Experimental 3D Printable Nema 17 Stepper Motor

STEPPER MOTOR: This experimental build is my first attempt at making a practical 3D printable stepper motor. The goal was to create a 3D printable stepper motor that could directly replace a Nema 17 stepper motor in a RepRap style 3D printer.

This motor has been demonstrated to work directly with a standard Allegro 4988 bipolar stepper driver in the 12 volt range, and has a native resolution of 240 steps/revolution in the full-step mode. It runs best, however, in the half-step mode, with an effective resolution of 480 steps/revolution. It has the standard Nema 17 hole pattern for M3 machine screws. It cost around $5USD in parts and took 3.25 hours to print. Motor designed by David Hartkop for the Pueblo City-County Library District's Idea Factory makerspace programming.

THE APPROACH: Rather than designing a rotor with high angular resolution, this motor uses a single magnet and four electromagnets. The electromagnets are arranged in diagonal pairs, which are wired in series to provide a total of two coil circuits for the motor. The motor has a native resolution of 4 steps per revolution in the full-step mode. This low resolution stepper motor then drives the input of a split-annulus orbital gearbox, which provides a 60:1 gear reduction. Effort was made to match the impedance of a 0.6W bipolar stepper motor using 33 gage magnet wire. Each of four electromagnets was wound with 75 feet of wire, though exact turns were not considered.

RESULTS: This motor works, but is mostly a curiosity at this point. The torque is suitable for driving a RepRap printer's X or Y stages but the coils were found to easily overheat. I recommend attaching a heat sink to the exposed ends of the four electromagnets.

IDEAS FOR IMPROVEMENT:

1. Use steel nuts for iron pole pieces placed around the rotor, gets coils closer to magnet
2. Use large washer or steel square as flux-return over open ends of electromagnets
3. Place heat sink on electromagnets
4. Use looser, lower-friction bearings
5. Use silicone based lubrication vs. petroleum grease which will degrade the ABS.
6. Use a higher-diameter diametrically polarized magnet for a rotor for better torque.