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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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Cylindrical Grinding
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Cylindrical Grinding
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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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Ultrasonic Machining
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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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Ultrasonic Machining
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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