Machine tool that uses a rotating abrasive grinding wheel to change the shape or dimensions of a hard, usually metallic, workpiece. Grinding is the most accurate of all the basic machining processes. All grinding machines use a wheel made from one of the manufactured abrasives, silicon carbide or aluminum oxide. To grind a cylindrical form, the workpiece rotates as it is fed against the grinding wheel. To grind an internal surface, a small wheel moves inside the hollow of the workpiece, which is gripped in a rotating chuck. On a surface grinder, the workpiece is held in place on a table that moves under the rotating abrasive wheel