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le workpieces are being machined. With continuouspath, or contouring, systems the position of the tool relative to the workpiece is specified by a series of coordinates, and the control system is designed to follow a path between these points by interpolation. Some machines follow a straightline path (linear interpolation)。 others follow a curved path (circular or parabolic interpolation). Numerical control can be applied to motions along or about any axis, but two or threeaxis control systems are the most mon. In general, verticalmilling machines and lathes utilize continuouspath, or contouring control. Verticaldrilling machines jig borers, and small milling machines often use positional control. One sophisticated form of NC machine is known as the machining center. This machine is generally a verticalmilling machine with several axes of control and with automatic toolchanging facilities. The tools are usually held in a rotary magazine, and tool changes are manded by the punched tape. Thus, with a machining center a plicated workpiece can be pletely machined on all faces except the base through a bination of milling, drilling, boring, facing, reaming, and tapping operations. This type of system is therefore most suitable for the batch production of main ponents. A further refinement o