Quote Originally Posted by Hobgoblin View Post
High precision with zero backlash
What sort of accuracy are you expecting and how do you propose to get zero backlash, or is that meant to read 'near enough zero'?

How wide is the machine, about 600mm between the bearings? It appears from your initial sketch that you are only using one ballscrew to move the gantry which I'm confident you will regret since you describe this as a CNC Mill, not router, implying you wish to use it to cut metals.

Quote Originally Posted by boldford View Post
Wouldn't the frame be stiffer with a few diagonal members added?

Yes, it would be significantly stronger. If the frame is stiff enough without (which it's not) then that just means smaller extrusion could have been used with diagonal pieces for the same strength but lower cost. Still, they can be added at a later date.