Definitions

servo-control

Numerical control

Numerical control or numerically controlled (NC) machine tools are machines that are automatically operated by commands encoded on a digital medium. NC machines were first developed soon after World War II and made it possible for large quantities of the desired components to be very precisely and efficiently produced (machined) in a reliable repetitive manner. These early machines were often fed instructions which were punched onto paper tape or Hollerith cards, although Mylar (or polyester) tapes became popular because of their high durability and quality. In the 1960s, NC machines largely gave way to CNC, or computer numerical control, machines. (GE had its NC 550 workhorse for many years until they came out with their first CNC (model 1050) in August 1974.)

Numerical Control (NC) was the forerunner of today's Computer Numerical 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 of a multi-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 work envelope) 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 state-of-the-art methods of the late 1940s inspired John Parsons , President of the Parsons Works of Traverse City, Michigan, to propose that a by-the-numbers technique (commonly used by machinists of that era) be placed under servo control with positional data generated by a computer, thereby providing much more data than would be practical by means 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.

Mr. Parsons' project was enjoined by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology (MIT) 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 almost constantly on the programmed contour and would spend very little of its time making non-cutting moves.

In the MIT 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 accurately the poly-line would approximate the actual curve. Thus, MIT retained separation of programming from operations while redefining the servo control as interpolative, rather than discretionary, positioning. MIT 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 MIT 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 reminiscent of a musician playing on a piano that has been modified to record the keystrokes on a paper chart which can be read by a player piano to reproduce the music. The popular novel Player Piano was 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/playback because 1) the latter put the machinists who were union members in charge of program production, thus union strikes could result in unacceptable delays in military production, and 2) numerical control demonstrated the capability of producing complex parts that were not possible by the conventional manual methods used in the record/playback technique. Although point number 1 may have seemed more important at the time, point number 2 actually had much more profound implications to the history of technology and to human productivity.

The electronic files used to control NC and CNC machines are often in a format called G-code, after Gerber Scientific Instruments , a manufacturer of photoplotters and developer of the file format. The X-Y two-dimensional motion of photoplotters was extended to include the third Z axis, and along with special codes, allows milling machines to be steered in more than three axes. Many of the lines of text in the control files start with the ASCII letter G, thus the name; however, there are other commands that start with the letter D and M, as well as X and Y for coordinates. The file format became so widely used that it has been embodied in an EIA standard.

Today

An entire manufacturing technology 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 many variants, even a variant that is essentially record/playback. The latter of which are known in the industry as "teach lathes".

In addition, powerful and well-crafted human/machine interfaces allow the machine operator to prepare programs by means of interactive displays which request only the definition of the machining operation and its required parameters (such as a "pocket" and its dimensions) and not the actual tool paths with all the calculations that are there required. Anyone who knows machining concepts and blueprint interpretation can produce programs at the machine without the need for CAD/CAM. Nonetheless, the vast majority of programs are now produced with the aid of CAD/CAM and, for most users, CNC today (for all its gigahertz microprocessors and megabytes of real time kernel software) is conceptually little different from the first NC demonstrated by MIT in 1952.

If there is a notable difference in concept, it is that CNC is no longer just for the spindle/cutting tool process of stock removal. It is for any processes that can be carried on machine tool motion platforms and that benefit from the separation of programming from operations, that is, from the CAD/CAM technology. These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and plasma cutting, bending, spinning, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing (PnP), sawing and undoubtedly, the industrial processes of tomorrow.

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