I've built this machine 100 different times in my head!!!

[Iwant1]

This also allows different pulley sizes to be used without changing the belt, which could come in useful.

Thanks Jonathan, good advice as I will be putting larger pulleys on the motor side. Now I can do it with the same belt.

Yep going for Hiwin, as I can't find any suppliers of new THK or NSK. I'm using 20mm profile rails and the carriages have 4 rows of balls.

I've redrawn the motor mount, but decided not to go with a full plate on the back of the z axis. Just extra weight for the gantry to carry. Can be easily added later if needed.