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南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位論文
計(jì)算機(jī)輔助設(shè)計(jì)與制造
CAD/CAM是表示計(jì)算機(jī)輔助設(shè)計(jì)和計(jì)算機(jī)輔助制造的專業(yè)術(shù)語(yǔ)。它是一種使用計(jì)算機(jī)完成某些設(shè)計(jì)和生成功能的技術(shù)。在生產(chǎn)企業(yè)里，人們通常把設(shè)計(jì)和制造是為兩項(xiàng)有著明顯不同職能的分工，而這項(xiàng)技術(shù)正朝著設(shè)計(jì)與制造的更大程度一體化方向發(fā)展。最終，CAD/CAM將會(huì)為未來(lái)的計(jì)算機(jī)集成工廠提供技術(shù)基礎(chǔ)。
計(jì)算機(jī)輔助設(shè)計(jì)（CAD）可定義為運(yùn)用計(jì)算機(jī)系統(tǒng)對(duì)設(shè)計(jì)的創(chuàng)意、修改、分析或優(yōu)化予以輔助。這些由硬件和軟件構(gòu)成的計(jì)算機(jī)系統(tǒng)，用于完成用戶公司要求的特定設(shè)計(jì)功能。CAD硬件通常包括：一臺(tái)計(jì)算機(jī)，一個(gè)或多個(gè)圖形顯示終端，鍵盤和其他外圍設(shè)備。CAD軟件包括各種計(jì)算機(jī)制圖程序，這些程序便于用戶公司完成設(shè)計(jì)職能，如：零部件的應(yīng)變分析，機(jī)構(gòu)的動(dòng)態(tài)響應(yīng)，熱傳輸計(jì)算和數(shù)控零件編程。由于用戶的生產(chǎn)流程、制造工藝和銷售市場(chǎng)方面的差異，應(yīng)用程序的配置也將因用戶而異。這些因素均導(dǎo)致對(duì)CAD系統(tǒng)要求的差異性。
計(jì)算機(jī)輔助制造（CAM）可定義為通過(guò)直接或間接與廠家生產(chǎn)資源相適應(yīng)的計(jì)算機(jī)界面，使用計(jì)算機(jī)系統(tǒng)來(lái)規(guī)劃、管理和控制制造工廠的運(yùn)行。正如定義所表示的那樣，CAM應(yīng)用程序可分為兩大類：
1. 計(jì)算機(jī)監(jiān)控程序；2. 制造程序。
二者之間的區(qū)別是理解計(jì)算機(jī)輔助制造的基礎(chǔ)。
計(jì)算機(jī)輔助制造的應(yīng)用程序，除了為監(jiān)控制造過(guò)程而直接使用計(jì)算機(jī)界面的應(yīng)用程序之外，還包括在工廠生產(chǎn)運(yùn)行過(guò)程中由計(jì)算機(jī)提供支持的間接應(yīng)用程序。在這些應(yīng)用程序中，計(jì)算機(jī)并不直接與制造過(guò)程相聯(lián)接。相反，在脫機(jī)狀態(tài)下，計(jì)算機(jī)可用來(lái)提供計(jì)劃書、進(jìn)度表、預(yù)報(bào)、指令和使廠家生產(chǎn)資源管理更加有效的信息資料。計(jì)算機(jī)和制造過(guò)程間的關(guān)系如下圖所示。圖中虛線用來(lái)說(shuō)明交流和控制處于脫即狀態(tài)下，需要人來(lái)完善界面。目前，CAM的應(yīng)用需要由人來(lái)為計(jì)算機(jī)輸入程序，解釋計(jì)算機(jī)的輸出，并采取所要求的措施。
生產(chǎn)操作
計(jì)算機(jī)
處理數(shù)據(jù)
控制信號(hào)
CAM用于生產(chǎn)支持
Notes:
1. CAD(computer-aided design) 計(jì)算機(jī)輔助設(shè)計(jì)
2. CAM(computer-aided manufacturing) 計(jì)算機(jī)輔助制造
3. computer monitoring and control 計(jì)算機(jī)監(jiān)控
