1355-計算機輔助設(shè)計與制造

1355-計算機輔助設(shè)計與制造,計算機輔助設(shè)計,制造 畢業(yè)設(shè)計（論文）外文翻譯題目 計算機輔助設(shè)計與制造專 業(yè) 名 稱 機械設(shè)計制造及其自動化班 級 學(xué) 號 068105309學(xué) 生 姓 名 鄧小剛指 導(dǎo) 教 師 姚坤弟填 表 日 期 2010 年 03 月 10 日1計算機輔助設(shè)計與制造CAD/CAM 是表示計算機輔助設(shè)計和計算機輔助制造的專業(yè)術(shù)語。它是一種使用計算機完成某些設(shè)計和生成功能的技術(shù)。在生產(chǎn)企業(yè)里，人們通常把設(shè)計和制造是為兩項有著明顯不同職能的分工，而這項技術(shù)正朝著設(shè)計與制造的更大程度一體化方向發(fā)展。最終，CAD/CAM 將會為未來的計算機集成工廠提供技術(shù)基礎(chǔ)。計算機輔助設(shè)計（CAD）可定義為運用計算機系統(tǒng)對設(shè)計的創(chuàng)意、修改、分析或優(yōu)化予以輔助。這些由硬件和軟件構(gòu)成的計算機系統(tǒng)，用于完成用戶公司要求的特定設(shè)計功能。CAD 硬件通常包括：一臺計算機，一個或多個圖形顯示終端，鍵盤和其他外圍設(shè)備。CAD 軟件包括各種計算機制圖程序，這些程序便于用戶公司完成設(shè)計職能，如：零部件的應(yīng)變分析，機構(gòu)的動態(tài)響應(yīng)，熱傳輸計算和數(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 CAD 系統(tǒng)要求的差異性。計算機輔助制造（CAM）可定義為通過直接或間接與廠家生產(chǎn)資源相適應(yīng)的計算機界面，使用計算機系統(tǒng)來規(guī)劃、管理和控制制造工廠的運行。正如定義所表示的那樣，CAM 應(yīng)用程序可分為兩大類：1. 計算機監(jiān)控程序；2. 制造程序。二者之間的區(qū)別是理解計算機輔助制造的基礎(chǔ)。計算機輔助制造的應(yīng)用程序，除了為監(jiān)控制造過程而直接使用計算機界面的應(yīng)用程序之外，還包括在工廠生產(chǎn)運行過程中由計算機提供支持的間接應(yīng)用程序。在這些應(yīng)用程序中，計算機并不直接與制造過程相聯(lián)接。相反，在脫機狀態(tài)下，計算機可用來提供計劃書、進度表、預(yù)報、指令和使廠家生產(chǎn)資源管理更加有效的信息資料。計算機和制造過程間的關(guān)系如下圖所示。圖中虛線用來說明交流和控制處于脫即狀態(tài)下，需要人來完善界面。目前，CAM 的應(yīng)用需2要由人來為計算機輸入程序，解釋計算機的輸出，并采取所要求的措施。處理數(shù)據(jù)控制信號CAM 用于生產(chǎn)支持Notes:1. CAD(computer-aided design) 計算機輔助設(shè)計2. CAM(computer-aided manufacturing) 計算機輔助制造3. computer monitoring and control 計算機監(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？許多刀具軌跡是簡單的但太復(fù)雜和昂貴以致于人們很難制造，對于這種情況，我們需要在計算機的幫助下來作數(shù)控部分程序。CAD/CAM 最基礎(chǔ)的概念是，我們可以用計算機輔助設(shè)計系統(tǒng)在計算機上畫出工件的幾何形狀。幾何形狀一旦完成，我們就可以用計算機輔助制造系統(tǒng)根據(jù) CAD 的幾何形狀，生成數(shù)控機床的刀具軌跡。利用 CAD 繪制所有路徑對數(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。對于這種型腔加工很可能將花幾個小時去制造代碼。然而，我們能夠利用 CAM 程序去計算機 生產(chǎn)操作3創(chuàng)造 NC 代碼在每分鐘內(nèi)。第二步：接著，這個模型就被輸入到 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，在這個例子中就是一個型腔。我們一定也能夠讓 CAM 系統(tǒng)如刀具的使用，材料的類型，進給量，切屑深度等信息。第三步：CAM 模型證實保證刀具軌跡的正確性。如果發(fā)現(xiàn)了一些錯誤，最簡單就是在某個位置上加以改正。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，它代表了計算機輔助程序。CAPP 是使用計算機輔助在數(shù)控刀具軌跡中的應(yīng)用。然而，CAPP 從來就沒有真正獲得廣泛的流傳和接受，而且至今我們很少聽到這個項目。取而代之的是在更多的市場上使用 CAD/CAM，使用計算機的思維來幫助生產(chǎn) NC 程序。不幸的是，因為 CAM 是一個整體技術(shù)與制造技術(shù)和自動化技術(shù)有著關(guān)聯(lián)的——不僅是軟件，而且使用CNC 機器工具。描述 CAD/CAM 組成及功能CAD 系統(tǒng)包括 CAD 設(shè)備及 CAM 設(shè)備——每個都有許多功能元件。它們將在短時間內(nèi)掃描這些元件目的是為了了解其工作的整個過程。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。4三維表面模型 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。三維實體模型這種當前高科技市場形式被使用必須通過所有的高端科技軟件。幾何形狀被看作是一個實體特征其包括許多方面。實體模型可以被切開以展示內(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)用其加工路線來加工幾何圖形，然后，所有的刀具路徑將是自動的產(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。我們能夠選擇簡單的幾何圖形和按照 CAM系統(tǒng)的說明在適當?shù)奈恢蒙线M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生成 G 和 M 代碼或會話式程序。在它們合適的環(huán)境下，有些系統(tǒng)產(chǎn)生中間代碼。而其他使用較標準代碼例如 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。每一個模型也可能使用不同的設(shè)備。例如，開始用簡單的設(shè)備，到后來用復(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程如下： ● 21/2-方向的機床● 3 個方向的機床● 加工表面的機床● 5 個方向混合的機床每種體現(xiàn)出高精度的設(shè)備不可能在所有的壞鏡下制造，一個工序很可能只要求三個方向的設(shè)備，而一個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￥5000，但是許多復(fù)雜模型將花費￥15000 甚至更多。因此，我們沒有必要購買這種高水準的軟件，因為我們不能完全發(fā)揮他們5的潛能。3. 幾何圖形及刀具軌跡我們必須理解一個重要的概念，即 CAD 所繪制的幾何形狀并不一定與 CNC機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為是非均勻有理 B 樣條曲線。事實上，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)利用鼠標單擊很容易產(chǎn)生。然而，一個標準的 CNC 加工刀具不能夠直接使用產(chǎn)生一個橢圓——它只能產(chǎn)生直線和圓弧。CAM 系統(tǒng)將順從于這種問題，通過估算用直線段代替曲線。CNC 機床刀具通常只能識別圓弧或直線。因此，CAM 系統(tǒng)必須估計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)會在真正的曲線兩側(cè)各生成一個幾何邊界，從而形成一個公差帶。