Robot Actuators

by LoboCNC Dec 18, 2018
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Fantastic design, thank you so much for designing it and sharing it.. I have bought all of the parts, well almost all, I do not seem to be able to find out information on the circuit boards used.. are they a stock standard issue?

Do you have the CAD files from the actuators/gears? I want to modify them but I can't with inventor.

Thanks in advance!

There are Parasolid format CAD files already posted. Can Inventor import those?

Is there anyway I can edit the dimensions of your files?

You would probably need Solidworks (and the original Solidworks files) in order to modify the original design dimensions.

Hi sir
thank you for your sharing ...
please I printed the all H20 parts ... with infill 35% ... The parts don't seem to assemble, PlanetCluster seems to be to big.
I read the instruction and I would like to know what you meant by :
...you can print the PlanetCluster gears and 97 or 98% (and the rest of the parts at 100%) ...
on which parameters in Cura or on printer i must set at 97 or 98%...
thank you

Just search for "cura scale tool" for instructions on how to scale your model. You'll want to use uniform scaling.

I saw your comment that this robot can lift approximately 200g.
I know specification of motors(45Ncm, 59Ncm ... etc) but don't know how to calculate the weight.
If you don't mind, please let me know how to calculate the weight robot can lift.

  • I'm also curious about the logic you used when you choice the motor. why 45Ncm for here, why 59Ncm for there...etc
    Thank you

Calculating the payload of a robot is kind of complicated. To a first order, you need to calculate the maximum torque load each joint sees (weight x horizontal distance) for both the payload and also the weight of each link of the arm that that joint supports. Then you've got to match that to the torque capability of the actuator (max motor torque x reduction ration x % efficiency). The % efficiency is usually the big unknown, especially with 3D printed gearing. Also, the maximum motor torque for stepper motors is highly dependent on the speed, so the max payload will also depend on the speed. And then, unless the arm is moving very slowly, the dynamic loads of all the joints moving can create additional loads on each joint. To figure out these, you need not only a full dynamic mathematical model of the arm, but also very accurate measures of the mass and inertia of each link of the arm. And all this has to be done for each joint of the arm. So as you work your way through these calculations, you'll see that the base joints need higher power actuators than the more distal joints for a given payload rating.

Great design! I ended up printing the gears with PETG and using magic lube as I do with my 3D printers. I got the first large one built and broke in, but once I added the airsoft BB’s, the unit locked up. I can move the bbs a bit but is this an issue with the track they are housed in not being sanded enough? The top plate is locked firmly in place and I’m not sure if I can get the bbs out again. Anyone have any info on this? Thanks!

If the top plate and the housing are still lined up to form a slot for the BB's, there is a hole in the bottom side of the housing that allows you to take a small rod and push the BB's back out of the slot. In general, I'd suggest test fitting the BB's in the bearing races without the gears in place so that you can make sure the bearing section runs smoothly. And the gears should also be tested and run in separately (as you've already done) so that you can be pretty confident of everything working in the final assembly.

I got it! It was the three planetary gears i was printing at 100%, which caused the whole track to drive upwards. I should have read the instructions more carefully and not think it was 97-98 infill. Duh lol. Works great now

Yup I found thebaccess hole, goodninfo on testing the bb race alone, that I haven’t tried yet. I suspect there’s an issue in the printing where one side is filling with bbs and causing the side to be higher, the top of the track looks a bit chewed. I may reprint with the .3 nozzle. Thanks

I got M3 flathead screws situation.
which milimeter should I use?

Oh, sorry, the flathead M3 screws for mounting the stepper motors should be 6mm long.

Thank you :)
What about
{ 4-40 flathead screws } at actuator? 6mm also?
and what about { 2-56 x 1/8" screws } at robot arm?
Should I purchase socket head screw?

For the 4-40 flatheads, 6mm (or 1/4") long should work fine. And yes, get socket heads for the 2-56 screws for mounting the Tic T500 boards

Thank you so much


I printed the H30 parts, all sliced with CURA at 100% scaling. The parts don't seem to assemble, PlanetCluster seems to be to big. Any with the same problem??

Depending on exactly how your printer & filament are printing, you may need to scale down the planet cluster gears. They are scaled for a printer that is perfectly accurate and any irregularities in printing will make them really hard to assemble. Try printing the cluster gears at 99% or 99.5%. Also note that you have to get the alignment marks on the gears lined up exactly with the marks on the stationary gear. I usually end up counting the ring gear teeth and pinion gear teeth between the adjacent planets to make sure the 3 planets are exactly equally spaced. If you are off by a tooth, the second level won't fit together.

hi! I have the same problem. I would like to know the real size.

