Blade micrometers can be calibrated with square or rectangular gage blocks when the blades are still flat and parallel. After much use, these blades will develop worn areas, particularly if you measure slots or grooves on cylindrical parts. In this case, use a pin gage to establish the correct readings on the good portion of the blade. The pin gage you use must be of a diameter which is smaller than the curvature of the worn area. Then take a measurement with the same pin gage in the worn area. (To find the worn areas, close the blades and hold them up to a light. You'll see the spots where the blades no longer meet.) The mathematical difference between the two readings will give you a correction factor which you add to any readings you now take in that worn area. Repeat this calibration often, because the correction factor will change with additional wear. When the damage is out of control, then you can return the blade mike to us for grinding and lapping. The blades will be returned to factory specs. If the blades chip or break, they can also be replaced.