Bonus tolerance is an additional
tolerance for a geometric control. Whenever a geometric tolerance is applied
to a feature of size, and it contains an MMC (or LMC) modifier in the tolerance portion
of the feature control frame, a bonus tolerance is permissible.
introduces bonus tolerance based on the MMC modifier. When the MMC modifier
is used in the tolerance portion of the feature control frame, it means
that the stated tolerance applies when the feature of size is at its maximum material
condition. When the actual mating size of the feature of size departs from MMC (towards
LMC), an increase in the stated tolerance— equal to the amount of the departure — is permitted. This increase, or extra tolerance, is called the bonus tolerance.
Figure 3-16 shows how bonus tolerance is calculated in a straightness application.
When the MMC modifier is used, it means that the geometric characteristic
can be verified with a fixed gage.
A functional gage is a gage
that is built to a fixed dimension (the virtual condition) of a part feature;
a part must fit into (or onto) the gage. A functional gage does not provide
a dimensional measurement; it only indicates if the part is or is not to
the print specification.
In Figure 1, the functional
gage is designed to the virtual condition (2.7) of the feature of size. Since the gage
opening is constant, the thinner the washer becomes, the more straightness
tolerance it could have and still pass through the gage.
Although a straightness control
is used for the example in Figure 1, the bonus tolerance concept applies
to any geometric control that uses the MMC (or LMC) modifier in the tolerance
portion of the feature control frame.
2 shows how to determine the amount of bonus tolerance permissible in an
application. The MMC modifier in the tolerance portion of the feature control
frame denotes that a bonus tolerance is permissible. The maximum amount
of bonus tolerance permissible is equal to the difference between the MMC
and the LMC of the toleranced feature of size.