True Position of Hole Pattern on 3D Curved Surface

[bucknast]

Hey all,

So I have a program for a panel cover, and the customer supplied drawing calls out hole pattern, with true position to A/B/C, and A/B/C are established from datum targets derived from the points at the intersection of the axis of some of the holes and the nominal surface of the part. This callout is confusing the hell out of myself and my quality guys. They're able to spit out some numbers from a faro arm, but I don't trust them.

I attached a watered down version of what I'm working with.

What's a better way to control the position of these holes on a drawing?

The part needs to mount to a frame underneath with an equally tight hole pattern (supposedly) and sit inside a pocket, so there's a surface profile requirement around the perimeter of the part of say .040".

Thanks!

 

[3DDave]

The common orienting and locating surface is the mutual mating surface, so that should be [A]. This gives both parts the same frame of reference. The holes can be located from that frame of reference so their differences in their measured locations can be compared in a useful manner.

Note that the axes of the holes are nearly or exactly parallel to the surface normals, so they don't really intersect.

Parts like this are often given multiple target points (4 would look like a good number) with X, Y, Z coordinates and a note to restrain the component to a fixture against those points, with some equalizing points to align the item and a couple more equalizing points to finalize the location.

For inspiration, look at the tooling that was used to place the holes in the mating frame.

 

[bucknast]

How would you call this out on the drawing?

 

[3DDave]

Look at ASME Y14.5-2009, figure 4-28, Contoured Surface as a Datum Feature.