Computer numerical control: Difference between revisions

Revision as of 03:09, 17 May 2005

Revision as of 03:40, 17 May 2005

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Mr. Parsons' project was enjoined by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology and redefined as interpolative positional control that caused the cutting tool to traverse a series of straight lines between defined points at a prescribed rate of travel. Thus, the cutting tool would be constantly on the programmed contour and would spend very little of its time making non-cutting moves.

In the M.I.T. scheme, a contour of constantly changing curvature was represented as a poly-line with the intersections between line segments being points on the curve, and the axial coordinates of these points were listed for execution in sequential order in the part program (much like the figure which results from connecting-the-dots in an activity book). The shorter the line segments the more closelyaccurately the poly-line approximatedwould approximate the originalactual curve. Thus, M.I.T. retained separation of programming from operations while redefining the servo control as interpolationinterpolative, rather than discretediscretionary, 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.

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 [[Kurt Vonnegut]] 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/playback because the latter put the machinist in charge of program production. This was the same machinist who was a union member; thus union strikes could result in delays in military production. Also, numerical control demonstrated the ability to produce complex parts that were not possible by 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 ParsonParsons concept or with record/playback, today this is a moot issue.

==Today==