CHAPTER SIX Fundamentals of Material Removal · CHAPTER SIX Fundamentals of Material Removal ... Its function is to eliminate friction between tool ... Tool angle of the tooth of

Production Technology Ch6: Fundamentals of Material Removal

MDP024 - Prepared By Dr. Mohamed Ahmed Awad - 123 -

CHAPTER SIXFundamentals of Material Removal

6.1 Introduction & Definitions

Material removal process (machining or material cutting) is

the process of removing layers of material from the surface

of the workpiece in order to obtain a machined part of the

required form, dimensions, and with the specified quality of

surface finish. Machining is the broad term used to describe

removal of material from a workpiece. Machining includes a

wide range of operations that remove material from a

workpiece in the form of chip.

6.2 Classification of Material Removal ProcessesMaterial removal processes may be classified into two main

groups; namely, traditional processes and non-traditional

processes, Fig. 6.1.

In traditional material removable processes, sharp edged,

wedged shaped cutting tool harder than the workpiece

material engages the workpiece to remove a layer of material

in the form of a chip. the material is sheared and deformed

under tremendous pressure. The deformed material then

seeks to relieve its stressed condition by fracturing and

flowing into the space above the tool in the form of a chip.



Traditional machining may be done manually or by using a

special machines. In manual machining the cutting tool is

held by hand and the cutting motion and power is provided

by the operator. In mechanical processes a machine is

designed and constructed to hold the cutting tool and the

workpiece and creates the relative cutting motion between

them.

Non-traditional material removal processes are diverse in

nature from the traditional ones by their characteristics

features, operations, fields of application, and design of its

machines. These new machining processes involve physical

phenomenon in metal removal where mechanical means are

applied in traditional ones. The most important characteristic

of the non-traditional processes is that the Material is

removed without contact between the tool and the

workpiece.

Fig. 6.1: Classification of material removal processes



6.3 Basic Requirements for Cutting processes.

In any cutting process (even in cutting an apple or peeling a

potatoes ), the following tasks must be done;

-Suitable cutting tool must be securely held.

-Workpiece must be securely clamped.

-Relative motions between tool and workpiece must be

created.

6.4 Mechanics of Cutting & Ship Formation

Fig. 6.2. shows a side view of the a tool wedge ( small

wedge shaped portion near to the cutting edge). Under the

action of the cutting force, the cutting edge penetrates the

workpiece.

Fig. 6.2: Mechanics of material removal



As a result of the advance of the tool wedge through the

workpice, the cutting force is transmitted along the shear

plane (ab) and thus the metal along this plane is subjected to

a shear force. If this force is quite enough to shear and

separate the material, the chip will begin to flow on the tool

surface, Fig. 6.3. If the workpiece material is brittle, like grey

cast iron or brass, there is a tendency for the shear action to

produce separate chip, as shown in Fig. 6.3, while soft

material will tend to form a more continuous chip.

Fig. 6.3: Effect of workpiece ductility on the deformed chip



6.5 Cutting Tool

Cutting tool is a very important element in machining

processes. Developing of harder tools leads to great

progress in metal cutting. During traditional machining, a

wide variety of cutting tools remove material in the form of

chips. Cutting tools are either single-point tools that use a

single cutting edge, or multi-point tools that have two or

more cutting edges, Fig. 6.4. A single-point tool often stays

in contact with the workpiece, while the cutting edges of

multi-point tools often enter and exit the workpiece.

Fig. 6.4: Single and multiple points cutting tool

Cutting tools are considered from two points of views; its

geometry, and its material.



6.5.1 Cutting Tool MaterialCutting tool material must possess the following properties;

otherwise it will not cut satisfactory:-

6.5.1.1 Properties

High Strength; the tool must withstand the mechanical

loads that will be subjected to during cutting process.

High Hardness; cutting tool must resist wear. This is

because chips will flow over the tool face. Moreover,

the tool edge, and the tool flank will be in contact with

the workpiece. The tool must also keep its hardness at

high temperature; (cutting temperature may exceeds

700 C)

Toughness; the tool should withstand the impact

forces that will be subjected to during cutting..

Generally, tool materials need to withstand high

temperatures, high forces, resist corrosion, etc..

Cutting tool are manufactured from a variety of

materials to suit different machining conditions. The

most common used material are given below :-



6.5.1.2 Types

High Carbon SteelHigh Carbon steel ( Tool steel ) of about 0.9 to 1.35%

carbon content is used in manufacturing hand cutting tool

(the tool which is held by hand). The hot hardness value is

low and this is the major factor in the tool life. The

maximum cutting speed is about 7 m/min. Tool steel has

Low cost and is suited to hand tools, and wood working.

Hand cutting tools such as; files, saws, chisel, markers,

are made from high carbon steel.

High Speed Steel (HSS)High-speed steel (HSS) tools are so named because they

were developed to cut at higher speeds. First produced in

the early 1900s. High-speed steels are the most highly

alloyed of the tool steels. They have good wear

resistance, and are relatively inexpensive.

Because of their high toughness and resistance to

fracture, high-speed steels are especially suitable for

weak tool, interrupted cuts, and for machine tool with low

stiffness that are subject to vibration and chatter.

It consists of alloyed steel with 14-22% tungsten, as well

as cobalt, molybdenum, chromium and vanadium. When

properly heat treated the tool properties will be improved

significantly The cobalt component gives the material a

hot hardness value much greater than Carbon Steels.



High speed steels account for the largest tonnage of tool

materials used today.

They are used in a wide variety of cutting operations

requiring complex tool shapes such as drills, reamers,

taps, and gear cutters It is used in all type of cutters, and

machine cutting tools such as; twist drill, turning tools,

milling cutters. Their basic limitation is the relatively low

cutting speeds when compared to carbide tool.

Cemented Carbides TipsTo meet the challenge of higher speed for higher

production rates, cemented carbide tips were introduced

in 1930. These tips are small pieces manufactured from

metals carbides, and mounted on steel shanks, Fig.6.5.

Fig. 6.5: fixation of cemented and ceramics tips

They exhibit a very high hardness., and used when the

cutting speed is very high. It is produced by sintering

grains of tungsten carbide in a cobalt matrix ( provides

toughness). Other materials are often included to increase



hardness, such as titanium, chrome, molybdenum, etc.

Compressive strength is high compared to tensile

strength; therefore the bits are often brazed or clamped to

steel shanks, or used as inserts in holders, Fig. 6.5. Hot

hardness properties are very good.

Ceramics TipsCeramic oxides, such as aluminum oxides are used to

manufacture the ceramic cutting tips. These tips exhibit

very high hot hardness properties. The tips are used as

inserts in special holders. It can be used for machining

most of metals. These tools are the best to be used in

finishing operation . Since there is no occurrence of

welding between the chip and the ceramic tips, the friction

is reduced and coolants are not needed.

DiamondsThe hardest substance of all known materials is diamond.

It has low friction, high wear resistance, and the ability to

maintain a sharp cutting edge. It is used when good

surface finish and dimensional accuracy are required,

particularly with soft nonferrous alloys and abrasive

nonmetallic materials. Special attention should be given to

proper mounting diamond crystal on the shank. Diamond

tools can be used satisfactorily at almost any speed, but

are suitable mostly for light, uninterrupted finishing cuts..

diamond is also used as diamond dust in a metal matrix

for grinding and lapping ( For example, this is used in the

finishing of tungsten carbide tools)



6.5.2 Cutting Tool Geometry

If a cutting tool is to shear or cut metal effectively it must

have three essential angles, otherwise it cannot cut

satisfactory These angles made the wedge shape of the tool

and are called rake angle (γ), clearance angle (α), and tool

angle (β), Figs, 6.6 to 6.9.

As shown in Fig. 6.6, the tool flank is the tool side facing the

workpiece. The tool face is the tool side over which the chip

is flow. The cutting edge is the intersection of the tool face

and tool flank.

