I am looking in to building a back-knife lathe CNC style like the one in this thread

My question is about beam strength, From what I can see he is using an aluminum tube. I am familiar with the lathe, but I can't see any of his support structure, so I don't know sizes or anything.

I don't have much experience with different beam types. so I'm not sure what's best. In my system I would have a 48" span. Should I use an aluminum tube?, 80/20?, what size?

For a lathe like this once the shape is roughed, the pressure is pretty consistent, while roughing it's knid of an impact. the force direction varies from downward to backward depending on the radius of what being turned.

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