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It Is Datum Shift - Not Bonus (#255)

(In accordance with the ASME Y14.5-2009 standard)

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One of the most difficult concepts for folks to understand is what happens to the tolerance when a MMB or LMB modifier is applied to a datum feature reference.  On this drawing datum feature C is referenced at MMB.  I was asked how this affects the position and profile tolerances.  Can it provide a "bonus" tolerance?  The answer is "NO!"  It can allow a shift of the datum feature on the datum feature simulator.

The second figure is of a possible fixture that could be used to inspect the position and profile of a surface tolerances.

The part is "grabbed" by datum feature A simulator. Although the simulator closes in on datum feature A, the part may still slide axially and rotate about the axis of the fixture. The part must slide to make contact with the datum feature B simulator and over the virtual condition pin that is the datum feature C simulator. The part may then rotate about the axis of datum feature A but is limited by the pin. So, inspection is allowed to rock the part on the fixture to bring the features into the profile and position tolerances. Because of the rule of simultaneous requirements, the part must meet all of the tolerances simultaneously. Clearly, this datum shift is not identical to a bonus tolerance on the profile and position tolerances. Only on the simplest of parts might a datum displacement act similar to a bonus tolerance.

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Back to Tips Tip added Apr 2011