This is a tool for making compression springs out of 1.75mm filament. It's kind of a proof-of-concept design, as it only makes one particular size (20mm outer radius, 20mm rise). The one in the pictures is an earlier prototype.

The idea is that you put one end of a piece of filament into the hole to hold it in place, then heat the filament using a heat gun or hair dryer just enough to make it soft. You wrap it around the tool in the grooves, then let it cool. There's a horizontal groove at the bottom to make the last loop flat if you want. Once it's cool, you can remove the spring from the tool and it will keep its shape.

It's best to make this out of higher-temp plastic. I used PETG, and made the spring out of PLA.