Sorry - I just haven't done anything with aluminium apart from one very small job where the problems were more to do with using a small cutter and getting clogged cutter flutes than machine loading. Most of my work to date has been with ply/mdf/hardwood/plastic - that's just what happens to have come along!

I want to do a bit of work on steel, but it might just be limited to spotting hole centres accurately to finish on a drilling machine. One day I shall have time to experiment with these things!

One thing to bear in mind, perhaps, is that because the tubes you are using are fairly chunky, there is quite a lot of stiffening in those welded joints without explicit diagonal braces. My own experience (again, anecdotal not rigorous!) is that most of the "trying to shake itself apart" loads are trying to move the whole bed plane fore and aft, needing the side frame triangulation. There are much smaller "lozenging" loads in the plane of the bed. I doubt if you need quite so much bracing across the bed, therefore. And it's difficult enough to get the bed support components flat as it is.