Yeah, yeah…. I know, I know, the SSL certificate expired again. I’m glad like 5 of you still e-mail me about site bugs. I changed my hosting options in January and I guess just I had to log in to click the button again, but had been too lazy to actually do so. Not like anyone reads websites any more, they just want me to stream working on stuff or make short form videos. All I ever do now is log into the admin panel and delete spam comments anyways!

Anyways, Operation IDIocracy and the New Robot Trap House basically ate my entire 2023. With that said, by about December 2023, I had unpacked enough of my tools and machinery to the point where making something was plausible again. I figured a new bot build would be a good stress test and will force me to unpack and organize more things as I needed them.

With that being said, I was also broke, so no new 30lbers! The idea was to see what I could bang together for Motorama using just my vast trove of random parts, with maybe a McMaster order thrown in. As expected, that lasted all of maybe 15 minutes.

Welcome Back to Stance

Almost a decade ago on this very website (scary), I built the world’s first, and somehow still only, dual 45-degree angled spinner called Stance Stance Revolution. It was built as a bit of an inside joke based on Counter Revolution from BattleBots’ 2015 reboot season (and RoboGames before that) and Plan X, which caused some Internet Consternation because its (reversible) weapon usually spun downwards. So let’s make a bot where you have no idea which way it’s spinning, but it’s alright because I don’t either!

It somehow kicked ass. While it never won anything outright, it had a terrifyingly good run at some of the old MassDestruction events held at the Artisan’s Asylum (then in Somerville) makerspace. After these events were done, I kind of just put the husk away, swearing I’d rebuild it at some point. Like 27 vans and 5 BattleBots season later, IT WAS TIME.

The plotting for SSR2 actually went back a long time to around late 2020, after I’d moved to the Old Robot Trap House, rallied the troops to build the first chassis of Overhaul 3, and then decided to skip BattleBots’ 2020 season due to my team being dispersed all over the country and travel being very difficult. So I was basically thinking of other things to build (the same thought turnovers lead to random projects such as the Omnibot reboot)

Here’s the first concept of the new Stance Stance Revolution I made around then. It’s just a solid blob model with no details, but it conveyed the vibe of version 2. The biggest change is to remedy a major problem that version 1 had: It didn’t even have stanced wheels. Basically I was thinking of making conical wheels, whether by a hot wire foam cutting jig or 3D printing little TPU cones or something, so the whole drivetrain sat at 45 degrees, in-line with the weapon disc on each side. Extreme camber.

I basically didn’t even think about or look at the concept until late last year, when I was considering my options for Motorama. I picked it back up again and decided to take it more seriously. One of the challenges was of course how to even drive the wheels?! I brainstormed and chicken scratched out some ideas, including putting motors parallel with the wheel face (so they, in the global coordinate frame, were 45 degrees angled up) and also using bevel gears built into the side of each wheel mating with a pinion embedded in the frame.

From there, I turned my attention to an old friend, worm gears.

Motors parallel to the wheels presented a packaging problem because they would stick out a lot, especially motors with gearboxes, and I’d likely have to raise them fairly high to fit them in the frame. The bevel gear idea had more merit, but I’d still need to run a geared motor and design something to split the power to both wheels, like a layshaft in the middle. Ideally I’d be able to arrange a motor long-ways in the robot and have it drive both wheels using a single shaft.

So why not strip two holes with one impact driver…. and handle both the 90 degree + 45 degree power transmission turnaround and the gear reduction in one? Open worm gears aren’t super commonly seen in robot fighting, I think more due to the need to precisely align two gears. The general lack of backdriving capability also could make your drivetrain more vulnerable to sudden torque loads, which could shear off the teeth. Nonetheless, I think this approach is under-loved and therefore the solution had more appeal to me.

Above, I’ve made a few worm gear toy models to play with both in CAD and IRL. I wanted to get a feel for what my sensitive variables are when it came to making the mounts for these. The bronze gear was ordered from SDP-SI and its matching worm gear was just a generated profile by Autodesk Inventor. It sits in a little frame to be made from whatever material I had loaded in the Ender flock at the time.

