For literally over a decade now, Motorama and the NERC Robot Conflict event associated with it has been a sort of winter robotting tradition for me, minus only a few years; in 2020, I was in the middle of moving and so didn’t go back north for a while, and there was no Motorama in 2021 for Reasons.
However, the last time I really fielded a bot was in 2018 with the outgoing 30haul; I just went in 2019 for funsies and to help out. This year, with the Reasons beginning to wind down and with everybody in the robotting community ready to get competitions going again, I decided to to take a break from vans and enter something again.
But I had one rule for myself: No spending money on the robot.
This bot, whatever it’s going to be, was just a short detour from Operation IDIocracy, and it was going to just be a shitpost entry. Cleaning all the grunge and getting the engine apart has taken longer than I was counting on, and on top of that, it’s cold so I kind of lose motivation to wrench on things. Robots can be done indoors.
I had to build this entry as much out of just the materials and parts in my midden. Screws are fine, buying motors and ESCs or ordering custom cut parts is not. So what am I going to make? Initially, I thought about an extremely dirt floor rendition of Stance Stance Revolution in a 30lber, maybe using lawn care blades as the weapons.
To this end, I have plenty of weapon motor candidates in the form of 40-60mm brushless motors from Overhaul. I actually had a “preview” sketch model of what the next SSR beetleweight was going to be:
Actual stanced wheels! I had a few bright ideas on how to drive those while I was mocking the design up. This is definitely going to happen at some point; however, now getting close to the new year, I was getting less inclined to make a 30lb SSR as it was looking like more and more effort. Effort was to be avoided.
So why not a 30lb Sadbot? After all, Sadbot is my most successful heavyweight. I had plenty of 3″ C channel and 1 x 3″ rectangular tube left over from Vantruck add-on candidates, which was perfectly half the size (height) of Sadbot’s 6 inch channel stock.
We begin with a master sketch of the outline of the bot which I used to drive Inventor’s “Frame Generator” design tool, which is more or less the equivalent of a Solidworks Weldment with its libraries of standardized profiles and extrusions.
Weight was going to be a serious concern; my first pass frame measured out at 21 pounds out of 30. So having the frame size be driven by placing components and easily adjustable was important. I was basically looking at stuffing a 12lber inside a ring of steel.
The chassis material inside the beltline was going to be good ol’ UHMW plastic, a staple of my high school, pre-machine tool building. It behaves like a very dense bowl of grits and is easily workable with woodworking and handheld tools. This was really shaping up to be some kind of accidental retro-build.
Adding to the retro vibe was picking some 18V mixed-heritage cheap drill motors out of my bucket of motors. I would occasionally over the years “sample” cheap cordless drills from Harbor Freight, Walmart, and the like, so I have probably over a dozen of these which are almost all interchangeable. The newest lithium battery models have been deviating from the classic conical gearbox shape as manufacturers try to make the drills smaller/lighter while having 2 speeds as a default option.
The drill motor is to actuate a pokey stick similar to sadbot, and I was planning on a simple chain drive to an axle above the frame.
For drive, I decided to keep digging through my motor pile and found some mild-wound 555 sized motors. They’re identical in size to the usual 550 size R/C car or truck motor (and drill motors) but are typically wound to run at higher voltages, like 24 volts. The ones I have are surplus from who knows when, and have a Kv of 450 RPM per volt (Compare this with the typical 1200 to 1500 of a 550 class drill motor). They can therefore be run with a single stage of open gears, trading speed for torque.
In this bot, space was more the issue than anything, and using a single-stage spur reduction let me push the motors into the UHMW frame rails to clean up space in the middle for the drill motor and eventual electronics
So why not brushless? Well, that would involve finding 4 matching motors and/or ESCs in this size range, which I didn’t have. And no spending money!
I began sizing up the gear drive using a 4 inch wheel as a reference, using the spacing of the 555 drive motors and the drill motors as a guide for what gear size I could use. I’m going to exercise some impure nostalgia here by just 3D printing the wheel and hub. The gear pitch is 24 DP (or about Module 1), since that’s what my 3D printer flock can consistently print and have it work out well.
Initially, I was designing this hub to use the same wheels that 30haul did, made using the same methods and having the same dimensions as the Vex hubs I was using – hence the 1″ nub sticking out of the gear. I was going to hole-saw some 1″ gum rubber sheets and retain them with standoffs in the manner of its “disposable” duallies.
However, on a trip to Harbor Freight, I serendipitously found a new avenue to explore…
My only real complaint about the gum rubber wheels for 30Haul was that the sheets don’t come in any thicker dimensions than 1″, at least not with my minimal familiarity with the rubber indutry. Ideally, I’d be able to get a single 3-4″ thick wad, or laminate a few sheets together and be able to cut out wheels for actual full-size Overhaul.
