【正文】 phisticated lathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a bination turret lathe and milling machine. Additional features are sometimes included by the versatility of their machines. Numerical Control One of the most fundamental concepts in the area of advanced manufacturing technologies 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, perhaps none is more prominent than limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major step away from human control of machine tools. Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader. Numerical control was developed to overe the limitation of huma n 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 longrun tooling costs are lower. The development 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 machines 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 plex 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. Like so many advanced technologies, NC was born in the laboratories of the Masschusetts Institute of Technology. The concept of NC was developed in early 1950s with funding provided by the 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 is straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated. 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 further development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. This instructional programs were written on punched paper, which was later to be replaced by magic 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 development. A major problem wad the fragility of the punched paper tape medium. It was mon for the paper tape containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each programmed instructions had to be return 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 shop floor environment and this kind of repeated use. This led to the development of a special magic 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 magic dots. The plastic tape was much stronger than the paper taps, which solved the problem 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 a program of instructions, it necessary to interrupt machining operations and make a new tape. It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, puter technology became a reality and soon solved the problem 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 control machine tools are tied, via a data transmission link, to a host puter. Programs for operating the machine tools are stored in the host puter and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend o a host puter. When the lost puter goes down, the machine tools also experience downtime. This