PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
Grinding
Section 15
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
80-2
Grinding
• Characteristics of an abrasive must be:
– Harder than material being ground
– Strong enough to withstand grinding pressures
– Heat-resistant so that it does not become dull at
grinding temperatures
– Friable (capable of fracturing) so when cutting
edges become dull, they will break off and
present new sharp surfaces to material being
ground
PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
Types of Abrasives
Unit 80
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
80-4
Objectives
• Describe the manufacture of aluminum
oxide and silicon carbide abrasives
• Select the proper grinding wheel for
each type of work material
• Discuss the applications of grinding
wheels and abrasive products
80-5
Abrasive Classes
• Natural abrasives
–
–
–
–
Sandstone, garnet, flint, emery, quartz, corundum
Used prior to early part of 20th century
Almost totally replaced by manufactured abrasives
Best natural abrasives is diamond (high cost)
• Manufactured abrasives
– Used because grain size, shape and purity can be
closely controlled
– Aluminum oxide, silicon, carbide, boron carbide,
cubic boron nitride and manufactured diamond
80-6
Aluminum Oxide
•
•
•
•
Most important abrasive
Make up 75% of grinding wheels
Used for high-tensile-strength materials
Manufactured with various degrees of
purity
– Hardness and brittleness increase as purity
increases
80-7
Aluminum Oxide Purities
• Regular aluminum oxide (Al2O3) at 94.5%
– Tough abrasive capable of withstanding abuse
– Grayish in color
– Used for grinding steel, tough bronzes, etc.
• Aluminum oxide at 97.5%
– Not as tough as regular but still gray in color
– Used in manufacture of grinding wheels for centerless,
cylindrical, and internal grinding of steel and cast iron
• Purest form of aluminum oxide
– White material that produces sharp cutting edge
– Used for grinding hardest steels and stellite
80-8
Manufacture of
Aluminum Oxide
• Made from bauxite ore
– Mined by open-pit method in Arkansas and
Guyana, Suriname, and French Guiana
– Calcined (powered form) in large furnace
– Mixed with coke screenings, iron borings
• Coke used to reduce impurities
– Electrodes lowered, coke heated and fusion of
bauxite starts then more bauxite and coke added
• When furnace full, shut off, let cool
• Material broken up and fed into crushers
80-9
Silicon Carbide
• Suited for grinding materials that have low
tensile strength and high density
• Harder and tougher than aluminum oxide
• Color varies from green to black
• Green used mainly for grinding cemented
carbides and other hard materials
• Black used for grinding cast iron and soft
nonferrous metals (also ceramics)
80-10
Manufacture of Silicon Carbide
• Mixture of silica sand and high-purity coke
heated in electric resistance furnace
– Sawdust added to produce porosity and to permit
gas to escape during operation
– Salt added to assist in removing impurities
• Time required for operation is 36 hours
• Cool for 12 hours, remove sidewalls where
unfused mixture falls to floor leaving silicon
carbide ingot
• Ingot crushed; resultant silicon carbide
treated, screened and graded
80-11
Zirconia-Aluminum Oxide
• First alloy abrasive produced
– Made by fusing zirconium oxide and aluminum
oxide at extremely high temperatures
– Contains about 40% zirconia
• Used for heavy-duty rough and finish
grinding in steel mills, for snagging in
foundries and for rapid rough and finish
grinding of welds
• Performance superior to standard aluminum
oxide (last 2 to 5 times longer)
80-12
Advantages of Zirconia-alumina
Over Standard Abrasives
•
•
•
•
Higher grain strength
Higher impact strength
Longer grain life
Maintains its shape and cutting ability under
high pressure and temperature
• Higher production per wheel or disk
• Less operator time spent changing wheels
or disks
80-13
Boron Carbide
• Hardest material manufactured with
exception of diamond
• Not suitable for use in grinding wheels
– Used only as loose abrasive and as cheap
substitute for diamond dust
• Manufacture of precision gages and sand blast
