current day center lathe. At the same time

another major machine tool to be invented was the milling machine by Eli Whitney in 1818.

The drill press was the next machine tool to be developed around 1840 by John Nasmyth.

Stephan Fitch designed the first turret lathe in 1845. It carried eight tools on a horizontally

mounted turret for producing screws. Christopher Spenser invented a completely automatic turret

lathe in 1869. This was the first form of automatic lathe utilizing cams for feeding the tool in and

out of the workpiece, thereby automating most of the machining tasks. Spenser is also credited

with the development of a multiple spindle lathe. Finally the surface grinder was developed

around 1880. This completed the development of almost all basic types of machine tools.

Over the years, basic machine tools have been refined by various attachments and

automation of their movements. The invention of various precision measurement techniques has

also helped in improving the accuracy and productivity of machine tools.

Manufacturing technology has undergone major technological changes through various

developments in microelectronics. The availability of computers and microprocessors has

completely changed the machine tool scenario by bringing in the flexibility, which was not

possible through conventional mechanisms. The development of Numerical Control in 1952

brought about a kind of flexibility to the metal cutting operations. At present a number of

manufacturing processes are making use of these principles in some form or the other. This

allows for just in time manufacturing leading to zero inventories, zero setup times and single

component batches without losing any advantages of mass manufacturing.

Of all the manufacturing processes available, metal removal is probably the most expensive

one. This is because a substantial amount of material is removed from the raw material in the

form of chips in order to achieve the required shape. Also, a lot of energy is expended in this

process. Hence the choice of material removal as an option for manufacturing is considered when

no other manufacturing process suits the purpose. However, invariably all components undergo a

material removal operation at some point. Machining of metals is basically adopted to get higher

surface finish, close tolerance, and complex geometric shapes, which are otherwise difficult to

obtain.

The machine used for removing excess material from the workpiece is called a machine tool

and can be defined as a device in which the energy is expended in removing the excess amount

of material from the workpiece in the form of fine chips. Machine tools are mother machines,

since without them no component can be finished and the success of the industrial revolution can

be mainly attributed to them.

The principle used in all machine tools is one of generating the surface required by providing

suitable relative motions between the cutting tool and the workpiece. The simplest surfaces to

generate are flat surfaces and internal or external cylindrical surfaces. For example, if a tool is

reciprocated backward and forward in a straight line (generatrix: the line generated by the cutting

motion) and a workpiece is incrementally fed beneath the tool in a direction at right angles to the

motion of the tool (directrix: the line generated from the feed motion), a flat surface will be

generated on the workpiece. Similarly a cylindrical surface can be generated by rotating the

workpiece (generatrix) and feeding the tool parallel to the axis of workpiece rotation (directrix) or

by rotating a tool (generatrix) and feeding (directrix) into the stationery workpiece, a cylindrical

surface will be produced.

The machine tools can be divided into three groups:

1. Those using single point cutting tools

2. Those using multipoint tools

3. Those using abrasive wheels

Importance of Machining

There are several methods of changing the geometry of bulk material to produce a

mechanical part:

1. By putting material together (+) … Welding, etc.

2. By moving material from one region to another (0) … Rolling, Forging, and Extrusion

3. By removing unnecessary material (-) … all metal cutting operations

Metal cutting or machining is the process of producing a workpiece by removing unwanted

material from a block of metal, in the form of chips. This process is most important since almost

all the products get their final shape and size by metal removal, either directly or indirectly. The

major drawback of the process is the lost of material in the form of chips. In spite of these

drawbacks, the machining process has the following characteristics:

1. They improve the dimensional accuracy and tolerances of the components produced by

other processes.

2. Machining processes can produce internal and external surface features, which are

difficult or not possible to be produced by other processes.

3. Specified surface characteristics or texture can be achieved on a part or whole of the

component.

It may be economical to produce a component by machining process.

Research in metal cutting did not start until approximately 70 years after the introduction of

the first machine tool. Up to nineteenth century, metal cutting was more of an art than a science.