First of all, I really like your design and I can't wait to use it in my own projects.
I have a question about the gear ratio. I wanted to know how to calculate the gear ratio for this kind of planetary gear set so I looked it up but couldn't find much about it. With the information I did find, I tried to calculte the ratio for this gear set but it's always around 70:1 and not 38.4:1. So my question is, how do you calculate the ratio fot this type of gear set?

I did some research on the subject a while ago and wrote a couple of ideas here: https://juangg-projects.blogspot.com/2018/02/split-ring-compound-epicyclicplanetary.html Just in case someone finds it useful.

Thanks - nice write up!

Calculating the reduction ratio is a bit of a mind bender. If we define R1 and the number of teeth on the lower ring gear, R2 as the number of teeth on the upper ring gear, S1 as the number of teeth on the lower (input) sun gear, P1 and the number of teeth on the lower planet gear and P1 as the number of teeth on the upper planet gear, then the reduction ratio between the input sun gear and the output upper ring gear is:

(1 + R1/S1) / (1- (R1P2)/(R2P1))

I saw the Harmonic gear reduction you did previously, is this an extension of that or is this just a helical planetary gear reduction? I see it attached onto your universal joint robotic arm and i get that backlash reduction is king, But have you tried running this filled with lapping compound or even toothpaste initially as a break in compound. Also what lube have you been using for this? a Molybdenum disulfide based lube? or maybe a dry ptfe or rulon J based coating?

Also have you noticed a difference in smoothness based off polished or unpolished BB's?

You may have answered these questions somewhere else or possibly in a comment that i missed so sorry before hand if you did.

These compound planetary gearheads are completely different from the harmonic reducer. Running them with lapping compound or toothpaste, as you suggest would be a good idea, but I mostly just ran them dry for a bit. Also, before assembly, I went over each tooth witha needle file to remove any nits or bumps. As for grease, I'm just using a med. weight lithium grease.

For the BB's I've used both regular ones (I think they are mostly ABS) and also some biodegradable ones, but the biodegradable ones were much softer. I also found a source for acetal BB's (http://www.craigballsales.com/ACETAL_c_286.html) which are definitely preferable. That said, I haven't noticed much practical difference between any of them when running the actuator.

The acetal BB's are interesting. I had done a bunch of testing on on caged and non caged 1/2" hex bearings a couple of years ago. I had found at the time they where surprisingly reliable, but that the BB's made a significant difference. With stop build day around the corner I should have some free time after work soon. Ill take a go at printing the arm out. Though im probably going to PC instead of pla, and probably a nylon 12 for the gears. Also on the spots that you used UNF bolts, is there a reason you choose to go that direction instead of using plastic forming screws?

Thread forming screws would probably be better, but I never have a good selection lying around, and unless you order them from McMaster or something, they aren't so commonly available for most folks.

So I printed 3 of the DiffPlanet30 for testing. I ran all three for 24 hour, in total, I did a few things in hopes to simulate a more realistic use case. (ie. 50% duty cycle, varying the speed and rotation, and had a light load on all 3) After looking at all of the planetarys it definitely seems to wearing quite a bit no matter what lubricant i used. Backlash on all the drives has increased, and not surprisingly the average current drawn by the motors has decreased as well. I believe alot of the play in the system can be related to wear on the inner and outer races of the bearing. Have you noticed a significant amount of wear on yours as well? Any suggestions or thoughts on how to possibly reduce the wear? It could be that the pc/abs blend i used wasn't the best choice, im thinking of going to a peek or ultem blend and trying that for the outer housing and the top ring gear. and possibly a nylon for the planet gears.

That's pretty much what I'd expect - that with initial usage, all the high spots will wear off and you'll end up with more backlash but higher efficiency. As you wear the gears down to smoother surfaces, however, I'd expect the rate of wear to decrease.

I'm not familiar with PC/ABS blends, but I've had pretty good luck using just plain old PLA. The only real drawback is if you get too much motor heating. Here is a really nice write-up on practicalities of 3D printed gears:


He rates nylon as the best choice (not surprising) followed by PLA then ABS. If POM weren't so difficulty to print (or so I hear), it'd be a great material to try. All that said,I don't think you're ever going to great life out of FDM printed gears, but for hobby and demonstration applications, you can get pretty far.

Arrgh. Finally broke down and milled open the big actuator. I could not get that fallen nut to let the gears move to get the slot in place to pop out the airsoft BB's. So, I had to crack into it without ruining too many parts. Used my CNC machine and got through but did slice two of the planetary gears. Oh well.

We really need a way to access that inner cavity for maintenance when the unit can't spin. I've had 3 occasions now cause me to get stuck. 1) When the stepper motor shaft is no longer turning the main sun gear due to wear/glue coming undone. 2) When the planetary gears get crushed due to speed thus it can't spin and 3) due to a fallen in nut.

