數(shù)控技術(shù)外文文獻翻譯、中英文翻譯、外文翻譯

數(shù)控技術(shù)外文文獻翻譯、中英文翻譯、外文翻譯,數(shù)控技術(shù),外文,文獻,翻譯,中英文 附錄一：  數(shù)控技術(shù) CNC 代表計算機數(shù)（字）控（制），自 20 世紀 70 年代以來一直受到人們的關(guān)注。此前它被稱作 NC，即數(shù)（字）控（制）。雖然大多數(shù)其他行業(yè)的人從來沒有聽說過這個名詞，但 CNC 以這樣或那樣的方式幾乎觸及每一種形式的制造過程。從事制造業(yè)工作的人，很可能經(jīng)常與 CNC 打交道。 CNC 之前 盡管以上表述不盡如此，但數(shù)控機床顯然代替（或參與）了現(xiàn)有的一些制造過程。舉最簡單的制造過程—鉆孔為例。 臺式鉆床當(dāng)然可用于加工孔。操作者可以將鉆頭裝入鉆夾頭中，鉆夾頭固定在鉆臺的主軸里；接著可以（手工）選擇所需要的轉(zhuǎn)速（通常通過切換皮帶輪改變主軸轉(zhuǎn)速），并啟動主軸旋轉(zhuǎn)；然后操作鉆軸手柄，將鉆頭鉆入被加工的工件。 顯而易見，使用臺式鉆床鉆孔，需要很多人為的干預(yù)。幾乎全過程的每一步都需要人參與！如果只有數(shù)量不多的孔或工件要加工，這種人為干預(yù)對制造廠來說可能還是可以接受的，但是，隨著生產(chǎn)數(shù)量的增加，由于操作乏味而產(chǎn)生疲勞的可能性也增大了。務(wù)必注意我們只是舉了最簡單的操作之一（鉆削）為例，還有更加復(fù)雜的加工方式，對普通機床操作員的技術(shù)水平要求非常高（技能要求高， 意味著操作失誤的可能性增大，易導(dǎo)致廢品的產(chǎn)生）。（我們通常稱被 CNC 代替的這種機床為普通機床。） 相比較起來，替代臺式鉆床的數(shù)控機床（可能是數(shù)控加工中心或數(shù)控鉆削攻絲中心）可以通過編寫程序完成這種操作，自動化程度極高。一度由臺鉆操作員做的各種操作現(xiàn)在都由數(shù)控機床完成，包括：將鉆頭裝入主軸里、啟動主軸、將工件定位在鉆頭下方、鉆孔、停止主軸。 CNC 的工作原理 也許你已經(jīng)做出猜想：普通機床需要操作員做的每一件工件，在數(shù)控機床上都是由程序控制的。一旦機床裝夾完畢并運行，數(shù)控機床很容易保持自動運行狀態(tài)。實際上，由于無事可做，數(shù)控操作員在冗長的生產(chǎn)運行過程中感到很無聊。在有些數(shù)控機床上，甚至工件裝夾的過程都是自動化的。讓我們來看一下具體的可編程功能。 運動控制 所有的數(shù)控機床都有一個共同點：它們都有兩個或兩個以上的可編程運動方向，稱之為軸。軸運動可以是直線的（沿直線），也可以是旋轉(zhuǎn)的（沿圓周軌跡）。表征數(shù)控機床復(fù)雜程度的首要指標之一是它有幾個軸。一般來說，軸數(shù)越多，機床越復(fù)雜。 任何數(shù)控機床的軸是為了產(chǎn)生加工過程所需的運動。在鉆削的例子中，這些 （三個）軸將刀具定位在待加工孔的上方（兩軸運動），然后加工孔（第三軸運動）。軸用字母命名，常用的直線軸名稱 X,Y,Z,常用的旋轉(zhuǎn)軸名稱為 A,B,C,它們與坐標系相關(guān)。 可編程附件 如果數(shù)控機床只能進行兩個方向以上的軸運動，則這種機床沒有什么優(yōu)勢。幾乎所有的數(shù)控機床在很多其他方面都是可編程的，具體的數(shù)控機床與其配置的可編程附件很有關(guān)系。而且，在全功能數(shù)控機床上，任何需要的功能都是可編程的。下面是一種機床（加工中心）的一些可編程功能的例子。 自動刀具交換裝置 多數(shù)加工中心的刀庫能夠容納很多刀具，需要時，刀具被自動裝到主軸上， 準備加工。 主軸轉(zhuǎn)速選擇和啟動|停止 主軸速度（轉(zhuǎn)|分鐘）的指定很簡單。主軸可正方向轉(zhuǎn)動，也可以反方向轉(zhuǎn)動。當(dāng)然，也可停止轉(zhuǎn)動。 冷卻液的啟動|停止 很多加工操作需要冷卻液，用于潤滑和冷卻，冷卻液的開和關(guān)可在加工循環(huán)中進行。 CNC 程序 想一想逐步發(fā)指令的方式，數(shù)控（CNC）加工程序只不過是另外一種指令的組合。其格式類似句子，數(shù)控系統(tǒng)按其編寫順序，一步一地執(zhí)行。 專門的 CNC 字用于表示機床執(zhí)行的操作。數(shù)控字以字母地址開頭（如 F 表示進給速度，表示主軸轉(zhuǎn)速，X，Y，Z 表示軸運動）。一組數(shù)控字以合理的方式寫在一起，就構(gòu)成了一個命令，命令很像句子。 CNC 控制器（數(shù)控系統(tǒng)） 數(shù)控系統(tǒng)對數(shù)控程序進行翻譯，按順序激活一連串的命令。數(shù)控系統(tǒng)閱讀程序時，會激活相應(yīng)的機床功能，產(chǎn)生軸運動，總而言之，遵照執(zhí)行程序中所給的指令。 除了翻譯數(shù)控程序，數(shù)控系統(tǒng)還有其他的作用。目前所有的數(shù)控系統(tǒng)都允許程序有錯時對其進行修改（編輯）。數(shù)控系統(tǒng)提供了專門的程序測試功能（如空運行）確保程序的正確性。數(shù)控系統(tǒng)允許指定獨立于程序的重要的操作員輸入， 如刀具長度值?？傊瑪?shù)控系統(tǒng)使機床所有功能都可控。 CAM 系統(tǒng) 對于一些簡單的應(yīng)用（如鉆孔），數(shù)控程序可手動編制。也就是說，編程員僅使用鉛筆、紙、計算器這些設(shè)備，坐下來編寫程序。而這正是開發(fā)簡單數(shù)控程序的最好飛的方法。 隨著數(shù)控機床的應(yīng)用越來越復(fù)雜，尤其是經(jīng)常需要編制新程序，這是手工編寫程序變得越來越困難。為簡化編程過程，人們采用計算機輔助制造（CAM）系統(tǒng)。CAM 系統(tǒng)是運用于計算機（通常是個人計算機）、幫助數(shù)控編程員進行編程的軟件。一般來說，CAM 系統(tǒng)能消除編程的枯燥和辛苦。 許多公司的 CAM 系統(tǒng)是和公司設(shè)計工程部開發(fā)的 CAD 圖紙一起工作的。這免除了在 CAM 系統(tǒng)里從新對工件的結(jié)構(gòu)進行定義。