1. GRINDING
• Grinding is the process of removing excess
material from the workpiece by the mechanical
action of abrasive particles that are held
together by an bond, generally in the form of
solid wheel.
• The wheel known as the grinding wheel is
rotated at high speeds, and when the surface of
the rotating wheel is brought in contact with the
workpiece, material is removed in the form of
fine chips.
• As the process removes very little material in the
from of fine chips, it is a finishing process
2. GRINDING
• The grinding wheel is usually in disk
shaped and is precisely balanced for high
rotational speeds.
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Chip formation
In grinding, the chips are small but are
formed by the same basic mechanism of
compression and shear. Burning chips are the
sparks observed during grinding with no cutting
fluid, because the chips have heat energy to
burn or melt in the atmosphere. The feeds and
depths of cut in grinding are small, while the
cutting speed is high.
Grinding may be classified as non-precision
or precision, according to purpose and
procedure.
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Non-precision grinding
The common forms are called, snagging
and off-hand grinding. Both are done primarily
to remove stock that can not be taken off as
conveniently by other methods. The work is
pressed hard against the wheel or vice versa.
The accuracy and surface finish are of secondary
importance.
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Precision grinding
Precision grinding is concerned with
producing good surface finishes and accurate
dimensions.
3 types of precision grinding exists
–External cylindrical grinding
–Internal cylindrical grinding
–Surface grinding
9. Types of grinding operation
1.SURFACE GRINDING
a. Horizantal spindle
b. vertical spindle
2.CYLINDRICAL GRINDING
a. Extranal
b. Internal
3.CENTERLESS GRINDING
a. Extranal
b. Internal
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Grinding wheel
A grinding wheel is made of abrasive
grains held together by a bond. These grains cut
like teeth when the wheel is revolved at high
speed and is brought to bear against a work
piece. The properties of a wheel that determine
how it acts are the kind and size of abrasive,
how closely the grains are packed together and
amount of the bonding material.
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Abrasive materials
Different abrassive materials are
appropriate for grinding different work material.
Abrasives are hard substances used in various
forms as tools for grinding and other surface
finishing operations. They are also able to cut
materials which are too hard for other tools and
give better finishes and hold closer tolerances.
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Types of abrasive materials
1. Aluminum Oxide (Al2O3) known as Alundum
or Aloxide. Various substances may be added
to enhance hardness, toughness, etc. Plain
Al2O3 is white, and used to grind: steel,
ferrous, high strength alloys.
2. Silicon Carbide (SiC) known in trade as
Carborundum and Crystalon. Harder than
Al2O3 but not as tough. Used to grind:
aluminum, brass, stainless steel, cast irons,
certain brittle ceramics.
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Common abrasive materials (continue)
3. Boron Nitride in the forms of single-crystal
cubic boron nitride (CBN) and
microcrystalline cubic boron nitride (MCBN)
under trade names such as Borazon or
Borpax. Used for hard materials such as
hardened tool steels and aerospace alloys.
4. Diamond, a pure form of carbon, both
natural and artificial. Used on hard materials
such as ceramics, cemented carbides and
glass.
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Grain size
Important parameter in determining
surface finish and material removal rate. Small
grit sizes produce better finishes, larger grit sizes
permit larger material removal rates. Also,
harder materials need smaller grain sizes to cut
effectively, while softer materials require larger
grit size.
Grain sizes used in grinding changes
between 8-250, whilw 8 is very coarse, but 250
is very fine.
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Bonding materials
To get wide range of properties needed in
grinding wheels, abrasive materials bonded
by using organic or ingorganic materials.
Inorganic bonds
1. Vitrified bond (V): Clay bond melted to a
porcelain or glass like consistency. It can be
made strong and rigid for heavy grinding and
not effected by water, oil, acids. Most grindig
wheels have vitrified bonds.
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Inorganic bonds (continue)
2. Slicate bond is essentially water glass
hardened by baking. It holds grains more
loosely than a vitrified bond and give closer
cut. Large wheels can be made more easily
with slicate bond. Usually used in situation
where heat generation must be minimized.
3. Metallic bond: Cubic boron nitride and
diamond abrassives are usually (but not
always) embeded in metallic bonds, for
utmost in strength and tendency to hold the
costly long-wearing grains.
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Organic bonds
1. Rubber bond is a flexible bond, used in
cutoff wheels.
2. Resinoid bond is a high strengt bond, used
for rough grinding and cutoff operations.
3. Shellac bond is relatively strong but not
rigid, used in applications that requires good
finishing.
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Wheel grade and wheel structure
The grade of a grinding wheel is a measure
of how strongly the grains are held by the bond.
Typical structure of a grinding wheel contains
abrasive grains, bond material and pores (air
gaps) Volumetric proportions can be expressed
as
Vg + Vb+ Vp = 1
If Vp relatively large and Vg relatively small, it is
called open structure.
