Hey Jonathan,

I did think about plating the back, but currently have the ballnut for the Y carriage elongated to attach to the both the back and front plates, I'll have a look at the gantry side plates, i did want to add in some angled brackets, but will look at your design again,

In terms of alignment, i was planning on bolting the 'x' rails to the 120x 80 and then bolting this through the table so there was some adjustment available with shims/ rizzla papers if required-! but agin may re think.

The rotating ball nuts are currently fixed to the carriage side plates with the 'fixed' bearings at each end...they just look similar to your ball nut design...

heres another rendering-