I bought one of Haimer's 3D Taster early last year.
Now, the dial on mine is graduated in inches, but the instruction manual that comes with it describes the shank as a 20mm shank. It also describes the probe ball tip as 2mm diameter.
I bought a dedicated 20mm R-8 collet to use with the Taster, and marked the shank and the collet so I could get a little bit more repeatability and not have to calibrate the concentricity every time I used the taster.
Well, that didn't work so well, plus trying to hold the Taster in the collet, and tighten the drawbar with just 2 hands, always offered the possibility of dropping the Taster onto the milling table. 3 hands were needed, one for the spindle brake lever, one to hold the Taster and the collet in the spindle, and one more hand to turn the drawbar wrench.
Plus, I had noticed that the 20mm collet always got sucked way up into the spindle.
So, I bought, from an eBay seller in the UK, an R-8 end mill holder for a 20mm shank end mill, and 7/16-20 UNF threads. Impossible to find here in the US, and most of the true metric ones were something like an M10 (12?) thread for the drawbar. What with shipping and the cost of the end mill holder, it was like $86 USD.
It finally arrived in the mail today, so I spent a couple of hours trying to calibrate the Taster in its new dedicated holder. I could not get enough travel in 2 of the 4 centering screws to get the Taster concentric to the spindle rotation. I tested the 20mm end mill holder with an indicator and it was very true, the indicator didn't wiggle a bit.
So I put the Taster back into the end mill holder and finally twigged to the fact that the 20mm bore was a bit too big for the Taster shank. ?What the Hell????
So i miked the Taster shank. It was not 20mm, it was 0.75000". Jesus H. Christ on a crutch! I got a 3/4" end mill holder, and popped the Taster shank into it. Slipped in as perfect as could be. Mounted the endmill into the mill spindle, and then it took about 20 minutes to get the Taster concentric within less than 0.0001".
So I wasted 2 hours tonight trying to calibrate the Taster, $86 on a 20mm end mill holder, and about $30 last year on a dedicated 20mm R-8 collet for the Taster.
This also explains why the 20mm R-8 collet would suck up so deep into the mill spindle. Fortunately, I never felt the need to torque it really tight, since it's just a measuring instrument, not a cutter.
Lesson learned, don't trust anything you haven't measured and verified yourself.
Now it occurs to me, I better measure the diameter of that probe ball on the end. It may be a 1/4" for all I know.
Oh well, no damage done, and I learned a lesson without breaking anything or getting hurt, so it was a cheap but effective learning experience.