shaun
I would suggest either way a stronger motor or screw need to be lower lead..
I believe the initial friction what causing the slips.. that's a very high lead for Z in my opinion...
mightbe just swap to a ballscrew which one has almost no friction...
you have the possible best couplers, they never slip... I mean if diameters are match and bolts normally tightened..
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for me similar happened..
I had the impression everything is fine..
til I had a job came up where very much Z movement was involved,
not like a 3d cut, but many tiny pockets... and I noticed the pattern is deepening as machine going along with the part..
after I changed the motor stronger... it stopped..
very possible you had the slip before, but it was so minimal you didn't noticed..
when you just cut trough parts you never notice minor slips..
heres an ebay link
SFU1605 Ball Screw L250MM ballscrew with SFU1605 Single ballnut for CNC | eBay
for this money don't worth to playing around with leadscrew..
leadscrew not cheaper, not more precise..
and leadscrews always has more friction than ballscrews..