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Before we present the basics of CAD,it is appropriate to give a brief history.CAD is a product of the computer era.It originated from early computer graphic systems to the development of interactive computer graphics.Such two systems include the Sage Project at the Massachusetts Institute of Technology(MIT)and Sketchpad.The Sage Project was aimed at developing CRT displays and operating systems.Sketchpad was developed under the Sage Project.A CRT display and light pen input were used to interact with the system.This coincidentally happened at about the same time that NC and APT(Automatically Programmed Tool)first appeared [1] .Later,X-Y plotters were used as the standard hard-copy output device for computer graphics.An interesting note is that an X-Y plotter has the same basic structure as a NC drilling machine except that a pen is substituted for the tool on NC spindle[ 2 ].
In the beginning,CAD systems were no more than graphics editor with some built-in design symbols.The geometry available to the user was limited to lines,circular arcs,and the combination of the two.The development of free-form curves and surfaces,such as Coon’s patch,Bezier’s patch,and B-spline,enables a CAD system to be used for sophisticated curves and surface design.Three-dimensional 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 also 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.It was in 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 applications.
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,and so on,has made it possible for small companies and even individuals to own and use CAD systems.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 modelers now run on PCs and the platform has become less of an issue.With the standard 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.
[1]This coincidentally happened at about the same time that NC and APT(Automatically Programmed Tool)first appeared.
CAD与初次出现的NC和APT（自动编程工具）碰巧同时问世。（coincidentally为插入语,at about the same time为介词短语，可译为“几乎”“同时”）
[2]An interesting note is that an X-Y plotter has the same basic structure as a NC drilling machine except that a pen is substituted for the tool on NC spindle.
一个有趣的现象是X-Y绘图仪与NC钻床具有相同的基本机构，除了绘图笔被NC机床上的主轴刀具替代之外。（the same...as可译为“与……相同”,as为介词，引导短语）
在讲述CAD（计算机辅助技术）的基本理论之前，先说说它的简史是比较合适的。CAD是计算机时代的产品。它从早期的计算机绘图系统发展到现在的交互式计算机图形学。两个这样的系统包括：麻省理工学院的Sage Project及Sketchpad。Sage Project旨在开发CRT显示器及操作系统。Sketchpad是在Sage Project下发展起来的。CRT显示器和光笔输入用于与系统进行交互操作。CAD与初次出现的NC（数字控制）和APT（自动编程工具）碰巧同时问世。后来,X-Y绘图仪作为计算机绘图的标准硬拷贝输出装置使用。一个有趣的现象是X-Y绘图仪与NC钻床具有相同的基本结构，除了绘图笔被NC机床上的主轴刀具替代之外。
开始,CAD系统仅仅是一个带有内置设计符号的绘图编辑器。供用户使用的几何元素只有直线、圆弧以及两者的组合。自由曲线及其曲面的发展，如昆氏嵌面、贝塞尔嵌面以及B样条曲线，使CAD系统可用于复杂曲线与曲面设计。三维CAD系统允许设计者步入三维设计空间。一个三维模型包含了NC刀具路径编程所需要的足够信息，因此，能够开发CAD与NC之间联系的系统。所谓交钥匙的CAD/CAM（计算机辅助制造）系统便是根据这一概念开发的，并从20世纪70年代至80年代流行起来。
20世纪70年代，三维实体建模的发明标志着CAD一个新时代的开始。过去的三维线框模型仅用其边界来表达一个物体。这在某种意义上是含糊的，一个简单的模型可能有几种解释，同时也无法获得模型的体积信息。实体建模包含完整的信息，因此，它们不仅可用于生成工程图，而且也可在同一模型上完成工程分析。后来，三维实体建模还开发了许多商业系统和研究系统。这些系统中相当多的是基于PADL和BUILD系统。尽管它们在表达上是强有力的，但仍然存在许多缺陷。例如，这种系统要有极强的计算能力。直到20世纪80年代中期，实体建模开始介入设计环境。今天，实体建模的应用如同绘图和线框模型应用一样普遍。
在个人计算机上,CAD已走向大众化。这种发展使CAD应用面更广且价格更合理。CAD原本作为一种工具仅被航空和其他主要工业企业使用。AutoCAD、VersaCAD、CADKEY等个人机CAD软件包的引入，使小型公司乃至个人可以拥有并使用CAD系统。到1988年为止，已销售10万个以上的个人机CAD软件包。今天，基于个人计算机的实体建模易于获得，并且越来越受欢迎。由于微型计算机的迅速发展，使得个人计算机能够承受实体建模需要的计算负荷，如今许多实体建模在个人机上运行，并且作为平台已不成为一个问题。随着标准图形用户界面(GUI)的发展,CAD系统可以很容易地从一台计算机传送至另一台计算机，且大多数CAD系统都能够在不同平台运行。大型计算机、工作站和基于个人计算机的CAD系统之间几乎没有区别。
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.Numerically controlled pipe-bending machines are able to operate directly from part programs generated by pipe-routing software.
