power-operated tool used for finishing or shaping metal parts, especially parts of other machines. An establishment that is equipped with such tools and specializes in such work is known as a machine shop. Machine tools operate by removing material from the workpiece, much as a sculptor works. Basic machining operations are: (1) turning, the shaping of a piece having a cylindrical or conical external contour; (2) facing, the shaping of a flat circular surface; (3) milling, the shaping of a flat or contoured surface; (4) drilling, the formation of a cylindrical hole in a workpiece; (5) boring, the finishing of an existing cylindrical hole, as one formed by drilling; (6) broaching, the production of a desired contour in a surface; (7) threading, the cutting of an external screw thread; and (8) tapping, the cutting of an internal screw thread. In addition there are operations such as sawing, grinding, gear cutting, polishing, buffing, and honing. The tools themselves vary in size from hand-held devices that can be used for drilling and grinding to large stationary tools that perform a number of operations. Many machine tools have a name that indicates their principal function, e.g., drill press, broach machine, milling machine, and jig borer. The lathe
can perform turning, facing, threading, drilling, and other operations. In order to withstand the great heat that this work generates, the materials used in machine tools must be extremely hard and durable. Thus, their working surfaces are made of such substances as high-speed steels, sintered carbides, and diamonds. To help dissipate the heat, the area of contact between the working surface and the workpiece is usually lubricated with a fluid that may also improve the finish of the workpiece's surface. Modern machine tools are often numerically or computer controlled; where a human operator can be distracted, and is limited by the speed of human reflexes, a numerically controlled machine is more reliable and accurate. See boring mill
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