ME 202: Manufacturing

Technologies

Metal Cutting

Prepared by

Prof. Dr. S. Engin KILIÇ

Given by

Dr. Volkan Esat

Middle East Technical University

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Outline

• Introduction

• Machining

• Chip Formation

• Cutting Tool Materials

• Tool Wear

• Taylor Tool Life Model

• Cutting Fluids

• General Machining Operations

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Machining

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Metal Cutting

Machining : Removal of material in the form of chips from the workpiece by shearing with a sharp tool.

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Terms and Definitions

Relative Motion

between tool and workpiece

Primary motion

Cutting motion

Cutting speed Feed rate

Feed motion

Depth of cut

Secondary motion Depth of cut

adjustment

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Metal Cutting

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Orthogonal Cutting

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Characteristic of Chips

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Continuous Chip

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Discontinuous Chip

• Machining brittle

materials

• Small rake angle

• Large depth of cut

• Machining ductile

materials at

– low cutting speed

– high feed

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Continuous Chip with BUE

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Milling

Slab Milling Face Milling End Milling

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Turning

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Turning

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Tool Geometry

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Tool Materials

Requirements:

• Strength

• Hot hardness

• Wear resistance

• Toughness

• Low friction

• Favorable cost

Classification:

• Carbon Tool Steels

• Medium Alloy Steels

• High Speed Steels

• Cast Cobalt Based Alloys

• Cemented Carbides

• Ceramics and Ultra-hard Materials

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Tool Materials - Steel

• Tool Steel

– Plain carbon steel of 0.9 to 1.3%

– Hardened and tempered

– Loses its toughness above ~200oC.

• High Speed Steel

– 18% Tungsten, 4% Cromium, 1% Vanadium

– Retains hardness up to ~600oC

– Compared to tool steels, cutting speed could be doubled for an equal life.

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Tool Materials – Cemented Carbide

• Produced by P/M.

• can be operated at cutting speeds 200 to

500% greater than those of HSS.

• Replaced HSS in many processes

• Made in the form of throwaway inserts.

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Throwaway Inserts

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Tool Materials - Ceramics

• Made of pure aluminum oxide by P/M

technology.

• Can be operated at 2-3 times the cutting

speed of WC w/o coolant.

• Are in the form of disposable tips.

• Are as hard as carbides but are more

brittle.

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Tool materials (Cont’d)

• Diamond

– Hardest material known

– Diamond maching is done at high speeds with fine

feeds.

– Produces excellent surface finish.

• Cubic Boron Nitride (CBN)

– Hardest man-made material.

– Retains its hardness at ~1000oC.

– Can be used to machine hard aero-space materials.

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Tool Materials – Coated Carbide

• A tough-, shock-resistant tool is coated with a thin, hard, crater-wear resistant material.

• TiC-coated tools are 2-3 times more wear resistant than an uncoated one.

– resulting 50-100% increase in cutting speed for the same tool life.

• Ceramic (Al2O3) coating permits 90%speed increase in machining of steel.

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Coated Carbide

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Hardness of Tool Materials

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Tool Wear

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Tool Life

• A tool is said to reach end of its life when a

further wear causes one or some of the

following:

– Loss of dimensional accuracy of the workpiece

– Excessive surface roughness on the workpiece

– Increased power requirement of the machine tool

– Physical loss of a cutting edge of the cutting tool

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Taylor’s Tool Life Model

CVT n

Frederick W. Taylor

(1856-1915)V: Cutting Speed [m/min]

T: Tool Life [min]

C: Cutting Speed for 1 min of Tool Life

n: Exponent

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Tool Life Exponent “n”

• HSS: n = 0.125

• Uncoated carbides: n = 0.25

• TiC inserts: n = 0.30

• Coated inserts: n = 0.40

• Ceramics: n = 0.60

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Cutting Fluids (Coolants)

• Coolants are used– To decrease tool operating temperature

– To improve cutting performance

• A good cutting fluid should act as– Lubricant

– Coolant removing heat from the cutting zone

• The coolant must be– Non-volatile

– Non-toxic

– Non-foaming

– Of high-flushing temperature

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Advantages of Cutting Fluids

• Tool life is increased.

• Surface finish is improved.

• BUE (Built-up Edge) is prevented.

• Machine tool power is reduced.

• Corrosion hazard is reduced.

• Chips are washed away and cutting

zone is kept clear.

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Machining Operations

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Machining Operations (Cont’d)

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Machining Operations (Cont’d)

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Machining Operations (Cont’d)