机械专业英语董建国第17章.pptVIP

Unit 17 TEXT READING MATERIAL Unit 17 TEXT The Design of Jigs and Fixtures Holding devices may be classified as jigs or fixtures. A jig is a work-piece locating and holding device which positions and guides or controls a cutting tool. A fixture is a work-piece locating and holding device used with machine tools, inspection, welding and assembly. It does not control the position of the tool or instrument which is being used. In many instances gauge blocks may be provided for setting the depth or position of the cutting tool. Fig. 17-1 Drilling jig The designer approaches the design project by first becoming fully aware of all critical dimensions and surfaces. If possible, he should also know how the part fits into the main assembly. Once he is familiar with the special functional requirements and dimensions, he collects the data which may have an effect on his design. Data such as type of material, physical properties, weight, and the characteristics of the raw material. Other information which could affect the type of fixture to be built is the production quantities, lot sizes, etc. Next the designer determines whether fixtures already in the plant can be altered and used for his job. He must also consider the possibility and advisability of using standard holding devices such as vises, parallels and clamps. Once it is decided that a series of operations are to be done with newly designed fixtures, the designer combines as many operations into one fixture as possible. Fig. 17-2 Milling fixture LEARN TO SPEAK Mr. Zhang: Good morning, Mr. Cheng. Mr. Cheng: Good morning, Mr. Zhang. Mr. Zhang: What are you doing? Mr. Cheng: I am machining a work-piece. Mr. Zhang: What’s this? Mr. Cheng: It is a fixture. Mr. Zhang: What can it do? Mr. Cheng: We can use it to locate and hold the work-piece when it is machined. Mr. Zhang: Is the fixture only be used to machine? Mr. Cheng: No. In fact there are many kinds of fixture and they can also be us