TAILIEUCHUNG - Process Planning Episode 8

Tham khảo tài liệu 'process planning episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Workholding devices 275 Figure Workpiece constrained by three pins Figure Further two pins used for constraint restricted. A second plane of reference is then assigned in which two further pins are located as illustrated in Fig. . These prevent rotation in both dừections along the z axis and motion in one direction along the Y axis which means that a further three dừections of motion are constrained and a total of eight dữections of motion are now constrained. A third and final plane is then designated which is perpendicular to both previous planes and a single locating pin placed in it. This restricts motion in one direction along the X axis as illustrated in Fig. . This leaves motion 276 Process Planning Figure Final pin used for 3-2-1 location principle in only three dứections unconsttained. Although additional pins could be used to fully restrict motion this is unpractical as it would at worst prevent loading and unloading of the workpiece into the jig fixture. Therefore the clamping system must be designed in such a way as to prevent motion in the remaining three directions in addition to resisting the forces experienced during the machining operation. When using locating pins there are three basic principles that must be applied known as the principles of pin location ASTME 1962 1. Use the minimum number of locating pins necessary to locate in any one plane. The 3-2-1 principle determines the minimum number of pins requữed. Additional locating points should only be used if they have a useful purpose. 2. The location points should be positioned as far apart as possible in any one plane. 3. The planes used for the locating points should always be mutually perpendicular. Plane location There are a number of factors that must be considered before selecting location devices. The most important of these are the shape form of the workpiece the surface finish of the workpiece the type of clamping required. Workholding devices 277 Figure .

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