I have discovered that the z axis lead screw nuts were both very loosely mounted. When I tried to tighten up the mounting bolts, it was discovered that one of the bolts was stripped. The bolts are a very fine thread and screw into plastic so it doesn’t take much torque to strip the threads. There is also a lock washer between the mating surfaces for some reason.
I am tempted to ream out the holes and use bolts with lock nuts to attach the lead screw nuts to their mounting and maybe remove the lock washer between. Has anyone had to deal with this problem? I know it’s a cheap printer but it seems a poor way to attach something that has a lot of forces acting on it.
I have drilled out the threads on my printers and fitted nuts. Ideally the screws only need to be tightened to minimise vertical movement. If you can adjust to allow horizontal movement this will compensate for the lead screw runout. I used thread lock to fix the nuts in place. Remember the leadscrew is always under load in one direction due to weight so vertical free play is always minimal. The idea of allowing the z lead screw nut to float is to allow for the runout (bend) in the lead screw ,the horizontal float helps improve the print quality by reducing z banding.
Excellent point about the Z lead screw always being under load in one direction. I am going to loosen the nut to allow it to float more. Think I will use a dial indicator and check that there is no binding. Thank you for your reminder of how sometimes gravity is our friend!