I think I got it, it’s for adjusting the tension between X and Y.
Correct me if wrong.
EDIT: Uoppss.. And did the cad middle assembly wrong. It’s actually 27.5mm
So between 29 and 27.5 it’s just 1.5mm. Yup, don’t mind this thread… pretty sure it’s for the tension.
PS: I’m impressed by this new middle and the way the threaded rod fits now. Well done Ryan