Z axis error woes

[Swarfing]

ok imagine you are on one of those lifts at the seaside that take you up and down the cliff. You have a gantry across it when it is at the top of the hill and you level the roof off. You decide to move it along the rails a bit, is you table at the same height then? in you case it is the same except that on side of the rails is square with the gantry and the other side is on it's way down the hill.

[JAZZCNC]

No your wrong 2eOpoz the fact he's surfaced the bed means the distance from bed surface to the cutter tip is the same.

Yes what you say would apply if the bed was perfectly flat and level but because he's surfaced the bed he's effectively machined it into twist.
If the rails are not parallel the bed will be in twist as well after surfacing but the distance from spindle to bed will always be the same.

If the rails are not parallel you'll never be able to machine truely flat material without putting it into twist when clamped to the bed but the distance from tool to material surface will always be the same after clamping..!!

[Jonathan]

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!