I could probably get away with running singles for this bot, but I’m also keeping an eye out for thick chunks of rubber foam. And I found it by accident at Harbor Freight in the form of what I call the “Harbor Freight Yoga Mat”, actually a kneeling pad. It’s part number 56572 (as of now, anyway). It’s kinda exactly what I wanted – a big slab of rigid-feeling EVA foam. The texture felt right to make into wheels, and if the traction wasn’t good enough by itself, certainly I could coat them in latex or silicone (a long time favored builder trick still in active use at BattleBots!) for more traction.
So I violated my own rule, as usual, and spent money on the robot. One Harbor Freight Yoga Mat, for science!
I started prints of the hub design on a Markforged Mark Two (hi Markforged!) as well as a new-to-me Creality Ender 3 V2 (hi Naomi Wu!) that I picked up for $40 in a “Curated Wreckage” state. The pink material is PETG, which is my preference these days for non-critical and experimental prints that nevertheless will be used in the final application, despite my denial.
Those slots in the gears were added for a wheel retention feature that I dreamed up as well, and which will be seen shortly.
One issue I ran into with making the gum rubber wheels for 30Haul was keeping the hole saws centered with respect to each other. I decided to tunnel my way through the problem this time by opening up the center bore of the 1″ hole saw to fit on the 4″ hole saw’s larger arbor thread.
These cheap hole saw sets aren’t hardened meaningfully, so I blasted the bore out to 16mm with a metric step drill I had. 5/8″ probably would have been fine also. After this, I was able to jam both of the hole saws onto the larger arbor!
It was then just a drill press job away from making wheels that were at least concentric one one side; I didn’t want to go buy 2″ deep hole saw for this, so I had to just flip the Harbor Freight Yoga Mat around and find the pilot hole.
Here is what the deal is with the six slots. I didn’t want to just adhere the surface of the foam to the printed hub face because I thought the foam was just a bit lacking in integrity and would separate under the “skin”. I decided to add some interference elements, not unlike the standoffs of 30Haul, but without the intention that I could take it apart again.
So out came these ABS trim strips I bought for something way back when. I decided to use them as quasi dowel pins or driving keys. They get cut into chunks, pressed into the hub, and then trimmed with scissos.
With six slits cut into the foam with a knife, I could press these in with adhesive covering every surface, thus ensuring that plenty of surface area grips the foam on the inside.
E6000 contact cement is my go-to for these kinds of robot shenanigans. I slathered each edge of the “blades” and the hub face with it, and left it to cure under a weight.
That’s what one wheel looks like. The next day, I was sufficiently satisfied with the roundness and rigidity, and hit the go button on 6 more wheel hubs.
Alright, experiment over. Time to flesh out the rest of the design. I needed the drill motor shaft to stick into the center of the bot, so I decided on just using some fat standoffs.
The drill motor is mounted in a slightly unusual but also traditional way, using 4 of the of ball bearing clutch holes as mounting holes (and the other 4 still serving as clutch ring locking holes). All of these holes are to be tapped #10-32. This approach is stronger than using the two very small #4-#6 holes in the nosecone area.
The poker weapon drive is as simple as it could be. I’m using the same tactic as I used on Sadbot’s latest poker: Just welding a steel tube to a sprocket. The center bore is made for a 5/8″ ID bushing, which will just be a drilled piece of Delrin plastic in a 3/4″ drilled hole. The sprocket I plucked out of my Tomb of the Unknown Power Transmission Part is a 30 tooth #25 size.
25 isn’t my go-to for lifter/hammer style weapon in a 30lber. I’d much prefer #35, but I only had very small sprockets in #35 at the time, and something something no spending money. The same applies to the 5/8″ shaft. I would prefer at least 3/4″, but had some 5/8″ 4130 chromoly tube that I figured would stand a better chance than thin-wall 3/4″ regular buttery steel.
One of the next things to settle was where to put the weapon axle. I ran through a few options for this, including having it in-line with the top surface of the frame and making a half round shaped clamp to hold the axle in place.
The “overkill V2” iteration is where I decided I also wanted the ability to tension the chain, so let’s make a fancy set of towers that have slots so I can slide it back and forth for tensioning purposes?!
The third and final iteration is the “No, that’s too much effort” revision, which was a compromise. I needed more space between the sprockets anyway, so the axle had to move up. I decided to just make a simple block mount from the same offcuts that the frame will be made from, and use shaft collars or spacers to retain it axially instead of clamping
And that….. is all the CAD that I did before starting to cut metal and plastic. After all, I had 95% of the parts in house and ready after searching around for the day.
The electronics of this thing were to be equally found-object. Namely, I “found” two brushed Ragebridges, and decided I was going to break up one of my knockoff 18V Milwaukee Fuel batteries, which contain 2500mAh 18650s. The “9Ah” knockoff contains 15 cells (5S3P) and so I was going to run 7S and make two packs out of the one battery.
Next up: Fabrication, then the Motorama trip!