The filament I usually buy always comes on a plastic spool. But recently, I bought filament from some other source, and that one came rolled up in a zip-tied coil. Even though I do have empty spools, I did not feel like unwinding the entire coil just to re-wind it onto an empty standard spool... So I designed this simple contractible spool for loose filament coils, i.e. filament which comes rolled up without its own spool. The idea is to have a spool which can easily be contracted for inserting it into the filament coil and then expanded again to auto-center and auto-fit the coil from the inside. The construction of the spool is very simple, as it is built from only two different parts: the hub (2 of them are needed) and the fork arms (2 of them needed for each fork arm). The number of forks is configurable, as are the dimensions of the spool and the opening angle of the forks. Each fork has three 3.2mm holes on each side, which can be used to attach the inner (and outer when not printing) end of the filament. For the default configuration, the assembly is done by three M3x16mm screws and corresponding nylon nuts for each fork. The center hole in the hubs is 8.4mm, so the spool can easily be mounted onto an axis made from a standard M8 threaded rod. The fork arms have to holes for mounting them onto the hub, choose the one that better fits the needed overall diameter. The provided STL files are for filament coils with an inner diameter ranging from about 17cm to 23cm, so they should pretty well fit most of the coils. The hub STL files are for 3, 4, 5 and 6 fork arms. For most applications, I found the 3-fork spool to be sufficiently strong. The SCAD file is fully configurable, so if you need any other configuration, just modify the SCAD file accordingly and regenerate the STLs.When assembling the spool watch out that the hub wings are slightly off center, so you need to attache the fork arms on that side of the wing which goes straight through the center of the hub. Also make sure to use screw nuts with built-in nylon stops, as these will not loosen while the spool's contraction mechanism is used. I prefer to have all the holes are relatively tight. After the print is finished I use a corresponding drill bit to make the hole precisely have the diameter I want it to be. In this case, I use a 3.2mm drill bit for all small holes, and for the hub's center hole I use an 8.5mm drill bit.