4. MECHANICS OF CHIPS FORMATION
The geometry of actual machining operation is somewhat
complex.
There are two simplified geometric models of machining
process which neglect many of the geometric complexities,
describe the mechanics of the process accurately and play
active role in the analysis.
The orthogonal cutting model
The oblique cutting model
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5. MECHANICS OF CHIPS FORMATION
The orthogonal cutting model
Simplified 2-D model of machining that describes the mechanics of
machining fairly accurately
In this model wedge-shaped tool’s cutting edge is perpendicular to the
direction of cutting speed.
Tool in orthogonal cutting has only two angles
• Rake angle
• Clearance angle
The rake angle determines the direction that the chip follows
The clearance angle provides a small clearance between tool flank and
newly generated work surface.
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6. MECHANICS OF CHIPS FORMATION
Primary shear zone
As the tool is forced into the material the chip is formed by shear
deformation within a thin shear zone, referred to it as primary shear zone,
which is oriented at an angle Ø with the surface of work as shown in fig
given below.
Failure of material due to plastic deformation occur at the cutting edge of
tool, resulting in separation of chip from the parent material.
Bulk of mechanical energy of machining is consumed in this zone.
The thickness of shear zone is only a few thousands of an inch. Since it is
so thin, we can refer to it as plane
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7. MECHANICS OF CHIPS FORMATION
Secondary shear zone
Second shear action occurs in the chip after it has been
formed, referred to it as secondary shear.
This results from friction between chip and the tool as
the chip slide along the rake face of the tool.
Its effect increases with increased friction between tool
and chip
Type of material being machined
Cutting conditions of operation
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9. TYPES OF CHIP
Continuous chip
A long continuous chip will result when
work material is ductile
Cutting speed is high
Small feed and depth of cut
A sharp cutting edge
Low tool-chip friction
Good surface finish results when this type of chip is formed.
Turning tools are often equipped with chip breakers to solve the
problems of chip disposal when became long.
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10. TYPES OF CHIP
Discontinuous chip
A discontinuous chip will result when
Brittle work materials
Low cutting speeds
Large feed and depth of cut
High tool-chip friction
Form into separate segments as shown in fig
Impart an irregular texture to the machined surface
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11. TYPES OF CHIP
Continuous chip with built-up edge
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Ductile materials
Low-to-medium cutting speeds
Tool-chip friction causes portions of chip to adhere to
rake face near the cutting edge
BUE forms, then breaks off, cyclically
Much of the detached BUE is carried away with the chip,
sometimes taking portion of the rake face with it ,which
reduce life of tool.
Portion of the detached BUE that are not carried off with
the chip become imbedded in the newly work surface,
causes the surface to become rough.
12. TYPES OF CHIP
Serrated/segmented chip
These chips are semi continuous in the sense that they posses a saw-tooth
appearance that is produced by cyclical chip formation of alternating high
shear strain followed by low shear strain
These chips are associated with difficult-to-machine metals when they are
machined at higher cutting speeds e.g
Titanium alloy
Nickel-base super alloys
Austenitic stainless steels
steel
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