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Unit-6

Grinding

Theory:-

Grinding is a process of removing mater by the abrasive action of a revolving wheel on the

surface of a workpiece, in order to bring it to the required shape and size. It is very much similar to the

other machining operations. In this process, the wheel used for performing the grinding operation is

known as grinding wheel. It consists of sharp crystals called abrasives, held together by a binding

material or bond.

Classification Of Grinding Machines:-

Grinding machines are broadly classified into two types.

1. Rough grinders:-

Rough grinding machines removes stock without any consideration to the accuracy of parts.

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i. Floor stand grinder,

ii. Bench grinder,

iii. Portable grinder,

iv. Flexible shaft,

v. Swing fram,

vi. Abrasive belt,

2. Precession grinders:-

These machines finish parts to very accurate dimensions.

i. Cylindrical grinders

a) Centre type.

b) Centre less type.

ii. Internal grinders.

a) Chucking type.

b) Planetory type.

c) Centre less type.

3. Surface grinders:-

a) Reciprocating table.

I) Horizontal spindle,

II) Vertical spindle.

b) Rotating table

I) Horizontal,

II) Vertical.

4. Tool and cutter grinder

5. Special grinding machines

a) Crankshaft



b) Thread grinder

c) Cam grinder

d) Roll grinder

e) Valve grinder

Different types of abrasives:-

Grinding wheels are made of abrasive particles bonded together by means some suitable bond.

An abrasive is a hard material which can be used to cut or wear away other materials and also sharp

edges and corners.

There are two types of abrasive particles.

1. Natural abrasives:- These are produced by uncontrolled forces of nature. i.e,

a) Sand stone or solid quartz

b) Emery (50-60% crystalline al2o3+ironoxide)

c) Diamond

d) Garnet.

2. Artificial abrasives:- The Quality And Composition Of These Particles Can Be Easily Controlled And

Their Efficiency Is Far Better Than That Of Natural Abrasives.

a) Silicon carbide :-

It is available in variety of colors. A special variety of bluish green is very suitable for grinding tip tools.

The trade names of it are crystal on, electron.. etc.

b) Aluminum oxide :-

The trade names for 23are aloxite, alundum and borolon. Its special form is white al2o3 which when

pure, looks like brilliant white crystal. It is more suitable for tool steels and heat generation due to

grinding is low.

c) Boron nitrite:-

Boron nitrite grinding wheels are used to grind hardened and difficult to grind steels. These have long

life and high grinding ratio. These are low temperature better finish and quality of surface.

Bonds And Bonding Processes:-

A bond is a material that holds the abrasive grains together and desired shape in the form of wheel.

The most common bonds are:



1. Vitrified bond (V)

2. Silicate bond (S)

3. Shellac bond (E)

4. Rubber bond(R)

5. Bakelite or resinoid bond (B)

1. Vitrified bond:-

In this process, abrasive grains are mixed with fusible clay called feldspar and water. After drying it is cut

and trimmed to the required size and shape.

i. This type of bonds are very strong.

ii. Metal removal is very high

iii. The wheels are not affected by acids and alkalis.

2. Silicate bond:-

In this process, abrasives grains are mixed with sodium silicate. These wheels are much suitable for good

cutters, blades etc.

Advantages:-

i. This type of grinding wheel are free cutting action.

ii. Production process is quicker.

iii. Wheels are larger size.

iv. Heat generation is low and efficiency is high.

3. Shellac Bond:-

The abrasive grains are first coated with shellac by mixing in a seam heated mixer. This type of grinding

wheel are used for grinding cam shafts and other parts where a high polish is desired.

Advantages:-

This type of grinding wheel are very strong and high strength and elasticity.

4. Rubber Bond:-

The abrasive grains are mixed with pure rubber and sulphur. These wheels are used for parting

operations.



Advantages:-

I. This type of wheels are very hard and tough.

Ii. Wheels as thin as 0.125mm.

Iii. The wheels can be operated at 3000 to 5000 m/min.

5. Bakelite bond:-

The abrasive grains are mixed with powdered synthetic resin and a liquid solvent. Wheels are used in

foundries and billet shops because of ability to remove metal rapidly.

Advantages:-

I. This bond is very hard and strong.

Ii. These wheels can be operated at high speeds i.e, 3000 to 5000 m/min.

SELECTION OF GRINDING WHEELS :-

1. Properties of the materials to be machined like strength, hardness, toughness etc.

2. Surface finish required.

3. Amount of material to be removes.

4. Rigidity of the machine.

5. Use of coolant.

6. Wheel speed.

7. Grinding operation.

SPECIFICATIONS OF GRINDERS:-

1. Diameter and length of the centers,

2. Height of Centers above the table,

3. Length and breadth of table,

4. Maximum distance between the table and the wheel,

5. Wheel diameter,

6. Maximum length of the workpiece that can be ground,

7. Ranges and feeds type of drive ..etc.



TYPES OF GRINDING MACHINES:-

1. Plain cylindrical grinders:-

In this process the workpiece is held between two centers. One of these centers in the head stock and

other one in the tailstock. In operation the rotating work is traversed across the face of the rotating

grinding wheel is fed into the work by an amount equal to the depth of the cut. In this the head stock

center is not disturbed. It is the tailstock centre which is moved in or out manually or hydraulically to

insert and hold the work.

