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How is Datum B constraining the Position tolerance?

Jegley

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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  • 0

    Make -a- datum cylinder axis as long as humanly possible.   

    This drawing is terrible. 

    1: -A- datum is ambiguous as there's five interrupted cylinders.  Is the datum just the middle one, or all five features of size there?

    2: -B- datum cannot be a "width" between the same vectors like that.  I agree with Neil, that -B- datum should be defined and tied to the bottom extension line of that 1.00" dimension, clarifying the -B- datum as the one planar flange.

    The 1" protrusion/boss to the hub flange has no significance to B datum.  The perpendicularity should be only tied to that -B- datum flange face.

    If you are getting parts that are out of spec, using A to B, reverse the datums... making B primary and A secondary, and see how the true position comes out.  give that info to dumb engineer.

  • 0 in reply to louisd

    Hi Louis, the datum A is just the cylinder with the datum attached to it, not the rest of the interrupted cylinders. The very intelligent engineer did update the drawing now but I still don't think its right. Attached is how he drew it. When I try to add datum B pc-dmis won't let me. Also not quite sure about the dash in between datums on the feature control frame, never seen that before.

    I just checked the datums separately, but I think I should tell him that I cant check it how he has it called out. What do you think Louis?

  • 0 in reply to Jegley

    Okay after some research looks like this is a "common datum axis" I am going to try to construct a line using the two cylinders on my cmm and use that as the datum

  • 0 in reply to Jegley

    The datum is A-B not A|B.

    You need to define Datum A and B individually, and then create a compound datum.

  • 0 in reply to NinjaBadger

    beautiful. Thankyou. These parts were out before but now they are wayyy inside the tolerance. Thanks be to god.

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