Abrasive Processes

8/8/2019 Abrasive Processes

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Material removal by action ofhard, abrasiveparticles usually in the form of a bonded wheel

Generally used as finishing operations after partgeometry has been established by conventionalmachining

Grinding is most important abrasive process Other abrasive processes:

honing, lapping, superfinishing, polishing, and buffing


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Definition of an abrasive A small, non-metallic hard particle having

sharp edges and an irregular shape.


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1. 2.

3.

4.


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Evidence of grinding dates back to 2000 BCwhen ancient Egyptian used to grind largestones for pyramid construction.

Large water or wind driven grinding wheelsused to make flour (in a gristmill) since themiddle ages in Europe.

The first modern bonded grinding wheel weremade in the 1800s in India to grind gemstones.


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Can be used on all types of materials

Some can produce extremely fine surfacefinishes, to 0.025 m (1 -in)

Some can hold dimensions to extremely closetolerances


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Grinding operations generally involvebonded abrasive discs.

Fine grinding, honing and polishing willsometimes involve non-bonded abrasives(loose). Examples: optical grinding (optical lenses, mirrors)

and cylinder bores of combustion engines.


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Abrasive material Grain size

Bonding material

Wheel grade Wheel structure


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1. High hardness2. Wear resistance3. Toughness4. Friability - capacity to fracture when cuttingedge dulls, so a new sharp edge is exposed.

Materials with poor friability become loaded with thestock material, cut poorly and generate heat.


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Aluminum oxide (Al2O3) Most common abrasive

Used to grind steel and otherferrous high-strength alloys

Silicon carbide (SiC) Harder than Al2O3 but not as

tough

Used on aluminum, brass,stainless steel, some cast ironsand certain ceramics


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Cubic boron nitride (cBN) Very hard, very expensive Used for hard materials such as

hardened tool steels and aerospacealloys

Diamond Even harder, very expensive Natural and synthetic Not suitable for grinding steels Used on hard, abrasive materials

such as ceramics, cemented carbidesand glass


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1. Cutting - grit projects far enough intosurface to form a chip - material is removed

2. Plowing - grit projects into work, but not farenough to cut - instead, surface isdeformed and energy is consumed, but nomaterial is removed

3. Rubbing - grit contacts surface but onlyrubbing friction occurs, thus consumingenergy, but no material is removed


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(a) cutting (b) plowing (c) rubbing


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Small grit sizes produce better finishes Larger grit sizes permit larger material

removal rates

Harder work materials require smaller grainsizes to cut effectively

Softer materials require larger grit sizes


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Grit size is measured using a screen meshprocedure Smaller grit sizes indicated by larger numbers in

the screen mesh procedure and vice versa

Grain sizes in grinding wheels typically rangebetween 8 (very coarse) and 250 (very fine)


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Must withstand centrifugal forces and hightemperatures

Must resist shattering during shock loadingof wheel

Must hold abrasive grains rigidly in placefor cutting yet allow worn grains to bedislodged to expose new sharp grains


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1. Vitrified baked clay and ceramic. General purposewheels.2. Silicate sodium silicate. Minimizes heat generation.3. Rubber most flexible. Good for cutoff wheels.4. Resinoid thermosets. Good strength for cutoff

wheels.

5. Shellac not rigid. Good for surface finish.6. Metallic bronze bond for diamond or cBN matrix.


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Refers to the relative spacing ofabrasive grains in wheel

In addition to abrasive grainsand bond material, grindingwheels contain air gaps orpores

Volumetric proportions of

grains, bond material and porescan be expressed as:

01.=++ pbg PPP


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Measured on a scale that ranges between"open" and "dense."

Open structure means Pp is relatively large andPg is relatively small - recommended whenclearance for chips must be provided

Dense structure means Pp

is relatively small andPg is larger - recommended to obtain bettersurface finish and dimensional control


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Indicates bond strength in retaining abrasivegrits during cutting

Depends on amount of bonding material inwheel structure (Pb)

Measured on a scale ranging between softand hard Soft wheels lose grains readily - used for low

material removal rates and hard work materials

Hard wheels retain grains - used for high stockremoval rates and soft work materials


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Standard grinding wheel shapes: (a) straight, (b) recessedtwo sides, (c) metal wheel frame with abrasive bonded tooutside circumference, (d) abrasive cutoff wheel.


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Standard grinding wheel marking systemused to designate abrasive type, grit size,grade, structure, and bond material


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Usually a bonded wheel of abrasive material

is used to grind material off of the workpiece.

Slow but exact process to create a very highquality surface finish.

(a) horizontal spindle with reciprocatingworktable,(b) horizontal spindle with rotatingworktable,

(c) vertical spindle with reciprocatingworktable,(d) vertical spindle with rotating worktable.


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Similar to using a lathe, a rotating tool(grinding wheel) is held to a rotatingworkpiece.

(a) External (b) Internal


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The feed direction in grinding can be:

Traverse Feed Plunge Feed


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Tool post grinders are often mounted onlathes to perform additional work:


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In centerless grinding, the stock materialsown centre is found. Allows for moreprecision grinding. (ie. The work is notheld to the machines center.)

