Calculate GD&T bonus tolerance, feature size at MMC, or actual feature size for internal and external features in mm, cm, or inches.
Bonus Tolerance Formula
The following formulas are used to calculate GD&T bonus tolerance (the size departure from MMC toward LMC). Which one applies depends on whether the feature is internal (hole/slot) or external (pin/shaft).
Internal feature (hole/slot): BT = AV - FV External feature (pin/shaft): BT = FV - AV
Variables:
- BT is the bonus tolerance (additional geometric tolerance gained due to departure from MMC)
- FV is the feature size at maximum material condition (MMC)
- AV is the actual feature size
To calculate the bonus tolerance, find the departure of the actual size from MMC in the direction of least material condition (LMC). For an internal feature, this is actual − MMC. For an external feature, this is MMC − actual. If the actual size goes past MMC in the wrong direction, the feature is out of size and there is no valid “bonus” amount.
What is a Bonus Tolerance?
A bonus tolerance is an additional tolerance allowed in a geometric dimensioning and tolerancing (GD&T) system when a feature of size is controlled at maximum material condition (MMC). As the produced feature departs from MMC toward least material condition (LMC), additional tolerance is gained for applicable geometric controls (commonly position and orientation controls on features of size, and also runout controls when applied to features of size). This can increase manufacturing flexibility and potentially reduce cost.
How to Calculate Bonus Tolerance?
The following steps outline how to calculate the Bonus Tolerance using the given formula:
- First, determine the feature type: internal (hole/slot) or external (pin/shaft).
- Next, determine the feature size at maximum material condition (MMC) (FV).
- Next, determine the actual feature size (AV).
- Next, insert the values into the appropriate formula: for internal features, BT = AV − FV; for external features, BT = FV − AV.
- Finally, calculate the Bonus Tolerance (BT).
- After calculating the result, check that the result is not negative (a negative value indicates the feature has gone past MMC in the wrong direction and is out of size).
Example Problem:
Use the following variables as an example problem to test your knowledge:
Feature size at maximum material condition (MMC) (FV) = 10
Actual feature size (AV) = 9.8
Feature type = External feature (pin/shaft)
