Quote Originally Posted by Web Goblin View Post
80 x 80 x 6 box section can still distort.
Yep, anything will distort. It's a matter of how much and the effect it has.

Heavy frame = good, as long as you don't have to move it!

For a machine purely for woods it does seem a bit excessive, however you seem to want to use it for milling aluminium too in which case it pays to make it as strong as you can.

What sort of tolerances do you require as that it what determines if machining the rail surfaces perfectly parallel is necessary? You can compensate for it perfectly well in software.