This is a fan mount for a Bulldog extruder, although technically it would work with any NEMA 17 motor. It snaps onto the back of the NEMA motor. This is designed for a tight fit -- and it is tight, but it is removable. I included the DesignSpark file so you may modify it for a looser fit, or whatever else you need. It is designed for a 40mm fan. The Bulldog extruder motor does get hot, which may lead to filament grinding during longer builds. This design helps mitigate that heat. I should note that the Bulldog Extruders do come with a mount and built-in fan. As you can see from the photos, my setup uses a custom mount, so I needed another fan mount. Enjoy