Using hole pattern as orienting datum feature

[MattBD]

I haven't been a member here long, so please excuse the efflux of questions. I am trying to get my boss to send our team to a GD&T course, but that hasn't happened yet.

Anyway, I have a cylindrical disc that is mounting to the face of another part. Similarly, there are parts that will rest on this fixture that are kind of swept triangle-shaped.

I want to control the orientation of the cutouts relative to the holes for locating pins (only two pins will be used, but this part may be rotated for other applications, so there are four holes.

I have the flat bottom of the part as datum A (with a flatness callout). It makes the most sense to me to use the axis of the circular pattern created by the four holes as datum B. I feel like I should be able to create a center plane between two diagonally opposed holes and call that datum C. The four holes would be positioned to A and B, and the remaining countersunk holes and cutouts would be positioned to A, B, and C (since orientation relative to the four center holes matters).

If I can't create datum plane C in that manner, can someone suggest another method? Also, what should the profile of the cutouts be relative to? The cutouts would already be positioned to A, B, and C - I am guessing that the profile would be the same way.

http://i5.tinypic.com/42jf1q0.jpg

Thanks for being a sounding board in my quest to get things right the first time.

-Matt

 

[ewh]

You don't need to define a plane between two opposing holes - just label one hole as datum C. That will take care of any subsequent clocking.
If you use a profile tolerance on the slots, you don't need to control them with true position.

 

[CheckerRon]

Matt: EWH has given you good advice regarding using one hole for a clocking datum C, and controlling the triangular feature shapes by a profile tolerance. All the triangular feature dimensions and their locating dimensions will be basic, and the profile tolerance box to be attached to the feature by leader and have an "all around" circle at the elbow of the leader. Regarding datum B, the four hole pattern itself is a good Datum B, in accordance with para 4.5.8 and Figure 4-22 of ASME Y14.5M-1994.