Fig. 6.6: Pictorial view of tool wedge and tool angle



RAKE ANGLE (γ): Angle between tool face and the

perpendicular to the cutting motion. Its functions are to

facilitate chip removal, reduce cutting force, and improve

surface finish. The measure of rake angle depend on the

material of the cutting tool, material of the workpiece, and the

type of cutting operation. Rake angle may take a positive

value up to 30 degrees, but unfortunately in some cases a

negative value must be assigned to it.

CLEARANCE ANGLE (α): Angle between tool flank and

workpiece. Its function is to eliminate friction between tool

and workpiece. Its measure is about of 4 to 6 degrees.

Cutting tool could not cut without the existence of clearance

angle.

TOOL ANGLE (β): Angle between tool face and the tool

flank. It determines the strength of the tool.

Note: It must be noted that the sum of the three angles;rake, clearance, tool angles is equal to ninety degrees.

γ+ α + β =90 degrees Eq.6.1



Fig. 6.7: Cutting tool geometry

Fig. 6.8: Tool angle of the tooth of the blade of a hacksaw

Fig. 6.9: Tool angle of the tooth of the file



6.6 Cutting Motions

Relative motion between cutting tool and workpiece may be

analyzed into three motions; namely, depth of cut motion,

cutting motion, feed motion.

Fig. 6.10: Relative cutting motions between tool & workpiece

6.6.1 Depth of Cut (a)It is a motion given in the direction perpendicular to the

machined surface, Fig. 6.10. It sets up before the start of

cutting to determine the size of the machined parts.

Increasing the depth of cut results in increasing rate of metal

removal, and consequently power consumption

6.6.2 Cutting motion (cutting Speed)It is the motion, which causes the chip removal, Fig.6.10.

Increasing cutting speed results in improving workpiece

surface finish, but reduce the tool life. The value of the



cutting speed is comparatively high ranging from less than

5m/min in manual work up to to about 3000m/min.

6.6.3 Feed Motion (Feed Rate)It is the motion, which copies the cutting action on the

machined surface., Fig.6.10. Increasing the feed motion

results in increasing the rate of metal removal, but on the

account of workpiece surface quality. Feed motion is a slow

motion 1 mm/min up to about 100mm/min

Note: Cutting motion, and feed motion are givensimultaneous during cutting process.

The principle of generating a machined surface by a

combination of tool and workpiece movements is illustrated

in Fig.6.10. generation of plane surface is achieved by a

reciprocating motion of the tool along the length of the

workpiece (cutting motion), together with a linear motion of

the workpiece in a perpendicular direction to the tool path.

Prior to the start of machining process, the tool is made to

touch the workpiece surface, Fig. 6.11a. The tool is then

drawn away from the workpiece to a clear position and the

tool is approach to the work perpendicular to its surface by

the amount of the depth of cut, Figs. 6.11b, and 6.11c. The

reciprocating motion (cutting motion), and the feed motion is

engaged simultaneously to generate the required surface,

Fig 6.11d.



Machines are always equipped by means to change the

cutting and feed motion values. This is to adapt for the

different workpiece materials, different workpiece sizes and

different cutting operations.

:

Fig. 6.11: Setting the cutting motion for shaping plane surface

It is important to mention that the type of the cutting motion

and feed motion diverse from operation to another, i.e.it may

linear or rotary or reciprocating motion. Also the cutting feed

motion may be given either to the cutting tool or to the

workpiece. Fig. 6.12 demonstrates the cutting motion for

some machining operations



Fig. 6.12: Cutting motions for different cutting operations



6.7 Tool wear and tool lifecutting tools are subjected to high localized forces, high

temperatures, sliding of the chip along the rake face, and

sliding of the tool flank along the freshly cut surface. These

conditions induce tool wear, which, in turn, adversely affects

tool life, the quality of the machined surface, its dimensional

accuracy, and consequently the economics of cutting

operations. Tool wear is generally a gradual process, much

like the wear of the tip of an ordinary pencil. The rate of tool

wear depends on tool and workpiece materials, tool shape,

cutting fluids, process parameters (such as cutting speed,

feed, and depth of cut), and machine-tool characteristics.

