machinability of metals
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Machinability of Metals. Unit 28. 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. Machinability. - PowerPoint PPT PresentationTRANSCRIPT
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Krar • Gill • Smid
Technology of Machine Tools6th Edition
Machinability of MetalsUnit 28
28-2
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
28-3
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)
28-4
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
28-5
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
28-6
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
28-7
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
28-8
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
28-9
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)
28-10
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
28-11
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)
28-12
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
28-13
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
28-14
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
28-15
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
28-16
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
28-17
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
28-18
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
28-19
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
28-20
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
28-21
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
28-22
Factors Affecting Surface Finish
• Feed rate• Nose radius of tool• Cutting speed• Rigidity of machining operation• Temperature generated during machining
process
28-23
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
28-24
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
28-25
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