GM17 stepper hack

I have thought for some time that the best thing to drive an extruder with would be a small stepper with a gearbox. The reason being is that a stepper motor has close to zero efficiency when moving slowly. Power is speed multiplied by torque, so as speed increases the efficiency increases until the torque falls away due to inductance. A gearbox allows a much smaller stepper to be used because it can be run faster producing more power.

I had a look for steppers with gearboxes, but they seem to be ridiculously expensive. An alternative idea was to replace the DC motor in a gear motor with a small stepper. I couldn't find one with the correct ratio though until Solarbotics started selling replacement gears for the GM17. They allow the standard ratio of 1:228 to be changed to 1:104 or 1:51.



That makes the GM17 very flexible as they also do a magnetic shaft encoder with an integral H-bridge driver. Great for robotics, but it seems a bit under powered for an extruder.



The motor is about the same size, and has the same shaft, as the tiny steppers I got from Jameco for my first attempt at an alternative Z-axis.

I cut away the plastic cylinder that holds the motor and RepRapped an adapter flange to mount the stepper.



Here it is assembled: -


The small pinion gear is a push fit on the motor shaft, but I found that with the higher torque from the stepper I had to glue it on.

I can run the stepper up to 1000 steps / second in full step mode, with a 12V constant voltage bipolar drive. The step angle is 15° so that is 2500 RPM! It has very little torque at that speed, but it gets multiplied by the gear ratio of course.

At lower speeds the current increases and the motor gets way too hot at 12V, so it needs to be driven from a constant current drive. That is what I was intending to use anyway.

Jameco state the holding torque as 140 g.cm, so I have calculated the torque after the gearbox, assuming no losses as: -

Ratio	Max Speed	Max Torque
51	49 RPM	0.7 Nm
104	24 RPM	1.4 Nm
228	11 RPM	3.1 Nm

It seems remarkably high as NEMA23 steppers are only about 1 Nm. Note that the max speed is for about zero torque and the max torque is for about zero speed.

I attached it to a screw drive extruder and managed to extrude ABS at a rate equivalent to 0.5mm @ 19 mm/s with a step rate of 800 pps using the 1:51 gears.



So similar performance to a GM3 with these advantages: -

• No brushes to wear out.
• No shaft encoder and PID software.
• No RFI suppressor.
• Only needs step and direction pins on the controlling micro rather than two or three H-bridge controls and two quadrature inputs.
• The output shaft and final gear are one piece, whereas on the GM3 the plastic shaft is on a metal splined shaft that can slip.
• The clutch is one gear back from the output, so gives higher torque before slipping.
  

The interesting thing is that the projected torque figures indicate that it would be able to do a pinch wheel extruder with its original gear set. I will give that a go next.

I think the cost is about the same as a NEMA17. The advantage is it is smaller and lighter, the disadvantage is it would need separate bearings and a coupler. The NEMA17 will go a lot faster, but has less torque.