Prusa Core One modular auto rewind spool holder with a robust metal axle and ball bearings

Prusa Core One modular auto rewind spool holder using a spring, an adjustable friction clutch and a metal axle (to prevent axle bending) with a rewind capacity of 100cm.

Note: Unfortunately there is no easy way to make a reliable auto rewind spool holder which has the rewind mechanism integrated into the spool core. Therefore this is a complex model that should only be realized by experienced craftsmen with watchmaking skills. Please note that you will need DIY tools. At first you have to create a flat spot on the M6 screw with a file. Then drill a 2.1mm hole in that flat spot through the axle (M6 screw) and cut a M2.5 thread into the hole. Also at the inner end of the steel spring has to be drilled a 2.5mm hole without damaging the spring.

For a seamless function the filament spool has to be fixed on the spool holder hub with a nut.

Filament spool measures: Inner diameter 51 - 73mm, width 38 - 85mm

Update 12. October 2025: To clarify which parts should be downloaded/printed, I've reorganized the files and deleted the 52 mm inner diameter versions. They are replaced by the 51 mm inner diameter versions (used at Prusament spools).

It is recommended to tighten all fasteners of the rewind spool holder after a running in phase (~1 month). 

To develop this auto rewind spool holder for the core one right side spool box, i used my model of a ball bearing spool holder as base, which is based on the model from jmillerfo.

The dual PTFE path from the spool holder, to be seen on some pictures/videos, is available here. With this feature you can use the standard path to the Nextruder for filaments which can't be used with a MMU3, or use the MMU3 path from the spool holder as one feed to the MMU3. 

Changes made to my model:  added a functionel spring rewind system, added an adjustable friction clutch which can be adjusted with an adjustment nut and locked with a small nut. The result is this modular automatic rewind spool holder using a steel spring and ball bearings.

All blue parts need to bought somewhere, all grey parts are printable.

Printing instructions:

Use PETG for all parts exept the mount, i recommend a more heat resistant filament like PETG-GF / PETG-CF or ASA for the mount plate.

Print the hub with random seam position, otherwise you have to remove the seam by hand for smooth operation!

0.4 nozzle, 0.2 structural, 3 perimeters, infill 3D-Honeycomb

Supports are recommended for the Mount and the Spring Housing

For easy slicer setup use the .3mf files (most settings included). The spool mounting nuts *.3mf files have a stop at 2.6 mm for inserting magnets. If you don't want to use magnets, delete this stop in the slicer.

Not printable Parts needed: 

4 x M3 hex nuts (maybe there are some spare available form the kit upgrade)

2 x M6 thin hex nuts (sample)

1 x M6 Flat Head Screw 75mm long (sample)

3 x 608 ZZ Ball Bearing (sample)

1 x M2.5 countersunk screw 4mm (sample)

1 spring, taken from a key badge (sample)

Optional for every spool nut: 3 round neodym magnets 6x2mm (sample)

Preparation and Assembling:

Axle (M6 flat head screw) preparation:

Be careful with drilling and cutting the thread, lubricate the driller and thread cutter with oil, because the screw is hardened steel and not easy to treat.

Spring preparation: 

Assemble unit:

Take the Mount and push the 4 M3 hex nuts into the intended openings. If you have a problem with pushing in, use an M3 screw from the opposit side, drag the nuts into the recesses.

Now take the thin M6 nut and push it into the recession on the same side, press the mount against a table to make sure the nut is fully pressed in.

Take the prepared M6 flat head screw and screw it into the mount from the flat side. Tighten the screw by hand, do not apply exessive force. 

Now take the inner-bearing-seat and push a 608zz bearing into the recession until it sits firm.

Put the inner-bearing-seat assy onto the mount until the bearing touches the thin hex nut. Check the space between the mount and the seat part, it should be about 1mm, depending on the width of the used nut. If the space is less than 1mm use a thin (1mm) M6 washer and put it onto the thin hex nut to get more space.

Screw the inner-fixing-nut on until the bearing seat is fixed on the screw, use a 13mm open-end wrench (prusa kit tool), do not apply exessive power.

Be aware that the printed M6 thread of the inner, outer fixing nut and break adjuster nut may not be easy to screw on. I recommend recutting with a M6 thread cutter.

Compare the result with the picture below. Put a little bit of lubrication grease on the top of the bearing seat assy, apply it evenly over the entire surface. This ensures smooth clutch function.

Now take the adjustable break, the spring, the spring cover and housing, the break bearing

Push the spring through the hole of the break, see direction below

Put the break over the mount unit. The spring hole should match the hole in the screw

Take the M2.5 screw 4mm and fix the spring on the screw in a 90 degree angle

Drag the spring carefully through the hole to the outside and place the break on the inner bearing seat. Take the remaining spring through the slot in the break housing. Now rotate the axle counterclockwise and, at the same time, hold the spring in the housing down with two fingers. Rotate until the end hook of the spring hangs in.

Now the assy should look like below. Put the spring cover (black) onto the spring, the slot should be placed where the spring leaves the axle. Be careful, don't force it down before the slot is in the right position.

Now push the bearing into the spring housing and put it onto the break assy

In the next step screw the break adjuster nut on the screw (see picture below), then screw the adjuster lock nut on the screw, do not tighten yet.

We are now at an important step of the assembly, the break adjustment. Make a mark on the inner bearing seat (red part). Now hold the assy on the mount (black part) with the left hand, the clutch assy (orange/white parts) should look to you and the mark should be on top. 

1. Turn the clutch assy clockwise until you hear or feel that the clutch slips. 
2. Let the clutch turn back slowly and count the revolutions using the mark. It should turn back between 5 and 7 turns. 
3. If it turns back less than 5 turns, turn the adjustment nut clockwise one eighth of a turn, repeat step 1 and 2 until it turns back 5 to 7 turns.
4. If it does not slip at all or turns back more than 7 turns, turn the adjustment nut counterclockwise one eighth of a turn, repeat step 1 and 2 until it turns back 5 to 7 turns.
5. If it turns back as desired, hold the adjustment nut with a wrench and tighten the lock nut with another wrench until both nuts are secured.
6. Repeat step 1 and 2 once more to check if the setting is still ok. If not, unlock the nuts and adjust again using step 3 or 4.
7. If you are satisfyed with the clutch setting, put a suitable spacer (black part on top) on the screw. There are 4 spacer lenght available (9.8 / 10 / 10.2 / 10.4mm) to secure the hub. The hub must not be loose, but also not clamped.

Now mount the Assy to the printer using the original 4 screws.

Prepare the hub (yellow part), push the 608zz bearing fully into the recession, put the hub onto the mounted Assy and secure it with the outer retaining nut (black part) until the hub is firmly seated. Do not apply excessive force. Try steps 1 and 2 of the clutch adjustment again to check functionality.

Put the inner step ring and a step nut (blue parts) onto the hub, now the rewind spool holder should look like this:

Done! 

Attention: Avoid to turn the hub counterclockwise, this could damage the spring inside the assy!