# What GD&T methodology should I use to specify alignment of two holes perpendicular to a shaft?

A round shaft 120mm long has two holes; one 10 mm from the end, the second 80 mm from the end. Each hole is perpendicular to the shaft's length. And here's the problem: the center line of both holes must have 0° of angular alignment between the two. Evidently, I need to make this clearer as our vendor has provided parts with up to 5° of angular misalignment.

I've two thoughts one is to use the shaft's center-line in the upper view as a datum and then specify perpendicularity for each hole. The other thought is to make one of the holes diameter a datum and relate the other hole by parallelism. Neither of these really seem correct though.

• In theory you're describing parallelism. Can show it with section views. Using one of the holes as a datum, has the advantage that it translates more directly into an inspection procedure. For example, you could construct a go/no-go fixture with a dowel pin pressed into the fixture and and a slot, place the part onto the permanent pin, then check if a second pin can pass thru the other hole in the part and fall into the slot. It's effectively a test of projected tolerance. Using a slot keeps variation in one plane out of the picture. This kind of thing sometimes helps vendors. Commented Jan 5, 2022 at 19:08
• Thanks for the input, Pete. I think I'll go ahead with parallelism; defining the shaft as datum A; the first hole as datum B, related to A by perpendicularity; then the second hole will be specified parallel to datum B.
– Mick
Commented Jan 5, 2022 at 19:26
• I would show a radial view and the angle you need. Commented Jan 5, 2022 at 21:30