A tool for cutting or enlarging holes in hard materials. There are two classes of drills: those that have a rotary action, with a cutting edge or edges at the point and, usually, helical fluting along the shank; and those that work by percussive action, where repeated blows drive the drill into the material.
Rotary drillsRotary drills are commonly used in the home for wood, plastic, masonry, and sometimes metal. They were hand turned, but now electric drills are used for many applications. In metallurgy, the mechanical drilling machine or drill press is one of the most important machine tools, operating one or several drills at a time. As a lot of heat is generated, lubrication is very important. Most metallurgical drills are made of high-grade steel. Dentists' drills rotate at extremely high speeds, their tips (of tungsten carbide or diamond) being water-cooled; they are powered by an electric motor or by compressed air. Rotary drills are used for deeper oil-well drilling: a cutting bit is rotated at the end of a long, hollow drill pipe, new sections of pipe being added as drilling proceeds.
Percussive drillsPercussive 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.
See also rotary hammer drill.
Related category• TOOLS
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