Parallelism, cylinders vs 3d line. And why does +Y direction impact results

[[Ch...]]

I got a good one for you guys.  

I have a coupler with 8 bores.  Each bore is checked back to the center thru bore for parallelism.  

I need to be able to fully articulate why I have one bore that is always reporting more parallelism deviation, but there are a few weird caveats:

1. it is always the bore facing +Y.  Even if I rotate the part, the larger deviation is always the whichever bore facing the +Y direction.
2. It only impacts bores if I'm using cylinders.  If I use 3d lines to construct the bores, then all bores report roughly the same, regardless of if it is facing +y.   The cylinder and 3d line are both constructed by recalling the same circles. Forum Question 4.22.25 v.0.pptxForum Question 4.22.25 v.0.pptx
3. Datum A, the center datum bore, is constructed using two circles recalled into a cylinder. 

I'm looking for responses that will explain why it is only the bore facing +Y, and would I only see this when using a cylinder.  The answer should also be able to explain why we don't see this when using a 3d line.  

Forum Question 4.22.25 v.0.pptx

Edited April 22

[[Ma...]]

How you are getting that 3d line?

I would think of some shaft touching a part in that area. Like tilted part - that +Y part would be lower than -Y part

[[Ch...]]

Its sitting on a ground and squared mag block.  roundness and visual indicate no probe shanking.  

[[Ma...]]

Then i would suspect evaluation settings on that cylinder.

If you are sure it's not shanking, then it should be something on cylinder. I would make copy of a good cylinder - modify nominals to have that problematic cylinder remeasured on new feature to clarify some shenanigans within Calypso.

Once i got to situation, where i played with recalled feature ( circle ) and after changing back to normal measurable feature i could not change that feature to cylinder/circle/... like on normal feature - saved/reloaded program/Calypso - studied inspection file - there i found something, which could not be repairable without knowledge of file structure.

[[Ch...]]

We checked eval settings for every feature and characteristic used and all are identical.  

[[Cl...]]

I would measure a ring gage with the same strategy at the same location where its good and where its bad. See if you can duplicate it outside of the program.

Eliminating the part and fixture as variables. 

[[Ch...]]

My thoughts so far, is that neither answer can explain why the 3d line and cylinder(recalled from the same circles) have such different results.  If shanking or measurement strategy was at fault, then we should see the 3d line and cylinder behave the same.  But that's not what we see. 

 

 

[[Ma...]]

There wouldn't be any effect with OT on cylinder? I would check that not just on feature but in characteristic as well - you can set different evaluation even at characteristic - this option is double edged sword

Edited April 22

[[Ch...]]

Have you tried moving the part in your CMM volume and see if the issue continues ?

Im might also suggest making a copy of the program aligning to a different axis such as X and see if this behavior continues, could be an area of the scales that is the issue.

Could also be a scanning issue if you are scanning as it is changing directions in that axis.

 

 

What is the approx depth of these bores? and what is the total 'actual' depth you are measuring ??

 

 

[[Ch...]]

problem solved.  We found the cylinder depth in the feature window was larger than the rest of the cylinders. So it was causing projection error and leading to artificially high values for that one cylinder.   All actual measurement circles were set to the same height, but the problem cylinder had the depth set wrong.

 

Chris was the closest to being correct.  

[[Cl...]]

Akums razor.