Alright! Picking up from the last episode of stance, I was beginning to embark on the hyperfixation stage of designing. Basically, spitting things into a CAD model isn’t the part that really takes me forever (unless the model is extremely detailed or need simulation, I suppose) but the initial ideation and concept stone-turning.
A lot of this is just done by staring at the ceiling blankly for a while, or mulling over while I drive some place – much of the reason I seemingly enjoy driving up and down the entire Eastern Seaboard. When I taught 2.00Gokart, I called this “Grounding the design” in my lectures. It’s always easier once things condense because you start having actual dimensions and requirements and such!
For Stance, I started with a rudimentary driveshaft design for the worm gears to get some dimensions to work with. Next, I started hollowing the “block” out in a way that I can adjust the placement as the needs evolve. This took a few back and forths of moving the worm gear socket around (and changing its depth) as I dug up bearings I could use.
One of the first headaches from the brainstorming and cartoon model poking was the wheels. As mentioned previously, if I actually wanted to make 45 degree conical wheels, they had to get large. This to me seemed like a questionable benefit at best and a weight penalty pretty much by default.
To press on with the design, I decided to make a set of O-ring wheels like I have done for many bots before. These wheels will be used with soft (50A durometer) O-rings so they have some semblance of traction and shock absorption. If I found out I had weight or space later, maybe I’ll revise this to be a set of cut foam wheels.
Unusually, the O-ring groove was biased towards one side because they’re going to be touching the ground on the 45 degree. The “outside” was heavily flanged and the “inside” not so much. I usually specify O-ring wheels with a roughly 10% ‘stretch” so they don’t just flop off – this means specifying, for instance, a 2-3/4″ center circle O-ring for a 3 inch center circle wheel (which might end up around 3-3/16″ or so on the outside.
The bore was modeled as a hex shape with corner knockouts to prevent material bunching from the 3D print process affecting how the shape fits on the shaft.
i’m adding more sockets, caves, and spider holes for everything else here. The motor will sit in the middle between the two wheels and I’ll need to buy some shaft stock to make an extra-extra long dual shaft. No biggie there. On the ends with the worm screws, the shafts will ride in 3mm bearings embedded into the print.
With 3mm, 6mm, 8mm, and 10mm bearings already specified and it doesn’t even look like a robot yet, this is looking like it’ll become another Roll Cake or something. I can no longer design a bot that doesn’t use 57 tiny little ball bearings.
I’m choosing to retain the wheels with some “Poodle clips” or wide-shoulder E-clips, instead of a screw or nut and bolt setup. This will let me pop the wheels off quickly if need be, or… THEY’LL POP OFF QUICKLY IF NEED BE.
At this point, I was still entertaining the thought of the bot being made as two fully independent halves, which will be retained by some method. I started down the road of a dual dovetail joint retained by a pin doing down the longitudinal centerline, almost like a big Anderson PowerPole connector.
I’ve modeled stand-in side plates for mounting the weapon hubs here. They’ll change as needed later, and were for now just defined using the chassis “block” geometry.
I had enough of the design together to now make some comparisons to original Stance Stance. It occupies the same rough footprint, but the weapons are heavier and smaller in diameter (largely as a response to the modern meta of compact vertical spinners) and it has a lot more invested in the drivetrain. Stance Stance Revolution 1 used only two Fingertech Silver Spark motors for drive. How it was even able to move, I have no real idea.
Check out the preliminary weapon mount design as well. Two blocks, sandwiching the sidewall in the middle, with a single large shoulder bolt or similar running through it. The blocks will be attached to both sidewalls for rigidity.
It was also time to de-cartoon the blades, which were at first made for the visual only. I kept the basic shape, but I designed the counterweight side more functionally and used it to tune the center of mass.
Realistically, once you get within the 0.01″ mark…. I don’t think you’re even going to notice the benefits. Manufacturing tolerances alone will make any more precision not matter, and definitely the first few hits will ruin whatever you had going for you anyway. I got lucky and within a few “iterations” of changing the counterweight radius and angle, got the center axis to with 0.002″ of the coordinate frame.
I cooked up a hub to hold two 10mm bore type 6200 bearings. Severely overkill, but I already owned several. I probably could have saved a bit of weight here and used the smaller 6000 type bearing. The drive engagement is with 4 dowel pins on the inside of the blade bore (not visible here).
