lead screws vs threaded rod for z-axis in 3D Printer: Accuracy vs Precision

 lead screw (above) and threaded rod (below)

Threaded rod is, after all, not made to be used as a precise positioning tool. It is built to fasten and to remain stationary at all times.

from ProtoParadigm:

 

If a printer does use threaded rod for motion, the results will generally be better if the rods are suspended from the top  of the printer (as in a common Prusa design), supporting weight in tension rather than if the motors are on the bottom (see the now outdated Mendel design) holding the rods up and supporting weight in compression.  

Keeping the rods in tension rather than compression will minimize bowing and bending, though a bent rod can still cause problems.  While the deflection from compression may not be very visible, you can bet it's playing a role in your accuracy when your carriage is near the top.  Many printer designers minimize the compression issue by using very large diameter smooth rods that are more resistant to bending, but a tensile arrangement with or without larger rods is a better solution.

3ders: ditch the threaded rod and upgrade to a leadscrew for z-axis

Slic3r: Z Wobble and layer height setting