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Manufacturing is the industrial activity that changes the form
of raw materials to create products or in other word manufacturing
has been accurately defined as the activities that are performed in
the conversion of stuff to useful thing (product). It is the
application of physical and chemical processes to alter the
geometry, property and/or appearances of given starting material to
make part or product. The derivation of the word manufacture
reflects its original meaning: to make by hand. As the power of the
hand tool is limited, manufacturing is done largely by machinery
today. Manufacturing technology constitutes all methods used for
shaping the raw metal materials into a final product.The
development of new tool materials opened a new era for the
machining industry in which machine tool development took
place.
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Machining is the removal of the unwanted material (machining
allowance) from the work piece (WP), so as to obtain a finished
product of the desired size, shape, and surface quality. The
practice of removal of machining allowance through cutting
techniques was first adopted using simple handheld tools made from
bone, stick, or stone, which were replaced by bronze or iron tools.
Water, steam, and later electricity were used to drive such tools
in power-driven metal cutting machines (machine tools).
The importance of machining processes can be emphasized by the
fact that every product use in our daily life has undergone this
process either directly or indirectly. It is important to
understand the metal cutting process in order to make the best use
of it.
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In the metal working industries work piece of different shape,
dimension and materials are worked. The various working processes
to make these fall into two groups :
Metal Working Chip-less Process (Metal Forming) e.g. forging,
pressing , drawing etc.Chip Forming Process (Metal Cutting) e.g.
turning, drilling, milling etc.
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A Cutting tool may be used either for cutting apart, as with a
knife, or for removing chips. Part are produced by removing metal
mostly in the form of small chips.
All cutting tools can be divided into two groups they are:
Single point cutting tools Multi-Point cutting tools
Single point cutting tool having a wedge like action find, a
wide application on lathe, and slotting machine. Multi point
cutting tools are merely two or more single point tools arrange
together as a unit.TYPE OF CUTTING TOOLS
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SINGLE POINT CUTTING TOOLS FOR LATHE MACHINE
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MULTI POINT CUTTING TOOLS
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MULTI POINT CUTTING TOOLS
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MECHANICS OF CUTTING AND CHIP FORMATION In figure tool is
considered stationary, and the work-piece moves to the right. The
metal is severely compressed in the area in the front of cutting
tool. The metal in front of the tool rake face gets immediately
compressed, first elastically and plastically. This zone is
traditionally called shear zone.
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CHIP F0RMATIONThe metal in front of the tool rake face gets
immediately compressed first elastically and then plastically.The
actual separation of the metal starts as a yielding or fracture,
depending upon the cutting conditions, starting from the cutting
tool tip. The chip after sliding over the tool rake face would be
lifted away from the tool, and the resultant curvature of the chip
is termed as chip curl.
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Mechanics of cutting and Chip Formation Metal cutting as a slide
of card which would slide over one another as the wedge shape tool
moves under these cards as shown in figure.
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The chip is variable both in size and shape in actual
manufacturing practice. Study of chip is one of the most important
things in metal cutting.
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FOUR BASIC TYPES OF CHIP IN MACHININGWhether the cutting
condition can be, the chip produced may belong to one of the
following type :
Discontinuous chipContinuous chipContinuous chip with Built-up
Edge (BUE)Serrated chip
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Brittle work materialsLow cutting speedsLarge feed and depth of
cutHigh toolchip friction DISCONTINUOUS CHIPS
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DISCONTINUOUS CHIPSDiscontinuous ChipWhen brittle materials like
cast iron are cut, the deformed material gets fractured very easily
and thus the chip produced is in the form of discontinuous
segments.Cutting force becomes unstable with the variation
coinciding with the fracturing cycle.Higher depths of cut (large
chip thickness), low cutting speeds and small rake angles are
likely to produce discontinuous chips.
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Ductile work materialsHigh cutting speedsSmall feeds and
depthsSharp cutting edgeLow toolchip frictionCONTINUOUS CHIPS
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CONTINUOUS CHIPS Continuous chips are usually formed at high
rake angles and/or high cutting speeds.A good surface finish is
generally produced.Continuous chips are not always desirable,
particularly in automated machine tools,Tend to get tangled around
the toolOperation has to be stopped to clear away the chips.
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CONTINUOUS CHIP FORMATION
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Ductile materialsLowtomedium cutting speedsTool-chip friction
causes portions of chip to adhere to rake faceBUE forms, then
breaks off, cyclically
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BUE consists of layers of material from the workpiece that are
gradually deposited on the tool.
BUE then becomes unstable and eventually breaks up
BUE material is carried away on the tool side of the chip
The rest is deposited randomly on the workpiece surface.