Each machinist had his own method of machining metals, and grinding the cutting tools. Many of

these methods were primitive. A change in the existing trends of machining was noted at the

beginning of the twentieth century when the metal machining was viewed on scientific lines due to

many sided work of notable researchers like I.Thime of Russia and Franklin Winslow Taylor of

USA, etc.

According to Finnie, who published a historical review of work in metal cutting, early research

in metal cutting started with Cocquilhat in 1851 and was mainly directed toward measuring the

work required to remove a given volume of material in drilling. In 1873 Hartig presented

tabulations of the work required in cutting metal in a book that seems to have been the

authoritative work on the subject for several years.

The shavings, or swarf, removed during the cutting of metal are called chips, and the first

attempts to explain how chips are formed were made by Time in 1870 and famous French

Scientist Tresca in 1873. Some years later, in 1881, Mallock suggested correctly that the cutting

process was basically one of shearing the work material to form the chip and emphasized the

importance of the effect of friction occurring on the cutting-tool face as the chip was removed. He

produced drawings of chip formation from polished and etched specimens of partly formed chips;

in addition, he observed the effect of cutting lubricants, the effect of the tool sharpness on the

cutting process, and the reasons for instability in the cutting process that leads to undesirable

vibrations, or “Chatter”. Thime formulated the principal laws of metal machining. Many of these

observations are surprisingly close to the accepted modern theories and are still being repeated

more than 100 years later.

Taylor did some fundamental work on the tool-life and cutting speed relationship. Taylor was

mostly interested in empirical research and 30 years of the results of his experimental work was

published in transactions of ASME in 1907, which numbered to around 300 pages. Taylor was

interested in the application of piecework systems in machine shops, where a time allowance was

set for a particular job and a bonus was given to the workman performing his task in the allocated

time. To assist in the application of such a system Taylor investigated the effect of tool material

and cutting conditions on tool life during roughing operations. The principal object was to

determine the empirical laws that allow optimum cutting conditions to be established. In a

biographical note at the beginning of the paper it was reported that with a combination of the

development of a heat-treatment process to produce high-speed, steel cutting tools (known as

the Taylor-White process of heat treatment) and the results of empirical research to improve shop

methods, Taylor was able to improve shop methods, Taylor was able to increase the production

of the Bethlehem Steel Company’s machine shop by 500 percent. It is of interest to note that the

empirical law governing the relationship between cutting speed and tool life suggested by Taylor

is still used today and employed as the basis for many recent studies of machining economics.

One fundamental discovery made by Taylor was that the temperature existing at the tool cutting

edge controlled the tool wear rate. Due to this humble start people on the shop floor and the

production engineers realized the importance of doing the job on rationale basis rather based on

their intuition.

Since 1906 when Taylor’s paper was published, empirical and fundamental work on metal

cutting has gradually increased in volume. Ernst and Merchant have carried out most of the

fundamental work since the publication of the well-known paper in 1941, dealing with the

mechanics of the process. The main results of this more recent work are discussed in the later

chapters.

Classification of Material Removal Operations

The entire field of material removal may be divided into the following categories mainly in

terms of the size of the individual elements removed:

Cutting, Grinding, Special techniques.

Cutting operations involve the removal of macroscopic chips in the form of ribbons or

particles having a thickness of from about 0.025 mm (10-3 in) to 2.5 mm (10-1 in). A wide range of

kinematic arrangements briefly discussed in the next chapter are involved in cutting. Grinding

operations usually involve subdivision of the material removed into smaller particles that} in

cutting. Grinding chips will usually range in thickness from 0.0025 mm (10-4 in) to 0.25 mm (10-2

in). Other removal techniques such as electrochemical machining (ECM), electro discharge

machining (EDM), ultrasonic machining (USM), or electron beam machining (EBM) involve chips

of atomic or submicroscopic size.

