A brand new machine build. 2x4 size or something in that region. :)

[Jonathan]

When the machine is cutting parallel to X (with Y being the gantry axis) the cutting force applies a moment to the gantry, which will twist it, hence the gantry must be a structure with good torsional stiffness (in the XZ plane). The torsional stiffness of a beam is only good if the section is closed, so for a C section it is very poor. The same is true for an I-beam or plate, so I suggest changing it for RHS or multiple pieces of box section.

Torsion Equations

[JAZZCNC]

When the machine is cutting parallel to X (with Y being the gantry axis) the cutting force applies a moment to the gantry, which will twist it, hence the gantry must be a structure with good torsional stiffness (in the XZ plane). The torsional stiffness of a beam is only good if the section is closed, so for a C section it is very poor. The same is true for an I-beam or plate, so I suggest changing it for RHS or multiple pieces of box section.

Been here before haven't we Boy wonder.!! . . . . It depends on the Job it's going to do.?

If it's mainly wood or plastics then it will be perfectly fine.

If it's for meaningful cutting of Aluminium and steel then Yes it's much less than ideal, Has is a Gantry style machine really.
So with that I'd suspect it's main use will be softer materials in which case then it will be perfectly fine. RHS would also be unsuitable for Good Aluminium/Steel cutting machine so both are unsuitable in this case.

Another thing with UNP beam is that it doesn't take much to minimise any twist and few bracing plates will sort it.! The advantages with ease of build for a wood cutting machine are obvious and I know for a fact that it works on a Wood router with no detriment what so ever because I built my mates Panel router using one.! This worked 10hrs day 6 days week and did so for 2yrs producing 1000's of panels until recently when the building caught fire.!. . . MK2 will be bigger and better but will still probably use UNP beam.