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:
- Use steel nuts for iron pole pieces placed around the rotor, gets coils closer to magnet
- Use large washer or steel square as flux-return over open ends of electromagnets
- Place heat sink on electromagnets
- Use looser, lower-friction bearings
- Use silicone based lubrication vs. petroleum grease which will degrade the ABS.
- Use a higher-diameter diametrically polarized magnet for a rotor for better torque.
Preparing the Printed Gears
When first assembled, the gearbox will turn by finger but will be too rough for the low-resolution stepper to turn. The gears require lubrication and wearing in. I found the best way to do this is to first put grease into the gearbox, bolt it closed, and use a cordless drill set to high-speed with an appropriately sized socket to turn the rotor. Clamped the motor block in a vice and clamped the drill into place and essentially ran it for 6 minutes at high speed in the forward direction. After this, the action was smooth enough to rotate by magnetic force. The gearing remained very precise and without any discernible wiggle or backlash.
5 x 1/4-20 x 1 inch bolts
1 x 1/4-20 nut
1 x #6 x 3/4 inch bolt
1 x #6 nut
2 x skateboard bearings, type 608Z O.D.=22mm, I.D.=8mm, H=7mm
4 x #6 x 1 1/2 inch bolt
4 x 70 foot lengths of 33 gage copper magnet wire
1 x Neodymium ring magnet O.D=1/2”, I.D.=1/4”, H=1/2” polarization=diametric
Here is how I wired the motor and hooked it up to a stepper driver board.
Desired outer dimensions of the motor were found by measuring a Nema 17 stepper motor.
Skatboard bearings were found and mesured.
Bolt sizes for shaft and motor case were decided, holes scaled appropriately.
A diametrically polarized tube-type rare earth magnet was found and measured.
Planetary arrangement of gears was scaled to fit within desired package size.
System designed in Lighwave3D, mesh repair done with Netfabb Basic, and print arrangements made in Makerbot Desktop.
Exploded view of the motor
Wearing in the gear box using a high speed drill.
Video explanation of the inner workings of the 3D printed stepper motor.
Sources for this kind of magnet: