Ideally, it needs a structure that resists torsion moments - this is where my current machine fails - the gantry is a length of 44x44 structural aluminium which is ok in the beam but poor in tension as the centre box is only 18mmx18mm.

The torch and axis are not balanced and when accelerating it twists one way and the opposite on decelerating - causing ripples in the cuts.

What is ideal is a simple, light structure that ca withstand torsion or twist as well as support 3-4kg in the centre.

What that is, I have no idea, most machines just use box section because it's there I guess.

It needs a thick enough wall section to take 5-6mm screws firmly so I guess 5-6mm thick.

Without getting a structural engineer on the job, what stock item would be best suited here?