Dry Box with electric Auto-rewind Spool holder MMU2

I also had a homemade spool holder with a plastic spring. For this, a kind of friction clutch was needed so that the spring would not break. Friction was always an issue and never worked smoothly enough or too well so that the spring was not wound.

I was wondering if an electric variant would work better.

When purchasing the MMU2S I noticed that the stepper motor always has a certain resistance to turning, even when it is not being driven at all. Actually the ideal friction clutch that I was looking for (regardless of temperature and the like).

When unloading the filament, the MMU2S spools quite a bit of filament towards the spool (backwards). I didn't like the solution with the existing filament buffer, not because it wouldn't work, but because it was difficult to replace a filament.

This gave rise to the idea to create an automatic electric rewind spool system:

When the filament moves forward, the stepper motor is in rest position and acts like a magnetic friction.
When the filament goes back, the stepper motor is controlled to wind the filament back onto the spool.
The power of the motor is such that it does not independently pull the filament out of the MMU2S, so that the MMU2S is not accidentally unloaded.

A bonus is that you can also rewind the filament onto the spool via a push button without opening the dry box.

As a dry box I use the IKEA storage box SAMLA (39x28x28 cm/22 l) with matching lid and clips (8 pieces).
Two spools fit in this box, so you also need two stepper motors (Nema17, 37.35mm real thickness motor-house) for it  (US-17HS4401S).
This is a direct drive system (about 700mA current setting), so no difficult transmissions are required. You could go up to 1.5 A if needed.

There are of course various methods to control the motors.

For this purpose I designed the EasyBox with internal 6 stepper motor drivers, a position sensor for the stepper motor idler of the MMU2s, an Optical encoder for the stepper motor gear of the MMU2s and an magnetic coupler for the filament.

Without MMU2S, this dry box with motors can also be used, the motors then only serve as magnetic friction, no cables required.
You can always connect this to a stepper motor driver later, in order to have the manual rewind action, without opening the dry box.

The “Motor_Gondle_Ruler” is only for putting the motorbox on the proper place.

There is a pdf-document how to put the dry box together.

Tip in general: always cut the thicker filament tip off before your change/pullback your filament to the spool (eject filament option in the menu). Saves a lot of troubles. If it's stays in the MMU there is no trouble (your tips must be of course of the wright shape).

Change 17-04-2021:

Todays the newer spools are more slimmer: about 7 to 7.5 cm, instead of the old wider spools of 9 cm. So there is more room for a bigger motor : little bit more magnetic friction on unwind and more torque (4x) for the rewind force if needed. On the 700mA the motor will not pull the filament out the MMU by accident, but ones out the MMU it can pull the filament out the tube to the drybox (this tube is 3mm inner and 4mm outer diameter). So instead of a Nema 17 with 24 mm motor-house a Nema 17 with 38 mm motor-house (US-17HS4401S).

Change 15-04-2021:

Update of PDF manual to the newer spools, magnetic couplings and magnetic markers (MMU2s Material Marker from Florian).