When i got my hands on a small stepper motor from old scanner thought that i need to try it as an extruder motor... 

Downloaded the datasheet and the spec wasn’t promising: 96 step, 24V, 15ohm, small torque and the worst part... pull-in torque and speed significantly different to pull-out. 

Hmmm nice begining. But i didn’t want to buy any parts for experiments right now - got my eyes on bondtech dual-drive extruder gears and after making five different 3dprinters projects my hobby piggy bank is dry as an desert...

So... Designed a 21:1 planetary gearbox.

3d Printed all parts with PET filament, assembled and oiled everything. Without the motor attached there was an nasty friction when turning the gearbox so i’ve spinned it untill the friction got lot lower.

Mounted the motor and it was spinning... few times and got very hot - so hot that i didn’t bother to test it in my 3d printer... the current was set as low as the motor could turn... Epic fail :(