nozzles
– Used in ultrasonic machining applications
80-14
Manufacture of Boron Carbide
• Produced by dehydrated boric acid being
mixed with high-quality coke
– Mixture heated in horizontal steel closed
cylinder, hole for graphic electrode and hole for
escaping gases
• To remove air, dampen mixture with
kerosene to volatize and expel air
• High current at low voltage applied for
about 24 hours, then cooled
• Resultant product is hard, black lustrous
material
80-15
Cubic Boron Nitride (CBN)
• Synthetic abrasive has hardness properties
between silicon carbide and diamond
• Developed by General Electric Company in
1969
• Capable of withstanding grinding
temperatures up to 2500ºF
• Cool-cutting and chemically resistant to all
inorganic salts and organic compounds
• Capable of maintaining very close
tolerances
80-16
Manufacture of CBN
• Synthesized in crystal form from hexagonal
boron nitride with aid of catalyst, extreme
heat (2725ºF) and tremendous pressure
– Strong, hard, blocky crystalline structures with
sharp corners
• Two types
– Borozon CBN: uncoated abrasive used for
general-purpose grinding
– Boraxon Type II CBN: nickel-plated grains
used in resin bonds for general-purpose dry and
wet grinding
80-17
Manufactured Diamonds
• 1954, General Electric Company produced ManMadey diamonds in laboratory
• 1957, General Electric Company began
commercial production of diamonds
• First success involved carbon and iron sulfide in
granite tube closed with tantalum disks were
subjected to pressure of 66,536,750 psi and
temperatures between 2550ºF
– Temperatures must be high enough to melt metal
saturated with carbon and start diamond growth
• Industrial diamonds referred to as bort
80-18
Diamond Types
• Type RVG Diamond
– Elongated, friable crystal with rough edges
– Letters indicate it can be used with resinoid or
vitrified bond and used for grinding ultrahard
materials
• Tungsten carbide
• Silicon carbide
• Space-age alloys
– Used for wet or dry grinding
80-19
• Type MBG-II Diamond
–
–
–
–
Tough and block-shaped crystal
Not as friable as RVG type
Used in metal-bonded grinding wheels
Used for grinding cemented carbides, sapphires,
and ceramics as well as electrolytic grinding
• Type MBS Diamond
– Blocky, extremely tough crystal with smooth,
regular surface and not friable
– Used in metal-bonded saws to cut concrete,
marble, tile, granite, stone, and masonry
80-20
Ceramic Aluminum Oxide
• Known as SG abrasive, introduced by Norton
Company in 1988
• Outperforms conventional aluminum oxide
• Made by nonfused process
– Thousands of submicron-sized particles are sintered to
provide single abrasive grain of uniform shape and size
with more cutting edges that remain sharp
• SG abrasive well suited to CNC grinding
– Fewer wheel changes, less wheel dressing, higher
productivity and therefore lower labor costs
80-21
SG and CBN Abrasives
• Combination of technologies of CBN and
SG abrasives used to produce vitrified
CVSG abrasive grinding wheel
– Provides most of high material-removal rates
and low wheel wear of CBN yet can be trued
with single point diamond tools
• Free-cutting, allow increased depths of cuts
and feedrates, reduce burning, and lower
grinding costs
80-22
Advantages of SG Abrasives
Over Conventional Abrasives
• Last 5 to 10 times longer than conventional
wheels
• Metal-removal rates are doubled
• Heat damage to surface of very thin
workpieces reduced
• Grinding cycle time reduced
• Dressing time reduced as much as 80%
80-23
Abrasive Products
• After abrasives produced, formed into
products
– Grinding wheel
• Most important
• Abrasive material held together with suitable bond
• Material components are abrasive grain and bond;
other physical characteristics such as grade and
structure
– Coated abrasives
– Polishing and lapping powders
– Abrasive sticks
80-24
Basic Functions of
Grinding Wheels
1. Generation of cylindrical, flat and curved
surfaces
2. Removal of stock
3. Production of highly finished surfaces
4. Cutting-off operations
5. Production of sharp edges and points
80-25
Abrasive Grain
• Aluminum oxide or silicon carbide abrasive
used in most grinding wheels
