Strengthing a steel gantry.

[JAZZCNC]

It might help resonance but gluing a lump of wood inside would probably make more difference for the weight. Filling with an epoxy granite mix would be better if the steppers can take the extra weight. Or maybe epoxy granite poured around a wooden core. Or maybe just thicker steel :-)

Epoxy's Too expensive and messy.! . . . Can't beat Kiln dried Sand for dampening resonance, cheap as chips and removable if done correctly.

[routercnc]

Yes the stiffness does come from the outer surfaces, but by joining them together with diagonal or X-shaped bracing you would be passing the loads between different parts of the outer surfaces. This would be stiffer, especially torsionally.

Ideally you want to place the diagonals at 45degrees to the long axis, not 90degrees, but this is difficult in practice.

You could also make baffle plates to do the same thing (say 6mm thick), slightly smaller than the inner dimensions of the box section, with the corners chamfered to miss the rounded inner edge, then drop these in at intervals and secure around the edges with chemical metal.

The section needs to be reasonably large to enable this 'key-hole surgery' but I think with some long rods it could be done.

If you do this with the gantry on it's end with the lowest/first plate attached, you could then back-fill with sand up to a point, then drop in the next baffle and secure. Repeat until full.

Significant stiffness increase, extra damping, and no welding distortion.