Why probe calibration matters
A touch probe is only as accurate as its own calibration table, and every stylus ball has some amount of runout and deflection that shows up as a small but consistent measurement error. Skipping calibration means every part measurement, work offset, and tool setting the probe touches inherits that same hidden error, and it will not show up until a finished part fails inspection.
How the numbers are calculated
Probe error is the measured diameter of a known master ring or ball minus its actual certified diameter. Since a diameter measurement captures the probe touching both sides of the feature, only half of that error applies to any single-sided touch, so the calibration tip offset applied to the probe's radius correction is half the measured error.
Practical tips
Use a certified master with a diameter close to the features you will actually be measuring, since probe deflection is not perfectly linear across a wide size range. Recalibrate after a stylus change, a crash, or any time measurements start drifting from expected values, and keep the master itself clean and free of nicks, since a damaged master will calibrate the probe to match its own defect.