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Actual Mating Envelope

     Calculating Additional Tolerance Permitted when a Position Tolerance is Modified at MMC. 

An additional tolerance, often called Bonus tolerance, is permitted when a position tolerance is modified at maximum material condition (MMC) and the actual mating envelope of the feature has departed from the MMC. 

In these illustrations, the part is black and the actual mating envelope is red.
For an internal feature such as a hole or slot:

For an external feature such as a boss or tab:

Note: The Actual Mating Envelope must be oriented relative to the specified Datums. When an inspector merely uses the size of a feature to calculate the bonus tolerance, out of spec parts may be accepted. 

If a hole, for instance, has the following size and geometric control, and the hole measures .502. It would be incorrect to use a bonus tolerance of .003 (.502 - .499(MMC)) if the hole is not perfectly oriented to the Datums. If the hole is out of perpendicular to datum A by .002, for instance, the bonus that may be used is reduced by that amount. The bonus would be merely .001 and the allowable position tolerance = .016.

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