Lesson 2 CAD
课程2 CAD
Computer-aided design (CAD) involves the use of computers to create design drawings
and product models. Computer-aided design is usually associated with interactive
computer graphics, known as a CAD system. Computer-aided design systems are
powerful tools and are used in the design and geometric modeling of components and
products.
计算机辅助设计（CAD）涉及计算机来设计图纸、产品模型的使用。计算机辅助设计通常与交互式计算机图形学，称为CAD系统。计算机辅助设计系统是功能强大的工具，用于和零部件和产品几何造型设计。
Drawings are generated at workstations, and the design is displayed continuously on the
monitor in different colors for its various components. The designer can easily
conceptualize the part designed on the graphics screen and can consider alternative
designs or modify a particular design to meet specific design requirements. Using
powerful software such as CATIA (computer-aided three-dimensional interactive
applications), the design can .be subjected to engineering analysis and can identify
potential problems, such as excessive load, deflection, or interference at mating surfaces
during assembly. Information (such as a list of materials, specifications, and
manufacturing instructions) is also stored in the CAD database. Using this information, the
designer can analyze the manufacturing economics of alternatives.
图纸生成工作站，和设计的连续显示在监视器上不同的颜色，不同的部分。设计人员可以很容易地概念化的部分在图形屏幕设计可以考虑替代性设计或修改一个特定的设计，以满足特定的设计要求。采用功能强大的软件如（计算机辅助三维交互式应用程序），该设计可以。进行工程分析，可以找出潜在的问题，如过多的负载，偏转，或在装配过程中的干涉配合面。信息（如一组材料，规格，生产指令）也存储在CAD数据库。使用此信息，设计人员可以分析替代品的制造经济学。
1. Exchange Specifications
1。交换规范
Because of the availability of a variety of CAD systems with different characteristics
supplied by different vendors, effective communication and exchange of data between
these systems are essential. Drawing exchange format (DEX) was developed for use with
Autodesk and basically has become a standard because of the long-term success of this
software package. DEX is limited to transfer geometry information only. Similarly,
stereolithography (STL) formats are used to export three-dimensional geometries, initially
only to rapid prototyping system, but recently, it has become a format for data exchange
between different CAD systems.
因为一个具有不同特性的各种不同厂商提供的CAD系统的可用性，这些系统之间的数据的有效沟通和交流是必不可少的。图形交换格式（DEX）的开发使用Autodesk基本上已经成为一种标准，因为这个软件包的长期成功。DEX仅限于几何信息的传递。同样，光固化（STL）格式是用来出口的三维几何形状，最初只在快速成型系统，但最近，它已成为一个不同的CAD系统之间的数据交换格式。
The necessity for a single, neutral format for better compatibility and for the transfer of
more information than geometry alone is currently filled mainly by initial graphics
exchange specification (IGES). This is used for translation in two directions (in and out of
a system) and is also widely used for translation of three-dimensional line and surface
data. Because IGES is evolving, there are many variations of IGES.
为一个单一的必要性，更好的兼容性中立的格式和更多的信息比几何单独转让目前主要是由初始图形交换规范（IGES）填充。这是用于在两个方向的翻译（和一个系统），也被广泛用于翻译的三维线和表面数据。因为IGES是不断发展的，有许多不同的IGES。
Another useful format is a solid-model-based standard called the Product Data Exchange
Specification (PDES), which is based on the Standard for the Exchange of Product Model
Data (STEP) developed by the International Standard Organization. PDES allows
information on shape, design, manufacturing, quality assurance, testing, maintenance,
etc., to be transferred between CAD systems.
另一个有用的格式是基于实体模型的标准称为产品数据交换规范（PDE），这是基于产品模型数据交换标准（STEP）由国际标准组织开发。偏微分方程允许信息的形状，设计，制造，质量保证，测试，维护，等，是CAD系统之间转移。
2. Elements of CAD Systems
2。CAD系统的元素
The design process in a CAD system consists of the four stages described in this section.
设计在CAD系统的过程由四个阶段在本节中描述的。
1) Geometric modeling
1）的几何建模
In geometric modeling, a physical object or any of its parts is described mathematically or
analytically. The designer first constructs a geometric model by giving commands that
create or modify lines, surfaces, solids, dimensions, and text. Together, these propose an
accurate and complete two or three dimensional representation of the object. The results
of these commands are displayed and can be moved around on the screen, and any
section desired can be magnified to view details. These data are stored in the database
contained in computer memory.
在几何建模，物理对象或其任何部分的数学描述或分析的。设计师首先构建了一个几何模型的命令，创建或修改线，表面，固体，尺寸，和文本。在一起，这些建议的对象的准确和完整的两或三维表示。这些命令的结果显示，可以在屏幕上到处移动，和任何部分所需的可以放大查看详情。这些数据存储在数据库中的计算机内存。
The octree representation of a solid object is shown in Figure 2.1. It is a three-
dimensional analog to pixels on a television screen. Just as any area can be broken down
into quadrants, any volume can be broken down into octants, which are then identified as
solid, void, or partially filled. Partially filled voxels (from volume pixels) are broken into
smaller octants and are reclassified. With increasing resolution, exceptional part detail is
