Anet A8 single motor Z axis drive

This is a way to sync the Z-axis drive using a single motor.

Printed parts:
- plateL.stl - top left plate, the motor and idler are mounted here
- plateR.stl - top right plate, essentially a mirror image of the left plate, but without the motor and pulley.
- rod_plug.stl (print 2) - replaces the original guiding rod plugs on top plates
- rod_holder.stl (print 2) - holds the guiding rods from below, since the motors aren't there anymore
- tube_guard.stl (optional) - keeps my bowden tube fr