D-Bot "Rod Core" Triple Independent Z Mod

I started working on a 300x300 DBOT back in 2018 as a second printer after owning my first printer (Prusa MK3) for only couple months. I figured building a custom printer couldn't be that hard, right?

Well that was 2 years ago and now it's 2020. I haven't had much else do with my spare time so I've finally gotten around to working on this project and hopefully get it finished*.

Initially I built my printer with two Z motors connected to the same stepper driver. One motor on the left one on the right. Because I was going with the 300x300 variant I figured some extra support on the front would help so I bought an extra cross beam for the front and I had Z wheels in all 4 corners instead of just two. Everything seemed fairly sturdy.

One of the biggest issues I had with my DBOT during my early testing as it was originally built was binding on the Z axis. Second to that I could not get the bed level and that was probably due in part to the first issue.

Maybe it was over-constrained? Most likely I am just bad at math since I know I need to re-order some aluminum extrusions in a longer length to actually use the whole bed, but that's an issue for later.

The Duet supports the ability to use up to 4 Independent Z motors for True Bed Leveling. 3 points is generally enough though.

I initially tried just adding a third z motor and using rods instead of wheels but I still had binding and the True Bed Leveling couldn't correct for more than 1-2MM without binding up all 3 motors as everything was too stiffly mounted.

I spent a good amount of time browsing Thingiverse and other sites for ideas. I've credited everything I can remember looking at for inspiration by marking them as remix sources.

One major source of Inspiration was the RailCore Project:
https://railcore.org/

This bit in particular:
https://railcore.org/customization/levelling_washers.html
is where these parts come in.

The idea of using the spherical washers and having the bed attached to the printer in a relatively flexible manner seems like a genius idea to me but I wanted to work with what I had as much as possible (Sunken Cost Fallacy amirite?)

What I've made here is a Z axis lifter for the bed loosely based on the RailCore but using Linear Rods hence the name "Rod Core".

I used 12MM rods and bushings because I wanted strength and I'm more familiar with rods than I am with rails. It might be overkill but it's definitely sturdy.

For the rods and bushings you will need two each per lifter unit:

Misumi Straight Precision Linear Shaft - PSFJ12
THK Long Linear Bushing - LM12LUU

For my printer I used PSFJ12-455 Rods.

To attach the Bed Yoke to the Z-Yoke I used the follow parts (per lifter unit):

1x Spherical Washers:
https://www.amazon.com/gp/product/B009SX8JY4/

1x Rubber Washers:
https://www.amazon.com/gp/product/B07KCY6Y8H/

1x m3x45mm Socket Head Screw
2x m3 fender washers
1x m3 nyloc nut

I've attached the STEP and F3D files in case you want to modify them to use something else.

Sorry I don't have any pictures of them mounted. My work shop is embarrassingly messy at the moment but I have tested them and they seem to work as intended.

Print instructions

The parts were designed to not need supports you will need to use a knife, razer blade and/or deburring tool to cut out the slot where the m3 bolt will fit trough on the yokes. You'll also need remove the material blocking the holes for the top of the Z motor mount. I just shoved an M5 screwdriver thru the hole.

I used 4 perimeters and 40% infill. Supports should not be needed. A brim is probably a good idea. I used a 5mm brim myself.