Introduction
In view of the development of sophisticated technological equipment and machinery, there has been an increasing demand to manufacture complicated components of high accuracy in large quantities. Production of these components calls for machine tools which can be set up fairly rapidly without much attention. When a human operator operates a machine tool by turning the hand wheels in orderto cut a component, as on a milling machine, or to arrive at a machining position as on a boring machine, he cannot avoid making errors. These errors vary in degree and in position in each repetition. Moreover, he has to remove metal by steps making several small cuts and measuring as he proceeds, in order to ensure that he does not remove more than the desired material. If the machine tool can substitute for a part of the mental labour of the machine operator (attention devoted by the operator while carrying out the job), the component can be machined to the desired accuracy in a much shorter time. Then, it becomes necessary to feed the machine with the information related to the component to be machined on it.
With the advent of the production of sophisticated equipment and machinery, there has arisen a need to transfer instructions from the man to the machine through automatic devices. The control system used with such machines requires the instructions to be encoded in a suitable language, in a particular pattern and on a convenient medium, so that the machine can read the instructions automatically to perform the job.
The control of a machine tool by means of recorded information on punched tape or cards is known as numerical control, because information supplied to the control system consists of a series of numbers. The information stored in the punched tapes or cards can be read by automatic means and converted into electrical signals, which operate the electrically controlled servo-system. The use of the cards and the electrically controlled servo-system permits the slides of a machine tool to be driven simultaneously and at the appropriate speeds and direction so that complex shapes can be cut, often with a single operation, and without the need to reorient the workpiece. Thus the numerical control system accurately positions the cutter with respect to the workpiece, and in addition, it can operate a number of auxiliary facilities like vertical motion of the drill head etc. It also controls the switching functions relating to the management of the machine tool, like switching the coolant flow on and off indexing a turret head etc.
Numerical control can be applied to milling machines, lathes, grinding, boring machines, flame cutters, drilling machines, etc.
The principal advantages of a numerical control are the substantial reduction in the time required to set up the machine in preparation for doing the job, the reduction of the number of special jigs and fixtures required to machine a part, and the elimination or drastic reduction of time used to take trial cuts in order to obtain the required size. The amount of human error is also greatly reduced, thereby decreasing the scrap rate.