4. manufacturing support applications 生成支持應(yīng)用軟件
5. peripheral equipment 外圍設(shè)備（外設(shè)）
6. computer graphics 電腦制圖
什么是CAD/CAM軟件？
許多刀具軌跡是簡(jiǎn)單的但太復(fù)雜和昂貴以致于人們很難制造，對(duì)于這種情況，我們需要在計(jì)算機(jī)的幫助下來(lái)作數(shù)控部分程序。
CAD/CAM最基礎(chǔ)的概念是，我們可以用計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)在計(jì)算機(jī)上畫出工件的幾何形狀。幾何形狀一旦完成，我們就可以用計(jì)算機(jī)輔助制造系統(tǒng)根據(jù)CAD的幾何形狀，生成數(shù)控機(jī)床的刀具軌跡。
利用CAD繪制所有路徑對(duì)數(shù)控加工的路徑如下：
第一步：利用CAD繪制幾何圖形已被定義這種工件包括型腔加工。對(duì)于這種型腔加工很可能將花幾個(gè)小時(shí)去制造代碼。然而，我們能夠利用CAM程序去創(chuàng)造NC代碼在每分鐘內(nèi)。
第二步：接著，這個(gè)模型就被輸入到CAM模塊中。然后，我們就選擇合適的幾何形狀并定義要生成的刀具軌跡類型，在這個(gè)例子中就是一個(gè)型腔。我們一定也能夠讓CAM系統(tǒng)如刀具的使用，材料的類型，進(jìn)給量，切屑深度等信息。
第三步：CAM模型證實(shí)保證刀具軌跡的正確性。如果發(fā)現(xiàn)了一些錯(cuò)誤，最簡(jiǎn)單就是在某個(gè)位置上加以改正。
第四步：CAD/CAM模塊最終產(chǎn)生的是NC代碼。通過(guò)模型的后處理則可以生成適用于特定CNC控制器的NC代碼。
我們?nèi)∽煮w的首字母即CAPP，它代表了計(jì)算機(jī)輔助程序。CAPP是使用計(jì)算機(jī)輔助在數(shù)控刀具軌跡中的應(yīng)用。然而，CAPP從來(lái)就沒(méi)有真正獲得廣泛的流傳和接受，而且至今我們很少聽(tīng)到這個(gè)項(xiàng)目。取而代之的是在更多的市場(chǎng)上使用CAD/CAM，使用計(jì)算機(jī)的思維來(lái)幫助生產(chǎn)NC程序。不幸的是，因?yàn)镃AM是一個(gè)整體技術(shù)與制造技術(shù)和自動(dòng)化技術(shù)有著關(guān)聯(lián)的——不僅是軟件，而且使用CNC機(jī)器工具。
描述CAD/CAM組成及功能
CAD系統(tǒng)包括CAD設(shè)備及CAM設(shè)備——每個(gè)都有許多功能元件。它們將在短時(shí)間內(nèi)掃描這些元件目的是為了了解其工作的整個(gè)過(guò)程。
1. CAD模型
系統(tǒng)的CAD部分用于生成可作為CAD模型的幾何形狀。CAD模型是工件幾何形狀的電子描述，她在數(shù)學(xué)上是十分精確的。不論是獨(dú)立的CAD系統(tǒng)還是作為CAD/CAM軟件包的一部分的CAD系統(tǒng)，往往都可以在幾個(gè)不同的層次上混合使用。
兩維線條圖
幾何圖形被體現(xiàn)出兩個(gè)方向，就象一張清單，Z軸深度將不得不與CAM相關(guān)。
三維線框模型
通過(guò)將代表邊界的元素連接早一起，就可以在三維空間中表現(xiàn)幾何形狀。雖然線框圖難以想象,但可以得到CAM所需的所有的Z向信息。
三維表面模型
它與線框模型十分相似，所不同的是，在線框模型間覆蓋了一層薄薄的外皮，從而使模型更形象。另外，模型型腔是空的，完成表面模型即可。
三維實(shí)體模型
這種當(dāng)前高科技市場(chǎng)形式被使用必須通過(guò)所有的高端科技軟件。幾何形狀被看作是一個(gè)實(shí)體特征其包括許多方面。實(shí)體模型可以被切開(kāi)以展示內(nèi)部特征，而不僅僅是擁有一層表皮。
2. CAM模型
