Mechanics of chips formation

3. MECHANICS OF CHIPS
FORMATION
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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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8. TYPES OF CHIP
√ Continuous chip
√ Discontinuous chip
√ Continuous chip with built-up edge
√ Serrated chips
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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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