【正文】 machine the part. The designer must work closely with the manufacturing people to establish some standards for design. CAD allows different layers (or levels) to be created. This allows the designer to put different potions of the part geometry on different layers. For example, imagine a simple part that has a 1/2inch slot milled in it and four holes drilled through it. The designer would put the slot on one layer of the drawing and the holes on another layer. This will allow other software to take the part geometry form each layer and assign different tools to it. Think of different layers as different machining operation that would have different tool assigned to them. Layers are like transparency. If we put all of the total transparencies on top of each other, we see the total part. But we can still take any one transparency and isolate some of the part geometry. The CAD system will print the blueprint so it appears there is only one layer, or individual layers can be printed. The designer also dimensions the part. This is quite automatic. The designer chooses where he/she wants dimensions placed and the puter places the actual dimensions. Dimensions would be on a different layer than part geometry. The puter allows designs to be viewed and tested before the actual part is even manufactured. The part can be tested make sure it fits with any mating pars. CAD system can check for interference of parts, which can save a lot of wasted machining and development time. CAD systems can stresstest part to see if they will meet the strength requirements of the application. The graphics abilities of CAD system also allow 3D viewing of part from any angle. Integrated CAD The emphasis of the chapter is integrated CAD/CAM. The integration aspect is acplished when the design and analysis modules are united into one package. The design geometry entered by the designer can be used by the analysis programs and by the manufacturing, inspection, inventory, and other aspects of the product lift cycle. The manifestation of integrating software is recognized when the user can sit down at a puter terminal, design a part, view it on the screen, analyze it for design requirements, and hit the proverbial manufacturing button the keyboard to have the part automatically manufactured, inspected, and delivered. Various stages of integration do exist, but none is as plete as the example just mentioned. Integration implies automation. Automation implies that the hardware and software can create a CAD database and use it to analyze and manufacture the part totally automatically, with little or no human intervention. The question as to whether human intervention is