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Materials

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Math
2D Shaft Bonus Tolerance
GDT dimensioning of holes (and shafts) provides a powerful method for increasing inspection yield without trial and error fitting or binning. This method is what is known as bonus tolerancing. Figure 2.9 illustrates the possible size variation of the hole from the 2D Hole section. It does not, however, show the variation in position of the hole that is shown in Figure 2.10, for a nominal hole size.
Figure 2.9
 Figure 2.10
Combining the two types of variation, we get Figure 2.11, which shows the quite large envelope which results. If the hole becomes larger, why not widen the tolerance on the hole position since the mating part now has a larger target ? This is what bonus tolerancing provides.
Figure 2.11
Figure 2.12 is the same hole that we have been discussing, but with an added symbol in the feature control frame. The M with a circle around it stands for Maximum Material Condition (MMC). For a hole, this is the smallest possible size, and for a shaft, it is the largest. In other words, for mating holes and shafts, MMC is the tolerance condition where fit is most difficult
Figure 2.12
If the hole increases in size from MMC, leeway in the position of the hole becomes available. This addition to the positional tolerance zone diameter is defined as the amount that the hole diameter increases over MMC.

If no MMC symbol is present, the tolerance is assumed to apply at Regardless of Feature Size (RFS) and there is no bonus tolerance.

For a continuation of this discussion concerning virtual condition, please see 2D Bon Tol Example.

FREE Publications
Selecting the Right 3D Printer

Discover how to choose the right 3D printer for your needs and the key performance attributes to consider.

Mechanical Engineers Outlook

Guide for those interested in becoming a mechanical engineer. Includes qualifications, pay, and job duties.

Metal 3D Printing Design Guide

Direct Metal Laser Sintering (DMLS) 3D printing for parts with reduced cost and little waste.

CNC Machining Design Guide

Optimize your designs, reduce machining time, and lower your costs.

more publications