TYPES OF CONTROL FOR MACHINES
Three types of control are used for the operation of machines.
1. The first and simplest type of control is to station one or more workers at a machine to observe the production and activate the controls as needed.
2. The second type of control is to assist the human being by arranging the machine to control itself, partially or fully. This type of control uses pre-set stops, jigs, templates, cams, and other hardware attached to the machine. Some of these machines are semiautomatic, and others are fully automatic in their operation.
3. The third type of control operates a machine with the use of information stored on punched cards, punched tape, magnetic tape, or other nonhuman means for holding and programming information. This type of control is known as "numerical control," because numbers are involved. Only a minimum of human controlling assistance is needed. For some jobs, it will be advantageous to use numerical control rather than the hardware type of control. Numerical control may make a machine either semiautomatic or fully automatic.
Machines can be controlled with the use of information stored on tapes, or on other devices, which can store and make available information when needed. Suitable equipment is needed for interpreting the information on the tape and sending the signals to the machine at the proper time. Suitable devices must be installed on the machine to receive these signals and cause the machine to follow their commands. These devices actuate the controls on the machine. Numerical control is flexible in that tapes are easily changed for different jobs. When all of the units needed to provide the signals and to control a machine through a complete cycle of operation are combined, the combination is called a "system." Thus, numerical control is called as system.
The information needed for controlling the operation of a machine comes from the drawing of the desired part. Since numerical control locates all points on a drawing by their X and Y coordinates, it is preferable to have drawings dimensioned using this kind of coordinate system. This will eliminate the need for computing coordinates from conventional drawing dimensions. The axes for the coordinate dimensioning system should be located so that the coordinates of all points will be positive. Drawings should specify a curve by giving either its algebraic equation or a list of the coordinates of points through which the curve passes. A trained person with sufficient knowledge and skill must study a workpiece drawing and decide upon a manufacturing method or a sequence of steps for the complete machining cycle. He then must write the instructions for a complete machining cycle in the desired sequence. These instructions must then be translated to information, which can be stored on a tape, or other device. Information may be placed on a paper tape by means of punched holes.
Numerical control of machine tools can result in several important advantages. These advantages include faster production, because all machine functions can be programmed to operate at their most efficient rate and simultaneously where possible. Non-cutting periods can be reduced to a minimum, and thus machines can be utilized to their fullest extent. Since the time required to produce a workpiece remains constant, it can be accurately predicted for determining costs. Machines can be set up easier and faster. Higher quality workpieces can be produced, including obtaining closer dimensional tolerances. Less scrap is possible, because human mistakes can be almost entirely eliminated. Monotonous machine operating jobs are eliminated, and numerical control changes the skill required for machine tool operation. The special tooling required for a workpiece can be reduced, with resulting reductions in tool maintenance and tool storage costs. Engineering changes of parts can be made with less cost, since there is less special tooling to change. Making a new tape is relatively inexpensive. Lead time, or the time required to make the special tooling so that production of a part can be started, can be considerably shorter. When the production of the desired quantity of work-pieces has been completed, the tape used for numerical control can be easily stored for use again at some future time. An important limitation of numerical control is the relatively high cost of the machine tool and the additional equipment which is needed. Numerical control, if available, may result in a lower overall workpiece cost, especially when quantities to be produced are low and intermediate.