ok, first attempt using sketchup 8

[wilfy]

Once you include torsional stiffness in the calculation the single box section is much better than RSJ:



The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

Thank you for that, I thought that was what was said earlier, which is why I posted what I did, however I didn't realise rsj would be just as in accurate.

How do you join the 2 pieces of box out of interest, if its welding surely this can go just as wrong no???

[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.!