BTW, despite these problems, your design is gorgeous and these really are exciting actuators. I love the work you're doing. It's got me pretty excited to help you out.

Picture attached of milled out actuator.

Have you thought about milling into the ring to make a second exit hole that lines up with the one on the body? Perhaps you could then use a toothpick to work the balls out. Perhaps a hot iron could also form an exit hole.

Built the big and medium actuators. It was a ton of work to file and sand to get everything to fit correctly. Problem is I had one of the 4 depressed nuts fall into the casing when trying to screw on the arm because as you stick the machine screw in, if you push on the nut, it slips out. Gettng the airsoft BB slot to line up requires running the stepper motor, but I'll likely destroy the gears with the loose nut as I do that. The casing really needs an easy access panel. Any thoughts on a design for that?

Your medium actuator actually is more accessible with that base you have for it, so that actuator is much safer/easier to disassemble if anything goes wrong.

On getting all the gears to fit & run smoothly, yeah, it is a bit of work if you really want to minimize backlash. If you don't care so much you can print (or re-print) just the planet gears a little undersized for a looser fit all the way around.

Well, that's unfortunate. On mine, the nuts pressed pretty firmly into place. I should probably add to the instructions to superglue the nuts in place if they don't fit tightly, although redesigning it a 2-piece part, as you suggest, may make more sense.

Any way you could post the latest CAD you used with the helical gears? I want to try to modify the large actuator to use the same approach you used on the medium (H30) gear where you had the bottom base? That was a slick solution because you can then access from the bottom if anything goes wrong. All it would do is lift the height of that actuator by a bit (or not all). During debugging of the actuator I'd screw all the machine screws in to hold the base. Then when mounting to the basecone and shouldershell I would remove the machine screws and just sandwich both together using the same holes. That's pretty minimal modifications and could work.

I did post STEP CAD files (embedded in the ZIP files) but for some reason, Solidworks blew up the 30 and 40 pitch versions into absolutely huge files that are virtually unusable. I'm going to try to re-export as parasolid files.

Jeff, could you possibly post the CAD files? I want to get the NEMA 17 connected to the sun gear, but of the 10 NEMA 17's I have lying around, they all have a round shaft without the flat part. So, I'd like to modify the sun gear so it can fit. Thanks.

I'll post these shortly. In the meantime, I have posted the CAD for the full assembly of the robot arm (https://www.thingiverse.com/thing:3327968) that has all of the actuator parts in it.

WE-R2.4 Six-Axis Robot Arm
by LoboCNC

I modified the sun gear this morning to support a stepper without a flat shaft, added 2 M3 threaded set screw holes, but after printing realized it has straight gear teeth instead of the angled ones. Would love to get an updated copy of the CAD file that has the angled teeth. Although I may just go ahead and get the exact same steppers as you to make my life easier. I also think those set screws won't help much given that I can't make them too tight in an M3 threaded hole in PLA due to the lack of strength.

I attached a picture of how the print came out vs your angled teeth.

Oops! I originally build the robot model with actuators using straight rather than helical teeth, and I must not have ever updated the robot model using the new helical actuators. I'll post STEP files for the actuators on page for the Robot Actuators. The straight-teeth gearboxes will work, but the helical ones are much smoother.

REALLY NEAT! Thank you for taking all of the time to design this & for sharing it with the world. This is really cool.

WOW!This is absolutely brilliant!Congratulations on your design!

Thanks! I hope you can find some use for these.

Kudos!! Great work!! Thank you for inspiring!
Which driver do you recommend?
How do you conceal cables?

The driver I'm using is a Tic T500 from Pololu, However, its standard firmware doesn't support multiaxis coordinated motion, so I did my own version of the firmware. For anyone interested in re-flashing their Tic T500 firmware with my version, you can download it from:
I've also included documentation and a Windows test utility program.

As for the cable, as you can see in some of the photos, they aren't really concealed - they have to jump around each joint.

Thank you for sharing!

Wahoo ! You are a very good mecanicien (I'm one of us ;-)

Guys : There is A LOT OF WORK HERE !

Really neat...is that smaller motor the ones you can find in injet printers and the like??

Really cool robot build...very neat!

The particular motor I'm using is a 35BY48S011 that I bought surplus on ebay. With a minor modification to the bore of the sun gear, you could also use one of these flat NEMA14 motors which is actually a little lighter and has a little more torque: https://www.omc-stepperonline.com/nema-14-stepper-motor/round-nema-14-bipolar-09deg-5ncm-708ozin-05a-85v-%CF%8636x12mm-4-wires-14hr05-0504s.html