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BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TCHNOLOGY
FACULTY OF MECHANICAL AND INDUSTRIAL
ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
Manufacturing engineering II-laboratory report on tool
grinding

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Submitted to: - m/r Aragaw M.

Submission date: - 05 -10-2016E.C

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4. GRINDING MACHINE

4.1 Introduction
Grinding is a metal cutting operation like any other process of machining removing metal in
comparatively smaller volume. The cutting tool used is an abrasive wheel having many numbers of
cutting edges. The machine on which grinding the operation is performed is called a grinding
machine.
Grinding is done to obtain very high dimensional accuracy and better appearance. The accuracy of
grinding process is 0.000025mm. The amount of material removed from the work is very less.
4.2 Types of grinding machines
According to the accuracy of the work to be done on a grinding machine, they are classified as
1. Rough grinding machines
2. Precision grinding machines
4.2.1 Rough grinding machines
The rough grinding machines are used to remove stock with no reference to the accuracy of results.
Excess metal present on the cast parts and welded joints are removed by rough grinders. The main
types of rough grinders are:
1. Hand grinding machine
2. Bench grinding machine
3. Floor stands grinding machine
4. Flexible shaft grinding machine
5. Swing frame grinding machine
6. Abrasive belt grinding machine

4.2.2 Precision grinding machines

Precision grinders are used to finish parts to very accurate dimensions. The main types of precision
grinders are:
1. Cylindrical grinding machines
2. Internal grinding machines
3. Surface grinding machines
4. Tool and cutter grinding machines
5. Special grinding machines
4.2.3 Cylindrical grinding machine
Cylindrical grinders are generally used to grind external surfaces like cylinders, taper cylinders,
faces and shoulders of work. There are two types of cylindrical grinding machines and they are
1. External cylindrical grinding machines
2. Internal cylindrical grinding machines

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4.2.4 Surface grinding machines

Surface grinding machines are employed to finish plain or flat surfaces horizontally, vertically or at
any angle.
There are four different types of surface grinders. They are:
1. Horizontal spindle and reciprocating table type
2. Horizontal spindle and rotary table type
3. Vertical spindle and reciprocating table type
4. Vertical spindle and rotary table type

Horizontal spindle surface grinding machine

The majority of surface grinders are of horizontal spindle type. In the horizontal type of the
machine, grinding is performed by the abrasives on the periphery of the wheel. Though the area of
contact between the wheel and the work is small, the speed is uniform over the grinding surface and
the surface finish is good. The grinding wheel is mounted on a horizontal spindle and the table is
reciprocated to perform grinding operation.
Vertical spindle surface grinding machine
The face or sides of the wheel are used for grinding in the vertical type surface grinders. The area of
contact is large and stock can be removed quickly. But a criss-cross pattern of grinding scratches is
left on the work surface. Considering the quality of surface finish obtained, the horizontal spindle
type machines are widely used.

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4.5 Grinding machine operations

The process of grinding is the operation of removing excess material from metal parts by a grinding
wheel made of hard abrasives. The following operations are generally performed in a grinding
machine.
1. Cylindrical grinding
2. Taper grinding
3. Gear grinding
4. Thread grinding
4.5.1 Cylindrical grinding
Cylindrical grinding is performed by mounting and rotating the work between centres in a
cylindrical grinding machine. The work is fed longitudinally against the rotating grinding wheel to
perform grinding. The upper table of the grinding machine is set at 0° during the operation.
4.5.2 Taper grinding
Taper grinding on long workpieces can be done by swivelling the upper table. If the workpiece is
short, the wheelhead may be swivelled to the taper angle. Another method of grinding external
taper is to true the face of the grinding wheel by a diamond tool dresser to the required angle. In this
case, the table and the wheelhead are not swivelled.
4.5.3 Gear grinding
The teeth of gears are ground accurately on gear grinding machines for their shape. Gear grinding is
done by the generating process or by using a form grinding wheel. The generating process makes
use of two saucer shaped grinding wheels. These wheels are used to grind two faces of successive
teeth.