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 function 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(Figure 5-1).
Figure5-1 The block diagram of a CAD/CAM system
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 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 define what code is to be generated and how it is output to the machines.
(4)Communication enabling the programmer to use standard communications protocols or create his or her own.
当设计确定之后，制造才能开始。计算机在生产的许多方面扮演着一个重要角色。数控机床需要编制一个零件加工程序来加工零件；计算机技术起到辅助作用，在某些情况下实质上是自动生成零件加工程序。现代造船是从大张钢板上切下焊接钢板来制造船体的。计算机控制的火焰切割机经常用于执行此项任务，而且计算机用于计算最佳排料，以使边角废料最少。数控弯管机能够通过管路软件生成的零件加工程序进行直接操作。
印刷电路板装配过程也可通过计算机方法加以改进。质量是由计算机控制的自动检测装置保证的，该装置能检测到某个板子上的缺陷，并且能从装配线上剔除有缺陷的板子。计算机广泛用于绘制蚀刻到印刷电路板的布线图，并且生成数控钻床所需的零件加工程序。
最重要的制造功能之一是库存和生产控制。如果原始设计是在计算机上进行的，可直接获取材料需求清单。标准的计算机数据处理方法可组织这项工作流程，并且按需订购零件（见图5-1所示）。
图5-1 CAD/CAM结构框图
零件的几何形状需要计算大量刀位。零件加工编程软件通常是并入一个计算机辅助制造软件包中的。一些计算机辅助制造软件与计算机辅助设计软件合并成计算机辅助设计与制造工作站。计算机辅助制造软件可使用计算机辅助设计文件作为数据源，这样加快了编程的进程。
零件加工编程软件是一个用户界面友好的软件，这意味着程序员不必懂得计算机编程语言或它的操作系统。它用屏幕菜单引导使用者完成编程过程。数据可通过键盘、鼠标或功能键输入。有经验的程序员可使用计算机巨大的内置容量和诸如系列零件簇的先进技术来获取更高的生产力。
编程软件有一个动态图形数据库来支持实际加工顺序。这些顺序可被显示、编辑、串联或删除。无论是单一切削，还是采用CNC（计算机数字控制）机床固定循环加工，程序都可生成。该软件也会自动计算加工中所用的适当进给量和切削速度，生成一个刀具清单和定义刀具路径。
程序员为生成的每一个轮廓分配不同的图层或用图层构建卡具与夹具来获得一个完整的零件工装图。可以看到与在机床上进行实际加工一样的刀具运动过程。
使用零件加工编程软件，程序员能轻松解决三角学问题，以确定准确的刀具路径。当一个程序编好后，程序员可从个人计算机上通过通信线路用带有通信能力的内置软件将程序传送给数控机床。好的零件加工编程软件应具备如下条件：
1.建立用于特定机床或任务的加工参数和刀具。
2.定义几何模型及刀具路径。
3.生成代码，能让程序员确定生成的代码以及如何将代码输出到机床。
4.可使程序员使用标准的通信协议或生成他（她）自己的通信协议进行通信。