ok, first attempt using sketchup 8

[JAZZCNC]

Dean if your intention is still the same has before to only cut wood then ignore Jonathan he's talking over complicated bollocks again and it will easily do the job.

If you can get the Large box section for a tenner then obviously it's a no brainier and go for it.! . . Not disputing Box section won't be stronger but no where near has cheap or easy unless you can drop on something like this.

The RSJ will do the Job no problem the extra mass will help with cutting but does cost in acceleration and speeds.

I would how ever change the way you have the X axis bearings and use wider plate than the RSJ which bolts to it. I would then have gantry end plates which bolt to the RSJ and the bearings plates dropping down so the ballscrews can connect. This will help stiffen things up a bit.!! . . . Pretty much like I drew before.!