Machinability of Metals

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

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

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)

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

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

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

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

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 Beryllium-bronze (copper and beryllium), containing up to about 2% beryllium, is easily formed in the annealed condition. It has a high tensile strength and fatigue strength in the hardened condition. Beryllium-bronze is used for surgical instruments, bolts, nuts, and screws.

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

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

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

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

Factors Affecting Surface Finish Feed rate Nose radius of tool Cutting speed Rigidity of machining operation Temperature generated during machining process

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