Composite tolerancing a multi-feature hole pattern 1

[cm_eng]

I have a question regarding composite tolerancing that I cannot seem to find the answer to anywhere. I've come across this situation many times: I will have a part with multiple coaxial features, and I will have a pattern of those coaxial features. I attached a quick example below, where I have a pattern of thru holes with counter-bores. I understand that I can control the coaxiality of features using composite tolerancing. I can also control the tolerancing of a hole pattern using composite tolerancing. But it's entirely unclear to me how to do BOTH together. In the example I attached below: I want to control the coaxiality of the thru hole to the counter bore using composite tolerancing (it's critical that the each counter-bore is controlled by it's corresponding thru hole) , but then I ALSO want to control the pattern of thru-holes using composite tolerancing (it's critical that the thru holes are controlled respective to each other). I have no idea how to do this. The parts I'm designing are extreme precision and require very controlled tolerancing -- the counterbore/thru-hole is just an example to convey the problem. For example, in the attached image, how is my composite tolerance even interpreted? Is it defining the coaxiality of the counter bore to the thru hole? Is it defining the tolerance zone for the pattern of thru holes? Is it doing both? Neither? I would love some help on this problem. Cheers!

 

[chez311]

cm_eng,

You didn't mention which standard you are adhering to. If it is Y14.5-2009, there is the below figure *7-24 and accompanying section 7.4.2(a) for a pattern of holes/counterbores. My interpretation would be that the addition of extra frames would just be an extension of this - the tolerance zones for each frame of both the counterbore and hole would be coaxial and would move together.

*Added accompanying textual reference

 

[tim_member]

cm_eng,

If it's critical that each counterbore is controlled by its corresponding thru hole, then I don't think the approach you are trying to take is the best one, especially if the part requires extreme precision.

I would recommend to control each counterbore relative to the corresponding thru hole individually, as shown in Fig. 5-39 in Y14.5M-1994 or Fig. 7-26 in Y14.5-2009.

The thru holes, as a group, could still be controlled with a composite position tolerance, if required.

 

[cm_eng]

Tim, this is precisely what I was looking for. Thank you!

Cheers!