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4.5.4 Thread grinding

Thread grinding machines are used to grind threads accurately. The grinding wheel itself is shaped
to the thread profile. These formed grinding wheels have one or multi threads on them.
4.6 Grinding wheel
A grinding wheel is a multi-tooth cutter made up of many hard particles known as abrasives having
sharp edges. The abrasive grains are mixed with a suitable bond, which acts as a matrix to
manufacture grinding wheels.
According to construction, grinding wheels are classified under three categories.
1. Solid grinding wheels
2. Segmented grinding wheels
3. Mounted grinding wheels
4.6.1 Abrasives
Abrasives are used for grinding and polishing operations. It should have uniform physical
properties of hardness, toughness and resistance to fracture. Abrasive may be classified into two
principal groups.
1. Natural abrasives
2. Artificial abrasives

4.6.2 Natural abrasives

The natural abrasives are obtained from the Earth’s crust. They include sandstone, emery,
corundum and diamond. Sandstone is used as abrasive to grind softer materials only.
Emery is natural alumina. It contains aluminium oxide and iron oxide. Corundum is also a natural
aluminium oxide. It contains greater percentage of aluminium oxide than emery. Both emery and
corundum have a greater hardness and abrasive action than sandstone.
Diamond is the hardest available natural abrasive. It is used in making grinding wheels to grind
cemented carbide tools.
4.6.3 Artificial abrasives
Artificial abrasives are of two types.
1. Silicon carbide abrasives
2. Aluminium oxide abrasives
Silicon carbide
Silicon carbide is manufactured from 56 parts of silica, 34 parts of powdered coke, 2 parts of salt
and 12 parts of sawdust in a long rectangular electric furnace of the resistance type that is built of
loose brick work. There are two types of silicon carbide abrasives - green grit and black grit.
Silicon carbide is next to diamond in the order of hardness. But it is not tough enough as aluminium
oxide. It is used for grinding materials of low tensile strength such as cemented carbides, ceramic
materials, grey brass, bronze, copper, aluminium, vulcanized rubber etc.
This is manufactured under trade names of carborundum. It is denoted by the letter ‘S’.

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Aluminium oxide
Aluminium oxide is manufactured by heating mineral bauxite, silica, iron oxide, titanium oxide,
etc., mixed with ground coke and iron borings in arc type electric furnace. Aluminium oxide is
tough and not easily fractured, so it is better adapted to grinding materials of high tensile strength
such as most steels, carbon steels, high speed steels, and tough bronzes. This is denoted by the letter
‘A’.
4.6.4 Types of bonds
A bond is an adhesive substance that is employed to hold abrasive grains together in the form of
grinding wheels. There are several types of bonds. Different grinding wheels are manufactured by
mixing hard abrasives with suitable bonds. The table containing the types of wheels manufactured
using different types of bonds and their symbols is given below

4.6.5 Grain size, Grade and Structure

Grain size (Grit)
The grinding wheel is made up of thousands of abrasive grains. The grain size or grit number
indicates the size of the abrasive grains used in making a wheel, or the size of the cutting teeth.
Grain size is denoted by a number indicating the number of meshes per linear inch of the screen
through which the grains pass when they are graded. There are four different groups of the grain
size namely coarse, medium, fine and very fine. If the grit number is large, the size of the abrasive
is fine and a small grit number indicates a large grain of abrasive.

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Grade
The grade of a grinding wheel refers to the hardness with which the wheel holds the abrasive grains
in place. It does not refer to the hardness of the abrasive grains. The grade is indicated by a letter of
the English alphabet. The term ‘soft’ or ‘hard’ refers to the resistance a bond offers to disruption of
the abrasives. A wheel from which the abrasive grains can easily be dislodged is called soft whereas
the one, which holds the grains more securely, is called hard. The grade of the bond can be
classified in three categories.

Structure
The relative spacing occupied by the abrasives and the bond is referred to as structure. It is denoted
by the number and size of void spaces between grains. It may be ‘dense’ or ‘open’. Open structured
wheels are used to grind soft and ductile materials. Dense wheels are useful in grinding brittle
materials.

4.6.6 Standard marking system of grinding wheels

The Indian standard marking system for grinding wheels has been prepared with a view of
establishing a uniform system of marking of grinding wheels to designate their various
characteristics.
Prefix Manufacturer’s abrasive type symbol
First element (letter) Type of abrasive
Second element (number) Size of abrasive
Third element (letter) Grade of bond
Fourth element (number) Structure of the grinding wheel
Fifth element (letter) Type of bond
Suffix Manufacturer’s symbol

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