Question on GD&T added to print - Positional callout for the small holes

Emmyd

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#1
Hi all! We have received a print that GD&T callouts were added to and I'm not sure I agree/understand some of the callouts. In the drawing below - is the B datum just the one hole? And how would you measure the 0.2 positional tolerance back to itself? If the B datum is only the one hole, there is nothing controlling the rotation around B and if B is all the holes, how do you calculate the bonus? For these holes, in the past they are only used to move volumes of air through a cylinder and if they have positional tolerances at all on them the tolerances are large enough to not even have to measure. The way these holes are called out is that there are hard tooled pins/studs that must go through each hole. I've asked for assembly information before I hit back at the engineers.

Can anyone please help? I normally don't have problems understanding GD&T callouts, but this print has got me going crazy. I also have a problem with the positional callout for the large diameter. This would normally be a profile callout in my mind.

Thanks for any help!
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outdoorsNW

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#2
In cases of multiple holes, I have always seen the datum hole(s) called out independently of the rest of the holes. I read this as designating all 107 holes as the datum.

Additionally, while I have not worked with stacked position callouts, I also think the reference to datum B is a circular reference.

You need to go back to the designer and get the print fixed.
 

outdoorsNW

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#3
So I just looked up the composite control frame rules, the 107x callout is a composite position callout where the second row is a refinement of the top row. I am not going to go into all the details because I would likely do a poor job of explaining them, but the callout is definitely a circular callout with the datum B definition referencing datum B.

I suspect the intent is the one hole by the arrow is Datum B and the other 106 are following the callout, but you need to go back to the designer to be sure.
 

Emmyd

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#4
OutdoorsNW - thanks for the feedback!

That's what I was thinking as well. I'm trying to get some clarification on the function of this part in the assembly and will definitely be having a discussion with the designer.
If you interpret the print as one hole is the B datum, then I think there needs to be a tertiary datum C at least in one other hole opposite. The part is free to rotate around the single B datum hole and would be impossible to repeatably or reliably gage the part. I also am going to question the material condition as specified in the FCF - the top line is at MMC, bottom line is at LMC. The standard doesn't really say that these conditions can be mixed like that.
 

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