Auto-rewinder stand for Rubbermaid 21 cup container

Description

This is a free-standing auto-rewinder stand designed for use with the Prusa MMU2S multi-material unit. I designed this rewinder stand to meet the following criteria: * Must accommodate a spool 90mm wide by 200mm diameter, which is the size of a spool of Prusament as supplied with the MK3S kit. All other drybox rewinders I found cannot fit a spool that wide at full 200mm diameter. * Must work with low-friction PTFE feed tubes having 3mm inner diameter (not the too-tight 2mm inner diameter supplied with the MMU2S) _without_ pulling the filament out of the MMU2S after a retraction. * Must fit inside a Rubbermaid 21-cup (cloudy) food storage container. * Can stand alone without the drybox. My design [using a 16-cup Rubbermaid container](https://www.thingiverse.com/thing:4354340) also meets these requirements. While this 21-cup design is less convenient due to the filament threading through the cover, it offers better isolation from humidity, and a shorter feed path to the MMU2S due to the high-mounted PTFE fitting. For each rewinder stand, you will need in addition: * A [Rubbermaid 21-cup cloudy storage container with lid](https://www.amazon.com/dp/B00NP6T9JE/) (Amazon link for illustration; I found them cheaper at my local home supply store) * M6 PTFE tube fitting * M6 nut * Two 608 bearings (you got these with the MMU2S for the spool stands) Printing the auto-rewinder hub ------------------------------ The auto rewinder hub is the [Integrated Auto-Rewind Spool Holder](https://www.thingiverse.com/thing:3781815) by VincentGroenhuis, modified specifically for the requirements above. If printing the rewinder spring with PETG: Use the IntegratedRewinder STL file ending in "PETG". This prints 3 spring blades, each 1/11 the interior volume of the hub. If printing the rewinder spring with PLA: Use the IntegratedRewinder STL file ending in "PLA". This prints 5 spring blades, each 1/9 the interior volume of the hub. If printing the rewinder with ABS, use the STL file for PLA. The axle printed from the spool stand STL file has slots for a retainer ring in both of these positions, to accommodate a 3-blade spring or a 5-blade spring. Printing the stand ------------------ There are three STL files available for the stand, the only difference being the recycle symbol engraved into it: rubbermaid_21cup_spoolstand_PLA.stl rubbermaid_21cup_spoolstand_PETG.stl rubbermaid_21cup_spoolstand_ABS.stl The stand is straightforward to assemble. There are two side parts and two spacer parts. The spacers fit into slots in the side for alignment when gluing and clamping. I printed the stand with PLA. To bond the parts together, just paint a generous amount of acetone on each surface to be joined and clamp them together while still wet. Leave them clamped for a few hours. Before applying acetone to the other side and clamping it. Otherwise, for other materials, use whatever glue works best for the material you're printing. The STL file for the stand includes an axle with two retainer ring slots (for the two spring configurations possible), a spring retainer ring, and a washer. After printing the axle, the edge of the flat bottom surface may be sharp. Smooth it a bit with a flat file to prevent the edge from interfering with the spring coils. Then slip the retaining ring into the correct position so you don't lose it later. Assembly -------- **Rewinder spring:** The spring blades snap together in an obvious way. The blade with the teeth goes on the bottom of the stack, and the blade with the flat-sided hole goes at the top of the stack. **Axle:** Slide the retaining ring (washer with a break in it) over the shaft until it snaps into the indentation. **Axle+spring:** Slide the axle into the spring stack so that the flat-sided hole is against the retaining ring. The axle should stick out the other side. If it doesn't, you have the spring stack on the wrong end of the axle. **Rewinder hub:** Push a 608 bearing into the end of the hub shell (cylinder with diamond holes), and into the threaded end cap. You can use the bearings you got with the MMU2S kit for the spool stands. **Hub assembly** If the hub of your spool doesn't fit over the end cap petals, you'll need to assemble the hub in the spool. Otherwise you can assemble the hub and slip the spool over it. To assemble in the hub, put the appropriate spacer on the hub shell, and insert the shell into the spool so that the arrows on the shell point in the direction of the filament winding. Put a spacer on the other end of the shell. Then insert the axle+spring into the shell so the teeth of the spring mesh with the teeth inside the bottom of the shell. Screw the end cap on. If you push the axle toward the bottom of the shell and it bounces back a bit, the spring teeth aren't meshed. Wiggle and shake it a bit to settle it in. **Rubbermaid container cover:** Drill a 1/4 inch (or 6mm) hole at the "6 cups" or "1 liter" mark near the bottom of the container (which will be the top of the drybox). Attach the M6 PTFE fitting to the hole with an M6 nut. **Final assembly:** Place the washer on the end of the axle that doesn't have a built-in spacer. Grasping the ends of the axle with your fingers, drop the ends into the slots on the stand support arms. Put the stand with the mounted spool on the container lid. Thread the filament through the M6 fitting. Put a paper clip on it to keep it from retracting. Put a few packets of dessicant into the stand. Place the container over the lid stand and press it down. Auto Rewind Spool Holder modifications -------------------------------------- I significantly modified the Integrated Auto-Rewind Spool Holder by @VincentGroenhuis at https://www.thingiverse.com/thing:3781815 -- it works well for narrow spools, but a hub that accommodates a 90mm wide spool does not work well. With the 9 spring blades recommended for 90mm width, the spring allows the spool to unwind at least 7 turns before the spool starts slipping. Because a retraction is only about 1 turn of the spool, the spring still has most of its stored energy remaining, and often continues rewinding after retraction, pulling the filament right out of the MMU2S. This doesn't happen with the MMU2S default PTFE feed tubes having 2mm inner diameter due to the high friction, but I replaced all of my feed tubes (including the extruder feed tube) with 3mm inner diameter tubes to reduce friction along the entire filament path. So I had to modify the spring so to use 3 out of 11 blades (for a PETG spring) or 5 out of 9 blades (for a PLA spring), and design an axle with a retaining ring to hold the spring in place. This design has the advantage of needing less plastic to print the spring, instead of filling up the entire volume of the hub with spring blades. I also fixed a coplanar-surface bug in the OpenSCAD rewinder design, which was causing undesirable artifacts in the STL output. In the OpenSCAD source file, all of my changes are indicated by a comment starting with //am.

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