Quote Originally Posted by Davek0974 View Post
Aluminium was looked at, and is lighter, but stiffness suffers as a result and I want it rigid this time ;)
Aluminium has the huge advantage that it is usually quite straight. Steel box section can be quite horrendously bent, as far as CNC building goes, but still well within tolerance. Even slightly bent can make it hard to align a rack. I agree with Jazz, belt for a plasma cutter.