Possibly an out-of-spec belt — there was a run of them, then we switched to 520mm.
We’ve discussed an Acme screw upgrade — another option would be a linear slide, but that then gives up the wonderful safety aspects of belts (a crash just skips teeth rather than damaging the machine).
When I replied, I suggested that one could try adding a pair of M4 nuts as “jam” nuts to secure things — that said, the Z-axis is tricky to get right at first (at least that’s my experience) and requires a bit of fiddling, patience, and acquired familiarity.
I’ve had issues when trying to cut way down at the furthest extent, and the springs being too strong. You might try taking off one of the springs and seeing if that helps.
Was there a discussion about putting limit switches on all of the extents of all axis/rails?
Just if, for example, the extent settings in software are for some reason not adhered too?
Thanks.