Overview of the Cutting Process

The chisel (Fig. 1.13) was probably one of the first cutting tools used by man. The earliest

stone implements were undoubtedly blunt as shown in Fig. 1.13(a) but as experience was gained,

the importance of three basic angles became apparent-the rake angle ( ), the clearance angle (

), and the setting angle ( ) (Fig. l.l3 (b )). Modern tools have a wide variety of forms and their

geometry and kinematics differ considerably from those of the chisel. However, all of our modem

tools have effective rake, clearance, and setting angles.

Fig.1.13 Probable forms of early cutting implements

Certain superficial observations may be made by merely observing a metal cutting tool in

operation. These include:

1. The basic difference between the cutting of wood 'and metal. Formerly it was believed that

when metal was cut, the material merely split off in front of the tool as the tool advanced-

like the chip formed when an axe splits a log. When thickness of a metal chip is measured

and compared with the depth of layer removed. it is found that the chip is thicker than the

actual depth of layer removed and the chip correspondingly shortened.

2. There is essentially no flow of metal at right angles to the direction of chip flow. For the

purpose of simplifying the geometry involved in cutting, it is advantageous to start with a

two dimensional process. We then need to consider what happens in but one

representative plane. Although most cutting operations involve tools and processes, which

are not strictly two dimensional, many processes such as planing, sawing, and certain

turning operations are essentially two-dimensional. Later we will briefly discuss the

complications introduced by the three dimensional aspects of cutting tools.

3. Flow lines are evident on the side and back of a chip. These lines suggest that cutting

involves a shearing mechanism.

4. Some chips are in the form of a continuous ribbon while others are discontinuous, being

composed of individual segments.

5. The chip, tool, and workpiece are hot to the touch. Considerable thermal energy is

associated with the cutting process.

The significance of these observations

will be discussed in subsequent chapters.

A photomicrograph of a partially formed

chip reveals much concerning the cutting

process (Fig. 1.14). Such

photomicrographs are obtained in the

following manner. During the course of a

cutting operation, the tool is brought to a

sudden stop. Then the tool is carefully

removed leaving a partially formed chip

attached to the workpiece. The section of

the metal in the vicinity of the partially formed chip is cut from the workpiece and mounted in

plastic for convenience in handling. The mounted specimen is ground and polished to produce a

very smooth flat surface. Next, the polished surface is etched with a fluid such as a 1 percent

mixture of nitric acid in alcohol, which reacts at different rates with the different components of the

metal. The etched surface of the specimen is photographed through a microscope. Examination

of a photomicrograph of a partially formed chip such

as that of Fig. 1.2 reveals:

1. There is generally no crack extending in front

of the tool point.

2. A line separates the deformed and

undeformed regions. Line AB in Fig. 1.14

divides the work from the chip. The material

below this line is undeformed. The chip or

Fig.1.14 Photomicrograph of partially formed chip

Fig.1.15 Orthogonal (two dimensional) cutting

process

material above the line has been deformed by a concentrated shearing process. When

the line AB is projected perpendicular to the paper and parallel to itself, it describes what

is known as the shear plane (Fig. 1.15), making an angle with the direction of cut.

3. The chip is in intimate contact with the rake face of the tool from A to C and is subject to

a substantial shear stress sufficient to cause the

secondary subsurface to shear (as evident in Fig.

1.14 along the tool face).

4. The rate at which metal is deformed along shear

plane AB is high as a consequence of the thinness

of the region in which shear occurs.

5. A stationary nose or built-up edge (BUE) such as

that shown in Fig. 1.16 sometimes forms at the tip

of a tool and significantly alters the cutting

process. The BUE is one of the major sources of

surface roughness and also plays an important

role in tool wear. Its cause and significance will be

treated in detail in a later chapter.

The friction between the chip and the tool plays a significant role in the cutting process. This

friction may be reduced by:

1. Improved tool finish and sharpness of

the cutting edge.

2. Use of low-friction work or tool

materials.

3. Increased cutting speed (V).

4. Increased rake angle ( ).

5. Use of a cutting fluid.

When tool face friction is decreased there is a corresponding increase in shear angle and an

accompanying decrease in the thickness of the chip. The plastic strain in the chip decreases as

the shear angle increases (Fig. 1.17). Also, the length of the shear plane is seen to be

significantly decreased as the shear angle increases. The force along the shear plane will vary as

the area of the shear plane, assuming the shear stress on the shear plane remains constant.

