《机械专业英语》课件第21章.ppt

1、Unit 21TEXT LEARN TO SPEAK NEW WORDS AND EXPRESSIONS NOTES EXERCISES READING MATERIAL PROFESSIONAL WORDS AND PHRASES Computer Aided DesignA CAD system is basically a design tool in which the computer is used to analyze various aspects of a designed product.The CAD system supports the design process

2、at all levelsconceptual,preliminary,and final design.The designer can then test the product in various environmental conditions,such as temperature changes,or under different mechanical stresses.Although CAD systems do not necessarily involve computer graphics,the display of the designed object on a

3、 screen is one of the most valuable features of CAD systems.The picture of the object is usually displayed on the surface of a cathode-ray tube(CRT).Computer graphics enables the designer to study the object by rotating it on the computer screen,separating it into segments,enlarging a specific porti

4、on of the object in order to observe it in detail,and studying the motion of mechanisms with the aid of kinematic programs.Most CAD systems are using interactive graphics systems.Interactive graphics allows the user to interact directly with the computer in order to generate manipulate,and modify gr

5、aphic displays.Interactive graphics has become a valuable tool,if not a necessary prerequisite,of CAD systems.The end products of many CAD systems are drawings generated on a plotter interfaced with the computer.One of the most difficult problems in CAD drawings is the elimination of hidden lines.Th

6、e computer produces the drawing as wire frame diagram.Since the computer defines the object without regard to ones perspective,it will display all the objects surfaces,regardless of whether they are located on the side facing the viewer or on the back,which normally the eye cannot see.Various method

7、s are used to generate the drawing of the part on the computer screen.One method is to use a geometric modeling approach,in which fundamental shapes and basic elements are used to build the drawing.The lengths and radius of the elements can be modified.For example,a cylinder is a basic element,the s

8、ubtraction of a cylinder with a specific radius and length will create hole in the displayed part.Each variation,however,maintains the overall geometry of the part.Other CAD systems use group technology(GT)in the design of parts.Group technology is a method of coding and grouping parts on the basis

9、of similarities in function or structure or in the ways they are produced.Application of group technology can enable a company to reduce the number of parts in use and to make the production of parts and their movement in the plant efficient.Recently CAD systems are using the finite-element method(F

10、EM)of stress analysis.By this approach the object to be analyzed is represented by a model consisting of small elements,each of which has stress and deflection characteristics.The analysis requires the simultaneous solution of many equations;a task which is performed by the computer.The deflections

11、of the object can be displayed on the computer screen by generating animation of the model.With any of these methods,or others which are used,the CAD system generates at the design stage a single geometric data base which can be used in all phases of the design and later in the manufacturing,assembl

12、e,and inspection processes.Fig.21-1 shows an example of various types of modeling for CAD.Fig.21-1 Various types of modeling for CAD LEARN TO SPEAKMr.Zhang:Hello,Mr.Cheng.Mr.Cheng:Hi,Mr.Zhang.Mr.Zhang:I want to know some knowledge of the CAD.Could you tell me some about it?Mr.Cheng:No problem.What d

13、o you want to know?Mr.Zhang:What does the CAD mean?Mr.Cheng:Computer aided design(CAD)means the use of a computer to assist in the design of an individual part or a system.The design process usually involves computer graphics.Mr.Zhang:What is the CAD system?Mr.Cheng:It is basically a design tool in

14、which the computer is used to analyze aspects of a designed product.Mr.Zhang:What can the CAD system support?Mr.Cheng:It can support the design process at all levels:conceptual,preliminary,and final design.Mr.Zhang:Can the designer test the product by the CAD system?Mr.Cheng:Yes.It can do.Mr.Zhang:W

15、hat is one of the most valuable features of CAD systems?Mr.Cheng:The display of the designed object on a screen.Mr.Zhang:What systems are the most CAD systems using?Mr.Cheng:Interactive graphics systems.Mr.Zhang:Do you think the elimination of hidden lines is one of the most difficult problems in CA

16、D drawings?Mr.Cheng:Yes,I think so.Mr.Zhang:Oh,I have learned so much from you.Thanks a million.Mr.Cheng:At your service.NEW WORDS AND EXPRESSIONSconceptual kEn5septjuEl adj.概念的，构想的preliminary prI5lImInErI adj.预备的，初步的graphics 5rAfIks n.制图学，图形segment 5semEnt n.段，节environmental InvaIErEnment(E)l adj.周

17、围的，环境的subtraction sEb5trAkF(E)n n.减少perspective pE5spektIv n.透视，透视法，透视画geometric dVIE5metrIk adj.几何的，几何学的cylinder 5sIlIndE(r)n.圆筒，圆柱体animation AnI5meIF(E)n n.动画片，动画 computer aided design(CAD)计算机辅助设计cathode-ray tube(CRT)阴极射线管regardless of 不管，不顾group technology(GT)成组技术finite-element method(FEM)有限元方法 N

18、OTES1Since the computer defines the object without regard to ones perspective,it will display all the objects surfaces,regardless of whether they are located on the side facing the viewer or on the back,which normally the eye cannot see.由于计算机定义物体时没有考虑其透视效果，因此它会显示出物体的所有面，而不管这些面是位于面向观察者的一面，还是位于人眼通常无法看

