As we saw in the 2D section, 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. The following figures are from the 2D section, but now have a third dimension out of the page. Since the tolerance zones and MMC envelopes form perfect right circular cylinders, we can use projections of the 2D figures. 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.
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 2.12 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.
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. Click here for a step-by-step example of how bonus tolerancing works. The 3D case is the same except that all of the 2D figures are projected out of the page.
As in the 2D case, if no MMC symbol is present, the tolerance is assumed to apply at Regardless of Feature Size (RFS) and there is no bonus tolerance.
A 3D shaft is dimensioned and toleranced in the same way as a 3D hole. For a discussion of the mating of a hole with a shaft, please see 3D Virtual Condition.