Numerical control is a system of control that uses numerically coded instructions to operate motors and other devices that run a machine. The instructions are provided by either of the two binary coded decimal systems: the Electronic Industries Association (EIA) code, or the American Standard Code for Information Interchange (ASCII). ASCII-coded machine control units will not accept EIA coded instructions and vice versa. Increasingly, however, control units are being made to accept instructions in either code.

Numerical control instructions may be provided through a keyboard on a machine control unit (MCU), in the form of a punched tape, on magnetic tape, or directly from a computer. The current trend is to provide instructions directly from a computer.

Automatic operation by NC is readily adaptable to the operation of all metalworking machines. Lathes, milling machines, drill presses, boring machines, grinding machines, turret punches, flame or wire-cutting and welding machines, and even pipe benders are available with numerical controls.

There are two principal kinds of NC systems: (1) point-to-point and (2) contouring. Point-to-point control systems are simpler than contouring systems.

Point-to-point NC system is basically a positioning system. Its primary purpose is to move a tool or workpiece from one programmed point to another. Usually the machine function, such as a drilling operation, is also activated at each point by command from the NC program. Point-to-point systems are suitable for hole-machining operations such as drilling, countersinking, counterboring, reaming, boring, and tapping. Hole-punching machines, spot-welding machines, and assembly machines also use point-to-point NC systems.

Contouring NC systems can direct the tool or workpiece to move at any angle, and also along curved paths. Many contouring NC machines, such as lathes and vertical milling machines, are of the two-axis type. This type allows continuous-path contours to be machined only in the XY plane. Three-axis machines are capable of simultaneous cutting movements in all three dimensional shapes in die and mold cavities.