數(shù)控編程員只需指定要做的加工操作，CAM 系統(tǒng)會自動生成數(shù)控加工程序（極像手工編制的程序）。 DNC 系統(tǒng) 程序編制（手工或 CAM 編程）完畢，必須轉(zhuǎn)載到數(shù)控系統(tǒng)。雖然安裝調(diào)試人員可以將程序通過鍵盤輸入到數(shù)控系統(tǒng)，但是這好像在把昂貴的數(shù)控機床當(dāng)打字機用。如果數(shù)控程序是用 CAM 系統(tǒng)開發(fā)的，則程序已是文本格式。如果程序是手工編寫的，可使用常用的字處理器將程序輸入計算機（但多數(shù)公司使用專門的CNC 文本編輯器）。不管哪種方法，程序的格式都是可以直接傳送到數(shù)控機床的文本文件。這時就采用分布式數(shù)控系統(tǒng)（DNC). DNC 系統(tǒng)只不過是與一臺或多臺數(shù)控機床連網(wǎng)的一臺計算機。直到現(xiàn)在，還必須用相當(dāng)原始的串行通信協(xié)議（RS-232C)進行程序的傳輸。新型的數(shù)控系統(tǒng)具有先進的網(wǎng)絡(luò)通信功能，可用更通用的方式進行連網(wǎng)（如以太網(wǎng)）。不管用什么方法，數(shù)控程序在運行前必須首先裝載入數(shù)控機床。 當(dāng)數(shù)控機床在計算機監(jiān)控下工作時，它就被稱為計算機數(shù)控機床（CNC）。計算機是 CNC 機床的控制單元，它們內(nèi)嵌于數(shù)控機床中或者通過通信渠道與數(shù)控機床連接，當(dāng)程序員編程時，通過紙帶或磁盤將一些信息輸入，計算機將對一些必要的數(shù)據(jù)進行計算來完成工作。 當(dāng)今的系統(tǒng)都由計算機來控制數(shù)據(jù)，因而稱之為計算機數(shù)控機床（簡稱 CNC 機床）。 NC 和 CNC 系統(tǒng)兩者的工作原理一樣，僅僅是控制執(zhí)行的方式不同。新型的數(shù)控系統(tǒng)通常速度更快、功率更大、功能更全。 計算機數(shù)控機床的結(jié)構(gòu) CNC 機床結(jié)構(gòu)較為復(fù)雜。一般來說，任何 CNC 機床都由以下幾個單元組成： 計算機、控制系統(tǒng)、驅(qū)動電動機和換刀機構(gòu)裝置。 根據(jù) CNC 機床的結(jié)構(gòu)，CNC 機床以下列方式工作： （1） CNC 機床語言是一種用在計算機上的二進制符號編程語言，而不是用在 CNC 機床上。 （2） 當(dāng)操作者開始執(zhí)行循環(huán)時，計算機將二進制代碼翻譯成電脈沖，并自動地傳送給機床的控制單元。控制單元將發(fā)出的脈沖數(shù)與接收的脈沖數(shù)加以比較。 （3） 電動機每接收一個脈沖，就自動地將脈沖信號轉(zhuǎn)換成旋轉(zhuǎn)信號從而驅(qū)動主軸和絲杠，帶動主軸旋轉(zhuǎn)和拖板或工作臺的移動。銑床工作臺上的零件或車床轉(zhuǎn)塔刀架上的刀具被驅(qū)動到程序指定的位置。 1. 計算機 像所有其它計算機一樣，CNC 機器上的計算機也只使用 1 和 0，按照二進制原理運行，處理來自系統(tǒng)電路的精確時間脈沖信息。有兩個狀態(tài)，高電平為 1， 低電平為 0，1 和 0 的序列是計算機區(qū)別于所謂的機器語言的唯一狀態(tài)，也是計算機理解的唯一語言。編程時程序員不必關(guān)心機器語言，只需要簡單地運用一系列代碼和符號來表達有用的信息。機器內(nèi)的特殊軟件將程序編譯為機器語言，由伺服電動機帶動刀具流動。然而，機床程序的可用性依賴于機床控制系統(tǒng)中是否有計算機。如果用一個微型計算機進行程序設(shè)計，例如，半徑（一個相當(dāng)簡單的任務(wù)），計算機將計算刀具路徑上的全部位移點。如果機床上沒有微型計算機， 這個將是一個繁瑣的工作，因為程序員必須計算刀具路徑上的所有交點位置?，F(xiàn)代 CNC 機床的計算機使用 32 位處理器可以實現(xiàn)信息的快速準確處理。 2. 控制系統(tǒng) 在 NC/CNC 機床里有兩種類型的控制系統(tǒng)：開環(huán)和閉環(huán)。所使用的控制環(huán)的類型決定機床的整體精度。 開環(huán)控制系統(tǒng)不提供位置反饋信息給控制單元。移動脈沖由控制單元發(fā)出， 并被一種稱為步進電動機的特殊伺服電動機所接收。控制單元發(fā)送給步進電動機的脈沖數(shù)控制電動機的旋轉(zhuǎn)角度。然后步進電動機接收下一個運動命令。由于這個控制系統(tǒng)只計算脈沖，不能識別位置偏差，因此機床將繼續(xù)其不準確操作，直到有人發(fā)現(xiàn)錯誤為止。由于這個控制系統(tǒng)只計算脈沖而不能識別位置偏差, 因此機床將繼續(xù)其不準確操作,直到有人發(fā)現(xiàn)錯誤為止? 開環(huán)控制適用于載荷狀態(tài)沒有變化的場合，例如，數(shù)控鉆床。開環(huán)控制系統(tǒng)的好處是成本低，因為它不需要為位置反饋安裝額外的硬件及電子裝置。其缺點是很難檢測位置誤差。 在閉環(huán)控制系統(tǒng)中，由控制單元發(fā)出電脈沖并送給伺服電動機，使電動機跟隨每個脈沖旋轉(zhuǎn)。這些運動能被一個稱為傳感器的反饋裝置檢測并記錄下來。每移動一步，傳感器就發(fā)送一個信號送回到控制單元，并且將當(dāng)前驅(qū)動軸位置和程序中的設(shè)定位置相比較。當(dāng)發(fā)出的和接收的脈沖數(shù)相匹配時，控制單元開始為下一個運動發(fā)出脈沖。 閉環(huán)系統(tǒng)非常精確。大部分閉環(huán)系統(tǒng)具有誤差的自動補償功能，通過反饋裝置指示誤差并作出必要的調(diào)整，將拖板帶回到準確位置。閉環(huán)控制系統(tǒng)通常采用交流、直流或液壓伺服電動機。 在數(shù)控機床中，位置測量可通過直接或間接測量的方式來實現(xiàn)。在直接測量系統(tǒng)中，傳感器讀取裝在作直線運動的機床工作臺或拖板上的標尺刻度數(shù)。這種系統(tǒng)比較精確，因為標尺裝在機床內(nèi)，并且機構(gòu)中的后坐力（鄰近嚙合齒輪之間的作用）不是很明顯。 