Big Epoxy Granite machine, Which frame is better ?

[m_c]

I can't help feeling that the thermal expansion of epoxy concrete, typically twice as much as steel, is going to tie that machine in knots. I'm happy to be educated otherwise.

This was something I failed to find any kind of definitive answer to.
The conclusion I drew, was it can't be that big a concern, given there are some very big EG machines with linear rails mounted to them, with no apparent allowance for expansion. So if it's not off any concern over a couple metres, I doubt I'd worry about it on your typical home build.

As for John's build, what I took from his internal structure, was it was more about keeping things in place during the pour/cast, rather providing much in the way of structural support. Remember in that photo, the outer steel plates are the actual mold and designed to be removed after curing, so they can be reused.