Numerical control of machine tools may be defined as a method of automation in which various functions of machine tools are controlled by letters, numbers and symbols.
Basically a NC machine runs on a program fed to it. The program consists of precise instructions about the manufacturing methodology as well as the movements. For example, what tool is to be used, at what speed, at what feed and to move from which point to which point in what path, all these instructions are given. Since the program is the controlling point for production, the machine becomes versatile and can be used for any part. All the functions of an CNC machine tool are therefore controlled electronically, hydraulically or pneumatically.
In NC machine tools, one or more of the following functions may be automatic:
1) Starting and stopping of the machine tool spindle.
2) Controlling the spindle speed.
3) Positioning the tool tip at desired locations and guiding it along. Desired paths by automatic control of the motion of slides.
4) Controlling the rate of movement of the tool tip(i.e. feed rate)
5) Changing tools in the spindle.