advisable is still open. Certainly human intelligence is valuable in developing a design in geometry and functionality, recognizing certain patterns and shortings of products, and formulating manufacturing plans. It has been suggested that artificial intelligence (AI) techniques can substitute for much of the human design/manufacturing input. AI research has had success in the areas of pattern recognition, planning, natural language understanding, robotic control, fault diagnosis, and other applications. The most well recognize area of AI is the development of expert system. Expert systems simulate human expertise by storing a knowledge base of data about a specific domain. The architecture of an expert system is shown in Figure . The knowledge base contains knowledge about a specific problem domain and facts stating the problem. The inference engine is the program control which searches te facts about the problem to satisfy the knowledge base ad above the problem計(jì)算機(jī)輔助設(shè)計(jì)一個(gè)好的機(jī)械設(shè)計(jì)產(chǎn)品必須保證產(chǎn)品的部分或全部功能在產(chǎn)品壽命中能夠正確有效地實(shí)現(xiàn)。在設(shè)計(jì)中產(chǎn)品功能的研究包括：重力、強(qiáng)度、熱、溫度特性、運(yùn)動(dòng)學(xué)和動(dòng)力學(xué)。評(píng)價(jià)設(shè)計(jì)的好壞與否可以通過(guò)比較產(chǎn)品的實(shí)際性能與設(shè)計(jì)要求的差距來(lái)衡量。設(shè)計(jì)除了功能達(dá)到設(shè)計(jì)要求之外，其它方面的實(shí)際同樣也十分重要。比如說(shuō)經(jīng)濟(jì)性方面的要求。這就是說(shuō)最終的產(chǎn)品要近可能地接近設(shè)計(jì)要求。如果要求支撐設(shè)計(jì)能承受1000N/min的能力，然而設(shè)計(jì)成能承受2000N/min，則沒(méi)有必要也不經(jīng)濟(jì)。因此機(jī)械設(shè)計(jì)必須處理好產(chǎn)品的功能與其經(jīng)濟(jì)性之間的矛盾。產(chǎn)品的性能受零部件的結(jié)構(gòu)、材料屬性和人機(jī)環(huán)境的影響；產(chǎn)品的經(jīng)濟(jì)性則和材料的成本以及市場(chǎng)因素有關(guān)。在機(jī)械設(shè)計(jì)過(guò)程中70%的制造成本已經(jīng)決定了。這就是說(shuō)設(shè)計(jì)者把產(chǎn)品交給制造之前，絕大部分主要的成本已經(jīng)決定。比如：材料尺寸、公差、表賣弄粗糙度及過(guò)程成本的其它參數(shù)。因此，在加工過(guò)程中，我們?cè)谥圃爝^(guò)程中再努力也只能減少%30的花費(fèi)。這個(gè)比例說(shuō)明設(shè)計(jì)過(guò)程起著置關(guān)重要的作用。我們應(yīng)該清楚計(jì)算機(jī)能夠幫上很大的忙。計(jì)算機(jī)輔助設(shè)計(jì)對(duì)不同的人意味著不同的意思。對(duì)一些人來(lái)說(shuō)，它意味著計(jì)算機(jī)輔助繪圖和草繪，對(duì)另外一些人來(lái)說(shuō)，他則意味著完全自動(dòng)化設(shè)計(jì)，設(shè)計(jì)者只要規(guī)定設(shè)計(jì)的功能，則計(jì)算機(jī)能設(shè)計(jì)出滿足要求的產(chǎn)品，甚至經(jīng)過(guò)優(yōu)化的設(shè)計(jì)。以上則是計(jì)算機(jī)輔助設(shè)計(jì)。他的目標(biāo)就是通過(guò)人類的智慧實(shí)現(xiàn)后面敘述的一種情形。計(jì)算機(jī)輔助設(shè)計(jì)仍停留在計(jì)算機(jī)輔助繪圖階段。大多數(shù)簡(jiǎn)單的CAD只能進(jìn)行自動(dòng)草繪功能。更高級(jí)的系統(tǒng)呢功能進(jìn)行分析引導(dǎo)工程師得到必須平衡的關(guān)系。因?yàn)椴煌馁u主提供具有不同特點(diǎn)的計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)，這些系統(tǒng)之間具有各自不同的數(shù)據(jù)標(biāo)準(zhǔn)。他們之間的交流和數(shù)據(jù)轉(zhuǎn)化變得非常有意義。IGES在不同系統(tǒng)之間數(shù)據(jù)格式轉(zhuǎn)換中扮演了中間者的角色。不同軟件廠商只要提供與IGES格式相互轉(zhuǎn)換的模塊系統(tǒng)，各種計(jì)算機(jī)輔助設(shè)計(jì)軟件能導(dǎo)入和導(dǎo)出IGES格式的文件，同時(shí)這種格式的文件在3D圖形轉(zhuǎn)換種也有根廣泛的應(yīng)用。