在間接測量系統(tǒng)中，旋轉(zhuǎn)編碼器將旋轉(zhuǎn)運動轉(zhuǎn)換成直線運動。在這種測量系統(tǒng)中，后坐力在很大程度上會影響測量精度。采用多種傳感器的位置反饋機制主要是基于磁和光電原理。 3. 驅(qū)動電動機 驅(qū)動電動機控制數(shù)控或計算機數(shù)控設(shè)備上機器拖板的運動。它共有四種基本類型：步進電動機、直流伺服電動機、交流伺服電動機和液壓伺服電動機。 微機單元(MCU)送給步進電動機的脈沖數(shù)控制步進電動機的轉(zhuǎn)動角度。步進電動機將由微機單元（MCU）發(fā)出的一個數(shù)字脈沖轉(zhuǎn)換成一個小步轉(zhuǎn)動，步進電動機能夠傳遞一定數(shù)量的步數(shù)，微機單元（MCU）送給步進電動機的脈沖數(shù)控制步進電動機的轉(zhuǎn)動角度。步進電動機常應(yīng)用在傳動扭矩要求較低的場合。 步進電動機常應(yīng)用在開環(huán)控制系統(tǒng)中，而交流、直流或液壓伺服電動機常使用在閉環(huán)控制系統(tǒng)中。 直流伺服電動機是可以變速的電動機，其轉(zhuǎn)速隨供給電壓的變化而變化。它們常被用來驅(qū)動絲杠和齒輪傳動機構(gòu)。直流伺服電動機能夠提供比步進電動機更大的傳動扭矩。 交流伺服電動機是通過改變電壓頻率來控制轉(zhuǎn)速的。交流伺服電動機比直流伺服電動機的輸出功率更大，它們也被用來驅(qū)動絲杠和齒輪機構(gòu)。 液壓伺服電動機也是速度可調(diào)電動機。在應(yīng)用氣泵場合下，液壓伺服電動機能夠產(chǎn)生的功率和速度要比電氣伺服電動機更大更高。液壓泵將能量提供給由微機單元控制的閥門。 4. 換刀裝置 通常，加工一個零件需要使用幾把不同的刀具。加工過程中必須為下一步加工工序迅速換刀。為此，大部分數(shù)控或計算機數(shù)控機床配備有自動換刀裝置，如加工中心上的鏈式刀庫和車削中心的轉(zhuǎn)塔刀庫。在具有自動換刀裝置的大部分機床上,刀架和自動送刀裝置可以進行旋轉(zhuǎn), 正向或反向均可?較為典型的是, 自動換刀裝置會卡緊車床主軸內(nèi)的刀具, 將其拉出, 然后換上另一把刀具?在具有自動換刀裝置的大部分機床上，刀架和自動換刀裝置可以進行旋轉(zhuǎn)，正向反向均可。 換刀機構(gòu)有隨機換刀和順序換刀。在隨機換刀中，沒有刀具選擇的具體路徑。對于加工中心，當(dāng)程序調(diào)用刀具時，刀具處理裝置能自動檢索到處于等待位置的刀具。對于車削中心，回轉(zhuǎn)刀架自動地旋轉(zhuǎn)，將刀具帶到指定位置。 雖然各種數(shù)控機床的功能和應(yīng)用各不相同，但它們有著共同的優(yōu)點。這里是 數(shù)控設(shè)備提供的比較重要的幾個優(yōu)點. 各種數(shù)控機床的第一個優(yōu)點是自動化程度提高了。零件制造過程中的人工干預(yù)減少或者免除了。整個加工循環(huán)中，很多數(shù)控機床處于無人照看狀態(tài)，這使操作員被解放出來，可以干別的工作。數(shù)控機床用戶得到的幾個額外好處是：數(shù)控機床減小了操作員的疲勞程度，減少了人為誤差，工件加工時間一致而且可預(yù)測。由于機床在程序的控制下運行，與操作普通機床的機械師要求的技能水平相比， 對數(shù)控操作員的技能水平要求（與基本加工實踐相關(guān)）也降低了。 數(shù)控技術(shù)的第二個優(yōu)點是工件的一致性好，加工精度高?，F(xiàn)在的數(shù)控機床宣稱的精度以及重復(fù)定位精度幾乎令人難以置信。這意味著，一旦程序被驗證是正確的，可以很容易地加工出 2 個、10 個或 1 000 個相同的零件，而且它們的精度高，一致性好。 大多數(shù)數(shù)控機床的第三個優(yōu)點是柔性強。由于這些機床在程序的控制下工作，加工不同的工件易如在數(shù)控系統(tǒng)中裝載一個不同的程序而已。一旦程序驗證正確，并且運行一次，下次加工工件的時候，可以很方便地重新調(diào)用程序。這又帶來另一個好處——可以快速切換不同工件的加工。由于這些機床很容易調(diào)整并運行，也由于很容易裝載加工程序，因此機床的調(diào)試時間很短。 運動控制——CNC 的核心 任何數(shù)控機床最基本的功能是具有自動、精確、一致的運動控制。大多數(shù)普通機床完全運用機械裝置實現(xiàn)其所需的運動，而數(shù)控機床是以一種全新的方式控制機床的運動。所有的數(shù)控設(shè)備都有兩個或多個運動方向，稱為軸。這些軸沿著其長度方向精確、自動定位。最常用的兩類軸是直線軸（沿直線軌跡）和旋轉(zhuǎn)軸 （沿圓形軌跡）。 普通機床需通過旋轉(zhuǎn)搖柄和手輪產(chǎn)生運動，而數(shù)控機床通過編程指令產(chǎn)生運動。通常，幾乎所有的數(shù)控機床的運動類型（快速定位、直線插補和圓弧插補）、移動軸、移動距離以及移動速度（進給速度）都是可編程的。 數(shù)控系統(tǒng)中的 CNC 指令命令驅(qū)動電機旋轉(zhuǎn)某一精確的轉(zhuǎn)數(shù)，驅(qū)動電機的旋轉(zhuǎn)隨即使?jié)L珠絲杠旋轉(zhuǎn)，滾珠絲杠將旋轉(zhuǎn)運動轉(zhuǎn)換成直線軸（滑臺）運動。滑臺上的反饋裝置（直線光柵尺）使數(shù)控系統(tǒng)確認指令轉(zhuǎn)數(shù)已完成。 