MASSIVE 2.5M x 4M BED - GANTRY DESIGN HELP NEEDED!!

[Blackrat]

option 1 for me please :D, or even use a section that has enough wall thickness , remember this surface must be flat to have an accurate machine, if you weld a plate on, how will you flat it ?

and gearbox with rack and pinion

[D.C.]

Thompson have a ballscrew wizard here you might find useful:
http://www.thomsonlinear.com/website...ing_screws.php

If you don't mind a lot of welding, it might be worth considering a truss type design like pg 178 of this machine design document:
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf