Howdy fellas.
FWIW, adhesion can be an issue. Make sure there is no oil residue and I would roughen the surfaces before pouring. Better still would be to give the Epoxy Granite something to "bite" into. Since you're using mild steel weldments, you could just simply spot weld some pieces that will reach into the Epoxy Granite and give it something to grip onto.
You can make the material as thick as you like. Be careful on a lathe bed exactly how you set it up as the weight might warp the bed, which would be a hassle.
E/G is great fun to work with. Just be careful of the epoxy as it can cause an allergic reaction that gets worse each time. Gloves are a must, but working on a big project beware brushing up against it. You'll get a nasty belly rash that takes months to go away--DAMHIK!
Remember that while the material is somewhat strong/rigid (can be about 1/3 of cast iron), it is mostly there to dampen vibration. Make sure there's enough steel beef for the real strength. I would think your project should come out real nice.
I assume you'll use a VMC (you mention a Haas?) to machine the beds for your linear rails so they're properly aligned. I wonder if you might want to consider whether that material will move after it is machined? You might consult with a machinist on that. Many materials like that do move as stress is relieved. There may be some seasoning procedures you could do to minimize the movement.
Cheers,
BW
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