Grinding Wheel Marking System
Standard wheel – markings specify all the important wheel characteristics. The marking system comprises of seven symbols which are arranged in the following order.
Example (Marking system)
51 – A 46
Specification of grinding wheels
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.
32 A 46 H8V
Standard Grinding Wheel Marking System
Construction of the grinding wheel
In order to suit the grinding wheel for different work situations, the features such as abrasive, grain size, grade, structure and bonding materials can be varied. A grinding wheel consists of the abrasive that does the cutting, and the bond that holds the abrasive particles together.
Different Types of Abrasives used in Grinding Wheels
- Natural Abrasive
- Artificial Abrasive
The natural abrasives are emery and corundum,These are impure forms of aluminium oxide.
Artificial abrasives are silicon carbide and aluminium oxide.
The abrasives are selected depending upon the material being ground.
‘Brown’ aluminium oxide is used for general purpose grinding of tough materials.
‘White aluminium oxide is used for grinding ferrous and ferrous alloys.
‘Green’ silicon carbide is used for very hard materials with low tensile strength such as cemented carbides.
Grain Size or Grit Size of Grinding Wheel
The number indicating the size of the grit represents the number of openings in the sieve used to size the grain. The larger the grit size number, the finer the grit.
Grade of Grinding Wheel
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. In a soft wheel, the bond is weak and the grit is easily detached resulting in a high rate of wear.
Structure of Grinding Wheel
This indicates the amount of bond present between the individual abrasive grains and the closeness of the individual grains to each other. An open structure 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 closely structured wheel.
Bond of Grinding Wheel
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 making wheels.
Types of Bonds in Grinding Wheel
- Vitrified bond
- Silicate bond
- Shellac bond
- Rubber bond
- 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.
4. 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.