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致的刀具軌跡以及大量的直線段，而較大的公差帶將會產(chǎn)生較少的直線段，刀具軌跡也比較粗糙。每一個直線段將要求在 NC 程序有模塊代碼。因此，當使用這種技術(shù)時，NC 程序能夠擴大范圍。在加工表面時我們一定要細心，依靠計算機生成正確的刀具很容易，但在用球狀端銑刀進行曲面的精加工是須進一步估價。如果我們沒能注意到這種技術(shù)的局限性，那么精加工后工件的精度就會大打折扣。4. 刀具庫和材料庫6為了創(chuàng)建機械加工工藝，CAM 系統(tǒng)需要了解切割刀具的利用以及機械材料。CAM 注意的是通過提供可定制的刀具型號及類型。材料庫包括的信息是最優(yōu)化的切削速度以及進給量。CAM 使用這種信息聚在一起創(chuàng)建正確的刀具軌跡以及機床參數(shù)。這類刀具和材料庫的格式經(jīng)常是獨有的，這一點帶來了一些兼容問題。專用的工具庫及材料庫不容易修改或不容易使用其他的系統(tǒng)。進步的 CAM 開發(fā)者趨向于將刀具和材料庫生成數(shù)據(jù)庫文件，這樣就可以為其他的應(yīng)用者進行修改和定制提供方便。5. 檢驗及后置處理CAM 系統(tǒng)通常提供檢驗刀具軌跡是否正確性的功能。這可以通過加工操作的刀具中心線的簡單繪制或通過復(fù)雜的實體模型來實現(xiàn)。實體驗證通常是CAD/CAM 軟件公司已經(jīng)獲得許可的第三方軟件。然而，它可能是作為一個獨立的軟件包。后置處理器是一個軟件程序，他將通過的中間代碼格式化為使用于每個特定機床控制器的 NC 代碼。后置處理器通?？梢酝ㄟ^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，這個問題任就是一件十分耗時的事情。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 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編程工具（APT） 。自動編程工具是一種利用描述機床操作的程序語言工具，自動編程工具是一種標準的、有著好的文件能夠通過三個方向軟件的發(fā)展的促進。許多CAD/CAM 系統(tǒng)可以按照這種標準輸出文件，而（由此生成的）CAM 文件以后也可以被后置處理程序和校驗軟件使用。有時會有這樣的情況，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床之中。這是理想的解決方案，然而7目前尚無任何標準管理這種交換。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。這種程序確實有些作用，但是，程序員必須花相當多的時間去搞清楚模型的設(shè)計意圖，而且還要將刀具路徑從幾何形狀中分離出來?？傮w來說，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)展趨勢在整個工業(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)計領(lǐng)域，實際上今天所有的產(chǎn)品都是用 CAD 軟件設(shè)計出來的——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)計者與 CAD/CAM 程序員之間一直以來的爭論，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 CAD/CAM 系統(tǒng)中，就會產(chǎn)生很大的爭論。工程師創(chuàng)建 CAD 模型逐步形成一種模式，商業(yè)就會更加充裕。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準。標準的接受度越高，擁有該軟件的公司的投資回報率就會越高。反對意見來自于小的組織，他們沒有必要或者沒有資源同時擁有昂貴的符合工業(yè)標準的 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又被翻譯成更為標準的版本。不論選擇什么樣的方法，各種組織和個人往往都會竭盡保護某種技術(shù)。如果他們檢查發(fā)現(xiàn)有巨大的效果，花時間去學(xué)習(xí)它以及吸收科學(xué)知識。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)出具有無法抵抗的證據(jù)來證實更好的方法。這是一次十分痛苦的轉(zhuǎn)變，當然如果我們能夠看見我們的將8來，這是不可能發(fā)生的事情。但是事實上，我們不可能總是預(yù)測支配的科學(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)最標準，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/CAMCAD/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）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.Process dataControl signalsCAM for manufacturing supportWhat 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.computer Manufacturing operationsThe 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，級數(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 FunctionsCAD/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 ModuleThe 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 ModuleThe 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 machiningEach 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. toolpathOne 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 librariesTo 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，獨占的，私有的）and can present some portability issues. Proprietary（輕便，移動）tool and material files cannot be easily modified or used on another system. More progressive （ 改革論者，進步論者，前進的）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-processorCAM 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，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)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 programming language used to describe machining operations. APT is an open standard that is well documented and can be accessed by third-party software developers. A number of CAD/CAM systems can export to this standard, and the CAM file