I'll try one more time.

First of all true position is applicable not only to circles. I'll try to give an example that resembles closer our situation- true position for a slot. If we have TP tolerance of 1.0 and the two ends of the actual axis of the slot are at 0.6 and -0.1 from the center line what do you report 0.7, 0.6 or 1.2? This is the analogy. Evaluating equal bilateral profile tolerance comes down to checking if the points are between +/-0.5, but if you need to compare it to the total tolerance band of 1.0 you will have to double the max deviation

For the profile tolerance you could plot the readings from the surface inspection on a single axis representing the +/-0.5 zone and draw circles through the zero point and look at their diameter. The biggest will represent the actual. I do not know why they've decided to use diameter for TP (radius is fine with me and we would've avoided all this talk), but diameter requires that you double the distance from nominal.

Lastly, TP and Surface Profile are under the category Location Tolerances according to Al Neumann (just another similarity I guess).

George K.

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