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How is Datum B constraining the Position tolerance?

Hi guys. I have the attached print and they are saying the position tolerance is out. I am measuring on my CMM using a cylinder for Datum A and then for Datum B using a midplane constructed between planes on each of the faces at the top that make the 1.000 dimension. I decided to run it without Datum B and I get the exact same result, or with just one face instead of the midplane and get the exact same. Got me thinking, what is datum B even constraining for the position tolerance? I am thinking that datum A constrains the orientation and "concentricity" of the two cylinders. So datum B would be an extra orientation constraint? but since Datum A is first, the orientation of datum A would trump that of datum B making Datum B useless. I am probably wrong, still new to this any advice is welcome. Thank you

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  • I'm not the most well-versed in Position, and I feel like this is a question for your design engineer.  I feel like if he asked for position to A and B then B would be necessary.  

    To me, this looks like some sort of axle and hub assembly.  I can't tell from the print if this assembly is machined as a unit or if the hub is welded to the axle.  

    Either way, I interpret the callout to control location and orientation.  The B datum would be what holds the perpendicularity of the shaft to the hub...

    In my mind....but like I said... I'm not the most well-versed in position .

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