附錄二： CNC CNC stands for Computerized Numerical Control and has been around since the early1970s. prior to this, it was called NC,for numerical control. While people in most walks of life have never heard of this term, CNC has touched almost every form of manufacturing process in one way or another. If you'll be working in manufacturing, it's likely that you'll be dealing with CNC on a regular basis. Before CNC While there are exceptions to this statement,CNC machines typically replace (or work in conjunction with) some existing manufacturing processes. Take one of the simplest manufacturing processes,drilling holes,for example. A drill press can of course be used to machine holes. A person can place a drill in the drill chuck that is secured in the spindle of the drill press. They can then (manually) select the desired speed for rotation (commonly by switching belt pulleys), and activate the spindle. Then they manually pull on the quill lever to drive the drill into the workpiece being machined. As you can easily see, there is a lot of manual intervention required to use a drill press to holes. A person is required to do something almost every step along the way! While this manual intervention may be acceptable for manufacturing companies if but a small number of holes workpieces must be machined, as quantities grow, so does the likelihood for fatigue due to the tediousness of the operation. And do note that we've used one of the simplest machining operations(drilling) for our example. There are more complicated machining operations that would require a much higher skill level (and increase the potential for mistakes resulting in scrap workpieces) of the person running the conventional machine tool. (We commonly refer to style of machine that CNC is replacing as the conventional machine.) By comparison, the CNC equivalent for a drill press (possibly a CNC machining center or CNC drilling & tapping center) can be programmed to perform this operation in a much more automatic fashion. Everything that the drill press operator was doing manually will now be done by the CNC machine, including:placing the drill in the spindle, activating the spindle,positioning the workpiece under the drill, machining the hole, and turning off the spindle. How CNC works As you might already have guessed,everything that an operator would be required to do with conventional machine tools is programmable with CNC machines. Once the machine is setup and running, a CNC machine is quite simple to keep running. In fact CNC operators tend to get quite bored during lengthy production runs because there is so little to do. With some CNC machines, some of the specific programmable functions. Motion control All CNC machine types share this commonalty: They all have two or more programmable directions of motion called axes. An axis of motion can be linear(along a straight line) or rotary(along a circular path). One of the first specifications that imply a CNC machine's complexity is how many axes it has. Generally speaking, the more axes, the more complex the machine. The axes of any CNC machine are