I like it. The usual way mills are is to have the x slide inverted but that means that the distance between the rails (doves on a mill) is small, your design spreads it out giving support over a wider area. One problem with that might be that flex in the rails be amplified but I suspect for a machine this size it would be fine and would probably be offset by the other advantages. You could even have 4 rails if you think you can align them OK.
The only other thing I saw was the gap between the x,y stage and the upright, this would not seem to be needed, or is it to allow materials to hang over the edge of the bed a little?
Good luck with it!
Graham |