The motor is a random nose hair shaver motor or something that I got in big sacks off AliExpress. One of my more recent habits is just going to AliExpress and searching for abject parts – sort price by cheapest and find the you-pull-it junkyard part equivalent. There’s a lot of sellers who sell harvested parts like from some broken Xiaomi drone. That’s how I scored a bag of drone motors for about $1.90 each, by searching “brushless motor” and sorting by price increasing. Trashcopter and its friends are basically the outcome of that search!

These motors were built like regular 1806 class R/C outrunner motors, but had an extra long shaft which extended out both sides. It seemed almost ideal to use for the drivetrain, so I decided to design around it. I think they were like $1.50 each or something! Clearly removed from some device, with wires of inconsistent lengths and color orders. They could certainly have less exciting retirements.

The assembly was helpful in deciding if I wanted to make a fully integrated “print-in-place” hub plus gear, such as out of nylon, or order properly manufactured gears and then machine them. This decision was also dependent on how the wheels will actually get mounted to the gears (or shafts).

For instance, if I could position the gear very close to the wheel, then it might be worth designing the wheel hub with a worm gear stump on it. If it HAS to go through a shaft, then I’d rather buy molded or machined gears and eliminate one source of precision loss (3D printed parts are never perfect, you can only design around the inherent unevenness).

To answer this, I had to think more about the chassis layout and see how real part dimensions will completely destroy my blob model and make me start from scratch.

I began a new sketch model where I imported the parts in question and just rotated and clicked & dragged everything into place for a visual. The chassis was defined from a rectangular block that had chunks successively slice off each end until I liked how it looked.

This immediately showed me that the biggest problem I’ll face is the diameter of the worm gear is going to forcefully drive how big the wheels are… and my goodness did they have to get BIG.

If I tried to make the wheels smaller, the worm gears ride so low as to be basically outside the bot. The worm gears I had in mind were 30 teeth; I clearly had to go to 20 teeth or even smaller in order to make this fit with wheels that weren’t 5.5″ in diameter.

And yes, I briefly did a design study on What if I gave up on life, a.k.a just used big foamy chunks like Susquehanna Boxcar but with straight wheels. This obviously gave me plenty of volume for everything and was perfectly reasonable, if I wanted to give up on life.

I decided to back down to a 20 tooth worm gear instead. This is kind of on the edge of what ratio I thought would make the bot driveable instead of twitchy or squirrely-fast, but it did allow the gear to be packaged fully inside the chassis while retaining reasonable ground clearance.

The earlier question of “Could I 3D print the worm gear and an integrated wheel hub” was answered and it turns out no I could not because of the need to clear the motor diameter. A dead shaft in this case would actually have fairly little support on its bottom side as well, so rigidity would be horrendous. I decided that it was better to keep the worm drive and motor enclosed inside the bot as well, for alignment and cleanliness reasons. This basically made decision to use a live (driven) shaft hung from two bearings.

So, with this in mind, I went ahead and placed an order for a big sack of worms and worm gears from AliExpress. I bought a metric set of trial parts to begin with, so it was easy to find substitutes on the Chinesium Market. Dimensions for these gear families are pretty standardized with only minor differences, so while the order was arriving (about 2 weeks) I went ahead and pushed the design itself, leaving some slop space in case part dimensions had to change.

To make the shafts, I was going to be lazy and use 3/8″ aluminum hex stock with one side machined into a partial rounded polygon at a 8mm diameter. This lets me use 8mm bore bearings which are super common, but also retains flat surfaces for fastening. The step from rounded to sharp will be the seat for the bearing.

With this approach in mind, it was time to start the detail design. I had enough information here using its positioning in the chassis mockup to drill deep into making the driveshaft assembly comprising the worm gear, shaft, means of attachment, and bearings. The general idea was to seat the motor and worm gears in pockets cut out of the chassis “block”, such that a plate that is mounted to the angled face keeps the gears in place and also doubles as the structure to mount the blade hub. If I remove the weapon and the side plate, then I can pull the gear assemblies out quickly.

In the next episode of Stance, designing the rest of the chassis and bringing it to a state where I’m ready to attack fabrication!