required for the purpose of causing the motions needed for the manufacturing process. In the drilling example, these axes would position then tool over the hole to be machined (in two axes) and machine the hole (with the third axis). Axes are named with letters.Common linear axis named X,Y,and Z. Common rotary names are A,B,and C. There are related to the coordinate system. Programmable accessories A CNC machine wouldn't be very helpful if all it could only move the workpiece in two or more axes. Almost all CNC machines are programmable in several other ways. The specific CNC machine type has a lot to do with its appropriate programmable accessories. Again,any required function will be programmable on full-blown CNC machine tools. Here are some examples for one machine type(machining centers). Automatic tool changer Most machining centers can hold many tools in a tool magazine. When required, the required tool can be automatically placed in spindle for machining. Spindle speed and activation The spindle speed(in revolutions per minute) can be easily specified and the spindle can be turned on in a forward or reverse direction.It can also,of course, be turned off. Coolant Many machining operations require coolant for lubrication and cooling purposes. Coolant can be turned on and off from within the machine cycle. The CNC program Think of giving any series of step-by-step instructions. A CNC program is nothing more than another kind of instruction set. It's written in sentence-like format and the control will execute it in sequential order,step by step. A special series of CNC words are used to communicate what the machine is intended to do. CNC words begin with letter address(like F for feedrate,S for spindle speed,and X,Y,and Z for axis motion). When placed together in a logical method, a group of CNC words make up a command that resemble a sentence. The CNC control The CNC control will interpret a CNC program and active the series of commands in sequential order. As it reads the program, the CNC control will activate the appropriate machine functions, cause axis motion, and in general, follow the instructions given in the program. Along with interpreting the CNC program, the CNC control has several other purposes. All current model CNC controls allow programs to be modified(edited) if mistakes are found. The CNC control allows special verification functions(like dry run) to confirm the correctness of the CNC program. The CNC control allows certain important operator inputs to be specified separate from the program, like tool length values. In general, the CNC control allows functions of the machine to be manipulated. What is a CAM system? For simple applications (like drilling holes),the