mprocesses notes 8

8/2/2019 MProcesses Notes 8

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Abrasive Machiningand Finishing

Manufacturing

Processes


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Outline

Units

Abrasives

GrindingGrinding Wheels

Grinding Process

Coated AbrasivesBelt Grinding

Honing

Lapping

Other Finishing Operations

Deburring Processes


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Abrasive Machining


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Abrasive Machining

Why a smooth surface?


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Abrasive Machining

Why a smooth surface?

Reduction in FrictionHeat - Bearings

Reduction in WearBushings/Bearings

AppearanceCar Body, Furniture

Clearance

Disk Head

SharpnessCutting Tools


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Abrasive Machining

How do we get a smoothsurface?


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Abrasive Machining

How do we get a smoothsurface?

Remove MaterialAbrasive Machining

FlattenBurnishing

Fill in VoidsAdd material

Paint

FinishWax


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Units

Meter (m)

Centimeter (cm) = .01 m

Millimeter (mm) = .001 m

Micrometer (m) = 10-6 m

Nanometer (nm) = 10-9 m

Angstrom () = 10-10 m


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Units

12872000 m meter

10-2 centimeter

10-6 micrometer

10-9 nanometer

10-10 angstrom


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Abrasives

Abrasives

Small, hard nonmetallic

particles with sharp edges andirregular shapes

Can remove small amounts ofmaterial, producing tiny chips

Abrasive processes can

produce fine surface finishesand accurate dimensionaltolerances


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Types of

AbrasivesConventional Abrasives

a. Aluminum oxide (Al2O3)

b. Silicon carbide (SiC)

Superabrasives

c. Cubic Boron Nitride (cBN)

d. Diamond

Abrasives are harder thanconventional tool materials


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Abrasive Factors

- Grain size

- Grain shape

- Hardness

- Friability (tendency to fracture)


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Abrasive Hardness and

Thermal Conductivity


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Grinding


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Example of a

Grinding Machine


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Types of Grinding

- Surface Grinding

- Cylindrical Grinding

- Internal Grinding

- Centerless Grinding

- Others

- Tool and cutter grinders

- Tool-post grinding

- Swing-frame grinders

- Bench grinders

- Creep-Feed Grinding


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Surface Grinding


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Cylindrical Grinding


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http://www.minnesotagrinding.com/ODGrinding400.jpg


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Internal Grinding


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Centerless Grinding


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Centerless Grinding


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Creep-Feed Grinding


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Bonded Abrasives/

Grinding WheelsBonded Abrasives

Most grinding wheels are made

of abrasive grains heldtogether by a bonding material

Types of bonding material:

Vitrified (glass)

Resinoid (thermosetting resin)

Rubber

Metal (the wheel itself is metal;the grains are bonded to itssurface


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Grinding Wheel

Components


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Grinding Wheel

Structure


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Grinding Process

Grinding

- Grains have irregular shapes

and random spacing

- Average rake angle is verynegative (about -60 or lower)

- Radial positions of grains vary- Cutting speed is very high (ca.

600 ft/min)


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Grinding Process


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Grinding Process

Grain force

((v/V)(d/D))(material strength)

Temperature rise

D1/4d3/4(V/v)1/2

Effects caused by grindingtemperature increase:

- Sparks

- Tempering

- Burning

- Heat Checking


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Grinding Wheel

WearTypes:

Attritious Grain Wear

Grains develop a wear flat

Grain Fracture

Necessary to produce sharpgrain edges

Bond Fracture

Allows dull grains to bedislodged from the wheel


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Grinding Wheel

Loading


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Truing and Dressing


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Cutting Fluids

- Remove heat

- Remove chips, grain fragments

and dislodged grains

- Are usually water-basedemulsions

- Are added by flood application


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Grinding Ratio

G = Volume of material removedVolume of wheel wear

Vary greatly (2-200 or higher)depending on the type of

wheel, grinding fluid, andprocess parameters

Higher forces decrease thegrinding ratio


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Grinding

Design Considerations:

- Design parts so that they can be heldsecurely

- Avoid interrupted surfaces if highdimensional accuracy is requiredbecause they can cause vibrations

- Ensure cylindrical parts are balanced andthick enough to minimize deflections

- Short pieces may be difficult to grindaccurately in centerless grinding becauseof limited support by the blade

- Parts requiring high accuracy formgrinding should be kept simple to preventfrequent wheel dressing

- Avoid small deep or blind holes or includea relief


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Ultrasonic Machining

Uses fine abrasive grains in aslurry to remove material from

brittle workpieces bymicrochipping and erosion

The tool vibrates at 20 kHz and a

low amplitude (.0125-.075 mm)which accelerates the grains toa high velocity

Can create very small holes andslots


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Rotary Ultrasonic

MachiningUses a rotating and vibrating tool

to remove material, as in face

milling

Diamond abrasives are

embedded in the tool surface

Effective at producing deep

holes in ceramic parts at highMRR


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Design Considerations:

- Avoid sharp profiles, corners

and radii; the slurry erodescorners off

- Allow for slight taper for holes

made this way- Support the exit end of holes

being formed with a backup

plate to prevent chipping of theholder


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Coated Abrasives

Coated Abrasives

Abrasive grains are deposited

on flexible backing; they aremore pointed than those ingrinding wheels

Common examples:sandpaper, emery


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Coated Abrasives


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Coated Abrasives

Belt Grinding

Uses coated abrasives in the

form of a belt; cutting speedsare about 2500-6000 ft/min

MicroreplicationAbrasives with a pyramidshape are placed in a

predetermined regular patternon the belt


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Belt Grinding


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Honing

Used mainly to improve thesurface finish of holes

Bonded abrasives calledstones are mounted on a

rotating mandrel; also used oncylindrical or flat surfaces andto remove sharp edges ontools


