1、外文资料 Numerical Control One of the most fundamental concepts in the area of advanced manufacturing technol-ogies is Numerical Control(NC). Prior to the advent of NC, all machine tools were manually operated and controlled. Among the many limitations associated with manual control machine tools. Perha
2、ps none is more prominent than the limitation of operator skills. With manual control, the quality of the product are directly related to and limited to the skills of the operator. Numerical Control represented the first major step away from human control of machine tools. Numerical Control means th
3、e control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions,
4、 known as a reader. 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 de
5、velopment of NC led to the development of several other innovations in manufacturing technology:(1) Electrical 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 c
6、an automatically produce a wide variety of parts, each involving an assortment of widely varied and comples 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 controlle
7、d machine tools and processes. 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 strai
8、ght cuts efficiently and effectively. However, curved paths were a problem because the machine tool han 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 had to be calculated. This problem led to the develop
9、ment in 1959 of the Auto matically Programmed Tools (APT) language. This a special programming language for NC that uses statements similar to English language to define the part geometry, descrile the cutting similar to English language to define the part geometry, describe the cutting tool configu
10、ration, and specify the necessary motions. The development of the APT language was a major step forward in the further 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
11、 on punched paper, which was later to be replaced by magnetic plastic tape. Atape reader was used to interpredt 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
12、NC at this point in its development. A major proble was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machjining process. This problem was exacerbated by the fact that each successive time a part was
13、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 separate times. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use. This led to the
14、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 proble
15、m of frequent tearing and breakage. However, it still left two other problems. The most important of these was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in aprogram of instructions, it was necessary to interrupt machin
16、ing operations and make a new tape. It was also still necessary to run the tape through the reader as many times as there were solved the problems of NC associated with punched paper and plastic tape. The development of a concept known as Direct Numerical Control(DNC) solved the paper and plastic tape problems associated with Numerical Control by simply eliminating tape as the medium for carrying the programmed instructions. In Direct Numerical C
