Hi, i considered that, but then thought to get round that, i would m,achine the underside and fit the second rail, after the machine is part built, fit the Y/Z axis and use that to machine the underside of the x Bar. It would be perfectly level to the top.

or
First make a level on the floor, then, Make a level the top of the x, with epoxy, spin the full machine over, level the bottom with epoxy.

Although, i would be more confident in option 1. because as long as your top surface is accurate, so will your bottom rail.

If i can do it this way, i can design a substantially better Y/Z Lighter and stronger. I know i will fubar bearings faster, but it would be an asceptable costs

With long braces on the guides, you would be able to minimise any undulation anyhow, there has got to be something else wrong, just can not see what at the moment.

Any suggestions anyone ?