Grinding

GRINDING
Lecture Notes
Prem Kumar Soni
Assistant Professor
LNCT Group of Colleges
8/9/2019Prem Kumar Soni LNCT Group of Colleges, Bhopal 1

Introduction
O To grind means to abrade, to wear away by
friction, or to sharpen.
O In manufacturing it refers to the removal of
metal by a rotating abrasive wheel. Wheel
action is similar to a milling cutter.
O The cutting wheel is composed of many small
grains bonded together, each one acting as a
miniature cutting point

Grinding Function
O Material removal by action of hard,
abrasive particles usually in the form of
a bonded wheel
O Grinding is most important abrasive
processes
O Other abrasive processes are
Honing, Lapping, Superfinishing,
Polishing, Buffing Etc.

Working Principle
O Material removal process in which
abrasive particles are contained in
a bonded grinding wheel that
operates at very high surface
speeds

Grinding Wheel

Wheel Specifications
•Abrasive material
•Grain size
•Wheel grade
•Wheel structure
•Bonding material

O A grinding wheel is specified by the standard
wheel markings like diameter of the wheel,
bore diameter of the wheel, thickness of the
wheel type (Shape) of the wheel.

1. Abrasive material
O The abrasives are selected depending upon the
material being ground.
O ‘Brown’ aluminum oxide is used for general
purpose grinding of tough materials.
O ‘White aluminum oxide is used for grinding
ferrous and ferrous alloys.
O ‘Green’ silicon carbide is used for very hard
materials with low tensile strength such as
cemented carbides.

The abrasive aggregate is selected according to the
hardness of the material being cut.
 Aluminum oxide (A)
 Silicon carbide (S)
 Ceramic (C)
 Diamond (D, MD, SD)
 Cubic boron nitride (CBN)
Grinding wheels with diamond or CBN grains are
called superabrasives. Grinding wheels with aluminum
oxide (corundum), silicon carbide, or ceramic grains
are called conventional abrasives.

Aluminum oxide (A)
O Aluminum oxide is used for its hardness and
strength.
O Many types of sandpaper use aluminum oxide
crystals.
O its low heat retention and low specific
heat make it widely used in grinding
operations, particularly cutoff tools.

Ceramics
Structural, including bricks, pipes, floor and roof
tiles
Refractories, such as kiln linings, gas fire
radiants, steel and glass making crucibles
Whitewares, including tableware, cookware,
wall tiles, pottery products and sanitary ware

Diamond
O Diamond is a solid form of the element
carbon with its atoms arranged in a crystal
structure called diamond cubic.
O The hardness of diamond and its ability to
cleave strongly depend on the crystal
orientation. Therefore, the crystallographic
structure of the diamond to be cut is analyzed
using X-ray diffraction to choose the optimal
cutting directions.

CBN (Cubic Boron Nitride)
O Boron nitride is a thermally and
chemically resistant refractory compound
of boron and nitrogen with the chemical
formula BN.
O It exists in various crystalline forms that
are isoelectronic to a similarly
structured carbon lattice.

Grain size
O The number indicating the size of the grit
represents the number of openings in the sieve
used to size the grain.
O The larger the grit size number, the finer the
grit.

O Friable grit breaks down more easily and is
useful for cutting harder materials. Grit
size typically runs from coarse (16 -24 grit),
medium (36 - 60 grit) and fine (80-120grit).
Superfine grits run from 150 and higher.
O Grinding wheels usually will be between 24 and
100 grit.

Wheel grade
O Grade indicates the strength of the bond and,
therefore, the ‘hardness’ of the wheel. In a
hard wheel the bond is strong, and securely
anchors the grit in place and, therefore,
reduces the rate of wear.
O In a soft wheel, the bond is weak and the grit
is easily detached resulting in a high rate of
wear.

From A (soft) to Z (hard), determines how
tightly the bond holds the abrasive.
O A to H for softer structure,
O I to P for moderately hard structure and
O Q to Z for hard structure.
Grade affects almost all considerations of
grinding, such as wheel speed, coolant
flow, maximum and minimum feed rates,
and grinding depth.

Wheel structure
O This indicates the amount of bond present
between the individual abrasive grains and the
closeness of the individual grains to each other.
O An open structure wheel will cut more freely. That
is, it will remove more metal in a given time and
produce less heat.
O It will not produce such a good finish as a closely
structured wheel.

O Spacing or structure, from 1 (densest) to 17 (least
dense).
O Density is the ratio of bond and abrasive to air space.
O A less-dense wheel will cut freely, and has a large
effect on surface finish.
O It is also able to take a deeper or wider cut with less
coolant, as the chip clearance on the wheel is greater.

O Wheel structure defines the wheel density
that is the abrasive and bond versus
airspace. Comparatively it is found a
wheel which is of less density cuts freely
and this plays a major effect on the
finishing of the surface.
O The above nature is due to better chip
clearance of the lesser density wheel. With
a wheel that is of less density, we can
obtain a wider or deeper cut, by means of
less coolant.

