Well, it's never too late to change one's mind. Found a very good deal on a pair of supported rails and some ballscrews. Therefore I redesigned my X-axis with the new linear bearings and all axis with b-screws.
Y- and Z-axis retain the ground axle with closed linear bearings.
Still waiting for the major components.
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I've seen rails mounted this way. Are there any problems with mounting them vertically?

Cheers
Patrik