181903: Production Technolog Technology y P M Agrawal
Chip formation – Ductile Material
Chip has two surfaces 1.
One is contact with the rake of the tool and has a shiny and burnished appearance caused by rubbing as the chip moves up the tool face.
2.
The other is the original surface of the workpiece. It has jagged, rough appearance, caused by the shearing mechanism.
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Types of chips produced in metal cutting Basic types of chips produced in orthogonal metal cutting 1.
continuous chip with narrow, straight, and primary shear zone
2.
continuous chip with built-up edge
3.
Segmented or discontinuous chip
Type of chips produced in metal cutting depends on
Properties of workpiece material (brittle or ductile etc.)
Cutting conditions:
Rake angle
Cutting velocity (low, medium or high)
Depth of cut
Feed rate
Cutting fluid (type of fluid and method of application)
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Type of cutting, i.e., continuous (turning, boring etc.) or intermittent (milling)
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Continuous Chips
A continuous ribbon of chip produced during machining of ductile materials such as wrought iron, mild steel, copper and aluminium. Involves shearing of workpiece material to form the chip and sliding of the chip along the rake face of the cutting tool. chip formation occurs in a single p ane, ex en ng rom e cu ng tool to the unmachined work surface. Primary deformation zone: Area where plastic deformation of the crystal structure and hence shear occurs Shear angle: Angle on which the chip separates from the metal
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Continuous Chips
Indicates steady state cutting conditions (most research conducted under these conditions) Considered ideal for efficient cutting action because it results in better surface finish Cutting conditions:
Ductile material
High cutting velocity
ower ee ra e
Larger positive rake angle
Use of cutting fluids as coolant and lubricant
While machining ductile metals at high speed, chips are deliberately broken into small segments of regular size and shape by using chip breakers mainly for convenience and reduction of chip-tool contact length
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Continuous Chips with BUE
Because of the high temperature, high pressure, and high frictional resistance against the flow of the chip along the chip-tool interface, small particles of metal begin adhering to the edge of the cutting tool while the chip shears away.
s e cu ng process con nues, more particles adhere to the cutting tool and a larger build-up results
The built-up edge increases in size and becomes more unstable. Eventually a point is reached where fragments are torn off.
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Continuous Chips with BUE
Portions of these fragments which break off, stick to both the chip and the workpiece.
These fragments adhere to and score the machined surface, resulting in a poor surface finish.
Study of formation of BUE is important: It is one of the principal factors affecting surface finish and can have considerable influence on cutting tool wear
Cutting conditions:
Ductile material
medium cutting velocity
Higher feed rate
Larger positive rake angle
cutting fluids is absent or inadequate
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Discontinuous Chips
During the formation of chip, the material undergoes severe strain.
If the workpiece material is brittle, fracture will occur in the primary deformation zone when the chip is only partly formed. Under these conditions the chip separates from the unmachined portion.
Cycle is repeated during the cutting operation, with the rupture of each segment occurring on the shear angle or plane.
Poor surface produced due to these successive ruptures
Generally, chip segments are either loosely attached to each other or totally fragmented.
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Discontinuous Chips
Cutting Conditions:
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Brittle metal such as cast iron and hard bronze are cut. Brittle workpiece materials lack the ductility to undergo the high shear strains and form continuous chips. Hard particles and impurities in the matrix of materials will act as stress-raisers and actively encourage chip breakage (gray cast iron having graphite flakes, inclusions of manganese sulfide in free machining steels) Very low cutting speeds (produced even if some ductile metals are cut at very low speeds and high feeds) Small or negative rake angles and heavy depth of cut
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Chip Breakers and Chip Control
Chips develop a curvature as they leave the workpiece surface in all cutting operations of metallic and non-metallic materials.
Factors affecting the chip curl are:
Distribution of stresses in the primary and secondary shear zones
Thermal effects
Work-hardening characteristics of the workpiece material
Cutting fluids
As the depth of cut decreases, the radius of curvature decreases and chip becomes curlier.
Cutting fluids can make chips become more curly, thus reducing the tool-chip contact area and concentrating the heat closer to the tip of the tool.
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Chip Breakers and Chip Control
Continuous and long chips are undesirable since they tend to become entangled, severely interfere the cutting operation, damage the workpiece surface, obstruct the coolant flow and hazardous.
Discontinuous chips are generally desired because they
are less dangerous for the operator
do not cause damage to workpiece surface and machine tool
can be easil removed from the work zone
can be easily handled and disposed after machining.
Ideal chip size to be broken is in the shape of either the letter C or the number 9 and fits within a 25-mm square space.
Three methods to produce the favourable discontinuous chip 1.
proper selection of cutting conditions
2.
use of chip breakers
3.
change in the work material properties
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Chip Breakers and Chip Control
Usual procedure is to break the chip intermittently with cutting tools that have a chip-breaker features.
There are three types of chip breakers 1.
Step type: A step is ground on the face of the tool along the cutting edge.
2.
Clamp type: A thin carbide plate or clamp is brazed or screwed on the face of the tool
3.
Groove type: A small grove is ground behind the cutting edge.
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Chip Breakers and Chip Control
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a.
Schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip.
b.
Chip breaker clamped on e ra e ace o a cu ng tool.
c.
Grooves in cutting tools acting as chip breakers
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Most modern cutting tools and inserts have built-in chipbreaker features of various design.
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