The temperature of a cutting tool may reach a high value particularly when a heavy cut is taken

at high speed. This is evident when the work or tool is touched, by the presence of temper colors

on the chip, work, or tool, or may even be evident due to the loss of hardness of the tool point

with an attendant loss of tool geometry and failure by excessive flow at the cutting edge.

Fig.1.16 Photomicrograph of partially formed chip showing large BUE

Fig.1.17 Effect of shear angle on chip thickness and length of shear plane for a given tool and undeformed chip thickness

The operational characteristics of a cutting tool are generally described by a single word-

machinability. There are three main aspects of machinability:

1. Tool life.

2. Surface finish.

3. Power required to cut

There are three regions of interest in the cutting

process. The first area shown in Fig. 1.18 extends

along the shear plane and is the boundary between

the deformed and undeformed material or the chip

and the work. The second area includes the interface

between the chip and the tool face, while the third area includes the finished or machined surface

and the material adjacent to that surface. We are primarily interested in the plastic deformation

characteristics of the material cut in the first area, the friction and wear characteristics of the tool-

work combination in the second area, and the surface roughness produced and surface integrity

in the finished surface constituting the third area.

Single-Point Tools:

Single-point tools are cutting tools having one cutting part (or chip producing element) and

one shank. They are commonly used in lathes, turret lathes, planers, shapers, boring machines

and similar machine tools. A typical single-point tool is illustrated in Fig. 1.2. The most important

features are the cutting edges and adjacent surfaces. These are shown in the figure and defined

as follows:

1. The face is the surface over which

the chip flows (Rake Face).

2. The flank is the tool surface or

surfaces over which the surface

produced on the workpiece

passes.

3. The cutting edge is that edge of

the face, which is intended to

perform cutting.

a) The tool major cutting edge is that entire part of the cutting edge which is intended to

be responsible for the transient surface on the workpiece. It is also known as side

cutting edge.

b) The tool minor cutting edge is the remainder of the cutting edge. It is also known as

end cutting edge.

Fig.1.2 A typical single point cutting tool

Side flank

End (auxiliary)Cutting edge

End flank

Nose

Side (main)Cutting edge

Fig.1.18 Principal Areas of interest in machining

4. The corner is the relatively small portion of the cutting edge at the junction of the major

and minor cutting edges; it may be curved or straight, or it may be the actual intersection

of these cutting edges.

A single-point cutting tool may be either right or left hand cut tool depending on the direction

of feed. In a right cut tool, the side cutting edge is on the side of the

thumb when the right hand is placed on the tool with the palm

downward and the fingers pointed towards the tool nose (Fig.1.3 b).

Such a tool will cut when fed from right to left as in a lathe in which the

tool moves from tailstock to headstock. A left cut tool is one in which

the side cutting edge is on the thumb side when the left hand is applied

(Fig.1.3 a). Such a tool will cut when fed from left to right.

Tool Geometry:

The most important features of cutting tool are the cutting edges,

the faces and the flanks. The tool geometry and the nomenclature of

cutting tools and even the single point tools, is a complicated subject. The difficulty is to find that

in which planes the angles are important and in which planes they can easily be measured.

To determine the orientation and inclination of the rake face and flank surfaces, a coordinate

system is essential, resulting in a set of planes with reference to which the orientations or

inclinations can be determined. In order to define the geometry of a single point tool, a few

convenient reference planes must be defined so as to locate its parameters.

Reference Planes

Case-I: When tool is not operating on the workpiece but held in position in the space (Tool in

Hand System).

A horizontal plane containing the base of the tool shank may be defined and termed as Base-

Plane. The second reference plane may be fixed up along the primary motion (feed) of the tool. It

can be named as longitudinal plane, X-X’ and is perpendicular to the base plane. The third

reference plane can be the one that is perpendicular to both of the above planes. It can be called

as traverse plane (Y-Y’). All the angles of the tool for the preparation of cutting tool are defined in

reference to this system of planes. This system of planes can be called Coordinate System of

Planes.