根據(jù)CAD模型提供的幾何形狀，CAM模塊用于創(chuàng)建加工工藝模型。例如，CAD模型可能包括一些特征，即凹槽型腔。我們可能應(yīng)用其加工路線來(lái)加工幾何圖形，然后，所有的刀具路徑將是自動(dòng)的產(chǎn)生凹槽行程。同樣，CAD模型也可包括幾何圖形應(yīng)該產(chǎn)生鉆的操作。我們能夠選擇簡(jiǎn)單的幾何圖形和按照CAM系統(tǒng)的說(shuō)明在適當(dāng)?shù)奈恢蒙线M(jìn)行鉆孔。
CAM系統(tǒng)將生成描述加工操作的普通中間代碼，這些代碼以后可被用來(lái)生成G和M代碼或會(huì)話式程序。在它們合適的環(huán)境下，有些系統(tǒng)產(chǎn)生中間代碼。而其他使用較標(biāo)準(zhǔn)代碼例如APT就是他們的中間文件。
CAM模塊也有多種類型和層次。首先，通常有些不同模型功能如銑、鉆、以及裝配，每一步工藝是唯一的，典型的模型有附加軟件。每一個(gè)模型也可能使用不同的設(shè)備。例如，開(kāi)始用簡(jiǎn)單的設(shè)備，到后來(lái)用復(fù)雜的、多方向的刀具軌跡路線，CAM模型中銑床加工通常加工的過(guò)程如下：
● 21/2-方向的機(jī)床
● 3個(gè)方向的機(jī)床
● 加工表面的機(jī)床
● 5個(gè)方向混合的機(jī)床
每種體現(xiàn)出高精度的設(shè)備不可能在所有的壞鏡下制造，一個(gè)工序很可能只要求三個(gè)方向的設(shè)備，而一個(gè)模型工序很可能需要全部的表面加工設(shè)備，而且可能需要五個(gè)方向的CAM軟件包混合這樣就有可能完成其加工表面。這種一流的軟件安裝很可能需要花費(fèi)￥5000，但是許多復(fù)雜模型將花費(fèi)￥15000甚至更多。因此，我們沒(méi)有必要購(gòu)買這種高水準(zhǔn)的軟件，因?yàn)槲覀儾荒芡耆l(fā)揮他們的潛能。
3. 幾何圖形及刀具軌跡
我們必須理解一個(gè)重要的概念，即CAD所繪制的幾何形狀并不一定與CNC機(jī)床加工出的幾何形狀完全一致。只要刀具軌跡是直線或圓弧，CNC機(jī)車就可以加工出非常精確的刀具軌跡。CAD系統(tǒng)也可能加工高精度幾何圖形如直線及圓弧，但是也可能加工許多不同層次的曲面，同樣許多這種曲面被認(rèn)為是非均勻有理B樣條曲線。事實(shí)上，NURBS曲線可以描繪出從直線或圓弧到復(fù)雜的表面的任何幾何形狀。
例如，就象幾何圖形為橢圓形，橢圓有一系列曲線，有著不同的環(huán)形弧，橢圓在CAD系統(tǒng)利用鼠標(biāo)單擊很容易產(chǎn)生。然而，一個(gè)標(biāo)準(zhǔn)的CNC加工刀具不能夠直接使用產(chǎn)生一個(gè)橢圓——它只能產(chǎn)生直線和圓弧。CAM系統(tǒng)將順從于這種問(wèn)題，通過(guò)估算用直線段代替曲線。
CNC機(jī)床刀具通常只能識(shí)別圓弧或直線。因此，CAM系統(tǒng)必須估計(jì)直線段代替曲線之間的公差帶。在這種情況下，就像橢圓刀具軌跡產(chǎn)生包含著用直線段代替曲線之間的公差帶。
CAM系統(tǒng)會(huì)在真正的曲線兩側(cè)各生成一個(gè)幾何邊界，從而形成一個(gè)公差帶。它將生成一道刀具軌跡線包含著少量的公差帶，結(jié)果這刀具軌跡將在數(shù)學(xué)理論上不正確——CAM系統(tǒng)只能估算表面，使用它最基本的方法是為了估算刀具軌跡包括兩維曲線及三維表面曲線。
有些CAM系統(tǒng)也有可能直接繪制直線段為圓弧曲線。這可能在程序中產(chǎn)生許多模塊導(dǎo)致表面的光滑。
這種出現(xiàn)能夠控制公差帶的大小，其目的是使的刀具軌跡更加精確，這是有必要的。較小的公差帶可以生成細(xì)致的刀具軌跡以及大量的直線段，而較大的公差帶將會(huì)產(chǎn)生較少的直線段，刀具軌跡也比較粗糙。每一個(gè)直線段將要求在NC程序有模塊代碼。因此，當(dāng)使用這種技術(shù)時(shí)，NC程序能夠擴(kuò)大范圍。
