First, after doing some analysis, it appeared that the 3mm pitch 6mm wide belt was unlikely to be able to carry the full torque from the motors. So I’ve switched to a 5mm pitch 15mm wide belt, which while still unable to carry the full torque indefinitely is only a factor of 2 or 3 off instead of a factor of 20 off. Secondly, I added a bearing opposite the upper pulley so that it is supported from both sides. The recommended belt tension for this belt works out to something like 120lb, which is a fair amount of cantilevering, even over the 16mm wide pulley. The updated CAD looks like:
And the newly added bearing can be seen in this section view:
I did a first test print of all these parts and put them together. While there were a few tweaks necessary for the second revision, it looks like this leg is probably usable.
Alert! I’m at Maker Faire Bay Area all weekend in the Mech Warfare area in Zone 2 (May 17-19, 2019 for you time travelers from the future). Drop by and say hi!
If you were left in suspense last time, yes, the robot can walk! Getting it to do so in a minimal way was relatively painless. What I found, which hadn’t happened in earlier iterations, is that many types of dynamic motions would cause the lower leg belts to jump a tooth. Needless to say, this was nearly universally fatal, as there is no direct position sensing of the lower leg. This robot is heavy enough that my simulacrum 3d-printed timing belt pulleys just don’t cut it.
Well, there wasn’t enough time to actually get better pulleys now, so I just tuned the walking to be slow and gentle enough that nothing went awry. Here’s the first bit of a 13 minute video I took of it walking around and shooting targets.
Now, that that was over with, I had a few minor things to finish up before heading out to Maker Faire. I made some covers for the motors to keep BBs out.
And I made a bracket so that I could attach the front and rear target panels to shoulder joints:
And here’s a glamour shot of the whole thing in fighting form!
Now that it was all ready, time to take it all back apart and pack it for shipping.
As mentioned last time, I needed to build a lot of gearboxes and new leg assemblies in a very short amount of time. So, I got to work.
I made a new fixture for holding stators to be extracted:
I turned down 8 more internal gears. To begin with, my mandrel had warped enough from the first gears that I had to add some heat set inserts to hold a cap to keep the gears on. Then on the last 2 gears, I got greedy, went too fast, and my lathe mandrel melted entirely.
So, I had to spend 12 hours printing another one to finish up the last two internal gears, although their roundness was debatable after their encounter with the mangled mandrel.
I also at this point machined out a bunch more rotors, but didn’t manage to capture any photos.
Now for some assembly:
At this point I was 3d printer limited, and when I got to starting assembly, I only had 7 sets printed. Thanks to some very generous help from Beat and Roxi (thank you triply in advance!) I had a second Prusa MK3 that was also working 24/7 on the problem.
Notice how now I’m up to 8!
When I went to put on the backplates, I discovered that due to tolerance stackup, some of the units were having trouble fitting. To move on quickly, I post-machined all the backplates to move the rotor bearing back a bit with a dremel, and then made a little bit of clearance for the sun gear holder screws.
And then, TADA!
Now, in parallel to all that, I also designed a new leg which would mount to the gearbox output. I wouldn’t have time to get a shoulder bracket made out of metal like I had before either, so I needed to design that for 3d printing too.
I made a few improvements this iteration. The biggest was that I added a tensioning mechanism inside the upper leg, so that tension could be increased after installing the lower leg. The old leg was nearly impossible to assemble without breaking it, and was just as difficult to disassemble. Also, I managed to have an actual order of assembly that was feasible and that an appropriate tool could fit in at all places at each stage of the process.
What I didn’t try to do was to try a more mini-Cheetah like geometry, or really optimize for mass or looks or anything. I was trying to get something which would likely work for the length of a Mech Warfare match in as few drafts as possible.
Of course, the first iteration wasn’t necessarily functional. It came off the press at something like 3am Friday morning. I spent the next 4 hours machining, debugging and squeezing until I found about a dozen problems or things that needed to be fixed. Then, straight back to the printer for a second try, and voila, two was all I needed this go around!
