I am using a TIGO TP20 setup (PCDMIS 2017 R2) and I am measuring a small cylindrical part (somehow resembling a very short tube). The outer Diameter being 9mm and inner 6 mm. I have it placed vertically on my fixture and trying to true position control the inner circle as to the outer. The problem is that the face I am having the part on Z+ has no rotational datum and as such I only level on the top plane and probing from Z+ the 2 circles. There is no room (part being 4 mm short) for cylinders.
In any case, this is a simple TP callout of 2 circles. The issue is that while without moving the part on the fixture I am getting repeatable results at a few microns in 2 axis resulting in a TP of less than 0.01mm, when I turn the part 180 degs I am getting 0.03 shift in Y axis. Not only that, but rotating the part continuously by 180 Degs, I am getting consistent repeatable results over and over with the same offset...
The circle even being small I am probing as least squares, 360degs full circles and 23 and 17 hits accordingly in order to have a better approach.
Nevertheless, the distance between the 2 circles should always be the same no matter the orientation / rotation which is left unconstrained (actually was constrained externally with a plane of the fixture). The call out is only requesting TP from Datum A (outer dia).
Maybe it is the leveling plane curling (flatness was 0.015-0.020 mm) due to the fixture grip (not really stressing it much tbh), but still... why have consistency between placements?
For me, this is the first time seeing this happening and I am thinking it has to do with the rotation that's left "unconstrained" and this should not be happening imo. Any ideas maybe?
I would not dismiss the distortion as being not relevant all factors need to be taken into consideration and the effect they can have on the centre point.
The tri-lobe error in the tp20 style trigger probe is just a consequence of the physical trigger seating on 3 locators, the only way to minimise is to keep the styli length as short as possible. the error can be verified by measuring a certified roundness reference standard.
I am not dismissing it, I am just convinced it is not due to the fixture. In fact I ended up blaming the distortion of the part in free state that can cause a circle projection on a bended plane to have 10 microns of shift.
The graphical analysis (form) should follow with your rotation of your part. If it stays in the same position but you have rotated the part, then the error does not come from the part, but the machine.