Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2562824.
This is a flexible coupling I designed for the Anycubic i3 mega to connect the threaded rods and the z-motors, because I had some issues with z-wobble. I replaced the rigid couplers with these ones, to compensate imperfections of the threaded rods.
To attach the couplers you need four DIN 934 M4 nuts (more or less the standard nuts). For attachement of the threaded rods you can use the headless screws of the rigid couplers. For the motor side with 5 mm bore you need longer m4 screws. On the right side of the printer, where the z-endstop switch is, you should use the headless screws of the rigid couplers on the upper side. If you use longer screws, that do not fit completely into the coupler the may collide with the endstop switch.
The couplers are quite flexible against bending in any direction and work like a spring the z-axis direction, but they are very stiff to torsion around the z-axis.
The coupler has a side with a 8 mm bore and a 5 mm bore. I think, it might also be useful for other printer models.