Proper Callout for Hole Centered on Internal Bore?

[mesa5]

I have a part that has an internal bore with a small hole at the bottom of the bore. I want the hole at the bottom to be geometrically concentric with the diameter of the bore, because the mating part locates off the bore and slides into the smaller hole. If the hole is too far off of the centerline, the mating part won't fit correctly.

What is the correct GD&T callout to locate the small hole? Common sense says concentricity, but everything I've read says to not confuse GD&T concentricity with actual geometric concentricity (or co-axialarity) and to avoid it like the plague. Runout doesn't seem correct because I (within reason) don't care about the shape of the bore, just the centerline location. Should the hole be true position relative to a datum set by the bore?

 

[pylfrm]

mesa5,

Is the mating part intended to have a clearance fit with both the larger bore and the smaller hole? If so, then you should probably be concerned with the actual surfaces of the features, not imaginary centerlines.

One way to accomplish this is to apply a position tolerance at MMC to the small hole, referencing the larger bore as primary (and only) datum feature at MMB. Both features would have diameter tolerances set based on the fit requirements and the tolerances of the mating part.


pylfrm

 

[mesa5]

There's a tight clearance fit between the bore and the mating part, and the hole is basically a viewing port for an optical component. So the mating part in the bore is well centered, but if the hole is off axis, it will interfere optically.

 

[3DDave]

Concentricity controls the median points (midpoints) of each section through the target feature to be within a certain distance of the datum feature axis. It would apply here. The reason to avoid it is that there is no simple gage to check it, so it's harder to check.

An alternate would be profile tolerance since you are concerned with a fixed boundary to avoid eclipsing the view port.

 

[Belanger]

Go with position. Profile is probably too much, because you said you don't care about form.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems