An abrasive is defined as a material of extreme hardness that is used to shape other materials by a grinding or abrading action. Abrasive materials may be used as loose grains, as grinding wheels, or as coatings on cloth or paper. They may be formed into ceramic cutting tools that are used for machining metal in the same way that ordinary machine tools are used. Because of their superior hardness and refractory properties, they have advantages in speed of operation, depth of cut, and smoothness of finish.
Abrasive products are used for cleaning and machining all types of metal, for grinding and polishing glass, for grinding logs to paper pulp, for cutting metals, glass, and cement, and for manufacturing many miscellaneous products such as brake linings and nonslip floor tile.
These may be classified in two groups, the natural and the synthetic (manufactured). The latter are by far the more extensively used, but in some specific applications natural materials still dominate.
The important natural abrasives are diamond (the hardest known material), corundum (a relatively pure, natural aluminum oxide, Al2O3), and emery (a less-pure Al2O3 with considerable amounts of iron). Other natural abrasives are garnet, an aluminosilicate mineral; feldspar, used in household cleansers; calcined clay; lime; chalk; and silica, SiO2, in its many forms — sandstone, sand (for grinding plate glass), flint, and diatomite.
The synthetic abrasive materials are silicon carbide SiC, aluminum oxide Al2O3, titanium carbide TiC, and boron carbide B4C. The synthesis of diamond puts this material in the category of manufactured abrasives. There are other carbides, as well as nitrides and cermets, which can be classified as abrasives but their use is special and limited.
Various grades of each type of synthetic abrasive are distinguishable by properties such as color, toughness, and friability. These differences are caused by variation in purity of materials and methods of processing.
The sized abrasive may be used as loose grains, as coatings on paper or cloth to make sandpaper and emery cloth, or as grains for bonding into wheels.
A variety of bonds is used in making abrasive wheels: vitrified or ceramic, essentially a glass or glass-plus crystals; sodium silicate; rubber; resinoid; shellac; and oxychloride. Each type of bond has its advantages. The more rigid ceramic bond is better for precision-grinding operations, and the tougher, resilient bonds, such as resinoid or rubber, are better for snagging and cutting operations.
Ceramic-bonded wheels are made by mixing the graded abrasive and binder, pressing to general size and shape, firing, and truing or finishing by grinding to exact dimensions.
Grinding wheels are specified by abrasive type, grain size (grit), grade or hardness, and bond type. The term hardness as applied to a wheel refers to its behavior in use and not to the hardness of the abrasive material itself.
Literally thousands of types of wheels are made with different combinations of characteristics, not to mention the multitude of sizes and shapes available; therefore, selecting the best grinding wheel for a given job is not simple.