primary datum as a sphere
 

primary datum as a sphere?
imagine the a tie rod end.
the sphere component has a shaft protrusion.
for functionality in the assembly, the sphere is the primary datum.
if i called out the diameter of the sphere as the datum would be it be a center point? or a set of perpendicular planes?
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i have a sketch showing what i want.
not sure how i post it.
perhaps you could use the protrusion as a datum feature, a, and the dia of the sphere as b. then you could make ref to axis a-b.
i could do that... but in the application the sphere is clamped and completely constrained. except for rotation.
specifying a sphere as a datum feature of size gives you a point in the center (vs the axis of a cylindrical feature of size), which has three mutually perpendicular planes centered at that point (there are two mutually perpendicular planes associated with the axis of a cylindrical feature of size). the planes are the datum planes from which measurements are taken.
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but this center point of the sphere defines how the remaining geometry is created.
i understand it is not a plane, but it is the most important location for assembly reasons.
how can i show an image in a post