Portable Apocalypse is my current bot in the beetleweight category, and it is an undercutter with a powerful single tooth disk. The design started out shortly after building Someone Else’s Problem v2.0, and it has largely wound up being a scaled up version of SEP v2.0 and v3.0. The initial version of the design was 3D printed from NylonG filament with UHMW armor to try to make it durable, but life happened and it never saw competition.
The first competing version of the design came shortly after the latest version of SEP when my build methods expanded to include laser and waterjet cut parts after new companies like SendCutSend started making it a lot easier to quote and order one-off parts. So in the long drought between events both bots went from 3D printed nylon frames to titanium frames. If you don’t count screws, there is only a small number of main components that need to be assembled to put the bot together. It all boils down to a top and bottom plates, a front and back panel, drive motors, weapon motor, wheel guard, and weapon assembly.
The main structure of the whole bot comes from its top and bottom plates. 2mm titanium works great for the wedges for D2’s, why not make your whole bot out of that? Metal purchased from Sackin Metals and laser cutting done by SendCutSend.com.
The front wall and motor mounting block is printed from NylonX plastic with threaded standoffs pressed in to attach to the base and top plates, and heat set inserts to make the final two attachment points for the weapon motor mount. The back panel is also NylonX, with standoffs to attach to the base and inserts to attach the wheel guard and power switch. The top and bottom plate screw down to put the printed layers in compression and make it less likely to break if it is hit.
The front and back panels get screwed onto the base plate, along with the drive and weapon motors. The wheel guard wraps around the entire back half of the bot and attaches at the front with nutstrips. The wheel guard also acts as an additional layer of protection for the 3D printed back wall to help blunt any impacts. The back is curved so that it can’t get stuck on a flat back edge.
The weapon motor is a Propdrive 3530 brushless motor, with a 3D printed pulley pressed directly onto the can and screwed into place. The drive is 25mm gear motor gearboxes mated to 2300kv 1806 brushless motors. I had to set my controller to 40% of max speed for the wheels, and that was still plenty fast.
The weapon shaft assembly consists of a 3/8″ shoulder bolt dead shaft, 7/8″ OD bearings, a 3/4 lb laser cut 1/4″ AR500 disk, and a 3D printed pulley to go with the 2L V-belt. The blade bearing still spins well after one event but feels less smooth, so probably going to be a consumable that I will want to change out between events.
I had two sizes of the 3D printed pulleys with me at the event so that I could gear the disk for 8,000 or 10,000 rpm depending on who my opponent would be, but I never ended up changing the pulley in the end.
Due to the space constraints inherent in making an undercutter there weren’t a lot of parts that were ideal for dealing with both axial and radial forces in a small package, so I decided to roll my own “flanged” bearing setup. There is a bearing in both the disk and the pulley, with a nested pair of washers squeezed between them to help handle axial loads. I can really tighten the shoulder bolt down hard to lock the inner races of the bearings together and make the frame nice and tight, while still leaving the outer races and the weapon disk free to spin. Slots in the 3D printed pulley let me put in square nuts to clamp the pulley to the disk.
I designed all of this so that with the exception of the weapon shaft and power switch I only need one size of hex wrench to fully assemble/disassemble the entire bot. All of the 6-32 screws for the frame use a 5/64″ hex drive, and all of the metric screws for the motors need a 2mm hex drive, but the two are so close together that I can use one hew wrench for all of it!
After some issues I had with earlier versions of SEP having the weapon flex enough to damage its own pulley, I designed it so that the disc is always directly over the weapon motor and pulley. This way even if the weapon flexes it just rubs on the pulley instead of having the chance to bite into the pulley and do some serious damage.
Portable Apocalypse has so far participated in one event, NERC’s 2019 Franklin Institute event. The first fight of the day was against Zer0, a modular bot. He had chosen to go with his vertical drum configuration, and I think he managed to take a chip out of my tooth during the fight. Aside from a few scratches on the wheel guard this was no significant damage from the fight.
Zer0 was looking a bit worse after our match. He tapped out, but not before he lost 3 of his wheels, severely bent one of his drive plates, and lost his his titanium wedge. I think that he changed from belt driven 4wd to direct driven 2wd for the rest of the event due to the damage I caused. This is exactly the kind of thing that I was hoping that Portable Apocalypse would be capable of when I was designing it.
The second match was against Thunder Child. Nate Franklin was a fantastic driver as usual, so he quickly managed to get me pinned up against a wall with my wheels off the ground, and it seems that my weapon motor doesn’t have enough power to flip me over from a dead stop, so I lost the match. On the bright side, if you have to lose a match, it might as well be to the eventual champ! No damage from the fight, just a fresh battery and on to the third match.
The third match was against Wumbo. I managed to do some damage to his wheels, then both of our weapons connected and took each other out. His weapon went out from a failed 3D printed pulley, mine from a failed soldering job rearing its head. It looks like my poor soldering job on the weapon ESC left some stray strands of wire that eventually got jostled into causing a short and killing the ESC. The match devolved to a pushing match and a judge’s decision that went to Wumbo, knocking me out of the tournament. Still no significant damage to the frame.
In the end, Portable Apocalypse went out with a 1-2 record. Overall I would call the event a win for the design and a loss for the driver. Even after 3 fights all it needed to go to round 4 was a fresh battery and a spare ESC.
Even before I got to the event I had already started on the next iteration of the design, and now with some real world experience I have made further improvements. With little real damage from the competition, the biggest issues I faced were the lack of space in the electronics compartment, my poor ability at soldering (I only have experience at through-hole soldering before this), and the difficulty of having to halfway disassemble the bot just to charge the battery.
The improvements in the new design include
- Going back to brushed drive motors for simplicity in preparing spares
- Longer and taller electronics compartment to make room for wires and padding
- Slightly larger weapon motor
- Slightly larger ESC with power leads pre-soldered so I can’t screw it up
- Electronics compartment lid plate in addition to the top plate so the battery can be changed without having to halfway disassemble the bot
- Rework of the mountings for the wheel guard and drive motors
If the new design holds up well I am even considering turning it into a kit to sell!