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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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    Datum B should be the larger flange surface only, NOT a combination of the two surfaces making up the 1.000 dimension.  In terms of what it constrains -

    • Datum A is a cylinder, constraining two rotations and two translations (think of this as levelling one axis and setting the origin of the other two axes).
    • Datum B is a plane which constrains the final translation (the origin along the axis that datum A was levelled to) .

    Because the Ø2.000 is co-axial to datum A, datum B has no influence over its position.

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  • 0

    Datum B should be the larger flange surface only, NOT a combination of the two surfaces making up the 1.000 dimension.  In terms of what it constrains -

    • Datum A is a cylinder, constraining two rotations and two translations (think of this as levelling one axis and setting the origin of the other two axes).
    • Datum B is a plane which constrains the final translation (the origin along the axis that datum A was levelled to) .

    Because the Ø2.000 is co-axial to datum A, datum B has no influence over its position.

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