The best part? The weapon retainment is also with a snap ring. A very large, beefy one, mind you, at 2 inches in diameter.
The CAD has reached the “Hard to look at without vomiting ” stage at this point. I mushed a few dimensions around to optimize for both weight, which was becoming a concern, and serviceability (such as tool access, how to drop a part without also dropping 5 other parts, etc).
It was around here that I decided to backtrack significantly and get rid of the “Must be able to split in half in the funniest way” design. The center wall took up very precious space in a small bot and I was unable to find a battery I found satisfactory to fit in the awkward remaining triangles. I simply backed the model up all the way to the beginning after the motor and gear sockets and removed the big angled cut. Some very very angry assembly mates later…
I blew up everything more recent than the cut and started by emptying out the center of the chassis. It was kind of a manual Shell operation, because the actual Shell operation was too complex to drive all the custom thicknesses and skip/ignore-this-area demands.
As it was going to be 3D printed anyways, I wasn’t extremely concerned with excessive solid areas but did want to try and expose as much “perimeter” as I could, as well as get ahead on making space to mount internal parts.
More details going into the frame now. I added a center cage to hold the battery I had in mind, a 850mAh 4S that I had one of and bought another of. I also added a serviceability change, which was ensuring the motor and worm assembly can be removed as one.
The support bearing sockets were turned into a mild snap-in-place socket shape, such that the flexibility of the plastic let me push the bearings past them but the socket shape otherwise keeps them braced against the gear driving load. The motor mounting flange was “cut open” and I’ll only use two screws to hang onto the motor. This was done so I could avoid needing to untighten the worms in order to dismantle the drivetrain – I could, in principle, make 4 or 5 of the same motor-and-dual-worm assembly and swap any in and out.
Other features added by now include fastening holes for the side plates, which will simply be attached using Plastite (thread-forming) screws. No heat-set inserts, no backing nuts, no helicoils. Just big threads.
Weight was well beyond a concern by this stage. I had to change the thicknesses and dimensions of several parts a few times.
Originally designed to be 3/16″ thick, the discs had to become 1/8″ thick AR500 steel. I changed weapon motors to 2830 sizes because the two 2836 size 450 helicopter motors were going to be too heavy by almost an ounce apiece.
The battery mentioned earlier was revised from 1000mAh to 850mAh for the same reason, and I ended up buying titanium bolts for the weapon shafts instead of steel.
im not going to spend money on this robot
The weapon drive itself was kind of a detail left for last, because there was really only 1 choice given the placement of everything. That was gear drive. Gear driven weapons are relatively rare because most people can locate the motors sensibly, but when you are this close….
Now, belt drive does have its advantages still even at this close-in distance because of shock absorption. To counter that, simply make the gears B I G G E. I designed these as metric Module 1.5 (or about 16 DP) in case I wanted to use a pre-made pinion. The big ring gear would be 3D printed from nylon and fastened to the blade hub’s back using a bolt pattern, shown above.
One of the “blocks” that held the weapon shaft bolt was turned into more of a spoon shape with an integrated motor mount flange. I decided this was going to be more rigid and position-determinate versus having the motor flange cut into the side plates and the weapon shaft bolt blocks being completely separate.
While specifying the gear stock I was going to make the pinions from, I also decided to make a quick dumb version that was to be 3D printed from nylon as well, but had a set screw hub using two backing nuts stuffed into a socket.
I wasn’t sure if this was going to make it into the final version, since I think I’d rather have some solid metal in a weapon drive hub. But it would allow me to just bang the weapon drive out quickly and test it. And it wouldn’t hurt to have around as spares anyway!
After the weapon drive gears were finalized, I massaged the mounting bracket a little more to allow for easier service access.
As a last touch, because I did this many, MANY times during the design process… I indicated which way on the bot was the front. Many patterns and mirrors were lost to doing it on the wrong end! I’ve also done a bit of weight reduction here, cutting away more areas of the material which will be covered up by the side plates anyway so I didn’t need any more plastic there. Alignment cutouts for the bottom and top plates can also be seen.
And finally, the completed design!
Those side and top & bottom cover plates are going to be made from carbon fiber made by CNCMadness. The discs will be from SendCutSend. I just about don’t have to lift a damn finger on this bot except to make the aluminum weapon hubs and the driveshafts!
From here, it was “Watch my Ender 3/5s do all the work”! On the next episode of Stance, the Buildening…