Here is the final part-set with all metal bits installed:
I drew and printed up the shoulder in a separate effort, but managed to capture no pictures of it whatsoever until I went to put it all together.
Now, here is a shoulder attached, with the upper leg motor and upper leg installed.
And from the other side:
And, the entire first leg:
After carefully managing my 3d printing queue 24/7 to get all the legs, shoulders, and gearboxes printed, here’s a picture of all the legs on at the same time!
After completing one gearbox, I needed to build at least 4 more of them to replace the lateral servos on Super Mega Microbot (2). So, I got to work. First, I disassembled 5 more BE8108 motors.
Then, I drilled out the rotors, this time using the mill at AA.
Next I removed the stators from their backing. This was painful enough last time, that I tried a new technique using the mill to do most of the work. Unfortunately, one of the stators was critically damaged during my initial experimentation. So, now down to 4 survivors.
I went and printed 5 copies of all the printed parts:
After finally getting the darned thing apart, and printing a new outer housing, I went about re-assembling the whole mechanism. This time, I tried to take care to make the future disassembly less painful.
To start with, I filed down the problematic outer bearing interfaces of the sun gear holder so that the bearings were a slip fit over them. These two interfaces don’t need to be particularly snug, so that was easy enough, if monotonous, to accomplish. I also machined out a some pockets around the magnet hole, to make it possible to just hot-glue the position magnet in place and more easily extract it.
Next, I re-installed the sun gear holder back in the rotor.
After that, I pressed the input bearing into the new planet input:
Then I went about installing the shaft output bearing into the planet output, the planet output into the output bearing, the planet shafts into the planet output, the planet bushings into the planets, and the planet bearings into the bushings.
Those got dropped onto the shafts, and the planet input was stuck into place.
After that, the screws were installed in the planet input, and the stator was fit onto the front housing, using a shrink fit again:
At this point, I aligned the rotor and pressed it and the primary shaft into place.
Now I used my paper strip alignment technique to get the rotor properly (or at least functionally) spaced from the stator.
At this point, the rotor still didn’t spin freely. Because of all the rework I’ve done, and my sloppiness in executing it, bits of the exploded bearings and other detritus had lodged themselves against the rotor and stator. The problematic pieces were small, sub 5 thousandths, but still plenty enough to cause the rotor to hang when spinning. These I carefully extracted under a microscope with a pair of tweezers.
At this point, I had a gearbox that spun freely and seemed mostly correct!
So, last time I had a functioning gear-train, I just needed to disassemble everything in order to replace the outer housing. As I was putting things together, I realized that several custom fixtures would likely be needed in order to disassemble various parts cleanly. Here, I made a giant 3D printed cylinder that the front housing and planet output would bolt to, and then the central shaft could be pressed out.
I somewhat skimped on the printing… even at 25% infill it took something like 14 hours, however the shaft should be relatively easy to extract since it is only held via a mild press fit with the output shaft bearing. Or at least, it would have been had my mild press fit not ended up being tighter than desired, and had retaining compound not seeped over there.
My first attempt at removal just resulted in my carefully constructed fixture delaminating and the entire planet output and output bearing pushing into the assembly. The shaft didn’t budge at all on the shaft output bearing. Then, rather than wait for an even longer print, I installed all the washers on each of the bolts to better distribute the load across the fixture. At that point, removal was accomplished…. kinda. The fixture didn’t fail this time, but the planet input in the gearbox shredded. Fortunately, that was enough to remove the rotor. I could just print a new planet input, and toss the shaft which now appeared very well welded to its bearing.
Rest of the disassembly
At this point, the rotor was removed from the stator, but there was still a fractional planet input attached to it, with the planet input bearing very securely fastened to the sun gear holder, and the rotor bearing, even more securely fastened to the sun gear holder.
First, I shredded the planet input with a pair of pliers.
