Grinding wheels become inefficient due to two main causes known as loading and glazing.
Loading of Grinding Wheel
When soft materials such as aluminium, copper, lead etc. are ground, the metal particles get clogged in the pores of the wheel. This condition is called loading.
Glazing of Grinding Wheel
When a surface of the wheel develops a smooth and shining appearance, it is said to be glazed. This indicates that the wheel is blunt, i.e. the abrasive grains are not sharp. When such grinding wheels are used, there is a tendency to exert extra pressure in order to make the wheels cut.
Excessive pressure on the grinding wheel will lead to the fracture of the wheel, excessive heating of the wheel, weakening of bonding of the wheel and bursting of the wheel.
Dressing of Grinding Wheel
The purpose of dressing is to restore the correct cutting action of the wheel. Dressing removes the clogs on the surface of the wheel and the blunt grains of the abrasive, exposing the new sharp abrasive grains of the wheel which can be cut and brought to shape efficiently.
Truing of Grinding Wheel
Truing refers to the shaping of the wheel to make it run concentric with the axis. When a new grinding wheel is mounted, it must be trued before use. The cutting surface of a new wheel may run out slightly due to the clearance between the bore and the machine spindle. Grinding wheels, which are in use, also can run out of true, due to uneven loading while grinding.
Dressing and truing are done at the same time. Same tool is used for both Dressing and Truing.
Grinding Wheel Dressers
1. Star Wheel Dresser
The wheel dressers used for off-hand grinders are star wheel dressers (Huntington type wheel dresser) and diamond dressers.
The star wheel dresser consists of a number of hardened star-shaped wheels mounted on a spindle at one end and a handle at the other end. While dressing, the star wheel is pressed against the face of the revolving grinding wheel. The star wheel revolves and digs into the surface of the grinding wheel. This releases the wheel loading and dull grains, exposing sharp new abrasive grains.
Star wheels are useful for pedestal grinders in which a precision finish is not expected. Star wheel dressers should be used only on wheels which are large enough to take the load.
2. Diamond Dresser
Bench type off-hand grinders used for sharpening cutting tools are usually fitted with smaller and rather delicate wheels. These wheels are dressed and trued with diamond dressers. Diamond dressers consist of a small diamond mounted on a holder which can be held rigidly on the work-rest.
How to use a Grinding wheel dresser
For dressing and truing, the dresser is slowly brought in to contact with the wheel face and moved across. The finish obtained depends on the rate at which the dresser is moved across the face. For roughing, the dresser is moved faster. For fine finish, the dresser is moved slowly. Roughing will be efficient with a dresser that has a sharp point, while, for fine finishing, a blunt diamond dresser is more suitable.
Abrasive stick – When only a light dressing is required, abrasive sticks can also be used. There are abrasive materials made in the form of sticks for the convenience of handling. Diamond dressers, if moved too slowly, can glaze the wheel.