The NC System (as opposed to CNC system) uses fixed logical functions, those that are built in a permanently wired circuit within the control unit. These functions can not be changed by the programmer or the machine operator. Because of the fixed wiring of the control logic, the control logic the NC control system is synonymous with the term “hardwired” (in which most functions are implemented by electronic hardware based upon digital circuit technology.). The system can interpret a part program, but it does not allow any changes to the program, using the control features. All required changes must be made away from the control, typically in an office environment. Also the NC system requires the compulsory use of punched tapes for input of the program information. The NC controller interprets these instructions and then converts them into two types of control signals: motion control signals and miscellaneous control signals
A Typical Numerical Control System
Motion control signals are a series of electric pulse trains that are used to control the position and the speed of the machine table and spindle.
An obsolete Numerical Control OR Manual relay panel from the early 1960’s
Each pulse activates a motion of one basic length-unit (BLU) which is the minimum increment size of the NC control system. The typical increment for one BLU in the old generation of NC controls is 0.001in. (or 0.01 mm), while in modern CNC controls, the increment resolution can reach 0.0001 in. (or 0.001 mm). The number of pulses transmitted to each axis determines the incremental axis position, and the frequency of these pulses regulates the axis speed.
Miscellaneous control functions are a set of on/off signals to implement the control of the speed and direction of the spindle rotation, control of coolant supply, selection of cutting tool, automatic clamping and unclamping, etc.
