Drill, tool for cutting or enlarging holes in hard materials. Rotary drills are commonly used in the home for wood, plastic, masonry, and sometimes metal. In mettallurgy the mechanical drilling machine, or drill press, operates one or several drills at a time. Most metallurgical drills are of high-speed steel. Dentists' drills rotate at extremely high speeds, powered by an electric motor or by compressed air; their tips (of tungsten carbide or diamond) are water-cooled. Rotary drills are also used for deeper oil well drilling: A cutting bit is rotated at the end of along, hollow drill pipe, new sections of pipe being added as drilling proceeds. The percussive drills are used for rock-boring, for concrete and masonry, and for shallower oil well drilling. Rock drills are generally powered by compressed air, the tool rotating after each blow to increase cutting speed. The pneumatic drill familiar in city streets is also operated by compressed air. Ultrasonic drills are used for brittle materials; a rod attached to a transducer is placed against the surface, and to it are fed abrasive particles suspended in a cooling fluid. It is these particles that actually perform the cutting.