I made the threads with this http://www.thingiverse.com/thing:193647 . I like to keep these hubs on my spools for easy switching of filaments and it fits most of the spools i use including the cardboard ones. For very narrow spools you may need to trim some threads off by slicing a portion off the top of the bolt portion using 3d builder or other software or just by stopping the print. I like the look of the bearings whether needed or not , and I usually buy extras or salvage them from roller blades