(2) Computer Numerical Control

Computer numerical control (CNC) is the numerical control system in which a dedicated computer is built into the control to perform basic and advanced NC functions. CNC controls are also referred to as soft-wired NC systems because most of their control functions are implemented by the control software programs. CNC is a computer assisted process to control general purpose machines from instructions generated by a processor and stored in a memory system. It is a specific form of control system where position is the principal controlled variable. All numerical control machines manufactured since the seventies are of CNC type. The computer allows for the following: storage of additional programs, program editing, running of program from memory, machine and control diagnostics, special routines, inch/metric, incremental/absolute switchability.
CNC machines can be used as stand alone units or in a network of machines such as flexible machine centres. The controller uses a permanent resident program called an executive program to process the codes into the electrical pulses that control the machine. In any CNC machine, executive program resides in ROM and all the NC codes in RAM. The information in ROM is written into the electronic chips and cannot be erased and they become active whenever the machine is on. The contents in RAM are lost when the controller is turned off. Some use special type of RAM called CMOS memory, which retains its contents even when the power is turned off.