19、到的背面。此句中，since引导原因状语从句。since还可引导时间状语从句，要注意区分。2Group technology is a method of coding and grouping parts on the basis of similarities in function or structure or in the ways they are produced.成组技术是在零件具有相同功能、结构或加工方法的基础上对零件分组编码的一种方法。此句中，“成组技术是一种方法”是句子的主干，of coding and grouping parts on the basis of sim

20、ilarities in function or structure or in the ways they are produced 是介词短语做定语，修饰method。EXERCISES1Discuss the following questions in English:(1)What does CAD mean?Which work can it do?(2)What does CAM mean?Which work can it do?(3)If we want to use computer to design a product,which work do we have to

21、do?2Use English for replacing Chinese or use Chinese for replacing English.(1)计算机辅助设计技术是一种先进的设计技术。(2)计算机辅助制造可以大大提高产品质量。(3)成组技术是一种有效的机械加工技术。(4)Computer aided process planning(CAPP)is an efficient product machining method.(5)Computer aided testing(CAT)is a high efficiently product measuring method.(6)

22、We can rotate the object on the computer screen when we use CAD to design a product.3Choose the term which best matches the definition given in Column 2 from Column 1.Column 1(1)computer graphics (2)CRT (3)Interactive graphic (4)The computer defines the object without regard to ones perspective (5)U

23、sing the finite-element method (6)Computer aided manufacturing Column 2(a)it will display all the object surfaces.(b)the object to be analyzed is represented by a model consisting of small elements.(c)the display of the designed object on a screen.(d)allow the user to interact directly with the comp

24、uter.(e)means the use of a computer to assist in the manufacture of a part.(f)cathode-ray tube.4Translate the following passage into Chinese:CAD system can created and displayed graphic information on a cathode-ray tube(CRT),and it allows the designer to produce finished engineering drawings from si

25、ngle pensile sketches or from models and modify these drawings on the screen if they do not seem functional.From three-drawing,orthographic views,the designer can transform drawings into a three-dimensional views.This enables a designer to redesign a part on the screen,project how the part will oper

26、ate in use,and make successive design changes in a matter of minutes.READING MATERIAL CAD/CAM SystemsComputer aided design(CAD)means the use of a computer to assist in the design of an individual part or a system,such as an aircraft.The design process usually involves computer graphics.Computer aide

27、d manufacturing(CAM)means the use of a computer to assist in the manufacture of a part.CAM can be divided into two main classes:(1)On-line applications,namely,the use of the computer to control manufacturing systems in real time,such as CNC and AC systems of machine tools.(2)Off-line applications,na

28、mely,the use of the computer in production planning and non-real-time assistance in the manufacturing of parts.CAD/CAM is a unified software system,in which the CAD portion is interfaced inside the computer with the CAM system,shown as Fig.21-2 and Fig.21-3.The end result of current CAD/CAM systems

29、is usually a part program in the form of a list or a punched tape.In advanced CAD/CAM systems part programs can be directly fed into the control computers of CNC machines and inspection stations.Fig.21-2 Information flow chart in CAD/CAM application Fig.21-3 The block diagram of a CAD/CAM system The

30、 main concept of CAD/CAM systems is the generating of a common database which is used for all the design and manufacturing activities.These include specifications of the product,conceptual design,final design,drafting,manufacturing,and inspection.At each stage of this process,data can be added,modif

31、ied,used,and distributed over networks of terminals and computers.The single database provides a substantial reduction in human errors and a significant shortening of the time required from the introduction of a concept of a product to the manufacturing of the final physical product.The size and cap

32、ability of the required computer system depend on the complexity of the product.In the aerospace and aeronautics industry,where a complete aircraft can be designed with a CAD/CAM processor,the system must accommodate new data and changes in data arriving from a variety of users.Therefore these syste

33、ms must have a strong data management capability.By contrast,if a company designed simple products,the required CAD/CAM system would need only one computer terminal.Today the major users of CAD/CAM systems are the aerospace and automotive industries,but the declining price of these systems enlarge t

34、he number of other users.Advanced CAD/CAM systems include solid geometry modeling capability,in addition to the wire frame mode diagrams.The solid description is important when NC verification programs are included in the CAM system,and the changing status of the work-piece during machining simulati

35、on should be observed.In recent years CAD/CAM technology has improved industry productivity.It is a significant step toward the design of the factory of the future.PROFESSIONAL WORDS AND PHRASESunify 5ju:nIfaI vt.统一，使成一体database 5deItbeIs n.数据库，资料库drafting 5drB:ftIN n.制图substantial sEb5stAnF(E)l adj.实质的complexity kEm5pleksItI n.复杂性significant sI5nIfEkEnt adj.重大的aeronautics eErEJ5nC:tIks n.航空学declining adj.倾斜的，衰退中的 simulation sImjJ5leIF(E)n n.仿真，模拟 computer graphics 计算机绘图computer aided manufacturing(CAM)计算机辅助制造on-line 在线的，即时的real time 实时conceptual design 概念设计punched tape 穿孔带