I wouldn't worry too much about the flatness of aluminum extrusions. They are pretty flat. In fact, I see many manufacturers mounting rails directly on to extrusions. I also imagine that the rails are flatter than the extrusions, and that when they are bolted toghether, the rails may slightly pull the extrusions flatter (though this may be wishfull thinking). Regardless, I'd use the extrusions as they are.
I guess it boils down to how precise you want your machine to be.
Carlo |