A few changes from the previous status

April 2022 - reworked the X-U axis motor mount / bearing block mounts, as well as tool plates using cut and bent parts from SendCutSend April 2022 - swapped out X / Y / U bearing blocks from no preload to preload October 2022 - Present - experimenting with alternative extruder designs (small Nema8 motors, DC motors) March 2023 - swapped out the Duet 2 + Duex5 with a Duet 3 6HC

Had been wanting to try out the SendCutSend service, and stiffening up my X-U rail assembly was a good excuse to try it out. The vertical members (total quantity 4) are actually the all same plate - installed mirrored depeind on if they are on the high side or low side of the shared axis. Flat plates are aluminum, bent were in stainless. Total order price was a touch under $100 (~$10 a part). Idler pulleys are now fixed properly (using shoulder bolts that actual bear onto the surface). Tapped all the holes once I received the parts.
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Previously the X, Y, and U bearing blocks were all clearance blocks (no preload). Was able to change all four out for IKO blocks with preload - this stiffened up the system by a surprising margin, increasing my input shaping target by ~20Hz! (Measurements taken after installing the SendCutSend parts)

I picked up a really neat Nema8 with an attached 16:1 gearbox off eBay. While researching the motor, all of the extruder requirements seemed to check out - motor and output shaft could rotate at an appropriate speed with sufficient torque - all with a steps/mm value of 2292 (16x microstepping, typicality a standard value). The first crack at installing it into the printer (below left) worked pretty well - had the motor offset from the drive gear (experimentation has been with a single drive extruder, rather than dual extruder gears) using 2x printed spur gears, with the drive gear being a Bondtech BMG gear. Had some print artifacts resulting from surface seams on the printed gears, and eventually one of the printed gears slipped loose (didn't want to overtighten the set screw).
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For further testing, I put the motor inline with the drive gear (above right) using a shaft coupler I had on hand. A bit long in this setup, but has been working quite well! Verdict is out on if my hackish extruder designs are any better than a 'commercial' product, but I've been comparing to a BMG setup that doesn't seem to exhibit the extreme VFAs that others have experienced.

The Nema8 experimentation led me to try out the geared DC motor from a Stratasys Dimension machine, which I detailed here some:
https://forum.duet3d.com/topic/31642/duet-2-wifi-and-geckodrive-g320x-on-stepper-channels-10-and-11/11?_=1680448852416

While I was able to get it working, and print quality seemed solid, I think the Stratasys machine does some additional tricks to manage the load on the DC motor. I was finding with the Geckodrive (0-20A output) there was a fine line between the motor stalling/faulting on moderate extruder accelerations, and overheating the motor. I would typically see a 35°C thermal rise using the Geckodrive directly with the Duet 2, whereas in the Stratasys machine (all original controller boards) I would only see a 18°C thermal rise.
https://www.youtube.com/watch?v=vvajxcZIf6M

Finally, this past week I upgraded to a 1.02 rev of the Duet 3 6HC. Currently still running with 24V. Overall the swap went over smoothly.

I like how simple the pin names are in comparison to the Duet 2. Not that it was confusing, but something like io0.in is easy to understand and type. Motors and motion sound a touch different - almost like they resonate at a slightly higher frequency, with a little more consistency, but not really any louder? Only hiccup I had was wiring and sending signal to my heat bed SSR. I am using Out 0 to drive the SSR, but did not provide an input to Out 0 power in. VH contacts crimp fairly easily.

Extra temperature controlled exhaust fans.