Introduction to Bonus Tolerance

Definition

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.

This section 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.

Figure 2

Figure 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.