Add a Note to Expand a Pattern for Composite Tolerancing (#223)
(In accordance with Y14.5M-1994 standard)
PDF is Available with GD&T Reference Center Subscription.
Some folks think that composite tolerancing cannot be applied to a pattern of holes that have different sizes. When they have a group of holes of different sizes to control as a pattern it is not uncommon to locate one hole, make it a datum feature and control the other holes relative to that new datum feature. In this case, to guarantee a close fit, a refinement of perpendicularity is also required. Technically, this approach is OK. However, it does require a new set-up to inspect the part as well as the additional perpendicularity tolerance. The first drawing shows this approach.
Another approach is shown on the second drawing. Here all three holes are made a single pattern by the note, 3 HOLES. This approach gives the desired fit in the assembly while eliminating a set-up to establish an auxiliary datum reference frame and an additional inspection of the perpendicularity refinement on the larger hole.
