I have a bore that is Datum B. It has a perpendicularity callout to Datum A, which is a flat surface. So, I have to measure it as a cylinder. I have another bore that has a circular runout callout to AB. So do I have to measure B again as a circle? Because I would have to assign that as another datum to build the feature control frame. But if I just use B as a cylinder, then wouldn't it be measuring total runout and give me a different result than circular runout?
Here is what the ASME Standard says about Datum Features for Runout Tolerances (9.3.1):
"The datum axis for a runout tolerance may be established by a cylindrical datum feature of sufficient length, two or more cylindrical datum features having sufficient axial separation, or a cylindrical datum feature and a face at right angles to it. Features used as datum features for establishing axes should be functional, such as mounting features that establish an axis of rotation."
So, using a 2D circle as a datum reference would not be allowed.
Here is what the ASME Standard says about Datum Features for Runout Tolerances (9.3.1):
"The datum axis for a runout tolerance may be established by a cylindrical datum feature of sufficient length, two or more cylindrical datum features having sufficient axial separation, or a cylindrical datum feature and a face at right angles to it. Features used as datum features for establishing axes should be functional, such as mounting features that establish an axis of rotation."
So, using a 2D circle as a datum reference would not be allowed.
