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CAD and CAM
The engine lathe, one of the oldest metal removal machines, has a number of useful and highly desirable attributes. Today these lathes are used primarily in small shops where smaller quantities rather than large production runs are encountered.
In the beginning, CAD systems were no more than a graphics editor with some built-in design symbols. The geometry available to the user was limited to lines, circular arcs, and the combination of them. The development of free-form curves and surfaces, such as Coon’s patch, Bezier’s patch, and B-spline, enables a CAD systems allow a designer to move into the third dimension. Because a three-dimensional model contains enough information for NC cutter-path programming, the linkage between CAD and NC can be developed. So called turnkey CAD/CAM systems were developed based on this concept and became popular in the 1970s and 1980s.
The 1970s marked the beginning of a new era in CAD – the invention of three-dimensional solid modeling. In the past, three-dimensional, wire-frame models represented an object only by its bounding edges. They are ambiguous in the sense that several interpretations might be possible for a single model. There is also no way to find the volumetric information of a model. Solid models contain complete information; therefore, not only can they be used to produce engineering drawing, but engineering analysis can be performed on the same model as well. Later, many commercial systems and research systems were developed. Quite a few of these systems were based on the PADL and BUILD systems. Although they are powerful in representation, many deficiencies still exist. For example, such systems have extreme computation and resource (memory) requirements, an unconventional way of modeling objects and a lack of tolerance capability have all hindered CAD applications. It was not until the mid-1980s that solid modelers made their way into the design environment. Today, their use is as common as drafting and wire-frame model application.
CAD implementations on personal computers (PCs) have brought CAD to the masses. This development has made CAD available and affordable. CAD originally was a tool used only by aerospace and other major industrial corporation. The introduction of PC CAD packages, such as AutoCAD, VersaCAD, CADKEY systems, made small companies, even individuals, own and use CAD. By 1988, more than 100,000 PC CAD Packages had been sold. Today, PC-based solid modelers are available and are becoming increasingly popular. Because rapid developments in microcomputers have enabled PCs to carry the heavy computational load necessary for solid modeling, many solid graphics user interface (GUI) , CAD systems can be ported easily from one computer to another, most major CAD systems are able to run on a variety of platforms. There is little difference between mainframe, workstation, and PC-based CAD systems.
When a design has frozen, manufacturing can begin. Computers have an important role to play in many aspects of production. Numerically controlled (NC) machine tools need a part program to define the components being made; computer techniques exist to assist, and in some cases virtually automate the generation of part programs. Modern shipbuilding fabricates structures from welded steel plates that are cut from a large steel sheet. Computer-controlled flame cutters are often used for this task and the computer is used to calculate the optimum layout of the components to minimize waste metal.
Printed circuit board assembly can also be improved by computer methods. Quality is maintained by computer-controlled automatic test equipment that diagnoses faults in a particular board and rejects defective boards from the assembly line. Computers are used extensively to plot the artwork used to etch printed circuit boards and also to produce part programs for NC drilling machines.
One of the most important manufacturing functions is stock and production control. If the original design is done on a computer, obtaining lists of material requirements is straightforward. Standard computer data processing methods are employed to organize the work flow and order components when required.
Part geometry requires calculation of a large number of tool positions. Part programming software is usually incorporated into a family of CAM (Computer Aided Manufacturing) software. Some CAM software is associated with CAD (Computer Aided Design) software into CAD/CAM stations. Then the CAM software can use the CAD files as a source of data, which speeds up the programming process.
Part programming software is user-friendly, meaning that the programmer does not have to know the computer programming language or its operating system. It uses screen menus to lead the user through the programming process. Data can be entered via the keyboard, the mouse, or the function keys. Experienced programmers can use built-in macro capabilities and advanced techniques such as a family of parts to become even more productive.
Programming software has a dynamic graphics database to hold the actual machining sequences. These sequences can be viewed, edited, chained, or deleted. The programming can be accomplished whether single cuts or CNC machine canned cycle will be used. The software will also automatically calculate the proper feeds and speeds to be used during the machining, create a tooling list, and define the tool path.
Programmers can use different layers to associate with each profile being created or to construct clamps and fixtures to get a complete picture of the part setup. The tool motion can be seen as it will occur at the machine.
Using part programming software, the programmer can easily solve trigonometry problems to define an accurate tool path. When the program is done, the programmer can send it from the PC to the machine via a communication channel using built-in software with communications capability. Good part programming software is capable of:
（1）Establishing the machining parameters and tooling for a particular machine or job.
（2）Defining the geometry and tool path.
（3）Code generation, enabling the programmer to use standard communications protocols or create his or her own.
