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.
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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
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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
Production Technology Ch6: Fundamentals of Material Removal
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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
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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.
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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 :-
Production Technology Ch6: Fundamentals of Material Removal
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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.
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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
Production Technology Ch6: Fundamentals of Material Removal
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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)
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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
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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
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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
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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
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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.
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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
Production Technology Ch6: Fundamentals of Material Removal
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Fig. 6.12: Cutting motions for different cutting operations
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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
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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
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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.
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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:-
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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.
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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.