This is a collection of three different sized compound planetary gearboxes with compact form factors ideal for assembling into robot arms. All three sizes use 38.4:1 compound planetary gearing and include an integral slew bearing. The pancake configuration and convenient mounting surfaces make it easy to build your own robot arm by stacking the actuators a'la the UR3 or Sawyer industrial robots.
The largest actuator (for arm base joints) is about 3.4" in diameter and uses 20 pitch helical gears. It is designed for use with higher torque NEMA 17 stepper motors. The medium sized actuator is about 2.4" in diameter and uses 30 pitch helical gears and is designed for smaller NEMA 17 steppers. The smallest actuator is 2" in diameter and used 40 pitch helical gears and is designed for 35mm "tin can" style steppers. All three have an integral slew bearings that use 6mm plastic balls (airsoft BB's) to support the mounting surface for the next robot arm link.
(And for those of you curious about the teaser photos of the robot arm shown, It's posted here: https://www.thingiverse.com/thing:3327968)
UPDATE: I've posted CAD files for the different version in parasolid format.
The hard part in printing these gearheads is achieving low-backlash. If you don't care much about the backlash, you can print the PlanetCluster gears at a scale of 97 or 98% (and the rest of the parts at 100%) and the parts will go together without too much trouble. However, for a tight fitting assemble with little play, you need a very accurate printer and very accurately tuned extrusion settings.
One important slicer setting is the start point for the perimeters. You should use a random start point, otherwise you will get a noticeable seam in your gear teeth which will produce a distinct THUNK everytime you hit the seam.
After printing the parts, you will want to go over every tooth with a fine-toothed comb and scrape or file away and nits or lumps. You should also sand the bearing races smooth.
Lastly, when printing the smallest gearhead with 40 pitch teeth, you will get better results if you use a 0.3mm or 0.35mm nozzle and use a 0.3 - 0.35mm line width. The larger gearheads will print fine with a 0.4mm nozzle and a 0.4mm line width.
Large gearhead: Print one each of the "...H20.STL" parts, except you'll need 3 of the PlanetCluster parts.
Medium gearhead: Print one each of the "...H30.STL" parts, except you'll need 3 of the PlanetCluster parts.
Small gearhead: Print one each of the "...H40.STL" parts, except you'll need 3 of the PlanetCluster parts.
Start by pressing the 15 tooth sun gear onto the motor shaft. The smallest 15t sun is for a 35mm tin-can stepper with a 3mm diameter shaft, and will need a drop of superglue to hold it in place. The other two versions of the 15t sun gear have D-shaped holes and are designed for NEMA 17 steppers with 5mm diameter shafts with flats.
Screw the gearhead body to the motor:
For the smallest gearhead, you should use 2 4-40 pan head screws and may want to tap the mating holes in the body.
For the medium gearhead, the body and mounting plate are separated into two pieces. Mount the base plate to the motor using 4 M3 flathead screws and then screw the base plate to the body using 4 4-40 flathead screws.
For the large gearhead, screw the body to the motor using 4 M3 flathead screws.
Press nuts into the Ring48 piece:
For the small and medium gearheads, press 4 4-40 nuts into the pockets on the inside of the ring. Make sure the nuts are pressed all the way flush.
For the large gearhead, press 4 6-32 unts to the pockets on the inside of the ring. Make sure the nuts are pressed all the way flush.
Insert the 3 planet cluster gears between the sun and the stationary ring gear. Make sure that the fiducial marks on the top of the planet clusters align with the fidicial marks around the ring. It is vitally important that you use this alignment to get the proper phasing of all of the gears. If you don't get this right, the upper ring gear will not fit over the upper planet gears.
Insert the 18t upper sun gear over the extended hub of the lower sun gear. The upper sun just floats in place - its only purpose is to make sure the upper part of the planet cluster gears are pressed tightly into the upper ring gear.
Place the Ring48 gear over the upper part of the 3 planet cluster gears. If you've aligned everything right, it should slight into place with a little wiggling. If you can't get it slid into place, recheck your planet cluster alignment.
- At this point you will need to drive your stepper motor so that the bearing ball access notch on the ring aligns with the notch on the body. You will need a number of 6mm dia airsoft BB's (20 for the small, 26 for the medium and 39 for the large) for the integrated slew bearing. Shove a ball through the aligned notches into the bearing race. (They should snap into place.) Chase each ball around to the far side of the race with a pointy tool until the entire race is filled. To get the balls back out, you'll notice that there is a hole in the backside of the body that aligns with the ball notch. With the two notches aligned, position a ball at the notche and use a small diameter rod to poke the ball back out through the notch.
I recommend first test assembling everything and then running the motor for a while to wear in the gears and bearing. When things are running smoothly, disassemble and add grease to the gearbox and bearing to prolong the life. With some grease, I've found that the PLA gears are fairly durable. Note, however, that if your motor gets really hot, the gears will wear and/or deform under load very quickly.