Hybrid tolerancing

(continued from Dimensional tolerancing)

Hybrid tolerancing or Band-Aid tolerancing is a tolerancing method where the basic tolerancing is dimensional tolerancing. However, the designer has realized the shortcomings of dimensional tolerancing and has subsequently added some geometrical tolerances to fix the problem.

The problem with hybrid tolerancing is that the geometrical tolerances are added one at a time without considering the interaction between them and without laying them out in a consistent coordinate system.

Let us say that a designer has used dimensional tolerancing and has received shafts like the one in figure 5 on the dimensional tolerancing page.

He might then decide to use a parallelism tolerance like the one in figure 1 to ensure that the backside of the hub and the end face are parallel.

Figure 1: Shaft with parallelism tolerance

The green datum indicator identifies the backside of the hub as datum feature A. The red tolerance indicator requires the end face to be parallel to datum A within 0,2 mm. The meaning of the tolerance is shown in figure 2.

Figure 2: The meaning of the parallelism tolerance

The green datum A is a perfect plane that fits against datum feature A. The tolerance zone is limited by the two yellow planes that are parallel to datum A and separated by the tolerance value, 0,2 mm. The distance from the datum to the tolerance zone is not locked. This tolerance only controls the orientation, not the location.

Buth as it often happens with hybrid tolerancing, this tolerance does not really work as intended. As can be seen from figure 3, the allowable deviation of the toleranced end face relative to the axis of the shaft changes, depending on the deviation of datum feature A.

Figure 3: The problem with the parallelism tolerance

The cause of this problem is that the designer never thought about what the overall functional coordinate system for the shaft is, but instead added a geometrical tolerance to fix a particular shortcoming of the dimensional tolerancing. Presumably he will add a tolerance to keep the backside of the hub perpendicular to the shaft axis next.

The Band-Aid approach severly limits the advantages of using geometrical tolerancing. To get the full benefit, it is necessary to use systematic GPS tolerancing.

Henrik S. Nielsen