Adjustable-width Compact Spool Auto Rewinder for MMU (Metal Spring, Sliding Clutch, for Drybox)

Spool auto rewinder with

• metal spring

• adjustable sliding clutch

• compact design placed within spool

• adjustable width for spools between 67 and 128 mm 103 mm (see updates 20.07.2026 and 22.07.2026)

• intended for the limited space within the (Un)original Prusa (Heated) Drybox

For spools with a hole diameter > 51 mm. Perfect for 1kg and 2kg Prusament (see updates 20.07.2026 and 22.07.2026).

It is based on the Compact Spool Auto-Rewinder and it's stronger version remix.

Disclaimer

Please consider this model as a proof-of-concept.

I am happy with the overall design, but it still might need some tweaking of tolerances and did only very limited testing.

As I am not sure how soon I will be able to fine-tune and finalize it, I've added the complete Fusion360 design file and strongly encourage your input and remixes.

If you have any questions, feel free to leave a comment. I am happy to provide additional details on (printing or assembly) upon request.

Update 22.07.2026

I have added elongated versions of the interfaces.

• Using both elongated sides should cover 88 to 143 mm with a reasonable 15 mm overlap

• Mixing original and long variants should cover the 2kg Prusamen 123 mm with a reasonable 15 mm overlap. So if you have it printed already, one side should be enough to replace.

  • I'd suggest going with the elongated outer-threaded side as the clutch mechanism should be fairly easy to swap.

  • Screwing the nut on the axis within the inner-threaded elongated version could be a challenge.

Update 20.07.2026

It was brought to my attention that I must have made a miscalculation during design phase. The current model does NOT support 123 mm width 2kg Prusament spools. The current limits seems to be 103 mm - or if you minimize the overlap compromising stability maybe 108 mm - missing a 15 mm for 2kg spools.

I see some fairly straightforward ways to squeeze out 5 additional millimeters, but the remaining 5 would entail touching most internal parts of the mechanism and I am not yet sure how fragile it is going to get then.

Update 28.05.2026

I've been using the rewinder as-is in the Heated Drybox for almost a year now. Mostly though during single-filament printing as I have only recently finished to assemble my MMU3. In combination with the MMU3 it works very well for me without the need to change any tolerances.

I have learned that I have to set the adjustable clutch quite firmly near it's tightest position. Otherwise the spring may unload almost completely when it starts to glide during continuous pulls.

There are three things I am hoping to address in the near future:

• Create adaptors for filament spools with a hole diameter >> 51 mm and a width < 67 mm.

• Create a tool to better grab the clutch adjustment ring for setting the appropriate tension. Due to the miniaturised design, it only sticks out a few millimeters making it hard to tighten in by hand alone.

• As the rewinder has - by design - not a fixed width, this entails that the holders need to be moved sideways frequently (whenever I use spools with varying width). Hence, I am experimenting with spring-loaded clamps attaching the holders to the horizontal pipes in the drybox. 

Additional Parts

Due to the splitted body and further miniturization of the mechanism, a single auto-rewinder requires 6x bearings 10x15x4 mm (6700 / 61700).

Printing

I printed it from PETG on MK4S with mainly the standard settings

• 0.2 mm (STRUCTURAL) for the outer body and stands

• 0.1 mm (FAST DETAIL) for the interior

25 % infill and one additional perimeter layer.

Several parts need “smart fill” supports on vertical overhangs.