在加工表面時(shí)我們一定要細(xì)心，依靠計(jì)算機(jī)生成正確的刀具很容易，但在用球狀端銑刀進(jìn)行曲面的精加工是須進(jìn)一步估價(jià)。如果我們沒(méi)能注意到這種技術(shù)的局限性，那么精加工后工件的精度就會(huì)大打折扣。
4. 刀具庫(kù)和材料庫(kù)
為了創(chuàng)建機(jī)械加工工藝，CAM系統(tǒng)需要了解切割刀具的利用以及機(jī)械材料。CAM注意的是通過(guò)提供可定制的刀具型號(hào)及類型。材料庫(kù)包括的信息是最優(yōu)化的切削速度以及進(jìn)給量。CAM使用這種信息聚在一起創(chuàng)建正確的刀具軌跡以及機(jī)床參數(shù)。
這類刀具和材料庫(kù)的格式經(jīng)常是獨(dú)有的，這一點(diǎn)帶來(lái)了一些兼容問(wèn)題。專用的工具庫(kù)及材料庫(kù)不容易修改或不容易使用其他的系統(tǒng)。進(jìn)步的CAM開(kāi)發(fā)者趨向于將刀具和材料庫(kù)生成數(shù)據(jù)庫(kù)文件，這樣就可以為其他的應(yīng)用者進(jìn)行修改和定制提供方便。
5. 檢驗(yàn)及后置處理
CAM系統(tǒng)通常提供檢驗(yàn)刀具軌跡是否正確性的功能。這可以通過(guò)加工操作的刀具中心線的簡(jiǎn)單繪制或通過(guò)復(fù)雜的實(shí)體模型來(lái)實(shí)現(xiàn)。實(shí)體驗(yàn)證通常是CAD/CAM軟件公司已經(jīng)獲得許可的第三方軟件。然而，它可能是作為一個(gè)獨(dú)立的軟件包。
后置處理器是一個(gè)軟件程序，他將通過(guò)的中間代碼格式化為使用于每個(gè)特定機(jī)床控制器的NC代碼。后置處理器通?？梢酝ㄟ^(guò)模板和變量被定制為需要的樣式。
6. 便捷性
電子數(shù)據(jù)的便捷性是CAD/CAM系統(tǒng)唯一致命的弱點(diǎn)，這個(gè)問(wèn)題任就是一件十分耗時(shí)的事情。CAD文件創(chuàng)建了許多格式以及它們之間有許多步同的組成。利用CAD系統(tǒng)創(chuàng)建一個(gè)復(fù)雜的模型是比較昂貴的。因此，我們希望使模型的便捷性最大化，而使在不同系統(tǒng)中重新生成幾何模型的需求最小化。
CAM模型與手提式的CAD模型不同，我們通常不能夠發(fā)展一個(gè)CAM模型以及把它轉(zhuǎn)換成其他格式。唯一被廣泛接受的CAM模型交換版本就是自動(dòng)編程工具（APT）。自動(dòng)編程工具是一種利用描述機(jī)床操作的程序語(yǔ)言工具，自動(dòng)編程工具是一種標(biāo)準(zhǔn)的、有著好的文件能夠通過(guò)三個(gè)方向軟件的發(fā)展的促進(jìn)。許多CAD/CAM系統(tǒng)可以按照這種標(biāo)準(zhǔn)輸出文件，而（由此生成的）CAM文件以后也可以被后置處理程序和校驗(yàn)軟件使用。
有時(shí)會(huì)有這樣的情況，即特定的CAD/CAM系統(tǒng)生成的特有的中間文件不經(jīng)任何額外的后置處理就可以直接輸入到機(jī)床之中。這是理想的解決方案，然而目前尚無(wú)任何標(biāo)準(zhǔn)管理這種交換。
CAD/CAM模型交換的另一種方法是利用逆向后置處理器。逆向后置處理器可以從數(shù)控G&M代碼程序生成CAD/CAM模型。這種程序確實(shí)有些作用，但是，程序員必須花相當(dāng)多的時(shí)間去搞清楚模型的設(shè)計(jì)意圖，而且還要將刀具路徑從幾何形狀中分離出來(lái)?？傮w來(lái)說(shuō)，后置處理器的應(yīng)用具有一定的局限性。
軟件的組成及發(fā)展趨勢(shì)
在整個(gè)工業(yè)上，許多軟件包利用CAD或CAD/CAM。純CAD系統(tǒng)被應(yīng)用于所有的設(shè)計(jì)領(lǐng)域，實(shí)際上今天所有的產(chǎn)品都是用CAD軟件設(shè)計(jì)出來(lái)的——用紙筆繪的日子已經(jīng)一去不復(fù)返了。
