Machinability of Metals

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PowerPoint to accompany

Krar • Gill • Smid

Technology of Machine Tools6th Edition

Machinability of MetalsUnit 28

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Objectives• Explain the factors that affect the

machinability of metals• Describe the difference between

high-carbon steel and alloy steel.• Assess the effects of temperature

and cutting fluids on the surface finish produced

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Machinability

• Ease or difficulty with which metal can be machines

• Measured by length of cutting-tool life in minutes or by rate of stock removal in relation to cutting speed employed (depth of cut)

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Grain Structure

• Machinability of metal affected by its microstructure

• Ductility and shear strength modified greatly by operations such as annealing, normalizing and stress relieving

• Certain chemical and physical modifications of steel improve machinability– Addition of sulfur, lead, or sodium sulfite– Cold working, which modifies ductility

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Results of (Free-Machining) Modifications

• Three main machining characteristics become evident– Tool life is increased– Better surface finish produced– Lower power consumption required for

machining

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Low-Carbon (Machine) Steel

• Large areas of ferrite interspersed with small areas of pearlite– Ferrite: soft, high ductility and low strength– Pearlite: low ductility and high strength

• Combination of ferrite and iron carbide

• More desirable microstructure in steel is when pearlite well distributed instead of in layers

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High-Carbon (Tool) Steel

• Greater amount of pearlite because of higher carbon content– More difficult to machine steel efficiently

• Desirable to anneal these steels to alter microstructures– Improves machining qualities

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Alloy Steel

• Combinations of two or more metals• Generally slightly more difficult to machine

than low-or high-carbon steels• To improve machining qualities

– Combinations of sulfur and lead or sulfur and manganese in proper proportions added

– Combination of normalizing and annealing• Machining of stainless steel greatly eased

by addition of selenium

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Cast Iron• Consists generally of ferrite, iron carbide,

and free carbon• Microstructure controlled by addition of

alloys, method of casting, rate of cooling, and heat treating

• White cast iron cooled rapidly after casting– hard and brittle (formation of hard iron carbide)

• Gray cast iron cooled gradually– composed by compound pearlite, fine ferrite,

iron carbide and flakes of graphite (softer)

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Cast Iron

• Machining slightly difficult due to iron carbide and presence of sand on outer surface of casting

• Microstructure altered through annealing– Iron carbide broken down into graphitic carbon

and ferrite• Easier to machine

• Addition of silicon, sulfur and manganese gives cast iron different qualities

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Aluminum

• Pure aluminum generally more difficult to machine than aluminum alloys– Produces long stringy chips and harder on

cutting tool• Aluminum alloys

– Cut at high speeds, yield good surface finish– Hardened and tempered alloys easier to

machine– Silicon in alloy makes it difficult to machine

• Chips tear from work (poor surface)

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Copper• Heavy, soft, reddish-colored metal refined

from copper ore (copper sulfide)– High electrical and thermal conductivity– Good corrosion resistance and strength– Easily welded, brazed or soldered– Very ductile

• Anneal: heat at 1200º F and quench in water• Does not machine well: long chips clog

flutes of cutting tool– Coolant should be used to minimize heat

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Copper-Based Alloys: Brass• Alloy of copper and zinc with good corrosion

resistance, easily formed, machines, and cast• Several forms of brass

– Alpha brasses: up to 36% zinc, suitable for cold working

– Alpha 1 beta brasses: Contain 54%-62% copper and used in hot working

• Small amounts of tin or antimony added to minimize pitting effect of salt water

• Used for water and gas line fittings, tubings, tanks, radiator cores, and rivets

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Copper-Based Alloys: Bronze

• Alloys of copper and tin which contain up to 12% of principal alloying element– Exception: copper-zinc alloys

• Phosphor-bronze– 90% copper, 10% tin, and very small amount of

phosphorus– High strength, toughness, corrosion resistance– Used for lock washers, cotter pins, springs and

clutch discs

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Copper-Based Alloys: Bronze

• Silicon-bronze (copper-silicon alloy)– Contains less than 5% silicon– Strongest of work-hardenable copper alloys– Mechanical properties of machine steel and

corrosion resistance of copper– Used for tanks, pressure vessels, and hydraulic

pressure lines

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Copper-Based Alloys: Bronze

• Aluminum-bronze (copper-aluminum alloy)– Contains between 4% and 11% aluminum– Other elements added

• Iron and nickel (both up to 5%) increases strength• Silicon (up to 2%) improves machinability• Manganese promotes soundness in casting

– Good corrosion resistance and strength– Used for condenser tubes, pressure vessels, nuts

and bolts

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Copper-Based Alloys: Bronze

• Beryllium-bronze (copper and beryllium)– Contains up to 2% beryllium– Easily formed in annealed condition– High tensile strength and fatigue strength in

hardened condition– Used for surgical instruments, bolts, nuts, and

screws

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Effects of Temperature and Friction

• Heat created – Plastic deformation occurring in metal during

process of forming chip– Friction created by chips sliding along cutting-

tool face• Cutting temperature varies with each metal

and increases with cutting speed and rate of metal removal

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Effects of Temperature and Friction

• Greatest heat generated when ductile material of high tensile strength cut

• Lowest heat generated when soft material of low tensile strength cut

• Maximum temperature attained during cutting action– affects cutting-tool life, quality of surface

finish, rate of production and accuracy of workpiece

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High Heat

• Temperature of metal immediately ahead of cutting tool comes close to melting temperature of metal being cut

• High-speed cutting tools– Red hardness: turn red when cutting metal

• Occurs at temperatures above 900º F • Edge breaks down beginning at 1000º and higher

• Cemented-carbide cutting tools– Use efficiently up to 1600º F

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Friction

• Kept low as possible for efficient cutting action

• Increasing coefficient of friction gives greater possibility of built-up edge forming– Larger built-up edge, more friction– Results in breakdown of cutting edge and poor

surface finish• Can reduce friction at chip-tool interface

and help maintain efficient cutting temperatures if use good supply of cutting fluid

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Factors Affecting Surface Finish

• Feed rate• Nose radius of tool• Cutting speed• Rigidity of machining operation• Temperature generated during machining

process

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Surface Finish

• Direct relationship between temperature of workpiece and quality of surface finish– High temperature yields rough surface finish– Metal particles tend to adhere to cutting tool and

form built-up edge• Cooling work material reduces temperature

of cutting-tool edge– Result in better surface finish

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Effects of Cutting Fluids

• Perform three important functions– Reduce temperature of cutting action– Reduce friction of chips sliding along tool face– Decrease tool wear and increase tool life

• Three types of cutting fluids– Cutting oils– Emulsifiable (soluble) oils– Chemical (synthetic) cutting fluids

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Cutting Fluids

• Generally used for machining steel, alloy steel, brass and bronze with high-speed steel cutting tools

• Not used with cemented-carbide tools– If used, great quantities of cutting fluid are applied to

ensure uniform temperatures to prevent carbide inserts from cracking

• Not generally used with cast iron, aluminum, and magnesium alloys– Good results have been found in some cases