This is a Jig to wrap the perfect coil. After printing your jig you'll need to find a 3mm diameter and 30mm long rod. I used a nail that inserted in a drill an spun and polished with sandpaper. insert the rod in the hole, heat up the back end of the rod and put the rob to go a little inside your jig. grab a little of extra filament and heat it up and fill in the back end of the hole.