ManufProc2_4Abrasive

8/9/2019 ManufProc2_4Abrasive

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Chapter # 4

Abrasive Material Removal

Processes

1


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!hy Abrasive "rocesses are mportant

Can be used on all types of materials

$ome can produce e%tremely fine surface

finishes, to &'&() µm *1 µ+in

$ome can hold dimensions to e%tremely close

tolerances


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Grinding

Material removal process in which abrasiveparticles are contained in a bonded grinding

wheel that operates at very high surface

speeds

Grinding wheel usually dis- shaped and‑

precisely balanced for high rotational speeds


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.he Grinding !heel

Consists of abrasive particles and bondingmaterial

Abrasive particles accomplish cutting

/onding material holds particles in place

and establishes shape and structure ofwheel


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Grinding !heel "arameters

Abrasive material

Grain si0e

/onding material

!heel grade

!heel structure


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Abrasive Material "roperties

igh hardness

!ear resistance

.oughness

2riability + capacity to fracture when cutting

edge dulls, so a new sharp edge is e%posed


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.raditional Abrasive Materials

Aluminum o%ide *Al(O3 most common‑

abrasive

sed to grind steel and other ferrous

high strength alloys‑

$ilicon carbide *$iC harder than Al‑ (

O3 but not

as tough

sed on aluminum, brass, stainless steel,

some cast irons and certain ceramics


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5ewer Abrasive Materials

Cubic boron nitride *c/5 6 very hard, verye%pensive

$uitable for steels

sed for hard materials such as hardened

tool steels and aerospace alloys

Diamond 6 7ven harder, very e%pensive

Occur naturally and also made synthetically

5ot suitable for grinding steels

sed on hard, abrasive materials such as

ceramics, cemented carbides, and glass


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Grain $i0e

$mall grit si0es produce better finishes

8arger grit si0es permit larger material removal

rates

arder wor- materials re9uire smaller grain

si0es to cut effectively

$ofter materials re9uire larger grit si0es


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/onding Material "roperties

Must withstand centrifugal forces and hightemperatures

Must resist shattering during shoc- loading

of wheel

Must hold abrasive grains rigidly in place forcutting yet allow worn grains to be dislodged

to e%pose new sharp grains


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!heel $tructure

:efers to the relative spacing of abrasive grainsin wheel

n addition to abrasive grains and bond

material, grinding wheels contain air gaps or

pores ;olumetric proportions of grains, bond material,

and pores can be e%pressed as


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.ypical structure of a grinding wheel'

!heel $tructure


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!heel $tructure

Measured on a scale that ranges between


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!heel Grade

ndicates bond strength in retaining abrasive gritsduring cutting

=epends on amount of bonding material in

wheel structure *P b

Measured on a scale ranging between soft andhard

$oft wheels lose grains readily + used for

low material removal rates and hard wor-

materials ard wheels retain grains + used for high

stoc- removal rates and soft wor- materials


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$ome of the standard grinding wheel shapes *a straight, *b recessed

two sides, *c metal wheel frame with abrasive bonded to outside

circumference, *d abrasive cut off wheel'‑

Grinding !heel $hape


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$urface 2inish

Most grinding is performed to achieve goodsurface finish

/est surface finish is achieved by

$mall grain si0es

igher wheel speeds

=enser wheel structure > more grits per

wheel area


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.hree .ypes of Grain Action

Cutting + grit pro?ects far enough into surfaceto form a chip + material is removed

Plowing + grit pro?ects into wor-, but not far

enough to cut + instead, surface is deformed

and energy is consumed, but no material isremoved

Rubbing + grit contacts surface but only

rubbing friction occurs, thus consuming energy,

but no material is removed


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.emperatures at !or- $urface

Grinding is characteri0ed by high

temperatures and high friction, and most of

the energy remains in the ground surface,

resulting in high wor- surface temperatures

=amaging effects include

$urface burns and crac-s

Metallurgical damage immediately

beneath the surface

$oftening of the wor- surface if heat

treated

:esidual stresses in the wor- surface


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ow to :educe Grinding .emperatures

=ecrease infeed *depth of cut d :educe wheel speed v

:educe number of active grits per s9uare inch

on the grinding wheel C

ncrease wor- speed v w

se a grinding fluid

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Causes of !heel !ear

1. Grain fracture + when a portion of the grainbrea-s off, but the rest remains bonded in the

wheel

7dges of the fractured area become new

cutting edges .endency 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

=epends on wheel grade, among other

factors

sually occurs because grain has becomedull due to attritious wear, and resulting

cutting force becomes e%cessive


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Grinding :atio

ndicates slope of the wheel wear curve

where GR > grinding ratio V w > volume of wor-

material removed and V g > corresponding

volume of grinding wheel worn


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=ressing the !heel

Dressing + accomplished by rotating dis-,abrasive stic-, or another grinding wheel

held against the wheel being dressed as it

rotates

unctions /rea- off dulled grits to e%pose new

sharp grains

:emove chips clogged in wheel


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Application Guidelines

.o optimi0e surface finish, select $mall grit si0e and dense wheel structure

se higher wheel speeds *v and lower

wor- speeds *v w

$maller depths of cut *d and larger

wheel diameters *D will also help

.o ma%imi0e material removal rate, select

8arge grit si0e

More open wheel structure


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2or steel and most cast irons, use Aluminum o%ide as the abrasive

2or most nonferrous metals, use

$ilicon carbide as the abrasive

2or hardened tool steels and certain

aerospace alloys, use

Cubic boron nitride as the abrasive

2or hard abrasive materials *e'g', ceramics,cemented carbides, and glass use

=iamond as the abrasive


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2or soft metals, use 8arge grit si0e and harder grade wheel

2or hard metals, use

$mall grit si0e and softer grade wheel


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*a hori0ontal spindle with reciprocating wor-table, *b hori0ontal

spindle with rotating wor-table, *c vertical spindle with reciprocating

wor-table, *d vertical spindle with rotating wor-table'

2our .ypes of $urface Grinding


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$urface grinder with hori0ontal spindle and reciprocating wor-table

*most common grinder type'

$urface Grinder


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.wo types of cylindrical grinding *a e%ternal, and *b internal'

Cylindrical Grinding


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7%ternal centerless grinding'

Centerless Grinding


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Creep 2eed Grinding

=epths of cut 1&&& to 1&,&&& times greaterthan in conventional surface grinding

2eed rates reduced by about the same

proportion

Material removal rate and productivity areincreased in creep feed grinding because the

wheel is continuously cutting

n conventional surface grinding, wheel is

engaged in cutting for only a portion of thestro-e length


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Other Abrasive "rocesses

oning 8apping

$uperfinishing


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oning

Abrasive process performed by a set of bondedabrasive stic-s using a combination of

rotational and oscillatory motions

Common application is to finish the bores of

internal combustion engines $urface finishes of &'1( µm *) µ+in or better

Creates a characteristic cross hatched surface‑

that retains lubrication


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.he honing process *a the honing tool used for internal

bore surface, and *b cross hatched surface pattern‑

created by the action of the honing tool'

oning


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8apping

ses fluid suspension of very small abrasiveparticles between wor-piece and lap *tool

!a""ing com"ound + fluid with abrasives,

general appearance of a chal-y paste

A""lications optical lenses, metallic bearingsurfaces, gages


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.he lapping process in lens ma-ing'‑

8apping


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$uperfinishing on an e%ternal cylindrical surface'

$uperfinishing

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