計(jì)算機(jī)輔助設(shè)計(jì)及制造
在講述CAD的基本原理之前，先說(shuō)說(shuō)它的簡(jiǎn)史是比較合適的。CAD是計(jì)算機(jī)時(shí)代的產(chǎn)品。它從早期的計(jì)算機(jī)繪圖系統(tǒng)發(fā)展到現(xiàn)在的交互式計(jì)算機(jī)圖形學(xué)。兩個(gè)這樣的系統(tǒng)包括：麻省理工學(xué)院的Sage Project及Sketchpad。Sage Project旨在開(kāi)發(fā)CRT顯示器及操作系統(tǒng)。Sketchpad是在 Sage Project下發(fā)展起來(lái)的。CRT顯示和光筆輸入用于與系統(tǒng)進(jìn)行交互操作。
CAD與初次出現(xiàn)的NC和APT（自動(dòng)編程工具）碰巧同時(shí)問(wèn)世。后來(lái)，X—Y繪圖儀作為計(jì)算機(jī)繪圖的標(biāo)準(zhǔn)拷貝輸出裝置使用。一個(gè)有趣的現(xiàn)象是X—Y繪圖儀與NC鉆床具有相同的基本結(jié)構(gòu)，除啦繪圖筆被NC機(jī)床上的主軸的刀具代替之外。
開(kāi)始，CAD系統(tǒng)僅僅是一個(gè)帶有內(nèi)置設(shè)計(jì)符號(hào)的繪圖編輯器。供用戶使用的幾何元素只有直線、圓弧以及兩者的組合。自由曲線及曲面的發(fā)展，如昆氏嵌面、貝賽爾嵌面以及B樣條曲線，使得CAD可用于復(fù)雜曲線與曲面設(shè)計(jì)。三維CAD設(shè)計(jì)允許設(shè)計(jì)者進(jìn)入三維設(shè)計(jì)空間。由于一個(gè)三維模型包含啦NC刀具路徑編程所需要的足夠信息，所以能夠開(kāi)發(fā)CAD與NC之間聯(lián)系的系統(tǒng)。所謂交鑰匙的CAD/CAM系統(tǒng)便是根據(jù)這一概念開(kāi)發(fā)的，并從20世紀(jì)70年代至80年代流行起來(lái)。
20世紀(jì)70年代末，三維實(shí)體建模的發(fā)明標(biāo)志著CAD一個(gè)新時(shí)代的開(kāi)始。過(guò)去的三維線框模型僅用其邊界來(lái)表達(dá)一個(gè)物體。這在某種意義上是含糊的，一個(gè)簡(jiǎn)單的模型可能有幾種解釋。同時(shí)也無(wú)法獲得一個(gè)模型的體積信息。實(shí)體模型包含完整的信息，因此，他們不僅可用于生成工程圖，而且也可在同一模型上完成工程分析。后來(lái)，開(kāi)發(fā)了許多商業(yè)系統(tǒng)和研究系統(tǒng)。這些系統(tǒng)中相當(dāng)多的是基于PADL和BUICD系統(tǒng)。盡管他們?cè)诒磉_(dá)上是強(qiáng)有力的，但仍然存在許多缺陷。例如，這種系統(tǒng)要有極強(qiáng)的計(jì)算能力和內(nèi)存需求，非常規(guī)的物體建模實(shí)體建模開(kāi)始介入設(shè)計(jì)環(huán)境。今天實(shí)體建模的應(yīng)用如同繪圖和線框模型應(yīng)用一樣普遍。
在個(gè)人計(jì)算機(jī)上，CAD已走向大眾化。這種發(fā)展使CAD應(yīng)用面廣并且很經(jīng)濟(jì)。CAD原本作為一種工具僅被航空和其它主要工業(yè)企業(yè)使用。諸如AutoCAD，VersaCAD，CADKEY等個(gè)人機(jī)CAD軟件包的引入，使小型公司乃至個(gè)人可以擁有并使用CAD系統(tǒng)。到1988年為止已銷(xiāo)售10萬(wàn)個(gè)以上的PCCAD軟件包。今天，基于個(gè)人計(jì)算機(jī)的迅速發(fā)展使得個(gè)人計(jì)算機(jī)的實(shí)體建模的PCCAD易于獲得，并且銷(xiāo)售變得更為普及。由于微型計(jì)算機(jī)的迅速發(fā)展使得個(gè)人計(jì)算機(jī)能夠承受實(shí)體模型需要的大量計(jì)算負(fù)荷，所以如今許多實(shí)體模型在PC機(jī)上運(yùn)行，并且作為平臺(tái)已不成問(wèn)題。隨著標(biāo)準(zhǔn)圖形用戶界面的發(fā)展CAD系統(tǒng)可以很容易的從一臺(tái)計(jì)算機(jī)向另一臺(tái)計(jì)算機(jī)傳送，大多數(shù)CAD系統(tǒng)都能夠在不同的平臺(tái)上運(yùn)行。在大型計(jì)算機(jī)、工作站和基于個(gè)人計(jì)算機(jī)的CAD系統(tǒng)之間幾乎沒(méi)有區(qū)別。
