Numerical Control
  One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control.
  Controlling a machine tool using a punched tape or stored program is known as numerical control (NC). NC has been defined by the Electronic Industries Association (EIA) as " a system in which actions are controlled by the direct insertion of numerical dada at some point .the system must automatically interpret at least some portion of this data." the numerical data required to produce a part is known as a part program..
  A numerical control machine tool system contains a machine control unit (MCU) and the machine tool itself. The MCU is further divided into two elements: the data processing unit (DPU) and the control loops unit (CLU). The DPU processes the coded data from the tape or other media and passes information on the potions of each axis, required direction of motion, feed rate, and auxiliary function control signals to the CLU. The CLU operates the drive mechanisms of the machine, receives feed back signals concerning the actual position and v
elocity of each of the axes, and signals the completion of operation. The DPU sequentially reads the data. When each line has completed execution as noted by the CLU, anther line of data is read.
  A data processing units consists of some or all of the following parts:
  1) Data input device such as a paper tape reader, magnetic tape reader, RS232-C port, etc
  2) Data-reading circuits and parity-checking logic
  3) Decoding circuits for distributing data among the controlled axes 、
  4) An interpolator, which supplies machine-motion commands between data points for tool motion
  A control loops unit, on the other hand consists of the following:
  1) Position control loops for all the axes of motion, where each axis has a separate control
loop
  2) Velocity control loops, where feed control is required
  3) Deceleration and backlash take up circuits
  4) Auxiliary functions control, such as coolant on/off, gear change, spindle on/off control
  Geometric and kinematic data are typically fed from the DPU to the CLU.
  The CLU then governs the physical system based on the data from the DPU.
  Numerical control was developed to overcome the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:
  l Electric discharge machining
  2 Laser-cutting
  3 Electron beam welding
  Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.
  Historical Development of NC
  Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U.S. Air Force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively.
  However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter the straight lines making up the steps, the smoother is the curve. Each line segment in the steps shown in the close up in Fig.2.17 had to be calculated. This was a cumbersome approach that had to be overcome if NC was to develop further.
  This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its devel
opment.
  A major problem was the fragility of the punched paper tape medium. It was common for the paper containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. If it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shot floor environment and this kind of repeated use.
  This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and breakage. However, it still left two other problems.
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