It is possible to use computers instead of controller units in the numerical control machine tools thereby replacing much of the conventional N/C hardware with software. Such a machine could be self contained one having its own dedicated mini-computer to perform some or all of the basic numerical control functions [in which case it is known as computer numerical control (CNC)], or there may be a central computer controlling directly a system of several machine tools [in which case it is known as direct numerical control (DNC)]. Economic justification for their use is essential. The computer required for CNC is much smaller and much less expensive than that used in DNC system. The N/C machine in such systems has hardware like servo-amplifier, transducer units and interface components.
The output of the computer could be :
(i) Sequence of reference pukes for each slide/drive ; or
(ii) a binary word supplied at fixed time intervals.
In the first case, the number of pulses represents the position and the frequency of pulses represents feed-rate. These pulses could be either fed direct to servomotor or as a reference to a closed-loop digital system. It is used in CNC system. DNC system uses second technique and the control loop is closed through the computer itself as it compares the reference word with the feedback signal to determine the position error. The binary word, output of the computer is fed to a digital to analog converter, which supplies a voltage output proportional to the required axis feed.
As the data tape for producing a part is read only once in the computer controlled machines and stored in the computer memory the errors likely to occur in reading the N/C data tapes are avoided. The stored data can bs easily modified as editing features exist in computer controlled systems and the tape need not be sent for reprocessing as in case of conventional N/C systems which is rather a costly procedure. It may be mentioned that reprocessing of tapes can be necessary due to :
(i) mistakes in cutting conditions like changes in speeds and feeds on account of inconsistency in work piece and tool material ;
(ii) cutter compensation (actual size of tool dimension may be different from programmed size and not possible to be compensated by the appropriate manual devices on N/C controller) ;
(iii) or the tape may contain dimensional errors greater than the allowed tolerance.