Fig.1.4 Tool & Work in Coordinate system

Fig.1.3 Left & Right Cut tools

Fig.1.5 Tool & Work in Orthogonal system

Case-II: When the tool is operating on a workpiece fixed on the machine

In this case, tool reference planes are used as the reference, which may be again chosen as

three mutually perpendicular planes. The first plane is defined as the Base Plane as in Case-I.

In order to include the cutting action, the second plane can be defined as one, which has the

cutting edge. Thus it can be termed as Cutting Plane and is also perpendicular to the Base Plane.

The third reference plane is perpendicular to the two reference planes mentioned above and can

be termed as Orthogonal Plane. All the angles of the tool for analytical treatment are defined in

reference to this system of planes. This set of reference planes is called “Orthogonal System of

Planes”.

Tool Nomenclature Systems:

American Standards Association (ASA) System:

This is also known as Tool Geometry in Coordinate System or Tool in Hand System.

For simple turning this system is illustrated in Fig.1.6

Fig. 1.6 Tool angles in coordinate system

The following scheme is adopted to describe the tool signature in ASA system:

γy γx αy αx φe φs r

Back rake angle

Side rake angle

End relief angle

Side relief angle

End cutting edge angle

Side cutting edge angle

Nose radius

Orthogonal Rake System (ORS):

This is also known as DIN (German) System.

For simple turning this system is shown in Fig.1.7

Fig. 1.7 Tool angles in Orthogonal system

λ γ0 α0 α0’ φe φs r

Inclination angle

Orthogonal rake angle

End relief angle

Side relief angle

End cutting edge angle

Side cutting edge angle

Nose radius

Inter-relation between ASA system and ORS:

Sign Convention for various angles:

The angle of inclination is the angle

included between the cutting edge and a line

passing through the tool nose parallel to the

basic plane. The angle of inclination (also

called the slope angle) is considered to be positive if the

nose is the highest point of the cutting edge (Fig.1.8) .It

is equal to zero if the cutting edge is parallel to the basic

plane (Fig 1.8 b) and negative if the tool nose is the

lowest point of the cutting edge.

Fig.1.8 Angle of inclination of the side cutting edge

Fig.1.9 Sign convention for tool geometry

+ve X

+ve Y

+veve

xCot

yCot0Cot

If the rake plane staring from the tip of the tool is going downwards towards the shank, then

γy is +ve. Similarly, if the rake plane is going downwards starting from the end cutting edge is

going downwards towards the side cutting edge, then γx is +ve. Otherwise, they are considered

as –ve.

A single point cutting tool with positive rake angles is considered for simplicity. Start from a

point A on the rake face and travel vertically downwards (parallel to the velocity vector V, the

curvature of which may be ignored if the depth of cut is small compared with the work diameter)

to get point B. From B, travel along the axis of the tool to come out of the rake face at D, thus the

angle ADB representing γy. Similarly from point B, travel perpendicular to the tool axis to come

out of the rake face at C, thus angle ACB representing γx.

Draw the top-view of all the points, A, B, C and D to get a triangle ACD as shown in figure

below. Join the points C and D and extend, which represents a line with zero rake angle. Then

draw a line from A at an angle of φsd to cut the line CD at E. The line AE represents the side

cutting edge and thus the angle AEB represents λ .Now draw a line from point A perpendicular to

AE to cut the line CE at F. The angle AFB represents γ0. Draw a line perpendicular to CE and

passing through A to cut CE at H. Angle AHB represents the maximum rake angle. Now, it can be

said that the points O, B, G, C, F, H, D and E are coplanar. Therefore, the angle BFG is equal to

φsd.

