Alvin is right. You must key-in stylus dia. then check it using a standard. One other point is you must check to make sure the stylus is EXACTLY perpendicular to the part. Any variation from perpendicular will give you XY location error depending on where the stylus strikes the part (Z axis induced error.

Technically, you COULD use the OP sphere but it is a pain:
1) Save old setup then set CMM to MCS (machine coordinate system)
2) Using spherical stylus, OP as usual
3) Origin XYZ to last feature measured (which will be the OP ball)
4) Change to cylinder stylus and make sure it is PERPENDICULAR
5) Take a POINT on top of the OP Ball, origin Z, origin offset z= half OP ball dia (Z=-0.500 for a 1" OP ball)
6) Press OP button 4pts
7) Go to Z=.75mm (half your probe dia)
8) Take 3 equatorial hits at this height 120° apart
9) take last hit on top of ball exactly at X=0,Y=0
10) Recall old setup

A Pain Huh? Key in probe dia is easier....
www.qualityinspection.com

Pat McSwain