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7.Jigs and Fixtures

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    Jigs and Fixtures

    What are jigs and

    fixtures?

    Used to hold and guidework pieces

    Used to accuratelymanufacture duplicate

    parts

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    A jig is a device that holds and locates a piece

    of work and guides the tools that operateupon it.

    A fixture is a device designed and built forholding a particular piece of work formachining operations.

    Jigs and fixtures allow mass production ofconsistent quality. They also give added safetyby confining waste

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    Difference between Jig & Fixture

    Jig Fixture

    1. Used to guide and hold the workpiece 1.Only hold the workpiece

    2.Used to drilling and related operations

    only

    2. Used for milling,

    boxing,grinding,shaping

    3. It is lighter in weight & construction 3. It is heavier in weight & construction

    4.It reqires drill bushes 4. It require setting block

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    Degrees of freedom

    Figure 3-1. The twelve degrees of freedom

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    Degrees of freedom

    As shown in Figure 3-1, the twelve degrees of

    freedom all relate to the central axes of theworkpiece. Notice the six axial degrees of

    freedom and six radial degrees of freedom. Theaxial degrees of freedom permit straight-linemovement in both directions along the three

    principal axes, shown as x, y, and z. The radial

    degrees of freedom permit rotational movement,in both clockwise and counterclockwise radialdirections, around the same three axes.

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    A workpiece free in space canmove in an infinite number of

    directions. For analysis, thismotion can be broken downinto twelve directional

    movements, or "degrees offreedom." All twelve degreesof freedom must be restricted

    to ensure proper referencingof a workpiece.

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    Basic Categories of Jigs

    Clamps

    Chucks

    Vises Bushings

    Modular Fixtures

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    Figure 3-2.The three forms of location: plane, concentric, and radial.

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    Clamps

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    Figure 3-3.Solid, adjustable, and equalizing supports locate a workpiece from a flat surface.

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    Bushings

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    Figure 3-15.Examples of redundant location.

    Avoiding Redundant LocationAnother condition to avoid in workholder design is redundant, or

    duplicate, location. Redundant locators restrict the same degree of

    freedom more than once. The workpieces in Figure 3-15 show several

    examples. The part at (a) shows how a flat surface can be redundantly

    located. The part should be located on only one, not both, side surfaces.

    Since the sizes of parts can vary, within their tolerances, the likelihood ofall parts resting simultaneously on both surfaces is remote. The example

    at (b) points out the same problem with concentric diameters. Either

    diameter can locate the part, but not both.

    The example at (c) shows the difficulty with combining hole and

    surface location. Either locational method, locating from the holes or

    locating from the edges, works well if used alone. When the methods are

    used together, however, they cause a duplicate condition. The condition

    may result in parts that cannot be loaded or unloaded as intended.

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