CARSH (Compact Auto-Rewinding Spool Holder)

Main Features

• It an auto-rewinder, negating the need for a buffer
• It's compact, designed specifically for use inside the Prusa Original Enclosure
• It's strong enough to handle full spool rolls
• It requires minimal extra hardware
• It only requires PLA and PETG

How to use

This is an open-ended spool holder that locks into the Prusa-style spool holder connector. I designed this mainly to be used in the Original Prusa Enclosure for the MMU, in order to have the simplest and most reliable operation in the most compact space.

Basic operation:

• Set clutch tension
• Install spool
• Hold button and install into enclosure

Clutch tension is set by the dial, 0 being free-rotation, and 15 being full-engage. Ideal operation should allow slippage just as full spring tension is reached (it should not feel difficult to rotate at any point).

The filament spool can either be installed while the spool holder is in place, or while removed. Use the nuts to secure the spool and ensure proper centering.

Pressing the button on the end locks the entire assembly together to allow locking the spool holder in place. Once secure, it will disengage and allow normal rotation and auto-rewinding.

Required hardware

• 2x 608 bearings
• 2x M3x10 bolts
• 1x M5x25 bolt
• 1x M5 nut*

*can use regular M5 nut, or M5 nyloc nut – print the associated shaft model 

Print Instructions

All the required printed parts for full assembly are already combined into .3mf's:

• CARSH-PETG-combinedCCW.3mf OR CARSH-PETG-combinedCW.3mf
• CARSH-PLA-combined.3mf*
• CARSH-PLA-hub.3mf

*this has both versions of the shaft included – toggle part visibility if the nyloc version is preferred
 

For the PETG springs, only print the orientation for the use case where this will be installed. For example, if installing in the Prusa Enclosure in the standard top-right location, the spool will be rotating counter-clockwise as it feeds into the PTFE, so it will require the CCW-style spring. (the spring should be contracting around the shaft as it feeds filament)

The PLA hub is separated out into a separate .3mf specifically due to the need to print the thread portion at a lower temperature than normal in order to prevent edge curling. The .3mf is set up to use the default Prusament PLA settings for MK4 input shaping (225c running temp), with a custom Gcode at the 3.2mm height to change the temperature to 220c:

M104 S220

If needed, change this accordingly for your own print setup. The most important thing is that the nuts rotate freely on the hub. There should be no binding.

Note: If printing the .stl's individually, there are some key quality points to take into account (these are already set in the .3mf files):

CARSH-PLA-hub.stl

• Use "Stagger internal seams"

CARSH-PLA-matingplug.stl

• 5 perimeters (for 0.4mm nozzle)

CARSH-PLA-shaft.stl or CARSH-PLA-shaft(nyloc).stl

• 5 perimeters (for 0.4mm nozzle)
• May require helper pads to prevent lifting

PETG springs

• 0.15mm layer height

Tuning

As mentioned above, the dial should be set to allow slippage just as full spring tension is reached. This can be set without a spool installed, so I recommend determining the dial setting just by holding the mating plug and rotating the hub by hand. There should be no binding AT ALL – any extra force required to rotate means extra tension on the filament line, as well as extra force placed on the spring, leading to breakage.

As far as the assembly goes, the main component that will possibly need to be addressed over time is the tightness of the M5 screw. Due to the design, the tightness of this bolt is critical:

• Tight enough to keep the assembly from sagging
• Loose enough to prevent binding of the 608 bearing

I originally designed this using a regular M5 nut, but have added the nyloc nut as an additional option in hopes that this will negate any long-term re-tightening after the initial assembly is done.

Tips for extra smoothness/lifetime

These are not necessary for basic operation, but are my personal recommendations if you start to run into issues.

Use ceramic 608 bearings

• These have lower hold/running friction
• Not all bearings perform the same, and more $$$ does not mean less friction for this application
• My winner from testing 5 different brands:
  • Stetson Ceramic 608 RS
  • Beats out Bones Super 6 for less than half the price

Use PC instead of PETG for the springs

• Extends the lifetime of the springs
• Can also create a stiffer spring, depending on the specific material used

Sand the tops of the springs

• Depending on the surface quality of the print, there can be enough roughness to cause binding between the two halves
• The hub is designed with the slip plate to help mitigate this for the top spring
• The bottom spring is the most likely to cause binding with the underside of the top spring
• Dremel/rotary tool is recommended for sanding the bottom spring (due to the 6 tabs)

Extra Recommendations for Prusa Original Enclosure users

In order to make effective use of this for MMU operation in the enclosure, I am using a few key builds:

• Extra Spool Holder for Prusa Enclosure - Extended Length (by Kevin Barker)
• COBRA System for Original Prusa Enclosure Filament Guides (by Ludvig Broomé)
• External PTFE guide for Original Prusa Enclosure (by me!)

These allow proper mounting and positioning of the PTFE tubes for multiple spools in the enclosure. For the COBRA PTFE guides, I'm specifically using these 3:

• guide-1.stl
• guide-2.stl
• guide-3.stl

Assembly Instructions

1. Take both halves of the spring assembly and CA glue the edges together (6 small grooves). Let set until completely dry.

2. Glue the spring assembly into the spring housing (3 small groves). Let set until completely dry.

3. Take the Dial, find the tab that is aligned with the pointer. Take the Clutch, find the tab pocket that has the dot next to the 15. Align the Dial and the Clutch at these points to fit them together. Rotate back to 0.

4. Insert the spring assembly into the clutch. Set aside.

5. Take the Hub, insert 608 bearing into the large end. Snap the hub onto the clutch assembly by pressing the petals into the tabs. Ensure the 608 bearing does not fall out, and all petals are fully pressed in. Set assembly aside.

6. Prepare the shaft, 2nd 608 bearing, end plug, end cap, end cap springs, and 2x M3x10 bolts as shown.

7. Insert the end plug through the 608 bearing and into the shaft. Secure loosely with the 2x M3x10 bolts, do not tighten all the way.

8. Insert both end cap springs into the space inside the 608 bearing, and fit the end cap onto the plug.

9. Fully tighten both M3x10 bolts so that the heads are flush.

10. Insert M5 nut into slot in shaft. (NOTE: this will likely want to fall out for all subsequent steps, just ensure the shaft is kept in an orientation to prevent this)

11. Insert shaft assembly into open end of the Hub assembly. Push in all the way and keep the end cap engaged.

12. Prepare the Mating Plug by inserting the M5 bolt fully. (leave the allen key inserted for tightening later)

13. Align the springs with the center opening and note the flat section.

14. Insert the mating plug, with the flat lip aligned with the springs. Push down until the inner teeth are fully engaged with the shaft (note the close-up spacing). Keep pressure applied, and fully tighten the bolt (HAND TIGHT, do not use a wrench -- over tightening will cause binding).

15. Verify operation by setting the dial to ~8, then rotate the spool in the intended operational direction. If everything is working properly, the clutch should start slipping at a reasonable force, and there should be at least 2 full rotations of rewind.