181903: Production Technology

P M Agrawal

Chip formation – Ductile Material

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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.

Types of chips produced in metal cuttingBasic 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)

� Type of cutting, i.e., continuous (turning, boring etc.) or intermittent (milling)

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� 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 plane, extending from the cutting

Continuous Chips

plane, extending from the cutting 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

� Lower feed rate� Lower feed rate

� 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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� 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.

� As the cutting process continues,

Continuous Chips with BUE

� As the cutting process continues,

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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� 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

Discontinuous Chips

� 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:

� 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)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

� Geometry of the cutting tool � Geometry of the cutting tool

� 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 easily removed from the work zone� can be easily 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 toolon the face of the tool

3. Groove type: A small grove is ground behind the cutting edge.

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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

the rake face of a cutting

Chip Breakers and Chip Control

the rake face of a cutting

tool.

c. Grooves in cutting tools

acting as chip breakers

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� Most modern cutting tools and

inserts have built-in chip-

breaker features of various

design.

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