Originally Posted by
routalot
You might be able to find a machinist's precision level or you could seek a surface plate on which to adjust the rails to a common height.For a router,either would probably suffice,but for a mill intended to do very precise work it might not be enough.There have been machines which have been calibrated by laser to establish the amount of sagging that occurs as the turret moves across the gantry and on which there is a layer of compensation software that applies a correction factor.Not really a hobbyist's option though.