另一方面，CAD/CAM軟件包有著更多專利。CAD/CAM雖然小，但地位十分重要，它的應(yīng)用通常限制在加工和裝配業(yè)，其數(shù)量要比CAD小得多。
CAD/CAM系統(tǒng)包括CAD軟件設(shè)計(jì)以及CAM軟件去創(chuàng)建刀具軌跡和NC代碼。然而，普通的CAD模型相比于純CAD軟件比較弱及不精煉。這種不匹配造成了CAD設(shè)計(jì)者與CAD/CAM程序員之間一直以來(lái)的爭(zhēng)論，其主題是如何使CAD/CAM能夠融合。
如果先在業(yè)界一流的CAD系統(tǒng)上生成所有幾何形狀，然后再圖形輸入到某個(gè)CAD/CAM系統(tǒng)中，就會(huì)產(chǎn)生很大的爭(zhēng)論。工程師創(chuàng)建CAD模型逐步形成一種模式，商業(yè)就會(huì)更加充裕。幾何形狀能夠輸入到CAD/CAM軟件包中產(chǎn)生處理模型。因此，工業(yè)引導(dǎo)CAD軟件包走向不正式的標(biāo)準(zhǔn)。標(biāo)準(zhǔn)的接受度越高，擁有該軟件的公司的投資回報(bào)率就會(huì)越高。
反對(duì)意見(jiàn)來(lái)自于小的組織，他們沒(méi)有必要或者沒(méi)有資源同時(shí)擁有昂貴的符合工業(yè)標(biāo)準(zhǔn)的CAD軟件包以及CAD/CAM軟件包。他們往往需要根據(jù)紙上工程圖重畫幾何圖形，或者用并不完善的翻譯設(shè)備輸入模型。任何起源模型將結(jié)束走向于更高的正式的CAD/CAM文件。這類模型很可能在將來(lái)又被翻譯成更為標(biāo)準(zhǔn)的版本。
不論選擇什么樣的方法，各種組織和個(gè)人往往都會(huì)竭盡保護(hù)某種技術(shù)。如果他們檢查發(fā)現(xiàn)有巨大的效果，花時(shí)間去學(xué)習(xí)它以及吸收科學(xué)知識(shí)。然而，他將轉(zhuǎn)變成一種新的技術(shù)是很困難的事，即使它們體現(xiàn)出具有無(wú)法抵抗的證據(jù)來(lái)證實(shí)更好的方法。這是一次十分痛苦的轉(zhuǎn)變，當(dāng)然如果我們能夠看見(jiàn)我們的將來(lái)，這是不可能發(fā)生的事情。但是事實(shí)上，我們不可能總是預(yù)測(cè)支配的科學(xué)技術(shù)將在幾年內(nèi)走下坡路。
結(jié)果形成了技術(shù)壕溝，要從腳下消除這種壕溝將會(huì)十分困難和昂貴。大約只能保證，我們能夠發(fā)現(xiàn)去選擇技術(shù)出現(xiàn)最標(biāo)準(zhǔn)，即使不完美也要留住它.然而，如果發(fā)現(xiàn)走下坡路，我們將也更加適應(yīng)這位置。
Modern design and manufacturing
CAD/CAM
CAD/CAM is a term which means computer-aided design and computer-aided manufacturing. It is the technology concerned with the use of digital computers to perform certain functions in design and production. This technology is moving in the direction of greater integration（一體化）of design and manufacturing, two activities which have traditionally been treated as distinct（清楚的）and separate functions in a production firm. Ultimately, CAD/CAM will provide the technology base for the computer-integrated factory of the future.
