Low-Friction Spoolholder

I designed this spoolholder for my MMU3, since the included Prusa spoolholders were too wide to fit all five spools in the specific plastic bin I had. These are cheap to build in that they only require 14x 4.5mm ball bearings each, which cost under $10 USD for a 200 pack (I bought these). If printed correctly, this spoolholder should have significantly less friction than the ones included with the MMU3, so much so that the spool can spin freely for a good while when given a push.

I should note that this spoolholder doesn't have a PTFE tube holder or filament guide of any kind, as my intent was to use it in a drybox with the tubes attached to the wall of the bin (drybox setup is incomplete in the photo). I may consider adding a version with a filament guide in the future.

The design of this spoolholder is such that it can be loaded easily from the top, unlike most center-shaft spoolholders which require side loading. This is done by making the center roller removable; to load a roll of filament, simply slip the roller into the center hole of the spool and slot it into the grooves on the frame.

If you'd like to attach multiple of these spoolholders together, you can use x3 6-32 machine screws (length ~0.4" or less), and x4 of the same screws can be used to mount it in a drybox. Please note, the mounting screws screw into the plastic, meaning the head will be underneath the drybox. This means you'll need to either edit the file or bore out the holes in order to screw it onto a desk or workbench with the screws going into the desk. That said, if you comment with a specific screw diameter I'd be happy to upload an edited version of the Frame model.

Mounting holes are 51.6mm (left-right) by 124mm (front-back), sorry about the arbitrary dimensions :)

Print Settings

For material, I used PETG for everything. PLA might also work but I wouldn't recommend it.

Nozzle diameter is optimized for 0.4mm, as most walls are 1.6mm thick (4 perimeters)

Suggested layer height is as follows:

Bearing_Cage - 0.1mm

Roller_Cap - 0.15mm

Axle - 0.15mm

Roller - 0.15mm

Frame - 0.2mm

The only part that requires supports is the Roller, which needs a small ring supported around its base (in Prusaslicer use the “Support on build plate only” option). All part orientations in the STLs are as they should be printed.

Infill doesn't matter too much; most parts have thin enough walls to not need it. I used about 20-30% depending on the part.

I also suggest using a 1.1 extrusion multiplier on the Frame, this should give the thin sections more strength (not sure if it's really necessary though). It does cause a blob to accumulate on the nozzle after the first few flat layers, so pay attention to that. For the rest of the print blobbing wasn't an issue for me.

Alternatively, you can print the alternate Frame_Flat.stl, which doesn't have the cutouts.

Post-Processing

After printing, sand the ridges on the Axle piece where the bearings will rest. Even a little sanding can help a lot with reducing friction. I haven't found sanding the inner grooves on the Roller_Caps to be necessary, but feel free to do so if you can get the sandpaper in there.

Make sure to thoroughly deburr the Bearing_Cages so that the balls can pass through their cutouts without resistance.

Lubrication of the bearings I'd also recommend, if you have some lubricant on hand. Otherwise don't worry about it.

You may want to drill out and/or tap the holes on the Frame if you're planning on putting screws into the Frame; this avoids cracking when cutting threads with the screws. If you don't have a 6-32 tap (as was the case for me), try wiggling the screw back and forth while driving it in, the friction will melt the plastic and help prevent cracking (just watch out, the screw gets hot!)

Assembly

1. Lay both Roller_Cap parts down right side up (as they are in the STL), and place the Bearing_Cages into the middle upside down (flat side up).
2. Place 7 balls into the gaps of each Bearing_Cage. I like to hold the part sideways and drop half the balls in, then lay it flat and push the rest of the balls outwards into the grooves from the center hole with an Allen key.
3. Push the Axle down into the hole between the balls on one of the Roller_Caps
4. Screw on the Roller, holding the previous assembly together so the balls don't fall out.
5. Still holding everything together, flip the assembly upside down and push the Axle down into the second Roller_Cap. This should keep the balls from falling out while you screw the remaining parts together.
6. Spin the roller a little bit, it should be loose enough to rotate freely for ~1 second when given a good flick. If it seems stuck, try sanding some more or reprinting with different settings. Also check you are using 4.5mm ball bearings.
7. To load a spool, position the roller through the center hole of the filament spool and drop it onto the Frame, making sure the grooves in the Axle catch into those on the Frame.