position tolerance zone

[[Ri...]]

When I am measuring my parts and have a "true position" call out, I have several options for the shape of zone, I get significantly different results depending on which one I use. I guess my question is, when do you use diametral or rectangle for the shape of tolerance zone? I always assumed true position gave the actual position of a feature in relation to the called out datums? What am I missing that there are different types of true position measurements?

[[Ja...]]

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Your callout will have a zone modifier, or lack thereof, to explain the zone configuration. If it shows Ø before the tolerance, it is a diametral tolerance zone, if it shows SØ, it is a spherical tolerance zone for which you would choose sphere. If there is no zone modifier, it is a planar tolerance zone (only X, only Y, only Z). I don't know about rectangular zone, I never need to use it.

The direction, or plane, of the zone is determined by the blueprint (datum structure) and your alignment. If you have a bore along Z+ and a datum shank along X+, the only direction that makes sense to measure position would be along Y. Think of the geometry of the situation at hand to determine which zone to choose.