6.1 Numerical Control (NC):) Numerical Control is a technique for controlling machine toolso processes using coded command instructions. These coded command instructions are interpreted and converted by NC controller into two types of signals namely; motion control signals and miscellaneous control signals. Motion control signals are a series of electric pulse trains that are used to control the positions and the speed of the machine table and spindle. Miscellaneous control signals are set of ON/OFF signals to execute the spindle rotation and direction, control of coolant supply, selection of cutting tools, automatic clamping and unclamping, etc. In motion control signals, each pulse activates a motion of one basic length-unit (BLU).
6.2 Computer Numerical Control (CNC): CNC controls are soft-wired NC systems as control functions are controlled by software programs. Alternatively, Computer Numerical Control is the numerical control.system in which dedicated, stored program microprocessors are built into the control to perform basic and advanced NC functions. Control signals in CNC systems are in the form of binary words, where each word contains fixed number of bits, 32 bits or 64 bits are commonly used, representing different axial positions.
6.3 Direct Numerical Control (DNC) : Direct numerical simultaneously control the operations of a group of NC machine tools using a shared computer. Programming, editing part programs and downloading part programs to NC machines are main responsibilities of the computers in a NC system. Due to high cost of mainframe computers and introduction of CNC in 1970s, the DNC system couldn't become popular in industry. The capacity of stored memory and enhanced intelligence of the built-in, low cost, and dedicated computer replaced the desirable features of the DNC systems.
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