【正文】 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 fur ther development 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 magic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. 26Together, 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 was 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 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 tines. 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 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 tape, which solved the problem of frequent tearing and , 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 made even the most minor adjustments in a program of instructions, it was 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 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 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 an 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 on a host 27puter. When the host puter goes down, the machine tools also experience downtime. This problem led to the development of puter numerical control. 3 Turning The engine lathe, one of the oldest metal removal machines, has a number of useful and highly desirable attributes. Today these lathes are used primarily in small shops where smaller quantities rather than large production runs are encountered. The engine lathe has been replaced in today’s production shops by a wide variety of automatic lathes such as automatic of singlepoint tooling for maximum metal removal, and the use of form tools for finish on a par with the fastest processing equipment on the scene today. Tolerances for the engine lathe depend primarily on the skill of the operator. The design engineer must be careful in using tolerances of an experimental part that has been produced on the engine lathe by a skilled operator. In redesigning an experimental part for production, economical tolerances should be used. Turret Lathes Production machining equipment must be evaluated now, more than ever before, this criterion for establishing the production qualification of a specific method, the turret lathe merits a high rating. In designing for low quantities such as 100 or 200 parts, it is most economical to use the turret lathe. In achieving the optimum tolerances possible on the turrets lathe, the designer should strive for a minimum of operations. Automatic Screw Machines Generally, automatic screw machines fall into several categories。 singlespindle automatics, multiplespindle automatics and automatic chucking machines. Originally designed for rapid, automatic production of screws and similar threaded parts, the automatic screw machine has long since exceeded the confines of this narrow field, and today plays a vital role in the mass production of a variety of precision parts. Quantities play an important part in the economy of the parts machined on the automatic screw machine. Quantities less than on the automatic screw machine. The cost of the parts machined can be reduced if the minimum economical lot size is calculated and the proper machine is selected for these quantities. 28Automatic Tracer Lathes Since surface roughness depends greatly on material turned, tooling , and feeds and speeds employed, minimum tolerances that can be held on automatic tracer lathes are not necessarily the most economical tolerances. In some cases, tolerances of are held in continuous production using but one cut . groove width can be held to on some parts. Bores and singlepoint finishes can be held to . On highproduction runs where maximum output is desirable, a minimum tolerance of is economical on both diameter and length of turn.附錄 II車床 車床主要是為了進行車外圓、車端面和鏜孔等項工作而設(shè)計的機床。車削很少在其他種類的機床上進行，而且任何一種其他機床都不能像車床那樣方便地進行車削加工。由于車床還可以用來鉆孔和鉸孔，車床的多功能性可以使工件在一次安裝中完成幾種加工。因此，在生產(chǎn)中使用的各種車床比任何其他種類的機床都多。 車床的基本部件有：床身、主軸箱組件、尾座組件、溜板組件、絲杠和光杠。 床身是車床的基礎(chǔ)件。它能常是由經(jīng)過充分正火或時效處理的灰鑄鐵或者球墨鐵制成。它是一個堅固的剛性框架，所有其他基本部件都安裝在床身上。通常在床身上有內(nèi)外兩組平行的導(dǎo)軌。有些制造廠對全部四條導(dǎo)軌都采用導(dǎo)軌尖朝上的三角形導(dǎo)軌（即山形