Ah, I see what you mean now and it makes perfect sense. I don't have an abundance of metal working tools (yet) so making two perfectly parallel faces would have been a challenge, presumably that would normally be done by milling them parallel?

I considered the L shaped gantry design but I'm hoping to mount this machine vertically at some point and it seemed simpler to just use a piece of plate at the back and benefit from a better / simpler connection with the ball screw.

Right, on with the design...