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2.008 Design & Manufacturing

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1 2.008 Design & Manufacturing
II Spring 2004 Metal Cutting I 2.008-spring-2004 S.Kim

2 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 S.Kim

3 A lathe of pre WWII 2.008-spring-2004 S.Kim 3 HEADSTOCK SPINDLE

4 Material removal processes
▪ Cost :  ▪ Expensive $100 - $10,000 ▪ Quality:  ▪ Very high ▪ Flexibility: ▪ Any shape under the sun ▪ Rate:  ▪ Slow 2.008-spring S.Kim

5 Surface roughness by machining
Roughness (Ra) Process Flame cutting Snagging (coarse grinding) Sawing Planing, shaping Average application Less frequent application 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 S.Kim

6 (arithmetical average)
Machined Surface Flaw Waviness height Rough Medium Avg. Better than Avg. Fine Very fine Extremely fine Lay direction Roughness width Roughness height Waviness width Roughness-width cutoff Waviness height Waviness width Roughness- width cutoff Roughness height (arithmetical average) Roughness- width inch Lay 2.008-spring S.Kim

7 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 S.Kim

8 Cutting Tools 2.008-spring-2004 S.Kim 8 Top view Right side view
Tool shank Tool shank Rake face Rake face Side-cutting-edge Cutting edge angle Flank face Nose radius End-cutting-edge angle Side-cutting-edge Right side view Front view Back-rake angle End-relief Side-rake angle angle Side-relief angle Flank face 2.008-spring S.Kim

9 Cutting process modeling
 Methods: Modeling and Experiments  Key issues  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? 2.008-spring S.Kim

10 Orthogonal cutting in a lathe
Assume a hollow shaft Shear plane Shear angle To: depth of cut Rake angle 2.008-spring S.Kim

11 Cutting tool and workpiece..
Shiny surface Rough surface Secondary shear zone Chip Rake angle Tool face Tool Primary shear zone Flank Relief or clearance angle Shear plane Shear angle Workpiece 2.008-spring S.Kim

12 Varying rake angle α: - α α = 0 2.008-spring S.Kim

13 Basic cutting geometry
▪ We will use the orthogonal model Continuity Rake angle, α Chip V⋅to = Vc⋅tc tc: chip thickness to: depth of cut Tool Shear angle Cutting ratio: r <1 2.008-spring S.Kim

14 Velocity diagram in cutting zone
Law of sines 2.008-spring S.Kim

15 Forces and power ▪ FBD at the tool-workpiece contact
▪ What are the forces involved ▪ Thrust force, Ft ▪ Cutting force, Fc ▪ Resultant force, R ▪ Friction force, F ▪ Normal Force, N ▪ Shear Force, Fs, Fn 2.008-spring S.Kim

16 E. Merchant’s cutting diagram
Chip Tool Workpiece Source: Kalpajkian 2.008-spring S.Kim

17 FBD of Forces Friction Angle Typcially: 2.008-spring S.Kim

18 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 S.Kim

19 Shear angle (area of shear plane, shear strength)
2.008-spring S.Kim

20 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 S.Kim

21 Power shearing+ friction Power input Power for shearing
Specific energy for shearing MRR Power dissipated via friction Specific energy for friction Total specific energy Experimantal data 2.008-spring S.Kim

22 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 Kalpakjian 2.008-spring S.Kim

23 Example ▪ Consider the turning with a rod, from 1.25 inch diameter to 1.0. ▪ to; depth of cut, inch ▪ f; feed rate, 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 S.Kim

24 Cutting zone pictures continuous secondary shear BUE
serrated discontinuous Kalpakjian 2.008-spring S.Kim

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