CNC program can developed manually. That is ,a programmer will sit down to write the program armed only with pencil,paper, and calculator. Again, for simple applications,this may be the very best way to develop CNC programs. As applications get more complicated, and especially when new programs are required on a regular basis, writing programs manually becomes much more difficult.To simplify the programming process,a computer aided manufacturing (CAM) system can be used. A CAM system is a software program that runs on a computer(commonly a PC) that helps the CNC programmer with the programming process. Generally speaking, a CAM system will take the tediousness and drudgery out of programming. In many companies the CAM system will work with the computer aided design(CAD) drawing developed by the computer's design engineering department.This eliminates the need for redefining the workpiece configuration to the CAM system .The CNC programmer will simply specify the machining operations to be performed and the CAM system will create the CNC program(much like the manual programmer would have written) automatically. What is a DNC system? Once the program is developed (either manually or with a CAM system), it must be loaded into the CNC control. Tough the setup person could type the program right into the control, this would be like using the CNC machine as a very expensive typewrite. If the CNC program is developed with the help of a CAM system, then it is already in the form of a text file.If the program is written manually,it can be typed into any computer using a common word processor (though most companies use a special CNC text editor for this purpose). Either way, the program is in the form of a text file that can be transferred right into the CNC machine. A distributive numerical control (DNC) system is used for this purpose. A DNC system is nothing more than a computer that is networked with one or more CNC machines. Until only recently, rather crude serial communications protocol (RS-232C) had to be used for transferring programs. Newer controls have more current communications capabilities and can be networked in more conventional ways (Ethernet, etc.). Regardless of methods , the CNC program must of course be loaded into the CNC machine before it can be run. When Numerical Control is performed under computer supervision, it is called Computer Numerical Control (CNC). Computers are the control units of CNC machines. They are built in or linked to the machines via communications channels. When a programmer inputs some information in the program by tape and so on, the computer calculates all necessary data to get the job done. Today ’ s systems have computers control data, so