目前一種基于IGES格式的實(shí)體建模技術(shù)的數(shù)據(jù)格式（PDES）也得到很大的發(fā)展。這種格式的文件占用系統(tǒng)空間小、速度快、可靠性高。由于國(guó)外也有各種不同的標(biāo)準(zhǔn)存在，預(yù)計(jì)不久這些標(biāo)準(zhǔn)將導(dǎo)致一個(gè)國(guó)際標(biāo)準(zhǔn)規(guī)格，叫產(chǎn)品模型數(shù)據(jù)(STEP)的交換的標(biāo)準(zhǔn)。 硬件和軟件硬件在計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)之間存在很大的差別。服務(wù)器早就用于計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)。處理速度迅速增加的微型和小型計(jì)算機(jī)也可以進(jìn)行的計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的運(yùn)行。網(wǎng)絡(luò)一般被使用以便設(shè)計(jì)可以被中心儲(chǔ)存，數(shù)據(jù)可以被分給設(shè)計(jì)者。很多輸入裝置使用的非常廣泛。 普通I/O 設(shè)備包括：鍵盤，鼠標(biāo)，跟蹤球，數(shù)子化儀，控制桿，輔助鍵盤和光筆數(shù)字化儀。典型的輸出裝置通常是屏幕，繪圖儀，打印機(jī)、移動(dòng)硬盤和軟盤。有多種計(jì)算機(jī)輔助設(shè)計(jì)軟件提供給工作站和微型計(jì)算機(jī)。數(shù)據(jù)包在某種程度上以繪圖信息形式的交換文件變化。這些文件可以被輸入到其他軟件中幫助可編程的機(jī)床。計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的要素計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的設(shè)計(jì)過(guò)程由四級(jí)組成，如下描述。幾何學(xué)模型化。 在幾何學(xué)模型化過(guò)程中，將一個(gè)物體或者它的任何部分進(jìn)行分析后用數(shù)字描述。首先設(shè)計(jì)者給出命令建立或者修改能夠完成二維或三維的線條、表面、實(shí)體、尺寸和正文構(gòu)造幾何學(xué)模型。幾何學(xué)模型可以用三種不同的方式呈現(xiàn)。如下圖(線框內(nèi)；)，全部邊緣都象固體的線一樣可見(jiàn)。這幅圖像可能是含糊的，特別是復(fù)雜的形狀。不過(guò)，各種各樣的顏色一般用于物體的不同的部分，因此使物體更容易想象。在表面模型里，全部可見(jiàn)的表面被用模型顯示，并且在實(shí)體模型里，全部表面被顯示，但是內(nèi)部以數(shù)據(jù)的形式描述(，f) 設(shè)計(jì)分析和最優(yōu)化。設(shè)計(jì)幾何學(xué)特征有可能由分析壓力、疲勞、撓度、振動(dòng)組成。這階段溫度分布或者容忍。各種各樣復(fù)雜軟件包能計(jì)算出質(zhì)量的準(zhǔn)確并且快速的變化。設(shè)計(jì)評(píng)論和評(píng)估。設(shè)計(jì)過(guò)程忠一個(gè)重要的設(shè)計(jì)階段是檢查在各種各樣的組成部分之間是否存在任何干涉，是否有自由度，例如將要作為運(yùn)動(dòng)構(gòu)件的連桿。計(jì)算機(jī)軟件能利用動(dòng)畫模擬運(yùn)動(dòng)的構(gòu)件及其它運(yùn)動(dòng)狀態(tài)。在設(shè)計(jì)過(guò)程的評(píng)論和評(píng)估階段，零件已經(jīng)具有制造需要的準(zhǔn)確尺寸和公差。文檔編制和草繪。在那些先前階段完成之后，那些設(shè)計(jì)被自動(dòng)化草繪儀復(fù)制作為文檔編制和參考用，細(xì)節(jié)和圖紙也被沖印。計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)能繪制零件的剖視圖、局部視圖，并且能提取零件的各種各樣的視圖。以前，計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)里的大部分設(shè)計(jì)過(guò)程是在被連接了一臺(tái)計(jì)算機(jī)主機(jī)的工作站上進(jìn)行， 但是趨勢(shì)已經(jīng)迅速發(fā)生了強(qiáng)有力的改變，高性能和非常經(jīng)濟(jì)的32位臺(tái)式Unix工作站及性能更好的64 位的處理器也能運(yùn)行這些計(jì)算機(jī)輔助設(shè)計(jì)軟件。設(shè)計(jì)過(guò)程在深入研究計(jì)算機(jī)輔助設(shè)計(jì)之前，理解計(jì)算機(jī)輔助設(shè)計(jì)過(guò)程是非常重要的。計(jì)算機(jī)技術(shù)和它基于的理論一樣非常有價(jià)值。在通常一個(gè)設(shè)計(jì)的短的討論之后，計(jì)算機(jī)怎樣能幫助每個(gè)設(shè)計(jì)階段，我們將探索。希格利認(rèn)為一個(gè)反復(fù)的設(shè)計(jì)過(guò)程由6 階段組成的：l 1 需要的認(rèn)知l 2 具體問(wèn)題的確定l 3 問(wèn)題綜合l 4 分析、優(yōu)化l 6 決策、評(píng)價(jià)l 6 展示第一步，需要的認(rèn)知，起因由于一個(gè)需要。假例，因?yàn)槿剂隙倘?，需要一臺(tái)新的，更有效率噴氣發(fā)動(dòng)機(jī)。第二步，具體問(wèn)題的確定。確定噴氣發(fā)動(dòng)機(jī)的細(xì)節(jié)要知道推力、功率、允許的重量等等。有關(guān)一些這些說(shuō)明是限制條件；如，它們必須是設(shè)計(jì)要滿足的要求。噴氣發(fā)動(dòng)機(jī)重量限度可能是在飛機(jī)翼可