I appreciate you being devil's advocate. It's the best way to check a design and people's thinking. I think we see things in a very different manner as by your thinking all CNC mills would have racking issues with their single point of contact on the X and Y tables. On a mill the X and Y axes are 'wide' and heavy plates of steel which I've swapped out for 4 linear rails on the X and 2 on the Y. The Y axis load is supported across a wider area than a typical mill. Granted a decent mill has weight on its side for strength and rigidity so with a lighter weight rig you have to think differently. CNC routers with Y and Z travel on the gantry are notorious for poor accuracy. I haven't seen any sub £5k routers cut aluminium with any degree of accuracy or surface finish. By fixing the Z so it only does Z surely helps with rigidity? Where you would normally have to make a very sturdy single column for a typical mil, by utilising the gantry set up you make the Z a much simpler set up.

I'm only cutting small parts so wont be cutting at any table extremes. The longer Y is to get access to the bed to set it up because the gantry is in the way. This is the only reason I can see that mills are made with a single column - easier access. It makes more sense to me to have a gantry to support the Z axis. With the Z being the weakest link why would you only have 1 ball screw and not one each side of the spindle? I did think to have 2 ball screws on the X axis to support the longer Y table, but then dont you want the drive right where you are cutting? My design has you cutting right above or very close to the X and Y ball screws at all times.

I've not seen a design like mine, which is either because there's a fundamental flaw with it or it's a case of 'we always made them this way, so why change?'. I cant come up with a flaw when comparing to similar spec, sub £5k routers.