BUE results in poor surface finish
Reduced by increasing the rake angle and therefore decreasing
the depth of cut.
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Semi continuous - saw-tooth appearance
Cyclical chip forms with alternating high shear strain then low
shear strain
Associated with difficult-to-machine metals at high cutting
speeds
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Various chips produced in turning: (a) tightly curled chip; (b)
chip hits workpiece and breaks; (c) continuous chip moving away
from workpiece; and (d) chip hits tool shank and breaks off.
Source
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There are method of metal cutting, depending upon the
arrangement of the cutting edge with respect to the direction of
relative work-tool motion:Orthogonal cutting or two dimensional
cuttingOblique cutting or three dimensioning cuttingOrthogonal
cutting take place when the cutting face of the tool is 90 to the
line of action or path of the tool. If, however, the cutting face
is inclined at an angle less than (other than 90) 90 to the path of
tool, the cutting action is known as oblique. Orthogonal cutting or
two dimensional cutting
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In orthogonal cutting, the cutting edge of the cutting tool is
arrange perpendicular to the cutting velocity (V), where in oblique
cutting, it set at some angle other than 90 to the cutting velocity
(v), which gives an inclination angle . The analysis of oblique
cutting being very complex, the relatively simple arrangement of
orthogonal cutting is, therefor, widely used in theoretical and
experimental work
Oblique cutting is more practical while orthogonal cutting is
convenient for analysis.
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Orthogonal Cutting Model Simplified 2-D model of machining that
describes the mechanics of machining fairly accurately Chip
Thickness Ratiowhere r = chip thickness ratio; to = thickness of
the chip prior to chip formation; and tc = chip thickness after
separation
Chip thickness after cut always greater than before, so chip
ratio always less than 1.0
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= =Chip Thickness RatioBased on the geometric parameters of the
orthogonal model, the shear plane angle can be determined as: where
r = chip ratio, and = rake angle
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The current analysis is based on Merchant's thin shear plane
model considering the minimum energy principle. This model would be
applicable at very high cutting speeds, which are generally
practiced in production
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The tool is perfectly sharp and no contact along the clearance
face.The surface where shear is occurring is a plane.The cutting
edge is a straight line extending perpendicular to the direction of
motion and generates a plane surface as the work moves past it.The
chip does not flow to either side or no side spread.Uncut chip
thickness is constant.Width of the tool is greater than the width
of the work.A continuous chip is produced without any BUE.Work
moves with a uniform velocity.The stresses on the shear plane are
uniformly distributed
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Why should we know?Power requirement for the machine tool can be
calculatedDesign of stiffness, etc. for the machine
tolerancesWhether work piece can withstand the cutting force
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Fs = force which is the resistance to shear of the metal in
forming the chip. It acts along the shear plane.Ns = Normal to the
shear plane. This is the backup force on the chip provided by the
work piece.N = force at the tool tip interface acting normal to the
cutting face of the tool and is provided by the toolF = Is the
frictional resistance of the tool acting on the chip. It act
downward against the motion of the chip as it glides upward along
the tool face.R and R are equal in magnitude and opposite in
direction.
Fig. a
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The two orthogonal component (horizontal and vertical) FH and FV
of the resultant force R can be measure by using dynamometer. The
horizontal component is the cutting force FH and the vertical
component is the thrust force Fv. All these force can be
represented with the help of a circle known as Merchant force
circle Fig, b
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Here the two force triangle have been superimpose by placing the
two equal force R and R together. In the figure, is the angle of
friction. In this diagram, for the convenience the resultant force
have been moved to the point of the tool. Since the force FH and Fv
are at right angle to each other, their intersection lies on a
circle diameter R. The force F and N may be placed in the diagram
as shown to form the circle diagram. Now : firm fig. b and c
Fig. cForce Fs and Ns are right angle to each other1.
.2
..3
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from Fig. dFv4
..................5.6
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7
.8Putting the value of Ns (from eq. 3) in this both equations (7
and 8)...9
.10
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11
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Or
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To increase shear plane angle Increase the rake angle Reduce the
friction angle (or coefficient of friction)Effect of Higher Shear
Plane AngleHigher shear plane angle means smaller shear plane which
means lower shear force, cutting forces, power, and
temperatureEffect of shear plane angle : (a) higher with a
resulting lower shear plane area; (b) smaller with a corresponding
larger shear plane area. Note that the rake angle is larger in (a),
which tends to increase shear angle according to the Merchant
equation
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Where: D= Dia. Of jobN= Velocity of job or tool rev/min
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