Replace missing fan blades.One blade snapped off a 9-blade, 40mm extruder fan. I hate to trash a good fan, and considered applying an entire new ring of blades with the method loco uses here: http://www.thingiverse.com/thing:351835 But since only one blade was gone, I cut one replacement from a ring of blades. The file, blade ring.stl, was designed to fit over a blade hub with an OD of ~21.7mm. The blades have an OD of 36.5mm at their tips. Blade thickness is near 1mm. Suggest printing with the thinnest layers and slowest speed possible. Cut the blade(s) from the ring, and position it carefully. Rough up the mating surfaces, and attached it with 5-minute epoxy. After the glue is completely cured, file away any excess material. Extra weight could make the fan vibrate. Sharpen the blade edges a little, and generally smooth things out for good air flow.