Computer numerical control: Difference between revisions

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'''Numerical control''' or '''numerically controlled''' ('''NC''') [[machine tool]]s are machines that are automatically operated based onby commands that are received by their processing units. NC machines were first developed soon after [[World War II]] and made it possible for large quantities of the desired componentcomponents to be very precisely and efficiently produced ([[machining|machined]]) in a repeatedreliable fashionrepetitive manner. These early machines were often fed instructions which were punched onto [[paper tape]] or [[punch card]]s. In the [[1960s]], NC machines largely gave way to [[CNC]], or [[computer]] numerical control, machines.

[[de:Numerische Steuerung]]

[[ja:NC加工]]

Numerical controlControl (NC) iswas the precursor of computertoday's numericalComputer controlNumerical Control (CNC), which controls the automation of machine tools and the inherent tool processes for which they are designed. The CNC machine tool is the [[servo]] [[actuator]] of the [[CAD/CAM]] (Computer Assisted Design/Computer Assisted Manufacturing) technology both literally and figuratively.

CNC inherits from NC the essential character of by-the-numbers interpolation of transition points in the work envelope (the "[[Machine Coordinate System]]") of a mult-axis motion platform, based on the separation of programming from operations. The set of instructions, or "program" (usually an ASCII text file in which, in its simplest form, a line of text specifies the axial coordinates of a point in the Machine Coordinate System) 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 stored in the CNC memory and selected, the program is executed by pressing the appropriate key on the machine operator panel.

==Historical notes==

The need of the U.S. Air Force for templates more precise than could be obtained by methods which were 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.

HisMr. Parsons' project was taken overenjoined by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology and redefined as interpolated position control that hascaused the cutting tool to traverse a straight line between defined points at a prescribed rate of travel. Thus, the cutting tool would bewas constantly on the programmed contour and notspent spendingvery mostlittle of its time making non-cutting moves.

In the M.I.T. scheme, a contour of constantly changing curvature (that is, a spline) iswas 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 a child connecting the dotsfigure ofwhich aresults workbookfrom toconnecting-the-dots displayin aan figureactivity book). 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.

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 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 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 Parson concept or with record/playback, today this is a mutemoot issue.

==Today==