See what I mean Irving's much better than me. I can see it but I'm buggered if I can explain it :whistling:

With regard to cutting forces look here (ignore the spindle stuff)

http://rogercortesi.com/portf/spindle/spindle.html

Scroll down to the Niagara site as well and I'm sure there is a section on wood too, only just found it so haven't had a Reilly good look but there is plenty on metals

Going back to your machine have you looked at the 80 x 80 box stuff. over the long distance you need torsional stiffness, and as Irving pointed out lightness. (after all you have to move it) the box offers the best resistance to torsion, has equal strength in both axis and is light as there is no/little material in the center.

With the composite beam you have a lot of material (weight) close to the neutral axis (no stress) which again going back to parallel axis theory you want all the material at the extremes, like a tube or next best thing a box.

hope this helps