Transcript Machinability of Metals

Machinability of Metals
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Machinability
• Ease or difficulty with which metal can be
machined
• Measured by length of cutting-tool life in minutes
or by rate of stock removal in relation to cutting
speed employed
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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)
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High electrical and thermal conductivity
Good corrosion resistance and strength
Easily welded, brazed or soldered
Very ductile
• Does not machine well: long chips clog flutes of
cutting tool
• Coolant should be used to minimize heat
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Copper/Beryllium
• Heavy, hard, reddish-colored copper metal with
Beryllium added
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High electrical and thermal conductivity
Good corrosion resistance and strength
Can be welded
Somewhat ductile
Withstands high temperature
• Machines well
• Highly abrasive to HSS Tooling
• Coolant should be used to lubricate and minimize
tool wear
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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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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
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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
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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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