Computer numerical control: Difference between revisions

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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 it 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 polly-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 every NC machine tool to a select group from the military, the aerospace industry, the machine tool industry and the technical media in September, 1952.

At the time when M.I.T. was developing numerical control, engineers at General Motors were putting position transducers on the lead screws of a conventional engine lathe and recording the motion of the axes as the machinist put the machine through its paces to make a workpiece. The machine was also fitted with a servo system that took data from the recording to reproduce the same sequence of motion to produce a second, third and more parts. This technique is called record/playback and it is reminicent of a musician making music on a piano that has been modified to record his keystrokes on a paper chart to be read by a player piano to reproduce the music. The popular novel, "The Player Piano", is inspired by this machine. The author, Kirk[[Kurt Vonnegut]], who also wrote "Catch 22", was exposed to the machine when he worked as a publicist for General Electric. Record/playback is different from numerical control in that the program is produced by the machinist in the process of making the first part.

The Air Force wanted numerical control and not record feedback because the latter put the machinist in charge of program production. This was the same machinist who was in a union and who strikes could result in delays in military production. Also, numerical control represented the ability to produce parts that were not possible with conventional, manual means. The Air Force used its deep pockets to get its way and while American manufacturing may have been better served with the simpler Parson concept or with record/playback, by now this is a mute issue. An entire manufacturing process known as CAD/CAM has developed around the NC concept and, in addition, CNC with its powerful microprocessors and other enabling technologies proffered from the personal computing phenomenon has enabled the NC concept to branch into may variants, even a variant that is essentially record/playback. In the industry, these machines are called teach lathes. In additional, powerful and well crafted human/machine interfaces allow the machine operator to prepare programs from interactive screens based on defining the machining operations and its obvious parameters (like the dimensions of a pocket) and not the tool path with all the calculations that this requires. Anyone today, who knows machining and how to read a blueprint can produce program at the machine without the need for CAD/CAM. Nonetheless, the vast majority of programs are produced with CAD/CAM and for most users, CNC today, for all its Giga hertz microprocessors and megabytes of real time kernel software is conceptually no different from the first NC demonstrated by the M.I.T. in 1952.