【正文】 overe 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 longrun tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: Electrical discharge machining,Laser cutting,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 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 tolls 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 . Air Force. In its earliest stages, NC machines were able to made 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 10 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 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 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 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 magnetic 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 magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of 11 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 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 puter. When the host puter goes down,the machine tools also experience problem led to the development of puter numerical control.The engi。鏜孔和休用單刃刀具進(jìn)行精加工時(shí)。 在某些情況下，在連續(xù)生產(chǎn)過程中，只進(jìn)行一次切削加工時(shí)的公差可以達(dá)到 。為了在六角車床上獲得盡可能小的公差值，設(shè)計(jì)人員應(yīng)該盡量將加工工序的數(shù)目減至最少。應(yīng)用這個(gè)標(biāo)準(zhǔn)來評(píng)價(jià)具體的加工方法，六角車床可以獲得較高的質(zhì)量評(píng)定。在把試驗(yàn)伯重新設(shè)計(jì)為生產(chǎn)零件時(shí)，應(yīng)該選用經(jīng)濟(jì)的公差。普通車床的加偏差主要信賴于操作者的技術(shù)熟練程度?，F(xiàn)在，設(shè)計(jì)人員已經(jīng)熟知先利用單刃刀具去除大量的金屬余量，然后利用成型刀具獲得表面光潔度和精度這種加工方法的優(yōu)點(diǎn)?，F(xiàn)在，這些機(jī)床主要用在規(guī)模較小的工廠中，進(jìn)行小批量的生產(chǎn)，而不是進(jìn)行大批量的和產(chǎn)。這個(gè)問題目前正在解決之中，它是通過采用局部區(qū)域網(wǎng)絡(luò)將各個(gè)微機(jī)聯(lián)接起來，以得于更好地進(jìn)行數(shù)據(jù)管理。計(jì)算機(jī)數(shù)控解決了主計(jì)算機(jī)發(fā)生故障所帶來的問題，但是它產(chǎn)生了另一個(gè)被稱為數(shù)據(jù)管理的問題。這可以使得程序被輸入和存儲(chǔ)在每臺(tái)機(jī)床內(nèi)部。這兩種技術(shù)為計(jì)算機(jī)數(shù)控（CNC）的發(fā)打下了基礎(chǔ)。這個(gè)問題促使了計(jì)算機(jī)數(shù)字控制技術(shù)的產(chǎn)生。然而，它敢有著同其他信賴于主計(jì)算機(jī)技術(shù)一樣的局限性。當(dāng)需要時(shí)，通過數(shù)據(jù)傳輸線路提供給每臺(tái)機(jī)床。在直接數(shù)字控制中，幾臺(tái)機(jī)床通過數(shù)據(jù)傳輸線路聯(lián)接到一臺(tái)主計(jì)算機(jī)上。幸運(yùn)的是，計(jì)算機(jī)技術(shù)的實(shí)際應(yīng)用很快解決了數(shù)控技術(shù)中與穿孔紙帶和塑料帶有關(guān)的問題。即使對(duì)指令程序進(jìn)行最微小的調(diào)整，也必須中斷加工，制作一條新帶。然而，它仍然存在著兩個(gè)問題。在紙帶上通過采用一系列的小孔來載有編程指令，而在塑料帶上通過采用一系列的磁點(diǎn)瞇載有編程指令。易損壞的紙帶顯然不能承受嚴(yán)配的車間環(huán)境和這種重復(fù)使用。因此，這個(gè)問題變得很嚴(yán)重。在機(jī)械加工過程中，載有編程指令信息的紙帶斷裂和被撕壞是常見的事情。然而，在數(shù)控發(fā)展的這個(gè)階段中還存在著許多問題。指令程序?qū)懺诖┛准垘希ㄋ髞肀凰芰蠋〈?，采用帶閱讀機(jī)將寫在紙帶或磁帶上的指令給機(jī)器翻譯出來。最初的數(shù)控系統(tǒng)下今天應(yīng)用的數(shù)控系統(tǒng)是有很大差別的。這是一個(gè)專門適用于數(shù)控的編程語言，使用類似于英語的語句來定義零件的幾何形狀，描述切削刀具的形狀和規(guī)定必要的運(yùn)動(dòng)。臺(tái)階中的每一個(gè)線段都必須經(jīng)過計(jì)算。 然而，曲線軌跡成為機(jī)床加工的一個(gè)問題，在編程時(shí)應(yīng)該采用一系列的水平與豎直的臺(tái)階來生成曲線。數(shù)控這個(gè)概念是50年代初在美國空軍的資助下提出來的。數(shù)控可以使生產(chǎn)廠家承擔(dān)那些對(duì)于采用人工控制的機(jī)床和工藝來說，在經(jīng)濟(jì)上是不劃算的產(chǎn)品生產(chǎn)任務(wù)。數(shù)字控制還使得機(jī)床比它們采用有人工操的前輩們的用途更為廣泛。數(shù)字控制的機(jī)器比人工操縱的機(jī)器精度更高、生產(chǎn)出零件的一致性更好、生 3產(chǎn)速度更快、而且長期的工藝裝備成本更低。對(duì)于一臺(tái)數(shù)控機(jī)床，其上必須安有一個(gè)被稱為閱讀機(jī)的界面裝置，用來接受和解譯出編程指令。 數(shù)字控制意味著采用預(yù)先錄制的、存儲(chǔ)的符號(hào)指令來控制機(jī)床和其他制造系統(tǒng)。采用人工控制是，產(chǎn)品的質(zhì)量直接與操作者的技能有關(guān)。在數(shù)控技術(shù)出現(xiàn)之前，所有的機(jī)床都是由人工操縱和控制的。因此，為了減少或者完全不雇用這類熟練工人，六角車床、螺紋加工車床和其他類型的半自動(dòng)和自動(dòng)車床已經(jīng)很好地研制出來，并已經(jīng)在生產(chǎn)中得到廣泛應(yīng)用。此外，需要技術(shù)熟練的工人來操作普通車床，這種工人的工資高而且很難雇到。 雖然普通車床有很多用途，是很有用的機(jī)床，但是更換和調(diào)整刀具以及測量工件花費(fèi)很多時(shí)間