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Numerical control

Numerical control is a form of programmable auto­mation in which a machine is controlled by numbers (and other symbols) that have been coded on punched paper tape or an alternative storage medium. The initial appli­cation of numerical control was in the machine tool in­dustry, to control the position of a cutting tool relative to the work part being machined. The NC part program represents the set of machining instructions for the par­ticular part. The coded numbers in the program specify x-y-z coordinates in a Cartesian axis system, defining the various positions of the cutting tool in relation to the work part. By sequencing these positions in the program, the machine tool is directed to accomplish the machining of the part. A position feedback control system is used in most NC machines to verify that the coded instruc­tions have been correctly performed. Today a small com­puter is used as the controller in an NC machine tool. Since this form of numerical control is implemented by computer, it is called computer numerical control, or CNC. Another variation in the implementation of nu­merical control involves sending part programs over tel­ecommunications lines from a central computer to indi­vidual machine tools in the factory. This form of numeri­cal control is called direct numerical control, or DNC.

Many applications of numerical control have been de­veloped since its initial use to control machine .tools. Other machines using numerical control include compo­nent-insertion machines used in electronics assembly, drafting machines that prepare engineering drawings, coordinate measuring machines that perform accurate inspections of parts. In these applications coded numeri­cal data are employed to control the position of a tool or workhead relative to some object. Such machines are used to position electronic components (e.g., semiconductor chip modules) onto a printed circuit board (PCB). It is basically an x-y positioning table that moves the printed circuit board relative to the part-insertion head, which then places the individual component into position on the board. A typical printed circuit board has dozens of in­dividual components that must be placed on its surface; in many cases, the lead wires of the components must be inserted into small holes in the board, requiring great precision by the insertion machine. The program that controls the machine indicates which components are to be placed on the board and their locations. This informa­tion is contained in the product-design database and is typically communicated directly from the computer to the insertion machine.

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