Machinability of Metals

1. Machinability of Metals Unit 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

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)

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

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

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

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

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

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)

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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