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.
|
