16-02-2013, 10:20 AM
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
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
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
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
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
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)
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)
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
Does not machine well: long chips clog flutes of cutting tool
Coolant should be used to minimize heat
Copper/Beryllium
Heavy, hard, reddish-colored copper metal with Beryllium added
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
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
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
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