Then, I was able to get the input bearing off by cooling down the sun gear holder with the compressed air again.
However, the rotor bearing was too well pressed on to achieve that. So I ground it off with a cutoff wheel, after trying a few other things first. At this point, I was finally able to remove the sun gear holder from the rotor, and call my disassembly, as it were, complete!
When I first got the rotor in place, it was clearly not centered properly. Although much closer than in the plastic gearbox, it did interfere with the stator during a portion of a revolution. The first obvious problem was that the primary shaft wasn’t making it all the way through the front shaft bearing. That should have been an easy fix, but for two different very annoying reasons.
When I made the sun gear holder, one of the tweaks I made during my 3d printed iterations was to leave a hole where you could press on the primary shaft. That was intended to be used to install it all the way into the output bearing. Unfortunately, that hole is behind the position sensing magnet. The one that I superglued in place last time overly eagerly.
That magnet has a recess in the sun gear holder which nearly exactly encloses it, with maybe only a few thousandths around it in all directions. There was no way to grip the magnet at all. I tried soaking the junction in acetone for some time, I tried using the heat gun, and in the process ruined the 3d printed outer housing, but none of those loosened it up enough to allow another magnet to retrieve it.
Eventually I pressed on the primary shaft from the front of the motor, and used that to pop out the position sensing magnet.
Now that I had the magnet off, I could press the shaft back into place. Or at least I should have been able to. I made it into the output bearing, but then the pressing became inordinately difficult. I think even more so than in any of my test fits. This may have had two causes… one is that I was using a dowel for the main shaft which is slightly oversized, and second the retaining compound had seeped around into the interior of the bearing, and fractionally cured. Those combined made it both really hard to push the shaft in, and as I later discovered, impossible to press out.
Aligning the rotor and stator
At this point, I had the rotor installed seemingly properly, and it still was interfering with the stator. After much experimentation, including partially exploding the rotor bearing on the sun gear holder, I arrived at a technique that allowed me to align them properly and simultaneously discovered that my intended mechanism for registering the rotor to the sun gear holder was, at the least, sub-optimal.
The sun gear holder has 4 M3 flat head bolts which fasten it to the rotor. I had oversized those holes to 3.1mm, and put tapered countersinks on each. I had planned on the countersinks forcing the rotor to be centered. However, it didn’t look like that was working as I had expected. Each time I would loosen up the bolts and re-tighten them, the rotor would interfere with the stator in a different way. Eventually, I was able to center them by sliding slips of paper all around the stator between it and the rotor, then tightening the bolts down.
Then I spent a few hours, painstakingly under the microscope, picking out grains of steel that had managed to find their way to the rotor, most likely from when I partially exploded the rotor bearing.
At this point, the rotor did seem to be aligned properly to the stator, everything moved freely, and the gearbox worked as expected. I just need to disassemble everything in order to install a new outer housing to replace the one I destroyed while attempting to remove the position sensing magnet. That is its own story, for next time.
Both of these seem to have actually adhered to the tolerances I requested, so thankfully it won’t be too hard to fit everything together. However, getting everything together for the first time did involve a comedy of errors — a lack of planning for assembly order, a lack of foresight into how things would be *dis-assembled*, a stubbornly stuck shaft, and plenty of broken parts.
Sun gear holder assembly
The sun gear needed to be bolted into the rotor, and have a bearing installed that interfaces between the rotor and the back housing. That bearing interface had a similarly poor tolerance as the interior interface from last time, and thus the bearing needed to be pressed on. I had pressed one on as part of verifying that it would work at all, but then didn’t realize that the bolts to installed the holder to the rotor wouldn’t actually fit down. I ended up grinding a flat on each bolt so that it could fit around the bearing.
