Quote Originally Posted by JAZZCNC View Post
No your wrong 2eOpoz the fact he's surfaced the bed means the distance from bed surface to the cutter tip is the same.
Agreed. The cutter when surfacing moves along the same plane as when cutting, therefore surfacing eliminates that error. It makes no difference if you move the cutter or the bed, the relative motion is the same. It could be that part of the gantry or bed has slipped after surfacing. It's strange that the error appears to be so large, 3.8mm in 100mm. Have you tried measuring over a much greater distance, say 100mm?

Unfortunately that doesn't really help HankMcSpank!