當(dāng)設(shè)計(jì)確定之后，制造才能開(kāi)始。計(jì)算機(jī)在生產(chǎn)的許多方面扮演者一個(gè)重要角色。數(shù)控機(jī)床需要一個(gè)零件程序來(lái)表達(dá)被加工零件；計(jì)算機(jī)技術(shù)起到輔助作用，在謀些情況下實(shí)質(zhì)上是自動(dòng)生產(chǎn)零件加工程序?，F(xiàn)代造船是用從大張鋼板上切下并焊接鋼板來(lái)制造船體結(jié)構(gòu)的。計(jì)算機(jī)控制的火焰切割機(jī)經(jīng)常用于此項(xiàng)任務(wù)，而且計(jì)算機(jī)用于計(jì)算最佳排料，已是邊角廢料最少。
印刷電路板裝配過(guò)程也可以通過(guò)計(jì)算機(jī)方法加以改進(jìn)。質(zhì)量是由計(jì)算機(jī)控制的自動(dòng)檢測(cè)裝置來(lái)保證的，該裝置能在裝配線上檢測(cè)到某個(gè)有缺陷的板子，并且能從裝配線上刪除。計(jì)算機(jī)廣泛用于繪制蝕刻到印刷電路板上的布線圖，并且生成數(shù)控鉆床所需的零件程序。
最重要的功能之一是庫(kù)存和生產(chǎn)控制。如果原始設(shè)計(jì)是在計(jì)算機(jī)上進(jìn)行的，則獲取材料需求清單則是直截了當(dāng)?shù)?。?biāo)準(zhǔn)的計(jì)算機(jī)數(shù)據(jù)處理方法是用于組織這項(xiàng)工作流程，并且按需訂購(gòu)零件。
零件的幾何形狀需要計(jì)算大量刀位。零件編程軟件通常是并入一個(gè)計(jì)算機(jī)輔助制造軟件包中的。一些計(jì)算機(jī)輔助制造軟件與計(jì)算機(jī)輔助設(shè)計(jì)軟件合并成計(jì)算機(jī)輔助設(shè)計(jì)與制作工作站。計(jì)算機(jī)輔助制造軟件可使用計(jì)算機(jī)輔助設(shè)計(jì)文件作為數(shù)據(jù)源，這樣加快啦編程的進(jìn)程。
零件編程軟件是一個(gè)用戶界面友好的軟件，這意味著程序員不必懂的計(jì)算機(jī)編程語(yǔ)言或它的操作系統(tǒng)。它用屏幕菜單引導(dǎo)使用者完成編程過(guò)程。數(shù)據(jù)可通過(guò)鍵盤(pán)、鼠標(biāo)或功能鍵輸入。有經(jīng)驗(yàn)的程序員可使用計(jì)算機(jī)巨大的內(nèi)置內(nèi)容和諸如系列零件族的先進(jìn)技術(shù)來(lái)獲得更高的生產(chǎn)力。
編程軟件有一個(gè)動(dòng)態(tài)圖形數(shù)據(jù)庫(kù)來(lái)支持實(shí)際加工程序。這些程序可被顯示、編輯、串聯(lián)或刪除。無(wú)論是單一切削還是采用CNC機(jī)床固定循環(huán)加工，程序都可生成。該軟件也會(huì)自動(dòng)計(jì)算加工中所用的適當(dāng)進(jìn)給量和切削速度，生產(chǎn)一個(gè)刀具清單和定義刀具路徑。程序員為生成的每一個(gè)輪廓分配不同的圖層或用另外的圖層加入卡具與夾具獲得一個(gè)完整的零件工裝圖?？梢钥吹饺缤跈C(jī)床上進(jìn)行實(shí)際加工一樣的刀具運(yùn)動(dòng)過(guò)程。
使用零件編程軟件，程序員能夠很容易的解決三角學(xué)問(wèn)題，以確定準(zhǔn)確的刀具路徑。當(dāng)一個(gè)程序編好后，程序員可以從PC機(jī)上通過(guò)通信線路用帶有通信能力的內(nèi)置軟件將程序傳送給數(shù)控機(jī)床。好的零件編程軟件應(yīng)具備如下條件：
（1）建立加工參數(shù)以及用于特殊機(jī)床或任務(wù)的刀具。
（2）定義幾何模型及刀具路徑。
（3）生成代碼，能讓程序員確定將使用什么代碼以及如何輸出到機(jī)床。