An extuder motor indicator made from a 3D Printable Jet Engine's fan by CATIAV5FTW - scaled down to 25%, with a cone or without.
Link to this awesome printable jet engine here: https://www.thingiverse.com/thing:1327093
There are three versions:
With an integrated cone, with a separate cone with a spiral strip and without a cone. Each of them has a separate version with a brim around blades as the ones without a brim proved not printable for me - as they were lifting from the bed due to not enough surface to adhere (but it seems that it might be the property of the filament I wanted to use).
Versions with a brim have a 0.1 mm high brim helping with the adhesion. You can remove it carefully with an xacto knife later.
There's also a spacer to move the indicator further away from the motor, if your magnet is 3 mm thick (8x3 mm is a standard so you probably need this).
The spacer is supposed to fit inside the little cavity on the base of the Fan for easier positioning ;)
Recommended layer height: 0.05-0.1 mm.
You also need a 8x3 mm magnet and a CA glue (or similar) to glue the magnet, spacer and fan together (just be careful not to glue the fan to the extruder motor ;) ). You will also need to glue the spiral strip to a cone if you choose this version - get a toothpick to apply the glue and a lot of patience ;)