If Vg relatively large and Vp relatively small, it is
called dense structure.
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Wheel grade and wheel structure (continue)
Open structures are recommended in
situations where the clearance for chips must be
provided. Dense structures are used to obtain
better surface finish and dimensional control.
If Vb small relatively soft wheel, as Vb increase,
hardness is supplied.
Hard wheels used to achive high stock
removal rates and for grinding of relatively soft
work materials, while soft wheels generally used
for applications requiring low material removal
rates and grinding of hard work materials.
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Grinding wheel specifications
All grinding wheel manufacturers use
substantially the same standard wheel marking
system. This system uses numbers and letters to
specify abrasive type, grit size, grade structure,
and bonding material. However, properties of
the wheels are determined to a large extent by
the ways the wheels are made. The processes
vary from one plant to another, and wheels
carrying the same symbols but made by
different manufacturers are not necessarily
identical.
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American National Standard Institude’s marking
system for standard wheels
Prefix-Abbrasive type-Abrasive grain size-Grade-
Structure-Bond type-Manufacturer record
Ex: 51-A-36-L-5-V-23
Prefix:Manufacturer symbol indicating exact kind of
abrasive (use optional)
Abrassive type: A:Aluminum oxide
C:Silicon Carbide
B: Boron Nitride
D:Diamond
Grain size: coarse:8-24, medium:30-60, fine:70-180,
very fine: >220
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American National Standard Institude’s marking
system for standard wheels (continue)
Grade:Ranges from A-Z, where A represents soft, Z
represents hard wheel grade.
Structure: Scale isnumerical. 1: very dense and 15:very
open.
Bond type: B : Resinoid
E : Shellac
R : Rubber
S : Silicate
V : Vitrified
Manufacturer record: Manufacturer’s private marking
to idetify the wheel.
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Wheel shape and sizes
There are different wheel shapes that are
recognized as standard:
•Straight cylinders with or without recesses in
their sides
•Tapered two sides
•Straight cup
•Flaring cup
•Dish
•Saucer
Other shapes may be obtain as specials.
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Wheel shape and sizes (continue)
The principle dimensions that designate
the size of a grinding wheel are the outside
diameter, width, and hole diameter. Standard
wheel shapes are made in certain sizes only, but
the variety is large.
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Wheel wear
The overall wear of bonded abrasive is
caused by 3 distinct mechanism
Attritious wear: Sharp edges of an abrasive grain
become dull by attrition, developing flat areas.
Fracture of the grains:Portion of the grain breaks
off. The fractured area becomes new cutting
area
Fracture of the bond: Part of the grains fall out.
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Grain action
There are 3 types of grain action in grinding
1. Cutting:Grit can penetrate to the surface and
actually performs chip removing.
2. Plowing:Grit can penetrate to the surface
but can not perform cutting. The work
surface deformes.
3. Rubbing:Grits rubs to the surface, energy
consumed witout cutting.
67. Selection of grinding wheel
As per india standard –IS :1249-1958
1.CONSTANT FACTORS
• Type of work piece material to be ground
• Rate of metal removal from the work piece
• Area of contact
• Type of grinding machine
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JSS ACADEMY OF TECHNICAL EDUCATION,
BANGALORE
68. Selection of grinding wheel
2.VERIABLE FACTORS
• Wheel speed
• Work speed
• Condition of machine-capacity and rigidity
• Personal factors
69. Selection of grinding wheel
• Other factors
3.Type of bond to be used
4.Abrasive grain size, grade and structure
5.Whether grinding is wet or dry
70. Glazing ,
Glazing-
Which the abrasive particle s in the wheel have
become dull by attrition wear. there is
insufficient grain fracture and the grains are
not released by the bond. Thus causing the
cutting surface of the wheel to take a shinny
or like glass surface
71. loading
Loading is the wheeling of chips of the abrasive
grains or mechanical trapping of chips in the
pores of the grinding wheel.
Causes
-the work being too slow
-bond of wheel being too hard
-fast cut and deep to allow the chips carried
away by wheel
84. PARAMETERS OF GRINDING OPERATION
Speed : it is the peripheral speed of the work piece per
unit time (m/min).Cutting speed is grinding wheel is
the relative peripheral speed of the wheel with respect
to the workpiece. It is expressed in meter per minute
(mpm) or meter per second (mps).The cutting speed of
grinding wheel can be calculated asmpm (meter per
minute)
85. PARAMETERS OF GRINDING OPERATION
• Feed : it is the distance travelled by the tool
during each revolution of the work piece.
(mm/revolution).
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85
86. PARAMETERS OF GRINDING OPERATION
• Depth of cut : it is the perpendicular distance
measured from the original surface to the
machined surface of the work piece. (mm)
