Transcript Shear angle
2.008 Design & Manufacturing II Spring 2004 Metal Cutting I 2.008-spring-2004 S.Kim 1 Today, February 25th ▪ HW#2 due before the class, #3 out on the web after the class. ▪ Math Formulae, handout ▪ Lab groups fixed, and thank you. ▪ group report!!! ▪ Metal cutting demo ▪ Cutting physics 2.008-spring-2004 S.Kim 2 A lathe of pre WWII HEADSTOCK SPINDLE RAM LOCK TOOLPOST T-SLOT COMPOUND HANDWHEEL TALL STOCK LOCK LEVER TALL STOCK HANDWHEEL WAYS LEAD SCREW CROSS SLIDE CHIP PAN HANDWHEEL 2.008-spring-2004 S.Kim CARRIAGE HANDWHEE HANDWHEEL PEDESTAL TALL STOCK PEDESTAL 3 Material removal processes ▪ Cost : ▪ Expensive $100 $10,000 ▪ Quality: ▪ Very high ▪ Flexibility: ▪ Any shape under the sun ▪ Rate: ▪ Slow 2.008-spring-2004 S.Kim 4 Surface roughness by machining Roughness (Ra) Process Flame cutting Average application Snagging (coarse grinding) Less frequent application Sawing Planing, shaping Drilling Chemical machining Electrical-discharge machining Milling Broaching Reaming Electron-beam machining Laser machining Electrochemical machining Turning, boring Barrel finishing Electrochemical machining Roller burnishing Grinding Honing Electropolishing Polishing Lapping Superfinishing Kalpakjian 2.008-spring-2004 S.Kim 5 Machined Surface Flaw Waviness height Lay direction Roughness height Roughness width Waviness width Roughnesswidth cutoff Waviness width Waviness height Rough Medium Avg. Better than Avg. Fine Very fine Extremely fine Roughnesswidth cutoff Roughness height (arithmetical average) Roughnesswidth inch Lay 2.008-spring-2004 S.Kim 6 Cutting processes Why do we study cutting physics? Product quality: surface, tolerance Productivity: MRR , Tool wear Physics of cutting Mechanics Force, power Tool materials Design for manufacturing 2.008-spring-2004 S.Kim 7 Cutting Tools Top view Tool shank Tool shank Rake face Rake face Side-cutting-edge angle Cutting edge Flank face Nose radius End-cutting-edge Side-cutting-edge angle Right side view Front view Back-rake angle End-relief angle Side-rake angle Side-relief angle 2.008-spring-2004 S.Kim Flank face 8 Cutting process modeling Methods: Modeling and Experiments Key issues 2.008-spring-2004 S.Kim How does cutting work? What are the forces involved? What affect does material properties have? How do the above relate to power requirements, MRR, wear, surface? 9 Orthogonal cutting in a lathe Assume a hollow shaft Shear plane Shear angle To: depth of cut Rake angle 2.008-spring-2004 S.Kim 10 Cutting tool and workpiece.. Shiny surface Rough surface Chip Primary shear zone Secondary shear zone Rake angle Tool face Tool Flank Relief or clearance angle Shear plane 2.008-spring-2004 S.Kim Shear angle Workpiece 11 Varying rake angle α: -α 2.008-spring-2004 S.Kim α=0 12 Basic cutting geometry ▪ We will use the orthogonal model Rake angle, α Chip Continuity V⋅to = Vc⋅tc Tool tc: chip thickness to: depth of cut Shear angle Cutting ratio: r <1 2.008-spring-2004 S.Kim 13 Velocity diagram in cutting zone Law of sines 2.008-spring-2004 S.Kim 14 Forces and power ▪ FBD at the tool-workpiece contact ▪ What are the forces involved ▪ ▪ ▪ ▪ ▪ ▪ 2.008-spring-2004 S.Kim Thrust force, Cutting force, Resultant force, Friction force, Normal Force, Shear Force, Ft Fc R F N Fs, Fn 15 E. Merchant’s cutting diagram Chip Tool Workpiece Source: Kalpajkian 2.008-spring-2004 S.Kim 16 FBD of Forces Friction Angle Typcially: 2.008-spring-2004 S.Kim 17 Analysis of shear strain ▪ What does this mean: ▪ Low shear angle = large shear strain ▪ Merchant’s assumption: Shear angle adjusts to minimize cutting force or max. shear stress ▪ Can derive: 2.008-spring-2004 S.Kim 18 Shear angle (area of shear plane, shear strength) 2.008-spring-2004 S.Kim 19 Things to think about ▪ As rake angle decreases ior frict on increases ▪ Shear angle decreases ▪ Chip becomes thicker ▪ Thicker chip = more energy dissiipat on via shear ▪ More shear = more heat generation ▪ Temperature increase!!! 2.008-spring-2004 S.Kim 20 Power shearing+ friction Power input Power for shearing Specific energy for shearing Power dissipated via friction MRR Specific energy for friction Total specific energy Experimantal data 2.008-spring-2004 S.Kim 21 Specific energy (rough estimate) Approximate Energy Requirements in Cutting Operations (at drive motor, corrected for 80% efficiency; multiply by 1.25 for dull tools). Specific energy Material Aluminum alloys Cast irons Copper alloys High-temperature alloys Magnesium alloys Nickel alloys Refractory alloys Stainless steels Steels 2.008-spring-2004 S.Kim Kalpakjian 22 Example ▪ ▪ ▪ ▪ ▪ ▪ ▪ Consider the turning with a rod, from 1.25 inch diameter to 1.0. to; depth of cut, 0.005 inch f; feed rate, 0.025 inch/rev ω; spindle speed, 1000 rpm uf;spe ifc ic energy for i fr c it on us;specific energy for shear, 0.35 hp min/In3 1 hp = 550 ft.lbf/s ▪ ▪ ▪ ▪ ▪ ▪ How many passes? Tools speed? Time to make this part? V c max? Max power needed? Initial cutting force? 2.008-spring-2004 S.Kim 23 Cutting zone pictures continuous secondary shear serrated 2.008-spring-2004 S.Kim Kalpakjian BUE discontinuous 24