Qualification Accuracy

[[La...]]

Which reference sphere will qualify stylii with greater accuracy, 30mm that came with our Contura CMMs, or a 15mm we purchased from Zeiss? All are certified reference spheres.
Thank you to all who will chime in on this.

[[To...]]

I don't believe there is a difference, so I'm eager to hear what everyone else comes up with. I am willing to learn something new today.

[[Ca...]]

I'm not about qualification "accuracy", but I'd certainly consider the appropriateness of the sphere sizes. If given the option between 30 and 15mm sphere's to qualify an 8mm probe, I'd obviously choose the 30mm.

[[Gr...]]

Our practice here at Bronson & Bratton, is 5mm stylus and smaller we use a 8mm reference sphere anything larger than 5mm we use a 30mm reference sphere.
An interesting side bar here is how the Zeiss tech setup the reference spheres for our new CMM.
In May of this year we had a new Accura 2, Rds caa, XXT TL3 head delivered and set up to go along with the Contura G2, RDS, XXT TL1 we have.
He first set the environment to MM 8 decimal places and built the 30mm reference sphere using the "Ref. Sphere components".
Using this reference sphere he qualified a 5 mm stylus at A 0.0 B 0.0 then recorded the results.
Second step was to create a 8mm reference sphere using the geometry provided on the certs that came with it.
Now, using the 8mm reference sphere, he re-qualified the 5 mm stylus then recorded the new results.
From this point he kept modifying the sphere radius of the 8mm ref sphere in reference sphere management until the qualification results of the 5 mm stylus were identical whether we the 30mm or 8mm reference spheres.

[[Aa...]]

One thing to keep in mind is that when the Zeiss tech made adjustments to your machine for size results, it was using the Ref. Sphere that came with the machine. Essentially, they were calibrating the Ref. Sphere and the machine as a system. The actual calibrated size of the Ref. Sphere has an uncertainty that is ridiculously high for the accuracy of the machine. So long as that's the only sphere you use, and you're using it on that machine, that doesn't matter, since they were calibrated and adjusted as a system. But when you switch out Ref. Spheres, the size inaccuracy might be in the other direction, meaning adjustments made to the machine to account for that would be in the wrong direction. Just because you enter the calibrated size of the new Ref. Sphere in Calypso, that doesn't mean the machine will be adjusted correctly for the actual size.

I hope that makes sense and isn't too confusing.

[[Gr...]]

After the tech left I wanted to see if his adjustments would work across the board so to speak.
Took a 3.0x75.0 stylus and qualified it in the standard 1,2,3,4,5 positions using the 30mm ref sphere and recorded the R,S,X,Y,Z information.
I then re-qualified the same stylus using the 8mm reference sphere and recorded those results.
When all was said and done the discrepancies between the two sets of results were no more than .00003 in.

[[An...]]

I calibrate probes smaller than 2mm on an 8mm sphere, bigger than 2mm on a 25mm sphere.
I also believe that I have read this in a documentary by ZEISS!