Stepper-driven extruders have great consistency, but the cost of the motor and motor driver can be prohibitive. This is a simple, unregulated DC motor design that relies on excessively powerful torque from a 12V 30RPM gearmotor from Jaycar.It will drive 3mm PLA faster than I feel comfortable with.It uses the http://www.thingiverse.com/thing:2192 mendel_misc.inc include file.The motor's 4mm output shaft has a flat, and this is used to help anchor a knurled M4 nut with a 4mm hole drilled in it. Punching and solder are used to fill in the flat and anchor the nut to the output shaft.A couple of 16mm M8 washers and a 608 bearing are held on a 40mm M8 bolt with a wing nut. The wing nut must have one side filed flat to fit next to the motor. This bearing is movable due to deliberate slack in the mounting hole.The motor is inserted into the extruder body output shaft first, and the knurled nut is aligned with the filament hole. A 40mm M4 machine screw, two washers and a nut are used to clamp the motor in place.The unit then behaves much like a stepper-driven extruder from here, but can be driven by PWM 12V DC.