In my case I'm actually using the same type of reinforced belts that a 3d printer uses. In thinking about my problem, I think the issue is actually too little torque and not backlash / slop due to the belts. And relatedly it may be due to too little rotation needed by the stepper to make a visible change in focus. I am focusing in 1 step increments (with 32 microsteps enabled). I'm going to try to install the 5.18:1 stepper I bought when I was originally working on this. Unfortunately I just noticed that despite claiming to be "Nema 17", the bolt pattern in the front of the gearbox is *not* a nema 17 pattern. Sigh... I'll have to adjust the design and reprint (which is lengthy as the stepper mount is integrated into the mounting plate which is ~232mm x 70mm x 16mm (it needs to be strong). That one takes a while to print, though I do have a fast profile for a .6mm nozzle dialed in from printing PPE, so it shouldn't take quite as long as it did before.

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