Computer-aided design (CAD) can be defined as the use of computer systems to assist in the creation, modification, analysis, or optimization（最優(yōu)化）of a design. The computer systems consist of the hardware and software to perform the specialized design functions required by the particular user firm. The CAD hardware typically includes the computer, one or more graphics display terminals, keyboards, and other peripheral equipment. The CAD software consists of the computer programs to implement（實(shí)現(xiàn)，執(zhí)行）computer graphics to facilitate the engineering functions of the user company. Examples of these application programs include stress-strain（壓力-應(yīng)變）analysis of components（部件）, dynamic（動(dòng)態(tài)的）response of mechanisms, heat-transfer calculations, and numerical control part programming. The collection of application programs will vary from one user firm to the next because their product lines, manufacturing processes, and customer markets are different these factors give rise to differences in CAD system requirements.
Computer-aided manufacturing (CAM) can be defined as the use of computer systems to plan, manage, and control the operations of a manufacturing plant through either direct or indirect computer interface with the plant’s production resources. As indicated by the definition, the applications of computer-aided manufacturing fall into two broad categories:
1.computer monitoring and control.
2.manufacturing support applications.
The distinction between the two categories is fundamental to an understanding of computer-aided manufacturing.
In addition to the applications involving a direct computer-process interface（界面，接口）for the purpose of process monitoring and control, compute-aided manufacturing also includes indirect applications in which the computer serves a support role in the manufacturing operations of the plant. In these applications, the computer is not linked directly to the manufacturing process. Instead, the computer is used “off-line”（脫機(jī)）to provide plans, schedules, forecasts, instructions, and information by which the firm’s production resources can be managed more effectively. The form of the relationship between the computer and the process is represented symbolically in the figure given below. Dashed lines（虛線）are used to indicate that the communication and control link is an off-line connection, with human beings often required to consummate（使圓滿）the interface. However, human beings are presently required in the application either to provide input to the computer programs or to interpret the computer output and implement the required action.
Manufacturing operations
computer
Process data
Control signals
CAM for manufacturing support
What is CAD/CAM software?
Many toolpaths are simply too difficult and expensive to program manually. For these situations, we need the help of a computer to write an NC part program.
The fundamental concept of CAD/CAM is that we can use a Computer-Aided Drafting (CAD) system to draw the geometry of a workpiece on a computer. Once the geometry is completed, then we can use a computer-Aided Manufacturing (CAM) system to generate an NC toolpath based on the CAD geometry.
The progression（行進(jìn)，級(jí)數(shù) ）from a CAD drawing all the way to the working NC code is illustrated as follows:
Step 1: The geometry is defined in a CAD drawing. This workpiece contains a pocket to be machined. It might take several hours to manually write the code for this pocket（凹槽，型腔）. However, we can use a CAM program to create the NC code in a matter of minutes.
Step 2: The model is next imported into the CAM module. We can then select the proper geometry and define the style of toolpath to create, which in this case is a pocket. We must also tell the CAM system which tools to use, the type of material, feed, and depth of cut information.
