This thing keeps fingers, wire nips, pliers, and other assorted appendages out of the extruder fan; items which may be bad for fan blade health. I designed it after destroying my original Ultimaker fan with a pair of snips.I used this with http://www.thingiverse.com/thing:17768, but it should fit any fan shroud setup using the original 50 mm body and 40 mm hole pattern fan that comes with the Ultimaker kit. You will probably need slightly longer M3 screws to install this.