The Mrs aint gonna be happy about this little project!

[JAZZCNC]

This is causing me a few headaches. My plan is to join the Y 2 bearing mounts with a 20mm plate and mount the ballnut mount to that. Im also going to increase the with to 200mm

Alternatively, and this is going to mean major surgery, move the plate joining to 2 X screws together to the back of the black box section and sink them 20mm further in to X uprights. The ballnut mount can be mounted directly to the Z backplate.

I'd take the major surgery because this way will cause you a bigger problem than you realise.? Because your vertical that back void will fill with chips in no time at all. . . . Some of the chips fly up before they fall down.!!

Having the front open will mean they will bounce off back plate and escape.!

[r0bsk1]

Surgery is complete but there's been complications.
The 1610 screw in the pic was originally sized at 1190mm to fit between the 2 beams joining the uprights and mount on the backplate with the stepper on the back.
The screw is long enough to go through one side of the upright plates. However the ballnut on the 2510 screw is in the way of the ballscrew support and pulley.
Without getting a another screw with an extended pulley area my only option i can see is to mount the screw between the 2 beams on the other side of the back plate and create a access hole for the belt to pass through from the screw to the stepper.

[r0bsk1]

So some bypass surgery is now required!
I realised putting the belt through the backplate will put it at risk from swarf. I need to protected it from crap so my plan is to use a FK ballscrew support fixed to the outer of one of the Y uprights with a BF support at the other end mounted to the backplate. There's just about enough room if the FK support is sunk into the plate.
The belt from the stepper would have to come down diagonally due to the stepper being positioned to clear the X screw mount.

Temporary Y stepper mount

Also saves cutting a big ass hole in £100+ piece of tooling plate!
Ill draw it up next if Sketchup decides to open any time soon.