The front housing is slightly different than my original plan. I was initially going to machine the outer housing included into the front housing as one piece. However, having a shop do that turned out to be prohibitively expensive, both because of the need for a 4th axis, and also because of the depth of the part. It was looking to be something like $500 per piece at ~20 quantity. Thus, I simplified it into something that wasn’t very deep, didn’t need more than 3 axes, and had just the attachments necessary to maintain a rigid drive train. I figured I could for now continue 3d printing the outer housing.
That said, the front housing still has a lot attached to it. The output bearing fits in the central hole, the internal gear rests on the interior of the cylindrical protrusion, the stator rests on the outer surface of the cylindrical protrusion, and the outer housing bolts to outer recessed holes. Additionally, in the future, the position sensing board will fit into the end of the cylindrical protrusion.
The internal gear ended up closer to a slip fit, which meant I needed to use retaining compound if it was going to serve any practical purpose. That would also mean it would be really annoying to remove. Fortunately, I did manage to install the output bearing before using retaining compound on the internal gear, as it does need to be installed first. However, I didn’t manage to install retaining compound on the output bearing, which wasn’t exactly a slip fit, but did press out easily with my hands. That would come back to bite me a bit later on.
My next, relatively minor problem, occurred when I was test fitting the stator in place. That surface was within tolerance, however, the front housing still required cooling with an upside down duster before the stator could be installed easily. I got overly excited and installed it before having the outer housing installed, so had to take it off again. Then, after I had installed it again the second time, I realized I had not aligned the stator so that the phase wires lined up in the proper location on the outer housing. That, I just let slip, which turned out to be fine, since I had to tear it all apart later anyhow.
Next up, I installed the planet output shaft bearing. It was a very loose fit, so retaining compound was used, and possibly applied too sloppily as evidenced later. Also up were the planet gear shafts, bearings, planet gears with 3d printed bushing adapters, and the planet input with input bearing installed. Those all came together fine.
Next I installed the rotor which had the sun gear holder attached. It seemed to mostly fit, and looked like the rotor and stator would clear, so I got excited and super-glued the position sensing magnet onto the end of the sun gear holder. That way I’d be all set to power up the controller. Yeah, right, if only it were that easy!
Next up, my challenges in getting the rotor and stator aligned…
As discussed last time, the sun gear holders I had CNC machined unintentionally had a slightly undersized bore that the sun gear was going to fit into. The allowance was large enough, that there was no way I was going to press it into place as is. So, I decided to try a shrink fit, but before I did I wanted to do some math to verify that it was possible with the temperatures I could easily achieve and that I wasn’t going to explode (or even just fracture) the aluminum part from over-stressing it.
All the math
Referring again to the Machinery’s Handbook, it has a formula for a steel shaft / cast iron hub, and also for a steel shaft and hub, but nothing for a steel shaft and aluminum hub. I’ll just do a minimal derivation with some simplifying assumptions here to see how close it is.
First, I’ll assume a solid block of aluminum hub, and a solid shaft. In actuality, the hub is only 15mm external diameter, and the shaft (sun gear), is hollow with a 4mm internal diameter. Next, 6061 aluminum has a yield strength of 386MPa, I’ll aim for 200MPa maximum. The modulus of elasticity is 68.9GPa. That works out to 200MPa / 68.9GPa = 0.0029 mm of shrink per mm of diameter. Thus for an 8mm diameter shaft, I could have a maximum shrink of 0.023mm. My shaft is 7.998mm, which means that the holes should be no smaller than 7.975mm. I have some parts that big, but maybe only half of them. However, given that my simplifying assumptions were conservative, I’m probably in the right ballpark for all of them.
Giving it a try
I don’t have any dry ice on hand, but I do have compressed air canisters and a heat gun. So I pre-cooled the sun gear in the freezer. Then I heated up the sun gear holder with my heat gun set to maybe 260C. Next, I turned the compressed air canister upside down and performed a final cooling pass on the sun gear holder. After that I — very quickly — set the holder and gear on my press and forced them down together.
Granted, there is probably no way I am ever taking them apart now, but I guess I don’t have a pressing need to try.