ok, first attempt using sketchup 8

[deannos]

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I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.

I can weld , it wouldn't win any contests though, i would like to keep the RSJ for the X axis

How long is the piece of RSJ you've drawn for the gantry?

It measures out at 1715mm, but could be slightly longer.