Presentation on theme: "Chapter (7) MISCELLANEOUS WORKHOLDING FIXTURES 7.1. Workholding Devices for Turning 7.1.1. Collet chucks They are usually classified according to the method."— Presentation transcript:
Chapter (7) MISCELLANEOUS WORKHOLDING FIXTURES 7.1. Workholding Devices for Turning Collet chucks They are usually classified according to the method of closing the collet. -Push-out collet: The collet being closed by pushing it against a taper in the hood. It gives a good control of length. The hood must be removed when the collet is changed. - Pull-in collet: The collet is closed by pulling it into a taper in the hood. It gives a poor control of length. The hood does not need to be removed when the collet is changed.
It produces a good control of length, but wear of the many components of this collet produces concentricity errors. Very often a master collet is used, and variations of shape allowed for by fitting a liner Jaw chucks Are used for the early operations upon billets, castings and forgings. They are attached to the radial jaw slide by collar screws, for second- operation work and for irregular- shaped w.ps. Soft jaws are made from case-hardened mild steel.
Jaw chucks Are used to hold w.ps. From a bore.
Turning fixtures They are located and clamped to the m/c spindle, and carry location and clamping systems. The clamping system must secure the w.p. against the rotational forces as well as the cutting forces. Care must be taken to ensure that there are no projecting details that can cause accidents, and also to provide means of balancing the fixture.
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W F : ‘out-of-balance fixture’ force and acts at distance R F from the axis. W W : ‘out-of-balance w.p.’ force and acts at distance R W from the axis. The position and magnitude of the balancing force can be found as; W B R B = W F R F + W W R W
7.2. Workholding Devices for Grinding Surface grinding These fixtures are similar to milling fixtures but are more accurate Cylindrical grinding External and internal grinding can be done using a fixture that can be regarded as a more accurate version of a turning fixture. - The internal grinding fixture (shown in the next Fig.) is finally positioned up using a dial indicator from the ‘clocking diameter’, which is in the form of a groove to prevent it from being damaged. The fixture will be very carefully balanced.
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7.3. Workholding Devices for Assembly - A workholding device will be used for an assembly operation if it is awkward to do. For example, a clock mechanism involves many gear and spindle units that are sandwiched between two plates containing the pivot bearings. - An assembly fixture must be designed so that all the parts are located and held during the assembly, but the fixture must be removed easily when the operation is completed Welding Fixtures A welding fixture can be regarded as a special assembly fixture. The following special requirements must be considered; 1. The welder must have access to all the joints. 2. The fixture must be designed so that it can be moved about during the welding operation to allow the welder to use the most convenient welding technique (e.g. trunnion mounting). 3. The design must allow for the high, concentrated heat associated with welding. The position of the clamps and locators must be such that the heat does not damage them or cause errors. 4. Screw threads and similar fixture parts must be protected against weld splatter.