ASME Y14.5 2009 Section 7.53 Coaxial Pattern of Features of Size Question

[Zach Lankton]

Quick Question Regarding ASME Y14.5 2009 Section 7.53 Coaxial Pattern of Features of Size Question
Ref Fig 7-49 & Fig 7-51

Attached is a photo of the drawing section that I am looking for opinions on.

There are 4 different size holes that are coaxial.

Section 7.5.3.3 Excerpt:
"...Where holes are of different specified sizes and the same requirements apply to all holes, a single feature control symbol, supplemented by a notation such as TWO COAXIAL HOLES is used. See Fig. 7-51. The same tolerance zone relationships apply as for Fig. 7-49."

This is well covered by Sec 7.5.3.3 & Figure 7-51 in the standard where requirements are the same for each hole.

However, in my case the requirement for one of the holes is different.

The engineer chose to use 4 composite FCF's. The intent seems clear to me that the lower segment should control coaxiality of all 4 holes, with one hole needing to be held to a tighter tolerance zone.

Figure 7-49 shows a single composite tolerance zone. My interpretation has been that the axis of the tolerance zone is established the same but the tolerance zone at this hole location is smaller.

So my question is...
Does the current drawing make this intention clear or am I totally wrong? If so, is there a better way to annotate this intention?

Thanks in advance!
Zach

 

[chez311]

Evan,

I had your "translating zone" concept in the back of my mind (from the previous discussion on composite tolerance/custom DRF) I just didn't make the direct connection to this situation until you mentioned it, I was following that same line of thought without realizing it. The more examples I run into with composite tolerance the more that concept seems to stand up to scrutiny and line up with the standard - other interpretations may be viable but it seems to me they require additional concessions or extensions of concept past what is contained in the standard.

3DDave,

By Voelcker extensions I assume you mean the DOF constraints specified in a custom DRF correct? I do recall that Herb Voelcker championed this concept, I haven't heard anyone call them that though - interesting. I would tend to agree that the case of different size features/tolerance zones coupled with orientation requirements to one or more datums is problematic. I'm interested to see what Evan has to say on the topic - I know when I discussed it last it was mentioned that custom DRF might not be able to exactly replicate a composite tolerance, which seems to act more like an orientation tolerance. I wracked my brain - I think trying to handle this special case with either composite or multiple single segment is troublesome.