External centerless grinding


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Depths of cut 1000 to 10,000 times greaterthan in conventional surface grinding Feed rates reduced by about the same proportion

Material removal rate and productivity areincreased in creep feed grinding because thewheel is continuously cutting In conventional surface grinding, wheel is

engaged in cutting for only a portion of thestroke length


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(a) conventional surface grinding (b) creep feed grinding.

Creep Feed Grinding


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In grinding belts, the abrasive materialsare bonded to belts instead of largewheels.


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Bench & Tool grinders are used sharpen (orre-sharpen) tools by hand.

www.thorvie.comwww.tlc-direct.co.uk


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Honing Superfinishing

Lapping

Polishing

Buffing

By using afiner and finergrain, a

smoothersurface finishcan beachieved

Best


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Abrasive process performed by a set ofbondedabrasive sticks using a combination ofrotational and oscillatory motions Common application is to finish the bores of

internal combustion engines or gun barrels

Grit sizes range between 30 and 600

Surface finishes of 0.12 m (5 -in) or better Creates a characteristic cross-hatched

surface that retains lubrication


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(a) the honing tool used for internal bore surface, and (b)cross-hatched surface pattern created by the action of thehoning tool.


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In honing a set of abrasive sticks is used to

remove machining marks.

VIDEO Harley Davidson (HD) honing


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Similarities with honing: Uses bonded abrasive stick pressed against surface

to remove ~ 1um layer

Reciprocating motion with abrasive stick

Differences with honing: Shorter strokes

Higher frequencies

Lower pressures between tool and surface Smaller grit sizes


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Superfinishing on an external cylindrical surface.

Similar to honing, but a high frequency, short stroke is used.


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Uses fluid suspension of very small abrasiveparticles between workpiece and lap (tool)

Lapping compound aluminum oxide, emery,SiC, diamond

Typical grit sizes between 300 to 600

Applications: optical lenses, metallic bearing

surfaces, gages


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Hubble space telescope is a famous lappedinstrument. Error in flatness of the primary mirror was

2.2 microns. Shape off spec by 10 nm!


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Used to improvethe visualappearance of theworkpiece.

An abrasive issuspended in acompound or waxand applied with acloth/leather/felt.

www.pushcorp.com


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CLASS NOTES


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Size effect - small chip size energy toremove each unit volume of material is veryhigh Roughly 10 times higher for grinding compared

to conventional machining

Individual grains have extremely negativerake angles, resulting in low shear planeangles and high shear strains

Not all grits are engaged in actual cutting


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Wear is plotted as a function of volume of material removed,rather than as a function of time.

(1) Initial wear; (2) Steady state; (3) Failure


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Indicates slope of the wheel wear curve

whereGR= grinding ratioVw= volume of work material removedVg= corresponding volume of grinding

wheel worn

gV

VGR

W

=


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Grinding is characterized by high temperaturesand high friction. Most of energy remains in the ground surface,

resulting in high work surface temperatures

Surface Damage:1. Surface burns and cracks

2. Metallurgical damage immediately beneath the

surface3. Softening of the work surface if heat treated

4. Residual stresses in the work surface


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Decrease infeed (depth of cut) d Reduce wheel speed v

Reduce number of active grits per square inchon the grinding wheel C

Increase work speed vw Use a grinding fluid


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1. Grain fracture - when a portion of the grainbreaks off, but the rest remains bonded inthe wheel Edges of the fractured area become new cutting

edges Tendency to fracture is called friability


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2. Attritious wear - dulling of individual grains,resulting in flat spots and rounded edges Analogous to tool wear in conventional cutting

tool

Caused by similar mechanisms includingfriction, diffusion and chemical reactions


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3. Bond fracture - the individual grains arepulled out of the bonding material Depends on wheel grade

Usually occurs because grain has become dull

due to attritious wear and resulting cuttingforce becomes excessive


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1. Washing awaychips,

2. Keeping the wheelfrom becomingclogged,

3. Control of grinding

dust

4. Reduces operatingtemperatures


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INDEPENDENT STUDY


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Dressing - accomplished by rotating disk,abrasive stick, or another grinding wheelagainst the wheel being dressed as it rotatesto: Break off dulled grits to expose new sharp grains

Remove chips clogged in wheel

Required when wheel is in third region of wearcurve

Although dressing sharpens, it does not guaranteethe shape of the wheel


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Due to poor friability, the grinding wheelsoften get loaded with workpiece material.


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Truing - use of a diamond-pointed tool fed slowlyand precisely across wheel as it rotates Very light depth is taken (0.025 mm or less)

against the wheel

Not only sharpens wheel,

but restores cylindricalshape and ensuresstraightness acrossoutside perimeter


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To optimize surface finish, select Small grit size and dense wheel structure

Use higher wheel speeds (v) and lower workspeeds (vw)

Smaller depths of cut (d) and larger wheeldiameters (D) will also help

To maximize material removal rate, select

Large grit size More open wheel structure

Vitrified bond


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For soft metals, use Large grit size and harder grade wheel

For hard metals, use Small grit size and softer grade wheel

Abrasive Processes

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