Cylinder probe calibration issues

The clue here is "calibrating ABOVE the equator". What you are seeing is a difference between the RADIUS ground on the end of the stylus versus the diameter of the cylinder. I have three cylinder probes and all have LARGER diameters ground into the RADIUS than that of the cylinder. I was able to confirm this on an optical comparator at 50x. In all three cases the diameter of the radius is about 30 to 50 microns larger than the diameter of the cylinder. In all likelyhood... the center of the radius won't exactly align with the center of the cylinder. I haven't checked it on mine. I only use them on a manual Cordax 1805 with Measuremax. I NEVER use them on our FX7.10.7 under PC-DMIS. The results aren't accurate.

If you are only interested in Z readings, why are you using a cylinder stylus?

N8VES , Regardless of whether the diameter is larger near the end, the underlying problem is as I mentioned above.  When PC-DMIS performs a probe calibration, it is the hot-spot of the probe stylus that is instructed to take points.  For this particular stylus definition, that hot-spot is 1.5mm back from the flat bottom (imagine a sphere that is the same diameter as the cylinder with the bottom of that sphere sitting flush with the flat bottom of the cylinder).  The only way for it to behave in the way Rusty91307 is describing is for the length to be incorrectly defined.

ITEM:TIP3BY13MM_CYL_PART_268 M2
EffectiveWorkingLength 13.0
begintip
ribcount 10
color 80 80 80
cylinder 0 0 0 0 0 -2.5 3
cone 0 0 -2.5 3 0 0 -5.5 2.0
color 80 80 80
cylinder 0 0 -5.5 0 0 -7.5 2.0
cone 0 0 -7.5 2.0 0 0 -9.0 3.0
cylinder 0 0 -9.0 0 0 -12.8 3.0
cone 0 0 -12.8 3.0 0 0 -13 2.8   <-- Why is there a cone here? 
sphere 0 0 -11.5 3.0
hotspot 0 0 -11.5 0 0 1 3.0 3.0 shank
endtip

No Cone?
ITEM:TIP0.5BY15.25MMSHNK M2
begintip
ribcount 10
color 142 142 142
cylinder 0 0 0 0 0 -2.5 3
cone 0 0 -2.5 3 0 0 -7.5 1
cylinder 0 0 -7.5 0 0 -10 1
color 255 0 0
cylinder 0 0 -10 0 0 -15.25 0.5
sphere 0 0 -15.25 0.5
hotspot 0 0 -15.25 0 0 1 0.5 5.25 shank
endtip

Because there is a 0.2mm chamfer at the end of the stylus...

As I said before, the contact points when taking hits with this probe should be the hot spot.  You can only probe straight down (along the axis of the stylus - square to the flat) or radially (90° to the cylinder axis).

...Yup totally missed it went from 3-2-2-3  
I have only every used cylinder probes with a full radius on the tip though.

I checked the shop, there were 2 kinds of cylinder stylus: One with round end ( like a half sphere )and the other not

I also have only used radius tipped probes. That does make it a lot clearer, Neil, Thanks. I'll have to play with mine and see if I can get it to make contact in the hot spot zone. Will be interesting to see the results. I had a cylinder probe similar to the one you have pictured. Ruby instead of carbide. Originally it had a radius ball glued to the end of it and it fell off. If I can find it, I'll try to calibrate it in PC-DMIS.

pretty sure Neil has nailed the issue with a very good explanation. I didn't realize he was using a flat faced cylinder probe! thanks for the in-depth explanation! 

pretty sure Neil has nailed the issue with a very good explanation. I didn't realize he was using a flat faced cylinder probe! thanks for the in-depth explanation!