Spannerhands rewinder spool
Description
This is an auto-rewinding spool which fits the beautiful and useful [Spannerhands spool system](https://www.thingiverse.com/thing:2119644) (of which I have published a [remix](https://www.thingiverse.com/thing:4005831)). It allows the PTFE tube from a Spannerhands box to be directly connected to an MK3S/MMU2S printer PTFE tube without a filament buffer. This thing is a remix of Vincent Groenhuis [Integrated Auto-Rewind Spool Holder](https://www.thingiverse.com/thing:3781815) where I've used his parameterized SCAD file to make an 88 mm spool holder without rims and have added an axle and mounts which fit the Spannerhands box. The mounts are supposed to be printed in a flex material and diameter. The length of mounts and axle are chosen such that they are locked in place in the box. In addition, two (or four) flex rubber rings can be slid over the roll to produce increased friction against the spool (see below why you would want to do this). UPDATE 2020-06-26: After some experience, I have concluded that using the rubber rings tends to create a retraction force which is strong enough to create problems when the MMU hands over the filament to the extruder, so I've ceased using them. The principle of operation is that the serially cascaded springs inside the roll are fixed in one end by the axis which is, in turn, fixed to the box by the flex mounts. The springs are fixed in the other end to the roll by cogs. The roll connects to the spool through friction. When filament is extruded the spool turns the roll, increasing the tension in the springs until the spool starts to slip against the roll. From this point the tension in the springs will remain approximately constant. If the friction force at the point of slipping is sufficiently large, the tension in the springs will suffice to rewind the filament once it is ejected from the printer. Two 608 bearings are needed.
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