Abrasive wheels come in a variety of types and sizes, but their basic composition and characteristics are the same whatever their size, shape, or application. Knowing something about abrasive wheel composition can help you select the best grinding wheel for your application.
Cutting or grinding with an abrasive wheel is just as much a metal cutting process as milling, turning, filing, or sawing. The only difference is the abrasive wheel has thousands of tiny cutting edges instead of the few large ones the other processes possess. When cutting or grinding with an abrasive wheel, thousands of new cutting edges are exposed as the wheel wears down.
The simple technical description of an abrasive wheel is “a cutting tool consisting of abrasive grains held together by organic or inorganic bonds.” Abrasive grains are extremely hard substances such as aluminum oxide, zirconia alumina, and silicon carbide that are embedded in a matrix called the bond. The bond not only holds the abrasive grains together but also allows the mixture to be shaped to the desired wheel form and, after suitable treatment, to take on the necessary mechanical strength for its intended work.
Inorganic bonds include clay, glass, porcelain, sodium, silicate, magnesium oxychloride, and metal. Organic bonds include shellac, rubber, synthetic resin, or resinoid. Resinoid bonds are commonly used to make wheels for metalworking applications.
The degree of hardness possessed by the bond is called its grade. Grade also indicates the ability of the bond to hold the abrasive grains in place. A soft bond permits the grains to break away more readily than a hard one and should be used where the abrasive becomes rapidly dulled or blunt, i.e., when grinding hard materials. A hard bond retains the abrasive grains longer and should be used when grinding soft materials. A simple rule to follow is to use a hard wheel for soft materials and a soft wheel for hard materials. Remember that the “hardness” or “grade” of a wheel bears no relation to the abrasive material used in its construction. Therefore, a soft grade wheel can be made of the hardest abrasives, and a hard grade wheel can be made of the softest abrasives.
