Machine-tools are a class of metal removing machines such as lathes, millers and drillers. The basis of the cutting process is the movement of the cutting tool in relation to the material in a precise orientation and by a precise amount.
Traditional numerical control (NC) is based upon the movement being controlled via a pre-prepared punched paper tape. The development of microprocessors and compact computers have extended the sophistication of the control available, so that the term "computer numerical control" (CNC) is used. In practice, movements in all three dimensions are controlled.
The actual movement and monitoring of the controlled axes are carried out by motors and position transducers.
On the above basis numerical control machine-tools have been used for many years prior to the development of the microelectronics. The application of microcomputers allow for more sophisticated control. When metal is machined, its cutting properties can vary throughout the workpiece, particularly if it is a forging or casting. Microcomputers can add a further aspect of adaptive control by reacting to the current power consumption, torque, etc. of the driving motors.
Due to the nature of microcomputer systems a distributed processing approach can be adopted for the control of the various functions of a machine tool. This also allows a modular approach to the development of the hardware and software. In addition, greater operator interaction for unexpected situations is possible due to the work cycle not being restricted to preprogrammed punched paper tape.
Instead of being a substantial part of the cost of a machine tool, the use of microcomputers makes the numerical control cost less abd adds relatively little to the cost of the machine tool.
Some control systems are too complex for a single microprocessor. One approach is to use a bit slice microprocessor system whereby the codes of the data bus are broken into slices, each having the same number of bits (e.g. 16 bit into 4 slices of 4 bits). Each of the slices is then processed in a separate processor.
An alternative to bit slice microprocessor for complex systems is to use several microprocessors together.
Microcomputer A acts as a programmable interface between the machine tool and the system. It also handles tape and operator input and output. Microcomputer B calculates the axes motions as a function of time and hence the path of the cutter and microcomputer C controls the position of the feed axes. The three microcomputers share a common data memory. As more microcomputers are linked to the machine, a greater on-line processing capability is built up. For example, "worksurface programming" is the technique whereby the desired profile of the workpiece is specified and the size of the blank is entered. The control system works out the pattern of cuts necessary to produce the component. In some cases, this is displayed on the VDU as a check before the operator commits the machine.
The addition of VDUs machine tool control system allows a conversational approach which guides the operator when inputting the required data. The use of microcomputers has enabled the development of digital readout systems (with memory). The current position of all coordinates is displayed and in some cases the display can be switched at any time form one system of units to another and vice versa.

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