This compound planetary gearhead is an anti-backlash version of my earlier Robot Actuator (smallest version): https://www.thingiverse.com/thing:3293562 It has about 50% more torque, significantly less backlash, and also runs more smoothly. It is designed for a 35mm tin can stepper, and requires 35 4mm (or 5/32") balls for the integral slew bearing. It also has a 38.4:1 reduction ratio.
When printing small gears, even on a well tuned printer, the printing irregularities end up being significantly large compared to the tooth size, and produce a lot of friction. You can increase the clearances in the gear design to reduce friction, but then you end up with a lot of backlash. My solution here is to make the ring gears flexible in the radial direction but stiff torsionally. This springiness takes out the backlash and also allows the ring to expand to accommodate irregularities in the tooth profile.
When I get more time, I'll also do anti-backlash versions of the medium and large actuators.
To get as clean a print as possible, I printed these parts with the heated bed turned off (temperature fluctuation of the heated bed with can cause ridges in the print), and I aligned the seams so that it was easier to find and clean up the start/stop irregularities.
You will still probably want to go over each and every tooth with a needle file to remove any lumps or ridges. And on the 15t sun gear, you need to clean up the bore with a 3mm drill bit so that it slides nicely on the motor shaft.
You will also need to run some sandpaper in the bearing races to smooth out any lumps and to remove the seam ridge. I'd suggest test assembling the bearing section alone before assembling all the gears.
To complete the assembly, you will need:
1 35mm stepper 35PM048S8-08001 (Moon's)
4 4-40 nuts
2 4-40 x 1/4" screws
35 4mm or 5/32" balls (plastic or steel)
First put a tiny drop of superglue on the inside of the 15t sun gear and slide it onto the motor shaft. Make sure no glue gets into the motor bearing. Then assemble the motor onto the body of the gearhead with 2 4-40 screws.
Next, press the four 4-40 nuts into the upper ring gear. If they are not completely snug, secure them with superglue.
To assemble the gears, first note that each cluster gear needs to be aligned in a particular orientation to make all the teeth mesh properly. You'll notice that on the cluster gears, there is one tooth that has a bevel on the top. Mark this tooth with a sharpie (see 3rd photo) so that it is more visible. You'll also notice that on the body ring gear, at 120 degrees, there are three sets of adjacent teeth with beveled tops. Mark these with a sharpie as well. When you insert the cluster gears between the sun and ring, the one marked tooth on the cluster should go between the two marked teeth on the ring, as shown in the 4th photo.
Next, slide the 18t sun gear over the top of the 15t sun. It just kind of floats in place, acting as a spacer. Now slide the upper ring gear onto the three cluster gears.
Notice that there is a hexagonal hole in the end of the 15t sun gear. Use an Allen wrench to manually rotate your motor until the notches for inserting the bearing balls line up, as shown in the 5th photo. Push the balls into place, one-by-one, until the entire race is filled.
At his point, you want to run the gearhead for a while using either your motor (or using an Allen wrench in a drill) to help break in the gears. When everything is running smoothly, take it all apart and apply grease to all of the gears and to the bearing balls as you re-assemble. (Note there is a hole in the body that allows you to use a thin rod to poke the bearing balls back out of the bearing for disassembly.)