Z axis error woes

[Swarfing]

If the rails are not parallel then all he is doing machining it parallel to the twist. The bed will still drop as it moves along with the twist. I think you will find i am right on this one assuming that is what his problem is? Place an object on the bed at one end and check with a height gauge. Move the bed along and reposition the height gauge on the object and you will see it is different. The gauge will need to be set up off the machine.

[Jonathan]

If the rails are not parallel then

If the rails are not parallel then it wouldn't be able to move, assuming you mean rails parallel to each other. If not, rails parallel to what?

[Swarfing]

It will cost nothing to check the bearing holders placed on a rail supported across two supports of a known height surely. The exercise here is to eliminate what it is not so that what ever is left must be the issue?

[JAZZCNC]

Ok first Jonathan is correct.! We are talking about the planes the rails are on rather than being parallel to each other.

Think about it more and you'll see that after surfacing the bed theres no way the bed plane can differ from the rails plane.!!

Would we agree If the rails are level and correct and on equal planes along there entire length then the bed will be surfaced level and the distance from spindle to bed will be the same any where on the table.? . . . Well the same apply's if the rails are not on equal planes after surfacing.! The cutter as machined the bed into the same plane as the rails so therefore the distance from spindle to bed must be the same. . . . .Cant be anything but.!!

That said I'm not saying it's ok and everythings ok having it like this, it's bad because you can't machine anything truely flat and parallel but in this case it's correct or I'm(WE) are right that the distance will be the same.