This my first model for the Anet A8 (~replica) made in FreeCAD. On my Anet the Z-axises have a jitter due to the bad certernig of the step motors shafts or something else. Top end of the rods had no caps and I drawn it and printed. Should help in my case. I also provided the FreeCAD file to make possible to modify the diameters or hights. Good luck!