Grinding wheel-types of abrasives and their uses

 Construction of the Grinding Wheel

. different types of abrasive and their uses

. different grain sizes and their uses

. different grades of grinding wheels

. structure of a grinding wheel

In order to suit the grinding wheel for different work situations, the features can be varied such as abrasive, grainsize grade, structure and bonding materials

A grinding wheel consists of

The abrasive that does the cutting, and the bond that holds the abrasive particles together.

Abrasive

There are two types of abrasives.

Natural abrasive

Artificial abrasive

The natural abrasives are emery and corrundum, these are impure form of aluminum oxide.

Artificial abrasives are silicon carbide and aluminum oxide.

The abrasives are selected depending upon the material being ground.

'Brown' aluminum oxide is used for general purpose grinding of tough materials.

'White' aluminum oxide is used for grinding die steels.

'Green' silicon carbide is used  for very hard materials with low lensile strength such as cemented carbides.

Grain size

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

Grade

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, there fore, reduces the rate of wear. In a soft wheel, the bond is weak and the grit  is easily detached resulting in a high rate of wear.

Structure

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

Bond

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, and after suitable treatment to take on the necessary mechanical strength 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 materials used for marking wheels.

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 temparature conditions

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, cutlery etc.

Shellac bond

This is used for heavy duty, large diameter wheels where a fine finish is required. For example, the grinding of millrols.

Rubber bond

This is uesd  where a small degree of flexibility is required on the wheel as in the cutting off wheels.

Resinoid bond

This is used for high speed wheels. Such wheels are used in foundries for dressing casting. Resinoid bond wheels are also used for cutting off. Theya are strong enough to withstand considerable abuse.