Step 3: The CAM model is then verified to ensure that the toolpaths are correct. If any mistakes are found, it is simple to make changes at this point.
Step 4: The final product of CAD/CAM process is the NC code. The NC code is produced by post-processing（后處理）the model, the code is customized（定制，用戶化）to accommodate the particular variety of CNC control.
Another acronym that we may run into is CAPP, which stands for Computer-Aided Part Programming. CAPP is the process of using computers to aid in the programming of NC toolpaths. However, the acronym CAPP never really gained widespread acceptance, and today we seldom hear this term. Instead, the more marketable CAD/CAM is used to express the idea of using computers to help generate NC part programs. This is unfortunate because CAM is an entire group of technologies related to manufacturing design and automation-not just the software that is used to program CNC machine tools.
Description of CAD/CAM Components and Functions
CAD/CAM systems contain both CAD and CAM capabilities – each of which has a number of functional elements. It will help to take a short look at some of these elements in order to understand the entire process.
1. CAD Module
The CAD portion of the system is used to create the geometry as a CAD model. The CAD model is an electronic description of the workpiece geometry that is mathematically precise. The CAD system, whether stand alone or as part of a CAD/CAM package, tends to be available in several different levels of sophistication. （強(qiáng)詞奪理，混合）
2-D line drawings 兩維線條圖
Geometry is represented in two axes, much like drawing on a sheet of paper. Z-level depths will have to be added on the CAM end.
3-D wireframe models 三維線框模型
Geometry is represented in three-dimensional space by connecting elements that represent edges and boundaries. Wiregrames can be difficult to visualize（想象，形象化，顯現(xiàn)）, but all Z axis information is available for the CAM operations.
3-D surface models 三維表面模型
These are similar to wireframes except that a thin skin has been stretched over the wireframe model to aid in visualization. Inside, the model is empty. Complex contoured Surfaces are possible with surface models.
3-D solid modeling 三維實(shí)體模型
This is the current state of the market technology that is used by all high-end software. The geometry is represented as a solid feature that contains mass. Solid models can be sliced（切片，部分 ，片段）open to reveal internal features and not just a thin skin.
2. CAM Module
The CAM module is used to create the machining process model based upon the geometry supplied in the CAD model. For example, the CAD model may contain a feature that we recognize as a pocket .We could apply a pocketing routine to the geometry, and then all of the toolpaths would be automatically created to produce the pocket. Likewise, the CAD model（模子，鑄型）may contain geometry that should be produced with drilling operations. We can simply select the geometry and instruct the CAM system to drill holes at the selected locations.
The CAM system will generate a generic（一般的，普通的 ）intermediate（中間的，媒介）code that describes the machining operations, which can later be used to produce G & M code or conversational programs. Some systems create intermediate code in their own proprietary（所有的，私人擁有的 ） language, which others use open standards such as APT for their intermediate files.
The CAM modules also come in several classes and levels of sophistication. First, there is usually a different module available for milling, turning, wire EDM, and fabrication（裝配）. Each of the processes is unique enough that the modules are typically sold as add-ins（附加軟件）. Each module may also be available with different levels of capability. For example, CAM modules for milling are often broken into stages as follows, starting with very simple capabilities and ending with complex, multi-axis toolpaths :
● 21/2-axis machining
● Three-axis machining with fourth-axis positioning
● Surface machining
● Simultaneous five-axis machining
Each of these represents a higher level of capability that may not be needed in all manufacturing environments. A job shop might only require 3-axis capability. An aerospace contractor might need a sophisticated 5-axis CAM package that is capable of complex machining. This class of software might start at $5,000 per installation, but the most sophisticated modules can cost $15,000 or more. Therefore, there is no need to buy software at such a high level that we will not be able to use it to its full potential.
3.Geometry vs. toolpath
One important concept we must understand is that the geometry represented by the CAD drawing may not be exactly the same geometry that is produced on the CNC machine tool. CNC machine tools are equipped to produce very accurate toolpaths as long as the toolpaths are either straight lines or circular arcs. CAD systems are also capable of producing highly accurate geometry of straight line and circular arcs, but they can also produce a number of other classes of curves. Most often these curves are represented as Non-Uniform（不均勻的，不一致的）Rational Bezier Splines (NURBS) （非均勻有理B樣條）. NURBS curves can represent virtually any geometry, ranging from a straight line or circular arc to complex surfaces.