The inclination angle λ may be found in terms of the known angles as follows. Since triangles

OCE and BCD are similar:

F

A, BO

E

D

CG

H

Plan view looking in direction of V

Looking ┴ toplane ABC

Looking ┴ toplane ABD

Looking ┴ toplane ABF

Looking ┴ toplane ABE

BA

DA

B

B

A

C

B

A

F

E

Fig.1.10 Relationship between lathe tool angles

……………………………….…(i)

From equation (i), it can be seen that the angle of inclination can assume positive or negative

values depending upon the values of , and . A special situation would exist when the

cutting plane lies in the reference and is not inclined towards or away from the work surface. This

condition from the above equation can be derived by putting i.e. when

Similarly, by considering the similar triangles BDC and GFC, it can be written as:

…………..……………………..(ii)

From the above, it can also be written as:

= and

The angle of inclination has considerable practical significance and determines the direction

of chip flow relative to the workpiece. For positive values of , the cutting edge slopes away from

the workpiece and hence the chip flow would be directed away from the fresh machined surface.

On the other hand for negative , the cutting edge slopes towards the machined surface and can

lead to poor surface finish on account of the chip flow being directed towards the fresh machined

surface.

Relation between Orthogonal and Normal Rake angles:

Consider a point A on the cutting edge. Travel vertically downwards, perpendicular to the

base of the tool to reach point B. Travel along the projection of the cutting edge to reach point E,

which is also on the cutting edge. Angle AEB represents the inclination angle, .

From point B travel perpendicular to the line BE to reach a point F on the rake face. The

angle AFB represents the orthogonal rake angle . It can be seen that the plane AFB is

perpendicular to the plane AEB.

Draw a line BP so that it is perpendicular to the cutting edge. Now the plane PFB represents

a plane perpendicular to the cutting edge AE. The angle PFB represents the normal rake angle

. From the geometrical representation, it can be seen that the inclination between the planes

AFB and PFB is equal to . Now, a relation between the orthogonal rake angle and the normal

rake angle can be established as follows:

Positive and Negative Rake Tools:

The rake angles may be positive, zero or negative. Larger the rake angle, smaller the cutting

angle (and larger the shear angle) and lower the cutting force and power. However, since,

increasing the rake angle decreases the cutting angle, this leaves less metal at the point of the

tool to support the cutting edge and conduct away the heat. A practical rake angle represents a

compromise between a large angle for easier cutting and a small angle for tool strength. In

general, the rake angle is small for cutting hard materials and large for cutting soft and ductile

materials. An exception is brass, which is

machined with a small or negative rake

angle to prevent the tool from digging into

the work.

The use of negative rake angles

started with the employment of carbide

cutting tools. When we use positive rake angle, the force on the tool is directed towards the

cutting edge. Tending to chip or break it (Fig 1.12 a). Carbide being brittle lacks shock resistance

and will fail if positive rake angles are used with it. Using negative rake angles, directs the force

back into the body of the tool away from the cutting edge (Fig.1.12 b), which gives protection to

the cutting edge. The use of negative rake angle increases the cutting force. But at higher cutting

speeds, at which carbide tools are used, this increase in force is less than at normal cutting

P

A

B

A

P

B

E

F

Fig.1.11 Relationship between Orthogonal and Normal rake angles

Fig.1.12 Cutting with positive and negative rake tools

speeds. High cutting speeds are, therefore, always used with negative rake angles, which require

ample power of the machine tool.

The use of indexable inserts has also promoted the use of negative rake angles. An insert

with a negative rake angle has twice as many cutting edges as an equivalent positive rake angle

insert (as will be discussed ahead). So, to machine a given number of components, smaller

number of negative rake inserts is needed as compared to positive rake inserts.

The use of positive rake angles is recommended under the following conditions:

1. When machining low strength ferrous and non-ferrous materials and work hardening

materials.

2. When using low power machines.

3. When machining long shafts of small diameters.

4. When the setup lacks strength and rigidity.

5. When cutting at low speeds.

The use of negative rake angles is recommended under the following conditions:

1. When machining high strength alloys.

2. When there are heavy impact loads such as in interrupted machining.

3. For rigid setups and when cutting at high speeds.