• Each grain on working surface of grinding
wheel acts as separate cutting tool
– Removes small metal chip as passes over
surface of work
– As grain becomes dull, fractures and presents
new sharp cutting edge to material
• Fracturing action reduces heat of friction, producing
relatively cool cutting action
80-26
Grain Size
• Abrasive ingot (pig) removed from electric
furnace, crushed, grains cleaned and then
sized by passing them through screens
– Contain certain number of meshes or openings
per inch
8-grain
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
24-grain
60-grain
80-27
Commercial Grain size
Classification
Very
Coarse Coarse Medium
1
14
30
8
16
36
10
20
46
12
24
54
60
Fine
70
80
90
100
120
Very Fine Flour Size
150
280
180
320
220
400
240
500
600
80-28
Grain Sizes
• General applications for various grain sizes
– 8 to 54 for rough grinding operations
– 54 to 400 for precision grinding processes
– 320 to 2000 for ultra precision processes to
produce 2 to 4 µ (micron) finish or fine
80-29
Factors Affecting Selection of
Grain Sizes
1. Type of finish desire
2. Type of material being ground
3. Amount of material to be removed
4. Area of contact between wheel and
workpiece
80-30
Bond Types
• Function of bond is to hold abrasive grains
together in form of wheel
• Six common bond types used in grinding
wheel manufacture:
–
–
–
–
–
–
Vitrified
Resinoid
Rubber
Shellac
Silicate
Metal
80-31
Vitrified Bond
• Used on most grinding wheels
• Made of clay or feldspar
• Fuses at high temperature and when cooled
forms glassy bond around each grain
• Strong but break down readily on wheel
surface to expose new grains during grinding
• Bond suited for rapid removal of metal
• Not affected by water, oil, or acid
80-32
Resinoid Bond
• Synthetic resins used as bonding agents
• Generally operate at 9500 sf/min
• Wheels are cool-cutting and remove stock
rapidly
• Used for cutting-off operations, snagging,
and rough grinding, as well as for roll
grinding
80-33
Rubber Bond
• Produce high finishes
– Ball bearing races
• Used for thin cutoff wheels because of its
strength and flexibility
• Used also as regulating wheels on centerless
grinders
80-34
Shellac Bond
• Used for producing high finishes on parts
such as cutlery, cam shafts, and paper-mill
rolls
• Not suitable for rough or heavy grinding
80-35
Silicate Bond
• Not used to any extent in industry
• Used principally for large wheels and for
small wheels where necessary to keep heat
generation to minimum
• Bond (silicate of soda) releases abrasive
grains more rapidly than does vitrified bond
80-36
Metal Bond
• Generally nonferrous
• Used on diamond wheels and for
electrolytic grinding operations where
current must pass through wheel
80-37
Grade
• Defined as degree of strength with which
bond holds abrasive particles in bond setting
• Hard grade
– When bond posts very strong (retain abrasive
grains during grinding operation)
• Soft grade
– Grains released rapidly during grinding operation
• Wheel grade symbols indicated
alphabetically, from A (softest) to Z (hardest)
80-38
Factors that Determine the Grade
Selected for Particular Job
1.
2.
3.
4.
5.
6.
Hardness of material
Area of contact
Condition of machine
Speed of grinding wheel and workpiece
Rate of feed
Operator characteristics
80-39
Structure
• Space relationship of grain and bonding
material to the voids that separate them
– Density of wheel
• Dense structure has close grain spacing
• Open structure has relatively wide spacing
• Selection of wheel structure depends on type
of work required
• Indicated by numbers ranging from
1 (dense) to 15 (open)
80-40
Factors Affecting the Selection of
the Proper Wheel Structure
1. Type of material being ground
•
Soft material require greater chip clearance,
therefore open wheel
2. Area of contact
•
Greater area of contact, more open structure
3. Finish required
•
Dense wheels give better, accurate finish
4. Method of cooling
•
Open-structure wheels provide better supply
of coolant
80-41
Grinding Wheel Manufacture
• Most grinding wheels used for machine
shop operations are manufactured with
vitrified bonds
• Main operations in manufacture of vitrified
grinding wheels:
1.
