This is a small remix of my favourite dual Z mod for Tevo Tornado and
Creality CR-10 https://www.thingiverse.com/thing:2668969.
The design is symmetric so bearings used for upper holders are the same as bearing used for idler pulley. ( you will probably buy pack of 10 from china ;) )
You will need:
-600mm leadscrew and nut
-GT2 852mm Timing Belt
-3x 8/16/5 bearings
-2x M8 threaded rods
-8x M8 nuts
-7x M5x20 screws (idler and bottom holder)
-7x M5x35 screws (upper holder and gantry plate)
-11x M5 tee nuts
-8x M3x12 screw (leadscrew nut to gantry plate and bearing covers)
-4x M3 nut (leadscrew nut to gantry plate)
-4x 3mm washer (leadscrew nut to gantry plate)
-12x M3x8 screw (pulley and under bearing leadscrew locking ring)
You will need to print:
-8x BallNutM8.stl (or 516_Nut.stl)
How to install:
Move z axis all the way up and then mount the left upper holder. Align the holder in X direction first and then in Y direction by tightening upper bearing holder (bearing has a 2mm play in Y direction so it can be adjusted). Do the same for right upper holder. The right leadscrew is fixed to the frame only on top so you have to carefully align it to be perfectly parallel to Z axis! Tighten things up only when the leadscrews are perfectly aligned and there is no leadscrew bending! At last, you put belt on and then adjust its tension with idler pulley. Measure distance from X-axis to the bottom on both sides. Make sure the left side is equal to the right side. ADJUST ALL Z GANTRY WHEELS! When force needed to rotate wheels is the same for all wheels then all gantry wheels are correctly adjusted! This step is very important because there can be some play in one of the leadscrews nuts caused by misalignment of the wheels. If everything is aligned correctly leadscrews nuts will be equally pressed against the leadscrew (also main pulleys against top bearings) by gantry weight. The layers precision should be visibly improved.
If you have any questions just ask!