There are two basic regions of wear in a cutting tool: flank

wear and crater wear, Fig. 6.13. Flank wear is the most

important because it leads to loosen of the clearance angle,

increasing the rubbing between the tool flank and the

workpiece. Subsequently the wear is propagated and finally

the tool fail to cut. To prevent excessive tool wear, and

failure the tool must be re-sharpened (regrind). The time

between two successive tool regrind is called the tool life.

Fig. 6.13: Face and flank wear



6.8 Cutting fluidThe purposes of using cutting fluids on machining processes

are to cool and lubricate the tool bit and work piece that are

being machined, increase the life of the cutting tool, make a

smoother surface finish, and wash away chips. Cutting fluids

can be sprayed, dripped, wiped, or flooded onto the point

where the cutting action is taking place. Generally, cutting

fluids should only be used if the speed or cutting action

requires the use of cutting fluids. Flow of cutting fluid is

shown in Fig. 6.14.

Fig. 6.14: Flow of cutting fluid



6.9 Variables Affecting the Quality of Cutting ProcessesThe quality of the machined processes depends on:-

Tool material and geometry.

Work piece material.

Cutting conditions (such as speed, feed and

depth of cut).

Type of the cutting fluid.

Machine tool characteristics (such as stiffness

and damping).

Changing any of the above variables will affect:-

Type of produced chip.

Consumed energy in the cutting process.

Temperature rise.

Wear and failure of the tool.

Surface finish of the machined part.



6.10 Selection of Cutting Conditions

Every cutting process and workpiece material has optimal

cutting conditions that differ from other processes or

materials. Cutting conditions impact the rate of metal

removal, tool life, and the quality of machined surface.. The

machinist may adjust the cutting speed, feed rate, and the

depth of cut for each operation. The resulting conditions

determine the amount of metal removed, the rate of metal

removal, tool life, operation cost and the quality of the part.

Before proceeding in this section we have to differentiate

between two types of cuts; these are rough cut and finishcut. Commonly, the part is firstly rough cut and then finish

cut is followed. The objective of rough cut is to maximize the

possible rate of chip removal without regard to the quality

and accuracy of the machined part. This is to reduce the

machining cost. Small allowance is left for finish cut. The aim

of finish cut is to obtain the desired geometrical accuracy,

and surface quality of the machined part.

In rough cut, depth of cut and feed should be taken as high

as possible in the expense of the cutting speed. However,

very high speed should be employed in finish cut on the

account of feed, and depth of cut.

The following are some hints to be considered when assign

cutting conditions for a specific cutting operation:-



Depth of cut affects tool life the least, and cutting

speed affects it the most.

Increasing cutting speed results in improving the

surface quality of the machined part.

Cutting speed affects tool wear the most

Reducing the feed improves the surface quality.

Depth of cut has no effect on the surface quality.

Depth of affects the rate of material removal the most,

and cutting speed affects it the least.

Increasing rake angle improve the surface quality, but

reduce the strength of the cutting edge.

6.11 Advantages & Limitation of Material RemovalCompared to other types of manufacturing methods,

machining operations offer some unique advantages.

machining operations are capable of making parts that are

very precise. Very high dimensional accuracy, and high

surface quality could be obtained by machining operations.

However, the removal of material produces scrap, which can

be wasteful.



6.11.1 Advantages:

The advantages may be summarized in the following

points:-

More dimensional accuracy may be required than the

accuracy provided by casting, shaping, or in some

forming process alone.

High surface quality could be obtained.

Parts may have external and internal profiles that

cannot be produced by forming or shaping processes.

Heat treated parts may undergo distortion and require

additional finishing operations such as grinding.

Machining processes are suitable for large and small

batches.

6.11.2 Disadvantages

The machining operations usually include several

drawbacks; such as:

Removal processes waste material.

Require more energy and labor than forming and

shaping operations.

Removing a volume of material is time

consuming.

Costs of machining processes are relatively

high.

It may be very difficult to machine very

complicated shapes or very hard materials.

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CHAPTER SIX Fundamentals of Material Removal · CHAPTER SIX Fundamentals of Material Removal ... Its function is to eliminate friction between tool ... Tool angle of the tooth of