2.008-spring-2004 s.kim 1 2.008 design & manufacturing ii spring 2004 metal cutting i
TRANSCRIPT
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2.008-spring-2004S.Kim 1
2.008 Design & ManufacturingII
Spring 2004
Metal Cutting I
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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
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2.008-spring-2004 S.Kim 3
A lathe of pre WWII
HEADSTOCK SPINDLE
TOOLPOST T-SLOT
COMPOUND
HANDWHEEL
WAYS
RAM LOCKTALL STOCK
LOCK LEVER
TALL STOCK
HANDWHEEL
LEAD SCREW
CROSS SLIDE
HANDWHEEL
HANDWHEE
PEDESTAL
CARRIAGE
HANDWHEELTALL STOCK
PEDESTAL
CHIP PAN
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Material removal processes▪ Cost : ▪ Expensive $100 -
$10,000 ▪ Quality:
▪ Very high ▪ Flexibility:
▪ Any shape under the sun
▪ Rate: ▪ Slow
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Surface roughness by machining Roughness (Ra)
Process
Flame cutting
Snagging (coarse grinding)
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
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Average application
Less frequent application
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Machined Surface
RoughMediumAvg.Better than Avg.FineVery fineExtremely fine
inch
Waviness
height
Roughness
height
Roughness
width
Wavinessheight
Flaw
Waviness
width
Roughness height(arithmetical
average)
Roughness-width cutoff
Wavinesswidth
Roughness-width cutoff
Roughness-width
Lay direction
Lay
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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
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2.008-spring-2004 S.Kim 8
Cutting Tools
Top view
Tool shank
Rake face
Tool shank
Rake face
Cutting edge
Flank face
Side-cutting-edge angle
Nose radius
Side-cutting-edge
End-cutting-edge angle
Front view
Side-rakeangle
Side-relief
angle
Right side view Back-
rakeangle
End-reliefangle
Flank face
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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?
Cutting process modeling
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Orthogonal cutting in a lathe
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Assume a hollow shaft
Shear plane
Shear angle
To: depth of cut
Rake angle
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Cutting tool and workpiece..
Rough surface
Chip
Primary shear zone
Secondary shear zone
Shiny surface
Tool face
Flank
Relief orclearance angle
Tool
Shear plane Shear angle Workpiece
Rakeangle
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Varying rake angle α:
- α α = 0
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Basic cutting geometry
▪ We will use the orthogonal model
Continuity
Cutting ratio: r <1
V⋅to = Vc⋅tc
tc: chip thicknessto: depth of cut
Rake angle, α
Tool
Chip
Shear angle
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Velocity diagram in cutting zoneLaw of sines
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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
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E. Merchant’s cutting diagram
Source: Kalpajkian
Workpiece
Chip
Tool
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FBD of Forces
Friction Angle
Typcially:
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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:
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Shear angle
(area of shear plane, shear strength)
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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!!!
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Power
Power input
shearing+ friction
Power for shearingSpecific energy for shearing
Power dissipated via frictionSpecific energy for friction
Total specific energy
Experimantal data
MRR
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Specific energy (rough estimate)
Kalpakjian
Specific energy
Approximate Energy Requirements in Cutting
Operations (at drive motor, corrected for 80%
efficiency; multiply by 1.25 for dull tools).
Aluminum alloysCast ironsCopper alloysHigh-temperature alloysMagnesium alloysNickel alloysRefractory alloysStainless steelsSteels
Material
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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?
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Cutting zone pictures
Kalpakjian
continuous secondary shear BUE
serrated discontinuous