I found that the motor shaft cutout on the small gears was far too big for the 28BYJ-48 5 volt stepper motors from Amazon. I modified the gears.scad file to reduce the cutout diameter, reduce the width across the flats, and increased the contact area height to be the full gear height (originally around half gear height). The gears.scad file is attached.
The printer was a Creality Ender-3 Pro V2 using PLA+ filament, sliced in Cura.
Ah, I can’t attach a file. Here’s the modified section of gears.scad:
cylinder(r=shaft_r, h=999, center=true);
//chamfer the top/bottom for better fit
Thanks for this. I just thought I’d note that we’ve found the same issue recently with our printer. I always used some tape to sort it out, but @j.stirling is now working on a much neater solution that tightens the gear with a couple of small self-tapping screws. It’s on GitLab if you fancy trying it out - you can download the zip file of STLs from the “pipeline” bit just below the main description - look for the “build” artifact.