2.
3.
4.
Mixing
Molding
Shaving
Firing (Burning)
5.
6.
7.
8.
Truing
Bushing
Balancing
Speed Testing
80-42
• Mixing
– Correct proportions of abrasive grain and bond
carefully weighed and thoroughly mixed in
rotary power mixing machine
– Certain percentage of water added to moisten mix
• Molding
– Proper amount of mixture placed in steel mold
of desired wheel shape and compressed in
hydraulic press to form wheel slightly larger
than finished size
– Amount of pressure used varies with size of
wheel and structure required
80-43
• Shaving
– Some machines requires special wheel shapes
and recesses
• Shaped or shaved to size in green, or unburned, state
on shaving machine which resembles potter's wheel
• Firing (Burning)
– Green wheels carefully stacked on cars and
moved slowly through long kiln 250 to 300 ft
long with temperature held at ~2300ºF
– Takes about 5 days
– Causes bond to melt and form glassy case
around each grain producing hard wheel
80-44
• Truing
– Cured wheels mounted in special lathe
– Turned to required size and shape by hardenedsteel conical cutters, diamond tools, or special
grinding wheels
• Bushing
– Arbor hole in grinding wheel fitted with lead or
plastic-type bushing to fit specific spindle size
– Edges of bushing are trimmed to thickness of
wheel
80-45
• Balancing
– Remove vibration that may occur while wheel
revolving
– Small, shallow holes drilled in "light" side of
wheel and filled with lead to ensure proper
balance
• Speed Testing
– Wheels rotated in heavy, enclosed case and
revolved at speeds at least 50% above normal
operating speeds
– Ensures wheel will not break under normal
operating speeds and conditions
80-46
Standard Grinding
Wheel Shapes
• Nine standard grinding wheel shapes
established by:
– United States Department of Commerce
– Grinding Wheel Manufacturers
– Grinding Machine Manufacturers
• Dimensional sizes for each of the shapes
have also been standardized
– Table 80.3 in textbook
80-47
Mounted Grinding Wheels
• Driven by steel shank mounted in wheel
• Produced in variety of shapes for use with
jig grinders, internal grinders, portable
grinders, toolpost grinders, and flexible
shafts
• Manufactured in both aluminum oxide and
silicon carbide types
80-48
Grinding Wheel Markings
• Standard marking system chart used by
manufacturers to identify grinding wheels
– Information found on blotter of all small and
medium-size grinding wheels
– Stenciled on side of larger wheels
• Six positions in standard sequence
– Prefix is manufacturer's symbol and not always
used by all grinding wheel producers
• Marking system used only for aluminum
oxide and silicon carbide wheels, not
diamond wheels
80-49
Standard Marking System
opt. 1
2
3
51
36
L
A
4
5
5
6
V
23
V – Vitrified
Manufacturer's
size
GradeStructure
SGrain
–Type
Silicate
Abrasive
BondManufacturers
type
Manufacturers
R – Rubber
Soft
Medium
Hard
Asymbol
– Aluminum
Oxide
Dense to OpenRecord
private
indicating
exact
ASB –C Silicon
DE
FG
H–
I JRubber
K
LM
RSTUVWXYZ
RF
reinforced
Carbide
1 2Nmarking
3O4P5Q…..etc.
to identify wheel
kind of abrasive
B – Resinoid
(use optional)
BF – Resinoid reinforced
E – Shellac
O - Oxychloride
80-50
Selecting a Grinding Wheel for a
Specific Job: Example 1
It is required to rough surface grind a piece of SAE 1045 steel
using a straight wheel. Coolant is to be used.
Type of abrasive: steel to be ground, use aluminum oxide
Size of grain: surface not precision-finished, medium grain about 46 grit
Grade: medium-grade wheel will break down reasonably well, use grade J
Structure: steel of medium hardness, wheel should be
medium density: about 7
Bond type: operation standard surface grinding and since coolant is to be
used, a vitrified bond should be selected
After all factors have been considered, an A46-J7-V wheel
should be selected to rough-grind SAE 1045 steel.
