Computer numerical control: Difference between revisions

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CNC inherits from NC its essential character of by-the-numbers interpolation of transition points in the work envelope of a motion platform based on the separation of programming from operations. The program - usually an ASCII text file in which, in its simplest form, a line of text specifies the rectangular coordinate of a point in the work envelope of the machine determined by the stroke limits of the servo axes - is prepared from a blueprint or CAD file and transferred to the memory of the CNC via floppy drive, serial data interface or a network connection. Once in the CNC memory and selected for execution, pressing a key called NC Start or Cycle Start on the machine operator panel causes the CNC to execute the program.
 
==Historical notes==
The need of the U.S. Air Force for templates more precise than could be done by methods representative of the state of the art in the late 1940s inspired a gentlemen by the name of John Parsons, President of the Parsons Works of Traverse City, Michigan, to propose that a by-the-numbers technique commonly used in machining be put under servo control with position data generated by a computer (in order to have many more points that would defy the practicability of hand calculations). His concept was to machine to setpoints as guides for subsequent manual finishing, that is, to speed up a manual process so more points could be included. His project was taken over by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology and redefined as interpolated position control that has the cutting tool traverse a straight line between points at a prescribed rate of travel. Thus, the tool would be constantly on the programmed contour and not spending most of its time making non-cutting moves. In the M.I.T. scheme, a contour of constantly changing curvature (that is, a spline) is represented as a poly-line with the intersections between line segments being points on the curve and these points were listed in sequential order in the part program (like a child connecting the dots of a workbook to display a figure). The shorter the line segments the more closely the poly-line approximated the original curve. Thus, M.I.T. retained separation of programming from operations while redefining the servo control as interpolation rather than discrete positioning. M.I.T. demonstrated the first ever NC machine tool to a select group from the military, the aerospace industry, the machine tool industry and the technical media in September, 1952.