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PowerPoint to accompany

Krar • Gill • Smid

Technology of Machine Tools6th Edition

Metal-Cutting Technology

Section 8

27-2

Metal Cutting Technology

• Metals used in products must be machined efficiently to be economical

• Cutting metals efficiently requires– Knowledge of metal to be cut– How cutting tool material and its shape will

perform under various machining conditions

• Many new cutting-tool materials introduced in last few decades– Improved machine construction, higher cutting

speeds and increased productivity

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PowerPoint to accompany

Krar • Gill • Smid

Technology of Machine Tools6th Edition

Physics of Metal Cutting

Unit 27

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Objectives

• Define the various terms that apply to metal cutting

• Explain the flow patterns of metal as it is cut

• Recognize the three types of chips produced from various metals

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How Metal Is Cut

• Have used tools without understanding how metal is cut

• Prior thought held that metal ahead of cutting tool split (like ax splits wood)

• Since WWII, research conducted– Found metal compressed and flows up face of

cutting tool– Led to new cutting tools, speeds and feeds,

cutting-tool angles and clearances and cutting fluids

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Metal-Cutting Terminology

• Built-up edge– Layer of compressed metal which adheres to and

piles up on face of cuttingtool edge

• Chip-tool interface– Portion of face of cutting

tool on which chip slides as cut from metal

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27-7

Metal-Cutting Terminology

• Crystal elongation– distortion of crystal structure of work material

occurring during machining operation

• Deformed zone– Area in which work

material deformed during cutting

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Metal-Cutting Terminology

• Plastic deformation– Deformation of work material occurring in shear

zone during cutting action

• Plastic flow– Flow of metal that occurs on shear plane (extends

from cutting-tool edge to corner between chip and work surface)

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Metal-Cutting Terminology

• Rupture– Tear that occurs when brittle materials are cut

and chip breaks away from work surface

• Shear angle or plane– Angle of area of material where plastic

deformation occurs

• Shear zone– Area where plastic deformation of metal occurs– Along plane from cutting edge of tool to

original work surface

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Plastic Flow of Metal

• Study flat punches on ductile material– Stress pattern– Direction of material flow– Distortion created in metal– Used blocks of photoelastic materials

• Polarized light used to observe stress lines– Saw series of colored bands – isochromatics

• Tested three punch types: flat, narrow-faced, and knife-edge

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Flat Punch

• Flat punch forced into block of photoelastic material– Lines of constant maximum shear stress appear– Isochromatics (shape of stress lines)

• Appear as family of curves almost passing through corners of flat punch

• Greatest concentration occurs at each corner of punch• Larger circular stress lines appear farther away from

punch• Spacing relatively wide

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Narrow-Faced Punch

• Narrow-faced punch forced into block of photoelastic material– Stress lines concentrated

• Punch corners

• Where punch meets top surface of work

– Isochromatics spaced closer than with flat punch

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Knife-Edge Punch

• Knife-edge punch forced into block of photoelastic material– Isochromatics becomes series of circles tangent

to the two faces of punch– Flow of material occurs upward from point

toward free area along faces of punch

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When Cutting Tool Engages Workpiece

• Internal stresses are created• Compression occurs in work material because

of forces exerted by cutting tool• Concentration of stresses causes chip to shear

from material and flow along chip-tool interface– Since most metals ductile to some degree, plastic

flow occurs• Determines type of chip produced

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Chip Types

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Discontinuous (segmented) chip

Continuous chip

Continuous chipwith built-up edge

• Machining operations performed on lathes, milling machines, or similar machine tools produce ships of three basic types

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Type 1 - Discontinuous (Segmented) Chip

• Produced when brittle metals are cut• Point of cutting tool contacts metal

some compression occurs and chip begins to flow

• More cutting action produces more stress, metal compresses until rupture, and chip separates from unmachined portion

• Poor surface created

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Production of Type 1 Discontinuous Chip

• Conditions that favor the production– Brittle work material

– Small rake angle on the cutting tool

– Large chip thickness (coarse feed)

– Low cutting speed

– Excessive machine chatter

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Type 2 – Continuous Chip

• Continuous ribbon produced when flow of metal next to tool face not retarded by built-up edge or friction

• Ideal for efficient cutting action• Results in better surface finishes• Plastic flow as deformed metal slides on

great number of crystallographic slip planes– No fractures or ruptures occur due to ductile

nature

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Tool

Plane of Shear

Direction of Crystal Elongation

ShearAngle

Shear Zone

As Cutting action progresses,metal ahead of tool is compressed

with resultant deformation(elongation) of crystal structure.

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Conditions Favorable to Producing Type 2 Chip

• Ductile work material

• Small chip thickness (relatively fine feeds)

• Sharp cutting-tool edge

• Large rake angle on cutting tool

• High cutting speeds

• Cutting tool and work kept cool using cutting fluids

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Conditions Favorable to Producing Type 2 Chip

• Minimum resistance to chip flow– High polish on cutting-tool face– Use of cutting fluids– Use of cutting-tool materials which have low

coefficient of friction• Cemented carbides

– Free-machining materials• Those alloyed with lead, phosphor, and sulphur

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Type 3 - Continuous Chip with Built-Up Edge

• Low-carbon machine steel and high-carbon alloyed steels

• Low cutting speed with high-speed steel cutting tool

• Without use of cutting fluids

• Poor surface finish

chip

Built-upEdge

Tool

Finished Surface of Work

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Type 3 – Continuous Chip with Built-Up Edge

• Small particles of metal adhere to edge of tool– Build-up increases until becomes unstable and

breaks off– Portions stick to both chip and workpiece– Buildup and breakdown occur rapidly during

cutting action• Shortens cutting-tool life

– Fragments of build-up edge abrade tool flank– Cratering effect caused short distance back from

cutting edge where chip contacts tool face