80-51
Selecting a Grinding Wheel for a
Specific Job: Example 2
It is required to finish-grind a high-speed steel milling cutter
on the cutter and tool grinder.
Type of abrasive: cutter is steel, an aluminum oxide wheel should be used
Size of grain: must have a smooth finish, a medium to fine grain (60 grit)
Grade: cool-cutting wheel be used to prevent burning, medium-soft grade J
Structure: smooth cut, medium-dense wheel should be used, use a #6
Bond type: most cutter and tool grinders designed for standard speeds,
vitrified bond should be used; when speed excessive for wheel
size, a resinoid bond should be used
The wheel selected for this job (disregarding the manufacturer’s
prefix and records) should be A60-J6-V.
80-52
Precautions to Observe When Handling
and Storing Grinding Wheels
1. Never handle wheels carelessly
•
Treat them as precision instruments
2. Dry at a reasonable temperature
3. Store wheels properly
•
•
•
•
Straight or tapered wheels best stored on edge in
individual racks to prevent rolling
Thin, organic bonded wheels laid on flat horizontal
surface to prevent warping
Small cup and internal wheels put separately into
boxes, bins, or drawers
Large cup and cylindrical wheels should be stored on
flat sides with packing between wheels
80-53
Wheel Grade Faults
• Grade of grinding wheel most important and
difficult to select to suit workpiece material
and grinding operation
• Wise to start medium-grade wheel such a J
• Note performance, adjust grade until best
grinding conditions are reached
• Know characteristic of wheels that are too
soft or too hard
80-54
Characteristics That Indicate
Wheel Too Soft
1.
2.
3.
4.
5.
6.
Breaks down too fast
Poor surface finish
Cuts freely
Sparks out quickly
Difficult to maintain size
Scratches (fishtails)
80-55
Characteristics That Indicate
Wheel Too Hard
1.
2.
3.
4.
5.
6.
7.
8.
9.
Wheel glazes quickly
Loading (material ground fills voids)
Burned work surface
Squealing noise
Doesn't cut freely
Inaccurate work dimensions
Surface finish get progressively better
Won't spark out
Heat checks
80-56
Inspection of Wheels
• Inspect wheels after they have been received
– Damage might occur during transit
• Suspend and tap lightly with screwdriver
handle for small wheels or with wooden
mallet for larger wheels
– Vitrified or silicate wheels give clear, metallic
ring when sound
– Organic-bonded wheels give duller ring
– Cracked wheels do not produce ring
80-57
Diamond Wheels
• Used for grinding cemented carbides and
hard vitreous materials
• Manufactured in variety of shapes
– Straight, cup, dish, and thin cutoff wheels
• Wheels ½ in. diameter or less have diamond
particles throughout wheel, larger than ½ in.
made with diamond surface on grinding
face only
80-58
Diamond Grain Sizes
• Grain sizes range from 100 to 400
• Proportions of diamond and bond mixture
vary with application
– Diamond concentration identified by
letter A, B, or C
• C concentration contains four times number of
diamonds in an A concentration
• Mixture coated on grinding face of wheel in
thickness ranging from 1/32 to 1/4 in.
80-59
Three Types of Bonds for
Diamond Wheels
1. Resinoid-bonded
–
–
–
Give maximum cutting rate and require little
dressing
Remain sharp for long time and well suited to
grinding carbides
New development has been to coat diamond
particles with nickel plating before mixed
with resin

Reduces tendency to chip and results in coolergrinding, longer lasting wheels
80-60
• Metal bonds
– Generally nonferrous
– Particularly suited to offhand grinding and
cutting-off operations
– Holds form extremely well and does not wear
on radius work on small areas of contact
• Vitrified-bonded wheels
– Remove stock rapidly but require frequent
cleaning with boron carbide abrasive stick to
prevent loading
– Suited for offhand and surface grinding of
cemented carbides
80-61
Diamond Wheel Identification
• Method differs from that used for other
grinding wheels
ASD 100
R
75
B99 1/8*
Grit Size
Bond
24
120 Grade
400S
Abrasive
Modification
Concentration
Depth of
Bond Type
36 No
150
500shown
Diamond
to
designate
Res.
