3rd Z-Axis Stepper Parts for D-Bot by dotorg

Last crawled date: 3 years, 5 months ago

This is a pair of parts I designed / modified for D-Bot derived printers that will add a 3rd z-axis threaded rod connected to a 3rd stepper motor. The motor mount is a clone (or as close as I could get) to the D-Bot, so another print of the stock one would probably work fine. The bed mount is a from-scratch mount that fits the stock threaded rod nut (and anti-backlash, if used) but moves the nut farther out from the bed so it can align properly with a stepper mounted below it. The nut can mount either inserted into the body of the bracket, or inverted (as it is in my photo). The inverted position is still supported around it by a flange, and allows room for the anti-backlash nut to fit. The whole setup can mount either inside or outside the rear rails of the printer. I've ran it both ways, but moved it to the rear to avoid any potential snags from my bed wiring.
A lot of 3-support mechanisms (or customized 2-support) others have put together on here were focused on using toothed gears and belts to synchronize two or three threaded rods to a single stepper motor. While that mechanism works, it has some significant drawbacks, including the situation where a belt slips, making it far more complicated to get the Z axis back into alignment. It also leaves an exposed belt under the printer where things can catch on and potentially damage.
Using a 3rd stepper motor avoids those issues by driving all three rods equally. Depending on your controller, you may be able to run all three in parallel or serial off your controller. (I'm using a Duet Wifi, and run all three in serial.)
This is a forward-looking design in that I can add a Duet2x or 5x controller and go to three-way active bed leveling, where the controller drives each Z axis separately to live level the bed.
To keep the three steppers in precise sync long-term, I've got the printer calibrated to re-sync them at power up. The basic pattern is to have the printer home all three axis first. Then it lowers the bed 310mm quickly. At that point, it does two things -- it reduces the stepper current to a very low amount, and then does a slow additional move of 30mm. The build platform gantry catches on end-stops screwed on each stepper mount, causing the stepper in question to start skipping. Letting all three start skipping ensures all three go back into precise sync above the steppers. At that point, it cranks the stepper power back up and z-homes again.
For those interested in doing something similar, I have a corresponding part that bolts onto the top of the D-Bot motor mounts (including this one, which is a clone of it) that stops the gantry at precisely 40mm above the stepper motor mount. You can see it here: https://www.thingiverse.com/thing:2510053