Numerical ControlNumerical control (N/C) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job. When the job is changed, the program of instructions must be changed. The capability to change the program is what makes N/C suitable for low-volume and medium-volume production. It is much easier to write programs than to make major alterations of the processing equipment.There are two basic types of numerically controlled machine tools: pointtopoint and continuouspath (also called contouring). Pointtopoint machines use unsynchronized motors, with the result that the position of the machining head Can be assured only upon completion of a movement, or while only one motor is running. Machines of this type are principally used for straightline cuts or for drilling or boring.The N/C system consists of the following components: data input, the tape reader with the control unit, feedback devices, and the metalcutting machine tool or other type of N/C equipment.Data input, also called “mantocontrol link”, may be provided to the machine tool manually, or entirely by automatic means. Manual methods when used as the sole source of input data are restricted to a relatively small number of inputs. Examples of manually operated devices are keyboard dials, pushbuttons, switches, or thumbwheel selectors. These are located on a console near the machine. Dials ale analog devices usually connected to a synchronization-type resolver or potentiometer. In most cases, pushbuttons, switches and other similar types of selectors are digital input devices. Manual input requires that the operator set the controls for each operation. It is a slow and tedious process and is seldom justified except in elementary machining applications or in special cases.In practically all cases, information is automatically supplied to the control unit and the machine tool by cards, punched tapes, or by magnetic tape. Eightchannel punched paper tape is the most commonly used form of data input for conventional N/C systems. The coded instructions on the tape consist of sections of punched holes called blocks. Each block represents a machine function, a machining operation, or a combination of the two. The entire N/C program on a tape is made up of an accumulation of these successive data blocks, Programs resulting in long tapes all wound on reels like motion-picture film. Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop. Once installed, the tape is used again and again without further handling. In this case, the operator simply loads and unloads the parts. Punched tapes ale prepared on type writers with special tapepunching attachments or in tape punching units connected directly to a computer system. Tape production is rarely error-free. Errors may be initially caused by the part programmer, in card punching or compilation, or as a result of physical damage to the tape during handling, etc. Several trial runs are often necessary to remove all errors and produce an acceptable working tape.While the data on the tape is fed automatically, the actual programming steps ale done manually, Before the coded tape may be prepared, the programmer, often working with a planner or a process engineer, must select the appropriate N/C machine tool, determine the kind of material to be machined, calculate the speeds and feeds, and decide upon the type of tooling needed. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program. A program manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the drawing specifications. The control unit receives and stores all coded data until a complete block of information has been accumulated. It then interprets the coded instruction and directs the machine tool through the required motions.The function of the control unit may be better understood by comparing it to the action of a dial telephone, where, as each digit is dialed, it is stored. When the entire number has been dialed, the equipment becomes activated and the call is completed.Silicon photo diodes, located in the tape reader head on the control unit, detect light as it passes through the holes in the moving tape. The light beams are converted to electrical energy, which is amplified to further strengthen the signal. The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives.Some photoelectric devices are capable of reading at rates up to 1000 characters per second. High reading rates are necessary to maintain continuous