3D Printable 20:1 Cycloidal Gearbox for NEMA 17 Stepper Motors

Unlike a traditional gear based gearbox, a cycloidal gearbox uses a wobbling, eccentric disc instead of meshing teeth. It's compact, low-backlash, and packs a big reduction ratio into a small space, which is why it's great for robot joints. Now made for a motor as small as a NEMA 17, with a 20:1 reduction ratio.

I made this for a robotic arm (currently in progress as of todays date) which needs precision and strength. The design can be altered using CAD software like Fusion360. .3mf files are included.

40x52x7mm 6808 2RS Bearings

12x8x4mm 2RS Bearings

10x15x4 2RS Bearings

D4xM3x35mm Roller Bearing Shoulder Screw

Print all of the parts

Total 6 parts. This is just the gearbox, if you would like to apply this to your own project, download the 3mf files and model them using CAD software, I used fusion360. The output is delivered to the outer ring instead of the gear holes.

Print settings: (PLA)

0.12-0.2mm layer height

2 wall loops

Tree supports: Enabled for CamShaft, MountingBase, TopCap,

1. interface spacing : 0
2. Top Z Distance: 0.155mm

Infill: ≥ 10%

Beware of your printer's tolerances. Sometimes bearings may struggle to fit or be completely loose in certain parts. If this is the case, please take some time to calibrate your printer, and/or your filament for the best results.

Carefully Press the 12x8x4mm Bearings into the center of the discs. This May require a hammer, It is preferred to have a slightly tighter fit for longer use.

Press the 40x52x7mm Bearings into the outside of both caps. Then press the 10x15x4mm bearings into the center. For the bottom cap try to center it with the height of the bearing as much as you can.

You should have all of the parts prepped by now.

Insert the bottom cap into the bottom of the outer ring, with the center bearing's flush side facing up. There should be a gap between the center bearing and the bottom of the assembly.

At this point do not attach the top cap yet, it should look like the first picture (without the thing in the middle obviously)

After putting bearings in the gears, push 1 of the gears facing upwards, flush bearing side up. Then push the other gear flush side facing down, there should be a gap between both gears of roughly 0.5mm. To make sure you do it right, you shouldn't feel any friction when spinning the gears.

Position the gears as shown in the 2nd picture(top view), you should have a hole that runs through both gears, with a slight offset from the bottom disc. Make sure that the red circled notch is facing down

Once Positioned and oriented correctly, insert carefully into the outer ring and bottom cap assembly from step 4. By this stage you should be able to stick your fingers into 3 of the holes in a triangle and be able to spin the gears smoothly. Or you can try spinning the Camshaft and watch the gears wobble and move their holes slowly.

Optional Bonus: before adding the gearbox you can add silicone grease to help with the smoothness.

Same as the bottom cap, flush side facing into the gearbox, press the top cap into the top of the gearbox, if done incorrectly you can always pry it open using a flathead screwdriver or chisel.

make sure you can see through the holes, spin both sides to align them, you should have 6 holes in which you can put the screws all the way through

Add M3 nuts(nyloc or regular) (you may need to hammer them) to the hexagonal inserts on the bottom side of the mounting base.

Using 6 screws, from the top cap, insert them through the whole gearbox, the threaded part should be sicking out of the other end of the box.

If the previous steps were done right then you should be able to insert the screw fully through the gearbox without any resistance or friction.

Before mounting the Gearbox, mount your stepper motor using 4 M3 screws, insert them through the top of the mounting base.

After inserting all 6 through the gearbox, align them with the 6 holes on the mounting base (other side of the nuts) while also pressing the camshaft into the stepper motor's shaft.

Screw the bolts in place, make sure not to over-tighten and spin the outer ring, you should feel a fair amount of resistance but if you are able to, the cam should be spinning quite fast.

Once you turn on and run the motor the outer ring should be spinning quite slow but with a lot of torque.

This is what the finished result should be like. Obviously you cant see inside but the outer ring should be spinning as the gears wobble back and forth. This gearbox is being used in a joint for a robotic arm.