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Honing

Hole defects correctible by honing


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Superfinishing/

MicrohoningUses very low pressure and

short strokes


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Lapping

Used to enhance surface finish anddimensional accuracy of flat or

cylindrical surfaces; tolerances areon the order of .0004 mm; surfacefinish can be as smooth as .025-.1m; this improves the fit between

surfaces

Abrasive particles are embedded inthe lap or carried in a slurry

Pressures range from 7-140 kPadepending on workpiece hardness


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Lapping


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Example of a

Lapping Machine


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2- and 3-Body

Abrasion

2-body abrasion: grains are embedded in a surface

3-body abrasion: grains move freely between surfaces


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Lapping

MicrochippingClat

Crad

a

h

Plastic zone

Lateral cracks remove material

Radial cracks surface damage


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Lapping Finish

Grinding Lapping


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Types of Lapping

Single-sided lapping machine


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Types of Lapping

Upper lap rotation

Lower lap rotation

Rollingcylindrical workpieces

Upper laprotation

Lower laprotation

Cylindrical parts

Double-sided lapping

Cylindrical Lapping


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Lapping Process


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Examples of

Lapped Parts

The workpieces made of aluminum oxide were ringshaving 0.5 ID, 0.8 OD and 0.2 thickness. Its highhardness promotes a series of applications inmechanical engineering, such as bearings andseals.

Initial Ra = 0.65 mFinal Ra (after lapping) = 0.2 m


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Examples of

Lapped Parts

Hexoloy SiC is a new sintered alpha silicon carbidematerial designed specifically for optimum performance in

sliding contact applications. It is produced by pressurelesssintering ultra-pure sub-micron powder. This powder ismixed with non-oxide sintering aids, then formed into thedesired shapes by a variety of methods and consolidatedby sintering at temperatures above 2000 C (3632 F). Thesintering process results in single-phase, fine-grain SiCproduct that is very pure and uniform, with virtually noporosity. Whether used in corrosive environments,subjected to extreme wear and abrasive conditions, or

exposed to high temperatures, Hexoloy sintered alphasilicon carbide outperforms other advanced ceramics. Thiskind of ceramic material is ideal for applications such aschemical and slurry pump seals and bearings, nozzles,pump and valve trim and more.

Initial Ra = 0.053 mFinal Ra (after lapping) = 0.02 m.


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Examples of

Lapped Parts

Hardened steel W-1. The high content of Carbon allowshigh hardness to be achieved by hardening and alsoformation of carbide, which gives the high wear resistance.The dimensions for the parts made of W-1 were 0.8ODand 0.4 thickness (as seen in figure 3.3). The initialhardness of the steel was about 10-14 HRC.The parts were heat-treated and, after quenching in oil, theresulting hardness was 44 48 HRC. The steps followedfor the heat treatment were: 1) preheat oven to 1425-1500F; 2) place part in the oven for hour per inch ofthickness; 3) quench the part in oil; 4) test the hardness.

Initial Ra = 0.5 mFinal Ra (after lapping) = 0.1 m.


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Other Finishing

OperationsPolishing

Produces a smooth, reflective

surface finish; done with disksor belts with fine abrasivegrains

Electropolishing

Produces mirror-like surfaceson metals; the electrolyte

removes peaks and raisedareas faster than lower areas;also used for deburring


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Example of a Polishing

Machine


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Examples of

Polished Parts

Polished disk drive heads compared to the size of adime


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Polishing Results


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Polishing Results


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Magnetic Finishing

Magnetic Float Polishing

A magnetic field pulls on the

magnetic abrasive fluid, floating theworkpieces and pressing themagainst a drive shaft; forces arevery small and controllable so the

polish is very fine

Magnetic Field Assisted Polishing

The workpiece is rotated on aspindle and the magnetic fieldoscillates, producing vibrations inthe magnetic abrasive fluid


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Magnetic Finishing


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Abrasive Process

Capabilities


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Deburring

Burrs

Thin ridges (usually triangular)

that form on the workpieceedges during production; canbe detrimental to the part or its

function

Traditionally removed

manually; can account for up to10% of the part manufacturingcost


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Deburring Processes

- Manual (files and scrapers)

- Mechanical by cutting

- Wire brushing- Abrasive belts

- Ultrasonic machining

- Electropolishing

- Electrochemical Machining- Magnetic abrasive finishing

- Vibratory Finishing

- Shot blasting, abrasive blasting

- Abrasive flow machining

- Thermal energy (laser, plasma)


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Deburring Processes

Vibratory and Barrel Finishing

Abrasive pellets are placed in a

container with the workpiece;the container is vibrated ortumbled

Shot Blasting

Abrasive particles are

propelled at the workpiece athigh velocity by an air jet or awheel


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Deburring Processes

Abrasive Flow Machining

An putty-like substance withabrasive grains is forced aroundand through the workpiece;especially useful for pieces withinternal spaces that cannot bereached by other means

Thermal Energy

The workpiece is exposed to aninstantaneous combustion reaction;

the burrs heat up much morerapidly than the solid part and meltaway


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Summary

Abrasive processes offer a wayto increase surface finish and

dimensional accuracy

Deburring may be necessary for

proper part fit and function


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The End

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mprocesses notes 8

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