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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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Causes of !heel !ear
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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Causes of !heel !ear
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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Application Guidelines
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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Application Guidelines
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