I’ve only just started getting the parts together for my own MPCNC, but I thought I’d add a suggestion for the 25mm tube.
I purchased through a local stainless steel supplier a 6m length of milled stainless tube. It is VERY accurate. The nominal OD is 25mm (+/- <0.3mm over the 6mtr length) is 316 grade (extremely wear resistant) and very reasonably priced ($70AUD for the 6mtr length). Wall thickness is 1.6mm which is plenty rigid, 316 stainless is Hard as f’k.
I’ve set up the outer frame (corner blocked, feet, squared everything) at lengths of 1200*800, and there is no need for supports. The frame is very rigid.
I’ll start a thread in the builds section when I have more time. I have some more printing to do before I can complete the mechanical assembly. I’ll put together a build log and post something after that.
Let me know if anyone is interested in the details of the tube. I can offer details of the supplier and the exact name and specs of the tube if anyone wants to order. I’m fairly sure the steel shop is a national chain here in Australia.