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I have access to a rather large plasma table. I was thinking of using it to cut out a cnc from a single sheet of 1/2 hot rolled steel and use SBR20 linear rail kit from ebay. I already have a pretty nice spindle for my MPCNC that I'll be using.
I put together a quick sketch of my idea what do y'all think? I'm worried that z movement will be roughly 23" and I'll loose 7" to the spindle mount. However this is just a quick idea I had any advice or input is welcome.
Hot rolled steel isn't very flat to start with. Cutting a slab of it into pieces with a plasma cutter isn't going to help with that. But any surfaces you intend to mount rails to need to be extremely flat. Are you taking the rough assembly out for heat-treating and then having someone machine it flat? Or did you have some other plan that would make this work?
I didn't think of the surface finish on it, I guess I'll look into other materiel.
The surface finish isn't the main problem, it's the basic flatness of the material. If you put a straightedge on top of a piece of it, you'll see daylight in places where the metal dips down a little. That's enough to keep your rails from working correctly.
Those round SBR20 are not a good choice, as they are not very rigid. Profile linear rails like THK or Hiwin are a much better choice.