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.