they are called Computer Numerically Controlled Machines. For both NC and CNC systems, work principles are the same. Only the way in which the execution is controlled is different. Normally, new systems are faster, more powerful, and more versatile unit. The Construction of CNC Machines CNC machine tools are complex assemblies. However, in general, any CNC machine tool consists of the following units: computers, control systems, drive motors and tool changers. According to the construction of CNC machine tools, CNC machines work in the following manner: (1) The CNC machine language, which is a programming language of binary notation used on computers, is not used on CNC machines. (2) When the operator starts the execution cycle, the computer translates binary codes into electronic pulses that are automatically sent to the machine’s power units. The control units compare the number of pulses sent and received. (3) When the motors receive each pulse, they automatically transform the pulses into rotations that drive the spindle and lead screw, causing the spindle rotation and slide or table movement. The part on the milling machine table or the tool in the lathe turret is driven to the position specified by the program. 1. Computers As with all computers, the CNC machine computer works on binary principle using only two characters 1 and 0, for information processing precise time impulses from the circuit. There are two states, a state with voltage, 1, and a state without voltage, 0. Series of ones and zeroes are the only states that the computer distinguishes are called machine language, and it is the only language the computer understands. When creating the program, the programmer does not care about the machine language. He or she simply uses a list of codes and keys in the meaningful information.Special built-in software compiles the program into the machine language and the machine moves the tool by its servomotors. However, the programmability of the machine is dependent on whether there is a computer in the machine’s control. If there is a minicomputer programming, say, a radius (which is a rather simple task), the computer will calculate all the points on the tool path.On the machine without a minicomputer, this may prove to be a tedious task, since the programmer must calculate all the points of intersection on the tool path. Modern CNC machines use 32-bit processors in their computers that allow fast and accurate processing of information. 