Whydon't you put the horizontal plate of the X axis carriages under the gantry side plates. You will have a lot of unnecessary weight removed and by putting the ball nut support under the same plate you can shorten the ballscrew and the main frame (to the length of the rails). The gantry will slide above the ballscrew end blocks. It is also a good point to put the ballscrews closer to the working surface/plane.

I think you need a plate at the back of the gantry to crate a box. Even a thinner plate will increase rigidity.

Quote Originally Posted by danilius View Post
What I'm wondering is if there is an optimum distance between carriages for the gantry.
I don't think there is an optimum distance... The 20cm that you have in your project looks good to me. But I'm not an expert.