Metal
Vit.B
Note:
grade
hand hones
LowNumeral
= 25
Diamond
Section
=for
Resinoid
46 H R
180 L 500S
D = Mined
bond 1/8 1/4
Q H Pspecial
50= Metal
1/16
M
60
SD = Human-made
Example:
J220 N 600S
R modification.
J RV75
= Vitrified
80
ASD*= Armored
diamond
Manufacturer's
identification
L240
O 800S
THighLResinoild
T= 100 – 6 and 22symbol
100 N320 P1200S N
This symbol may be
100S 400 1500S sometimes omitted.
80-62
Cubic Boron Nitride Wheels
• Recognized as superior cutting tools for grinding
difficult-to-machine metals
• Have more than twice hardness of conventional
abrasives
– Also toughness to match so cutting edges stay sharp
longer with much slower wear rates
• Prolonged cutting capacity and high thermal
conductivity help prevent uncontrolled heat
buildup (reduce chances of glazing)
• Thermally and chemically stable at temperatures
above 1832ºF
80-63
Knoop
Hardness
Scale
Hardness of Various
Metals and Abrasives
7000
6000
5000
4000
3000
2000
1000
0
Diamond CBN Vanadium AL2O3 Tungsten High Steel Soft Steel
Carbide
Carbide HRC 65 HRB 85
80-64
Properties of Cubic Boron
Nitride Wheels
•
Contain four main properties necessary to
grind extremely hard or abrasive materials
at high metal-removal rates:
1.
2.
3.
4.
Hardness
Abrasion resistance
Compressive strength
Thermal conductivity
80-65
Wheel Selection
• Type of wheel selected and how used will
affect metal-removal rate (MRR) and life of
grinding wheel
• Generally affected by:
–
–
–
–
–
Type of grinding operation
Grinding conditions
Surface finish requirements
Shape and size of workpiece
Type of workpiece material
80-66
Wheel Selection Guidelines
• CBN grinding wheels available in complete
range of shapes and sizes
• Individually engineered wheels available to
suit specific systems
• Constructed with precision, preformed core
with abrasive portion on grinding face of
wheel
– Usually between 1/16 to 1/4 in. in depth
• Generally one readily available to suit any
grinding operation
80-67
General Guidelines for Using
CBN Grinding Wheels
1. Select bond
2. Specify normal wheel diameters and
widths
3. Choose largest abrasive mesh size that
produces desired finish
4. Choose wheels with optimum abrasive
concentration
80-68
Coated Abrasives
• Consist of flexible backing (cloth or paper) to
which abrasive grains have been bonded
• Two purposes
– Metal grinding and polishing
• Coarse-grit used for rapid removal of metal; fine
grits used for polishing
• Two types
– Natural: garnet, flint, and emery
– Manufactured: aluminum oxide, silicon carbide
80-69
Selection of Coated Abrasives
• Emery (natural abrasive)
– Black in appearance
– Used to manufacture emery cloth and emery
paper
– Grains not as sharp as artificial abrasives
– Generally used for polishing metal by hand
80-70
• Aluminum Oxide (manufactured abrasive)
–
–
–
–
Gray in appearance
Used for high-tensile-strength materials
Characterized by long life of cutting edges
Hand operations use 60-80 grit for roughing
and 120 to 180 grit for finishing
– Machine operations use 36-60 grit for roughing
and 80-120 grit for finishing operations
• Silicon Carbide (manufactured abrasive)
– Bluish-black in appearance
– Used for low-tensile-strength materials
– Selection of grit size same as aluminum oxide
80-71
Coated Abrasive Machining
• Over past few years, coated abrasive
machining has become widely used in
industry
– Improved abrasives and bonding material,
better grain structure, more uniform belt
splicing, and new polyester belt backing,
abrasive belt machining
• Now capable of grinding to less than .001in
tolerance and surface finish of to to 20 µin.