The table is usually made in two parts. The upper table carries the tailstock, headstock and

the workpiece and can be swivelled in a horizontal plane to a maximum of 10 on either side, along the

circular ways provided on the lower table. This enables grinding of tapered surfaces. The lower table is

mounted over horizontal guide ways to provide longitudinal traverse to the upper table and hence the

work.

The wheel head is usually mounted on horizontal cross ways on the bed and travels along these to

feed the wheel to the work. This movement is known as in feed.

2. Centreless cylindrical grinders:-

In this process the work is mounted between centers, is supported by a combination of grinding wheel,

a regulation wheel and a work rest table. The principle of centresless grinding is used for both external

grinding as well as internal grinding, many hollow cylindrical and tapered workpieces.



In operation, grinding operation is performed by the grinding wheel only, while the function of

the regulation wheel is to provide the required support to the workpiece. While it is pushed away by the

cutting pressure of the grinding wheel. This helps the workpiece to remain in contact worth the grinding

wheel. The regulating wheel essentially carries rubber bond and helps in the rotation of workpiece due

to friction.

3. Horizontal spindle reciprocating table surface grinder:-

The longitudinal feed to the work is given by reciprocating the table. In this process cutting is done on

the periphery of the straight wheel. The workpiece is usually held on a magnetic chuck on these



machines. The horizontal spindle machines are widely used in tool rooms. They are vastly used for

grinding flat surfaces. The machine size is designated by the dimensions of the working area of the table.

4. Vertical spindle reciprocating table surface grinders:-

The vertical spindle reciprocating table grinders the table, along with the workpiece reciprocates under

the wheel. The wheel covers all or a major portion of the width of the job. A manual or power feed can

be employed to feed the wheel-head vertically.



TOOL AND CUTTER GRINDERS:-

Base:-

The machine has box type, heavy and rigged base with precisely machined guide ways on it's surface.

Saddle:-

Saddle is mounted on the base. It can slide forward and backward along with the hardened guide ways

on the base.

Table:-

Table has two parts one over the other. The upper table serves as a worktable with t-slots. It can be

swivelled on the lower table. Lower table is mounted on the saddle. All table motions are manual no

power feeds are provided.

Wheel head:-

The wheel head is mounted on the column at the back of the machine. It can be raised and lowered and

can be swivelled through 360 about a vertical axis. Two wheels can be mounted on the motor shaft.

Normally straight or disc shaped wheel is mounted on one side and cup type or disc type wheel is

mounted on the second side of the motor with some attachments, tool and cutter grinder may be used

for surface, internal and cylindrical grinding of components. The attachments are:-



1. Universal work head.

2. External cylindrical grinding attachment.

3. Swivelling vice.

4. Tap grinding attachment.

5. Gear shaper cutters grinding attachment ..etc.

GRAIN SIZE:-

Grain size indicates the size of the abrasive grains used in making a grinding wheel. It is indicated

by non-depending on the grain size, the abrasive grains are classified as coarse, medium, fine and very

fine. For softer materials generally coarse grain size abrasives are used. Grinding hard materials wheels

are used for fast MRR and fine grinding wheel are used for finish the production.

Coarse - 8, 10, 12, 14, 16, 20, 24.

Medium - 30, 36, 46, 54, 60.

Fine - 70, 80, 100, 150, 180.

Very fine - 220, 240, 280, 400, 500, 600.

GRADE:-

The grade of a grinding wheel indicate the hardness with which the bond holds the abrasives grains. A

wheel is hard, when the grain are held tightly by the bond. The hardness depends upon the amount of

composition of bond material used. Grades are represented by alphabets.

Soft - A, B, C, D, E, F, G, H

Medium - I, J, K, L, M, N, O, P

Hard - Q, R, S, T, U, V, W, X, Y, Z.

Lapping:-

The tool used in lapping called a lap-first the lap is charged. Charging lap means the abrasive grains are

formed into its surface. Now this charged lap is rubbed against the work surface. A vehicle is applied to

improve the cutting action.

Laps are made soft cast iron, brass, copper, soft steel. Abrasives of silicon

carbide, aluminum oxide, diamond... Etc.



Advantages:-

1. Geometrically true surface can be produced.

2. Good surface finish is obtained.

3. Dimensional accuracy is very high.

Disadvantages:-

1. Low metal removal rate.

2. High operator skill is required.

3. High production cost.

HONING:-

Honing is an abrasing process employed in finishing internal surfaces. It is done with bonded abrasive

stone called hones. For precision honing, the workpiece is usually held in a fixture and the hone is

inserted into the hole to be finished. Then the hone is given a slow reciprocating motion as it rotates.

Coolants are essentially used during honing. They flush away small chips and abrasive particles and

maintain uniform temperature sulphurised mineral oil, lard oil mixed with kerosene is used as coolants.

Advantages:-

1. Improve the shape, size, accuracy and surface finish of the workpiece.

2. No limitation regarding length of workpiece.

Disadvantages:-

1. Cannot be used to finish flat surfaces.

THE END.



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