Qualifying on too large/small of a reference sphere

[[Jo...]]

In training they told me probes less than 1mm should qualify on an 8mm sphere, and larger probes on a 30mm sphere.  Does anyone have any wisdom as to why?  What exactly are the bad effects of qualifying on too large/small of a sphere? 

We are troubleshooting some wierd problems and realized we have been qualifying a 2.5mm probe on an 8mm reference sphere for a little while.  Sigma is always low.  Wondering how far I need to run this up the NCR tree..... 

[[Ma...]]

I think mainly due short probe shaft.

[[DW...]]

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 I would imagine qualifying 180° of a small stylus on a large sphere would lead to shaft shank out problems more times than not. 

[[Ch...]]

I agree with the posts above when using a small stylus (sub 1mm) on a large 30mm ref sphere. A small stylus on a large ref sphere runs the risk of shanking out if the shaft is stepped.  Example image below. If you can ensure this isn't occurring on your setup then I wouldn't think there should be any other limitations. I also don't think you should see any issues when using a large stylus (over 1mm) on a small ref sphere (8mm). Unless it is so large that it's contacting the shaft of the ref sphere itself when scanning from the side. Maybe others have experienced differently but I have used upwards of 5mm diameter styli on my 8mm sphere without issue at the 180/0 position. 

[[Ri...]]

Use a step stylus.

I use 20mm OAL

10mm L

and 5 ML

Qualify full 180° all the time on a 30mm Sphere on two Micuras.

[[Jo...]]

We *are* seeing significantly smaller sigma values when qualifying on the smaller sphere, but both values are well under an acceptable value for our measurements.  If this matters, I have not been able to see it metrologically yet.