Symmetry plane true position

[[Ja...]]

So I have a plane for datum A (single point probing -- flatness is about .0015 if that matters) and a slot parallel to datum A. I have to TP callout for the slot. I probe slot top and bottom planes separately using single points (flatness of both slot planes is about .001 if that matters) and then recall them into symmetry plane.
In the TP characteristic, when selecting this symmetry plane, should I constrain it to datum -A- or not? I am using only one datum (datum -A- for the primary datum in characteristics)

Note: I have tried it both ways and gotten different results. Using constrain shows values close to what it shows in its feature window. Without constrain, it shows a different value which is similar to if I use symmetry construction instead of symmetry plane for TP characteristic.

Can someone explain which approach (constrain or no constrain /// and //// symmetry plane or symmetry construction) is better and why?

[[SH...]]

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If you don't mind, can you share your feature control frame?

[[Ja...]]

My FCF is [ TP | .006 (MMC) | -A-] . The callout is under slot width dimension. Datum -A- is top plane parallel to the slot. Slot center plane is at .185 distance from Datum -A-.

[[Ro...]]

I have a group of about a hundred parts in a family that are datum to a symmetry plane in 2 directions (X & Y). i didn't have any problems doing that.

FYI, that's also when i started using Start alignments on all my parts, 3 single points, because on those parts taking a manual alignment was taking over 6 minutes due to the abundance of planes on interrupted surfaces.

[[Ja...]]

Sorry but I still did not get what you meant and what it had to do with my question. Can anyone explain how do you build your TP characteristic for a slot with a hole as its datum. (My FCF is located under slot width dimension as [TP | .006 (MMC) | D] and datum D is a hole on the same plane as slot see picture)

[[Da...]]

Your original post only mentions datum A which checks the slot parallel. You are now adding datum D which is a cylinder and thus it checks the slot for perpendicularity. I am guessing that the position is called out to both, I would use a symmetry plane for the A reference and a plane along the back of the slot for the D reference.

[[Ja...]]

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So the slot and datum D are manufactured at different stages. That's why I initially mentioned top plane(datum A) to be used in the TP characteristic for me to check the slot during its production. But now I have a complete part so I can check it against the actual datum D (NOTE: symmetry plane is parallel to the axis of datum -D- (theoretically)) . And the results are throwing me off. So I am trying to understand how will someone approach setting up this TP characteristic. (FYI: If it matters, I recall feature points to get symmetry plane)