To be honest I have no idea but 6mm steel sounds very thin - you can find beam deflection calls online easily enough, but I have some 5mm bar supporting a grow light (Tomatoes!) and that bounces like no-ones business; compare bar vs a 60mm box section 5mm thick and you'll see a hell of a difference. Perhaps the problem is that without reference to the machine construction its hard to visualise what you're trying to achieve - a couple of annotated photos could help.