The spool holder for my A1 mini is made of water pipe. I printed adapters to go in the spools. Sometimes when the Z travels downward, the filament gets slack and threatens to jump off the spool which could potentially cause a tangle. That may not be possible, but it bothers me so I came up with this contraption. I only learned today how to use Bambu Studio to set a pause so I could print over magnets. So I’m having a ball making refrigerator magnets and now this contraption. The base is taped to the table.
There is quite a few models out there, mainly originating on Thingiverse if I am not mistaken that offer a rewind…
Basically a spool holder with a printed spring that frees itself every now and then while making sure there is always a slight tension on the filament AND a rewind when it gets slack…
I won’t link them here as I won’t spoil your idea and hard work.
When I still had my vintage Prusa and a really long bowden system at some point I needed a rewind spool…
Mind you that was in the days of 3mm filament being the only size available…
I went like this, just to give you an idea you can improve on:
Simple spool holder.
One side with a printed nut to get the spool on.
Since the other side already doubled as the stand and spool holder I added a spring disk…
Simple disk with a few holes and pins to fit the holes in the various spools out there.
Just to link them together with no real care about backlash as this will be fixed…
On the side of the mount the disk had rim - to hold the spring from a thin 3m tape measure…
With the massive (by comparison) diameter of the disk there was only a few loops…
The inner mount of the spring was fixed to the spool holder, the other end left FREE in the big disk rim…
When filament is taken from the spool the spring winds up.
Since it is NOT fixed to the rim of the disk it will eventually start slipping.
With three or four turns on the rim it is already enough to rewind enough filament for whatever comes up during a print.
Just make sure the rim on the disk is a match for the length of the spring!
If I remember correctly I had mine with a diameter of around 10cm on the disk…
In case you already have a sturdy spool holder you can of course try the springs from the wider tape measures but I don’t think this is required as the thin ones are strong enough for the purpose…
filament gets tangled if one end gets free, but if a ‘loop’ comes off the spool, that can jam in something, and get tighter, so the extruder can not pull it through, so your solution would be useful. I’ve never tried inserting magnets, but plenty of 6mm nuts. I’m unsure if a steel nozzle would be a problem, but I could test it out, of course.
That sounds cool as hell. Mine has no rewind function at all as you can tell. It merely puts a little bit of drag on the spool and closes the gap where a loop could jump off. 3mm! Really? That was some filament! Tape measure spring? That was really thinking!
That’s what I was worried about: the nozzle grabbing the magnet. I don’t know if it was because of a snug fit or because the area of the tip of the nozzle is so small, but I did it over and over on my Mini and my P1S and it just printed right over them.
The biggest thing that makes it successful is the auto-pause (just made up at term). If you don’t already know how it works, it goes like this: After you slice the part where you want to embed something, click on the top of the vertical slider and drag it down just enough so that your keyboard arrow keys will be activated. Then, use your down arrow key to slowly get to the first layer that covers you magnet. Now, down arrow one move layer so that all you see is the cavity. Then, right click on the green ball at the top of the slider, and choose ‘add pause’. Then, you have to re-slice, then print. Grab a chair and be ready for the print head to pause and get out of your way. Drop in the magnet, make sure it’s pushed down all the way, click resume and watch it cover over your magnet. Of course one should have done some test prints to make sure your cavity is made right for the magnet, or nut, or whatever. Cool as hell in my book.
A simpler and less involved device might be the use of a piece of felt on the hub to induce friction. Tubing placed near the point of filament draw from the spool may help too.
For open spool machines, some reduction to “free spooling” is needed. For spools I place on my exterior holder (for TPU), I use tubing. But free spooling is less of a problem on the fixed height hot end printers.
I use a hardened steel nozzle, and it is defginitely attracted by a magnet. Brass, maybe stainless, and diamond/ruby, etc, maybe not. For inserting nuts, without pausing, you can always have a slot from the side. To retain the nut, I fill the slots with hot melt glue. If you allow a slightly larger hole, or a slot for the bolt, then that allows easy adjustment on site, if the spacing of the bolts was not accurate.
Feel free to implement and use it to your liking, it is an old idea 