Take, for example, the geometric entity that we call an ellipse（橢圓形）. An ellipse is a class of curve that is mathematically different from a circular arc. An ellipse is easily produced on a CAD system with the click of the mouse. However, a standard CNC machine tool cannot be use to directly problem an ellipse – it can only create lines and circular arcs. The CAM system will reconcile（使和解，使順從）this problem by estimating the curve with line segments.
CNC machine tools usually only understand circular arcs or straight lines. Therefore, the CAM system must estimate curved surfaces with line segments. The curve in this illustration is that of an ellipse, and the toolpath generated consists of tangent line segments that are contained within a tolerance zone.
The CAM system will generate a bounding geometry on either side of the true curve to form a tolerance zone. It will then produce a toolpath from the line segment that stays contained within the tolerance zone. The resulting toolpath will not be mathematically correct – the CAM system will only be able to estimate the surface. This basic method is used to produce estimated toolpaths for both 2-D curves and 3-D surface curves.
Some CAM programs also have the ability to convert the line segments into arc segments. This can reduce the number of blocks in the program and lead to smoother surfaces.
The programmer can control the size of the tolerance zone to create a toolpath that is as accurate as is needed. Smaller tolerance zones will produce finer toolpaths and more numerous line segments, while larger tolerance zones will produce fewer line segments and coarser（粗糙的） toolpaths. Each line segment will require a block of code in the NC program, so the NC part program can grow very large when using this technique.
We must use caution when machining surfaces. It is easy to rely on the computer to generate the correct tooolpath, but finished surfaces are further estimated during machining with ball end mills. If we do not pay attention to the limitations of these techniques, then the accuracy of the finished workpiece may be compromised（妥協(xié)，折衷）.
4.Tool and material libraries
To create the machining operations, the CAM system will need to know which cutting tools are available and what material we are machining. CAM systems take care of this by providing customizable （可定制的 ）libraries of cutting tools and materials. Tool libraries contain information about the shape and style of the tool. Material libraries contain information that is used to optimize（使最優(yōu)化）the cutting speeds and feeds. The CAM system uses this information together to create the correct toolpaths and machining parameters.（參數(shù)）
The format of these tool and material libraries is often proprietary（專利的，獨(dú)占的，私有的）and can present some portability issues.
Proprietary（輕便，移動(dòng)）tool and material files cannot be easily modified or used on another system. More progressive （ 改革論者，進(jìn)步論者，前進(jìn)的）CAM developers tend to produce their tool and material libraries as database files that can be easily modified and customized for other applications.
5.Verification and post-processor
CAM systems usually provide the ability to verify that the proposed toolpaths are correct. This can be via a simple backplot（背景繪制） of the tool centerline or via a sophisticated solid model of the machining operations. The solids verifications（確認(rèn)，查證）is often a third-party software that the CAD/CAM software company has licensed.（得到許可的 ） However, it may be available as a standalone package.
The post-processor is a software program that takes a generic intermediate code and formats the NC code for each particular machine tool control. The post-processor（后置處理器） can often be customized through templates（模板）and variables to provide the required customization. （用戶化，專用化，定制）
6.Portability 輕便，可帶的
Portability of electronic data is the Achilles` heel(唯一致命的弱點(diǎn))of CAD/CAM systems and continues to be a time-consuming concern. CAD files are created in a number of formats and have to be shared between many organizations. It is very expensive to create a complex model on a CAD system; therefore, we want to maximize the portability of our models and minimize the need for recreating the geometry on another system. DXF, DWG, IGES, SAT, STL and parasolids are a few of the common formats for CAD data exchange.
CAM process models are not nearly as portable as CAD models. We cannot usually take a CAM model developed in one system and transfer it to another platform. The only widely accepted standard for CAM model interchange is a version of Automatically Programmed Tool (APT). APT is a prog