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2.008 Design & ManufacturingII

Spring 2004

Metal Cutting I

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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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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

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

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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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

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