Different Alignments - why?

[[No...]]

Something I wanted to know for a long time:

Some time ago Zeiss put this in the release notes:
The text states that they originally had changed the base alignment calculation so that you'd get the same alignment in both cases.
But later they reversed the change, and now both cases again lead to different results.

But can somebody explain WHY the results are different? Why is there a difference if I only set the X and Z axes manually but select no primary and secondary datum features? The CAD screenshots are not very helpful as the orientation of the cylinder is not clear.

I'm using the second case a lot on the O-Inspect and never noticed any difference.

[[De...]]

Perhaps I am wrong, but it appears that in the first alignment, with a lack of a defined choice for a spatial or a planar rotation, the alignment is using the nominal space axis and rotation of the circle feature to define the orientation of the part.

In the second alignment, with a spatial and rotational axis chosen but no feature chosen, it appears it is using the machine Z and X axis respectively to define orientation of the part.

My guess is the intent is that given that it is a circle (a 2D feature) and not a cylinder (a 3D feature) they were going to remove the ability of a 2D feature to affect the spatial and or planar rotation of an alignment given that it is very difficult for the algorithm to determine the difference between an out of round condition or the circle being measured on an angle in relation to the cylinder which would produce an ellipse.

I assume the reason they changed it and then changed it back is due to some people relying on the original behavior and having it break when Zeiss decided to change it.

[[An...]]

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See attached.

Alignment_1.pdf

[[No...]]

That makes sense to me. So if I set the axes manually I'm telling Calypso to project the origin along them. Otherwise it's trying to extract the direction from the origin feature(s). Correct?
That explains why there was no difference on the O-Inspect. Because both directions were always the same.

[[De...]]

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Yes, I believe this is how it works.