Geometric Dimensioning and Tolerancing Bonus Tolerance – Example and Application of GD&T Bonus Tolerance

The concept of GD&T bonus tolerance is very important for a mechanical design engineer, especially for those who are designing parts fitting together such as a shaft and its mating component.

What is Bonus Tolerance?

Bonus tolerance is the additional tolerance available for a geometric control attached to a feature of size with MMC (maximum material condition) and LMC (least material condition) modifier.

What is MMC?

When a component consists of maximum amount of materials it could possibly carry then the component is said to be in MMC condition. For a shaft and hole base system, a shaft will be in MMC when it has the maximum diameter and a hole will be in MMC when it has minimum diameter.

Maximum Material Condition (MMC)

In the above figure the size of the hole can vary from 10.2 to 10.5. And at 10.2 it will be in MMC.

Example of Bonus Tolerance

Say, you have a drawing of a block with a hole like below:

bonus tolerance

Observe the GD&T control frame. The meaning of the GD&T symbol is: “a positional tolerance of 0.2 is applicable to the hole when it is at the maximum material condition (MMC)”.

You already know that MMC for the hole will have the diameter of 10.2 and at this condition there should have a positional tolerance of 0.2 means for inspecting the hole you should use a gauge of diameter 10. All the holes that allow the 10 diameter gauge to enter should pass the quality checks.

At MMC the maximum clearance available between the gauge and the hole is 0.2, so available tolerance at MMC is 0.2.

Now as per the given dimension the hole diameter can vary from 10.2 to 10.5, and as the hole size increase the tolerance available between the gauge and the hole also increases. At MMC the maximum available tolerance will be equal to

10.5 – gauge diameter = 10.5 – 10 = 0.5. Out of which specified positional tolerance is 0.2 so the bonus tolerance will be (0.5 – 0.2) = 0.3.

Conclusion

Bonus tolerance is a important concept in GD&T. Bonus tolerance will be available for the geometric dimensioning and tolerancing feature of size with MMC or LMC modifiers. The bonus tolerance reduces the number of rejected parts by increasing the tolerance zone.

Related Readings

GD&T Tutorial – Learn How to Identify the Feature of Size Dimension:This GD&T tutorial will talk about the concept of feature, feature of size, non-feature of size and also about the application and identification of the geometric dimensioning and Tolerancing concept in engineering drawing.

GD&T Tutorial on Projected Tolerance:This geometric dimensioning and Tolerancing or GD&T tutorial will discuss about projected tolerance, its application and benefits for preparing engineering drawing.