Numerically controlled (NC)machine tools were developed to fulfill the contourmachining requirements of complex aircraft parts and forming dies. The first NC machine tool was developed by Parsons Company and MIT in 1952 . The first-generation NC units used digital electronic circuits and did not contain any actual central processing unit; therefore, they were called NC or hardwired NC machine tools. In the 1970s, computer numerically controlled (CNC) machine tools were developed with minicomputers used as control units. With the advances in electronics and computer technology, current CNC systems use several high-performance microprocessors and programmable logical controllers that work in a parallel and coordinated fashion. Current CNC systems allow simultaneous servoposition and velocity control of all the axis monitoring of controller and machine tool performance, online part programming with graphical assistance, in-process cutting processmonitoring, and in-process part gauging for completely unmanned machining operations. Manufacturers offer most of these features as options.
COMPUTER NUMERICALLY CONTROLLED UNIT
A typical CNC machine tool has three fundamental units: the mechanical machine tool unit, power units (motors and power amplifiers), and the CNC unit. Here, a brief introduction of a CNC system from the user's point of view is presented.
Organization of a CNC Unit
A CNC unit of a machine tool consists of one or more central processing units (CPUs), input/output devices, operator interface devices, and programmable logical controllers.