2. Control systems There are two types of control systems on NC/CNC machines: the open loop and the closed loop. The type of control loop used determines the overall accuracy of the machine. The open-loop control system does not provide positioning feedback to the control unit. The movement pulses are sent out by the control and they are received by a special type of servomotor called a stepper motor.The number of pulses that the control sends to the stepper motor controls the amount of the rotation of the motor. The stepper motor then proceeds with the next movement command. Since this control system only counts pulses and cannot identify discrepancies in positioning, the machine will continue this inaccuracy until somebody finds the error. The open-loop control can be used in applications in which there is no change in load conditions, such as the NC drilling machine.The advantage of the open-loop control system is that it is less expensive, since it does not require the additional hardware and electrics needed for positioning feedback. The disadvantage is the difficulty of detecting a positioning error. In the closed-loop control system, the electronic movement pulses are sent from the control to the servomotor, enabling the motor to rotate with each pulse. The movements are detected and counted by a feedback device called a transducer. With each step of movement, a transducer sends a signal back to the control, which compares the current position of the driven axis with the programmed position. When the number of pulses sent and received matches, the control starts sending out pulses for the next movement. Closed-loop systems are very accurate. Most have an automatic compensation for error, since the feedback device indicates the error and the control makes the necessary adjustments to bring the slide back to the position. They use AC, DC or hydraulic servomotors. Position measurement in NC machines can be accomplished through direct or indirect methods. In direct measuring systems, a sensing device reads a graduated scale on the machine table or slide for linear movement. This system is more accurate because the scale is built into the machine and backlash (the play between two adjacent mating gear teeth) in the mechanisms is not significant. In indirect measuring systems, rotary encoders or resolves convert rotary movement to translation movement. In this system, backlash can significantly affect measurement accuracy. Position feedback mechanisms utilize various sensors that are based mainly on magnetic and photoelectric principles. 3. Drive Motors The drive motors control the machine slide movement on NC/CNC equipment. They come in four basic types: stepper motors, DC servomotors, AC servomotors and fluid servomotors. Stepper motors convert a digital pulse generated by the microcomputer unit (MCU) into a small step rotation. Stepper motors have a certain number of steps that they can travel. The number of pulses