can later be used by post-processors and verification software. There are some circumstances when the proprietary intermediate files created by certain CAD/CAM systems can be fed directly into a machine tool without any additional post-processing. This is an ideal solution, but there is not currently any standard governing this exchange.One other option for XAD/CAM model exchange is to use a reverse post-processor. A reverse post-processor can create a CAD/CAM model from a G &M-code of NC part program. These programs do work; however, the programmer must spend a considerable amount of time determining the design intent of the model and to separate the toolpaths from the geometry. Overall, reverse post-processing has very limited applications.Software issues and trendsThroughout industry, numerous software packages are used for CAD and CAD/CAM. Pure CAD systems are used in all areas of design, and virtually any product today is designed With CAD software-gone are the days of pencil and paper drawings.CAD/CAM software, on the other hand, is more specialized. CAD/CAM is a small but important niche（適當?shù)奈恢茫ヽonfined to machining and fabrication organizations, and it is found in much smaller numbers than its CAD big brother.CAD/CAM systems contain both the software for CAD design and the CAM software for creating toolpaths and NC code. However, the CAD portion is often weak and unrefined when compared to much of the leading pure CAD software. This mismatch sets up the classic（第一流的，標準的）argument between the CAD designers and the CAD/CAM programmer on what is the best way to approach CAD/CAM.A great argument can be made for creating all geometry on an industry-leading CAD system and then importing the geometry into a CAD/CAM system. A business is much better off if its engineers only have to create a CAD model one time and in one format. The geometry can then be imported into the CAD/CAM package for process modeling. Furthermore, industry-leading CAD software tends to set an unofficial standard. The greater the acceptance of the standard, the greater the return on investment for the businesses that own the software.The counter argument comes from small organizations that do not have the need or resources to own both an expensive, industry-standard CAD package and an expensive CAD/CAM package. They tend to have to redraw the geometry from the paper engineering drawing or import models with imperfect（有缺點的，未完成的 ） translators. Any original models will end up being stored as highly non-standardized CAD/CAM files. These models will have dubious（可疑的，不確定的）prospects（景色，前景，期望 ） of ever being translated to a more standardized version.Regardless of the path that is chosen, organizations and individuals tend to become entrenched（以壕溝防護） in a particular technology. If they have invested tremendous effort and time into learning and assimilating（吸收）a technology, then it becomes very difficult to change to a new technology, even when presented with overwhelming（壓倒性的，無法抵抗的） evidence of a better method. It can be quite painful to change. Of course, if we had a crystal ball and could see into the future, this would never happen; but the fact is that we cannot always predict what the dominant（有統(tǒng)治權(quán)的，占優(yōu)勢的）technology will be even a few years down the road.The result is technology entrenchment（塹墩）that can be very difficult and expensive to get out from under. About the only protection we can find is to select the technology that appears to be the most standardized (even if it is imperfect) and stay with it-then, if major changes appear down the road, we will be in a better position to adapt.