Bonding
O The bond is the substance which, when mixed
with abrasive grains, hold them together,
enabling the mixture to be shaped to the form
of the wheel,
O and after suitable treatment to take on the
necessary mechanical strength
O for its work. The degree of hardness possessed
by the bond is called the ‘grade’ of the wheel,
and indicates the ability of the bond to hold the
abrasive grains in the wheel.

There are several types of bonding used for
making wheels.
O Vitrified bond
O Silicate bond
O Shellac bond
O Rubber bond
O Resinoid bond

1. Vitrified bond
This is the most widely used bond. It has high
porosity and strength which makes this type of
wheel suitable for high rate of stock removal. It is
not adversely affected by water, acid, oils or
ordinary temperature conditions.
2. Silicate bond
Silicate wheels have a milder action and cut with
less harshness than vitrified wheels. For this
reason they are suitable for grinding fine edge
tools, cutters etc.

3. Shellac bond
This is used for heavy duty, large diameter wheels
where a fine finish is required. For example, the
grinding of mill rolls.
4. Rubber bond
This is used where a small degree of flexibility is
required on the wheel as in the cutting off wheels.
5. Resinoid bond
This is used for speed wheels. Such wheels are
used in foundries for dressing castings. Resinoid
bond wheels are also used for cutting off. They are
strong enough to withstand considerable abuse.

Bond name Bond symbol Bond description
Vitrified V
Glass-based; made
via vitrification of clays and feldspars
Resinoid B
Resin-based; made from plants or petroleum
distillates
Silicate S Silicate-based
Shellac E Shellac-based
Rubber R
Made from natural rubber or synthetic
rubber
Metal M Made from various alloys
Oxychloride O Made from an oxohalide

Types of Grinding Wheel
Cylinder or wheel ring
A cylinder wheel has no center mounting support but has a long & wide
surface. Their width is up to 12" and is used purely in horizontal or vertical
spindle grinders. This is used to produce flat surface, here we do grinding
with the ending face of the wheel.
Tapered Grinding wheels
Tapered Grinding wheel is a straight wheel that tapers externally towards
the midpoint of the wheel. As this pact is stronger than straight wheels, it
accepts advanced lateral loads. Straight wheel with tapered face is chiefly
used for gear teeth, grinding thread, etc.
Straight cup
This Straight cup wheels forms an option for cup wheels in cutter and tool
grinders, having an extra radial surface of grinding is favorable.

Straight Grinding wheels
Straight wheel are the most common mode of wheel that is
found on pedestal or bench grinders. This is used to gain on
several tools like chisels. The size of these wheels differs to
a great extent, width & diameter of its face obviously
depends on the category of its work, machines grinding
power.
Diamond Grinding Wheels
In diamond wheels industrial diamonds remain bonded to
the edge. This is used to grind hard materials like concrete,
gemstones & carbide tips. A slitting saw is designed for
slicing gemstones like hard materials.

Grinding Wheel Problems
1. The workpiece is burnt
Causes and solutions:
O In the feed too fast - Reduce the amount of
stock removed per pass.
O Dressing on the wheel is too fine - Use a
coarser dressing, more 'open'.
O Poor coolant direction - Adjust the coolant
direction into the grinding area.

2. Wheel loading and glazing
O Causes and solutions:
O Wheel grit is too fine - Try using a coarser grit
or softer grade wheel.
O Abrasive is too durable - Use a sharper, more
friable abrasive (white).
O Wheel is being dressed too slowly - Increase
the dress traverse.

3. Wheel chatter
Causes and solutions:
O Unsupported work - Increase the support.
O Machine vibration - Check the bearing condition.
O Cut rate is too heavy - Try reducing the cut rate.
O Wheel out of balance - Check and adjust.
O Spindle vibration - From worn spindle bearing or
an imbalance in flanges.
O Wheel out of trueness - Redress the wheel and
check the mounting.
O Glazed face of the wheel - Redress the wheel with
a sharp dressing.

4. Poor surface finish
O Wrong wheel dressing - Try a finer dressing,
slow down the dresser.
O Worn dressing tool - Inspect, rotate or replace
the dressing tool.
O Grit is too coarse - Use a finer grit size.
O Wheel is too hard - Use a softer grade wheel.
O Vibration/chatter - See problem #3 Wheel
Chatter.

5. Not holding on corners
O Grit size too large - Use a smaller grit size (the
maximum grit diameter should be less than 1.5
times the corner radius)
O Wrong wheel dress - Try a finer dress. Face
and side should be true to the wheel.
O Wrong wheel form - Worn out dresser. Use a
sharp dresser.

Application of grinding wheel
O Dressing the wheel refers to removing the current
layer of abrasive, so that a fresh and sharp surface is
exposed to the work surface.
O Trueing the wheel makes the grinding surface
parallel to the grinding table or other reference plane,
so that the entire grinding wheel is even and
produces an accurate surface.

Thank You

Grinding

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