The CNC is primarily steel box section,welded AND bolted! There are some new thoughts on the design, assembly and set up. I've not followed the classic route for a gantry CNC, I believe that there are major improvements that can be made to the 'normal' design which improve strength and accuracy both on the Y and Z axis. The X axis is probably the easiest one to strengthen.
On the Y and Z axis, the ball screws sit between the profiled rails, with the trucks outside the rails of the adjacent axis.

The Z axis is 20 mm ECO plate aluminium, built into a small but very strong arrangement. We had a large 4x4 parked on it at one point without any effect.