Deathcap: a Debut at DragonCon

Deathcap is a 3lb vertical drum spinner formerly named Dominant Mode.


While Dominant Mode performed pretty well at Bot Blast, I had some other designs and ideas for further improvements, and what better place to try them out than DragonCon’s Robot Battles?

Design Upgrades

The Drum

While titanium sparks were pretty, I wanted to upgrade my blades to tool steel (preferably to the “holy grail material” S7 :O). This meant I had to get thinner blades and shave some weight somewhere. I also had to come up with a better method of axial retention for the blades, since set screws won’t fit in thin blades. I also considered other types of drums (beater bars, bars welded on cylinders, etc), but I liked the idea of having sharp teeth that could bite, and wasn’t sure if there’d be enough time to make a completely new drum. I’ll save those crazy ideas for the next version 🙂

I did some quick density calculations and found that I would be able to have five 1/8″ S7 blades without adding too much weight to the drum. The next challenge was constraining the blades. I drafted a bunch of various designs, and eventually decided to try a crazy idea suggested by Jamison. The blades would slide over the hex drum into individual grooves. They would then twist so that the hex cut-outs in the blades would be rotated 30 degrees from the hex of the drum, thereby axially constraining the blades. The blades would then be constrained rotationally with the pins hammered into the corners of the hex. The hex-body of the drum otherwise stayed the same.

Drum Assembly
Drum Assembly

The Armor

I liked the spring-steel-over-aluminum armor combo, but the tempered spring steel didn’t allow for the small bend radius I needed with the armor thickness I wanted. This time, I wanted to try bending annealed 1075 spring steel and then hardening and tempering it myself in the ovens. That would allow me to shape the armor around the frame however I wanted. Designing the bent armor pieces were tricky. I initially tried to prototype the parts by laser cutting thin plastic sheets to test the bend and shape, but this turned out to be very difficult when translating to steel. In the end, I used the sheet metal design tools in SolidWorks with precisely designed bends in order to get the geometry to work.

Full CAD
Full CAD of Deathcap

As for the rest of the robot, I really liked how the drive was isolated from the rest of the frame, so I decided to keep the chassis design, allowing for a little more ground clearance this time. I also angled the top-back of the robot to allow for inverted driving. I switched the drive motors to the 1000rpm kitbot gearmotors, so the frame was able to shrink down a little. The last check was to make sure that, by design, the electronics would fit inside the robot (possibly with a little finagling!)


Almost all the frame parts are waterjet-made. I was able to access the MIT CSAIL waterjet this time (which is free!) and had come across all the material for free, so I decided to make a bunch of spares, in case I encounter BotBlast-level damage again.

Waterjet frame parts
Waterjet frame parts

The blades were cut out of 1/8″ S7 that Jamison found laying around the institute once upon a time. In order to save on precious material, I decided not to make spares of these, as they SHOULD be able to survive pretty well. I filed the corners of the blades just enough so that the pins needed to be hammered in a bit and assembled the whole drum, because I was so excited to see it together. Something to note though, this drum is a SERIOUS pain in the ass to assemble. Goal for the next drum design: come up with something that doesn’t require such a specific assembly procedure. Anyway, I think it looks pretty cool.



The blades were then sent to the ovens to be hardnened. Turns out, the corners that were filed pre-hardening are now too large for the pins and they’ve all become slip-fits :(. No matter. I just filed the remaining 3 corners to fit the other blades, but this was significantly more difficult with hardened blades. After some testing, I found that it was really easy to lose pins, and thus cause the blades to shift using only 3 pins per blade and having each blade constrained individually. I decided to use longer pins that tied either 2 or 3 blades together, which would hopefully distribute the forces a little, and make it more difficult for pins to fall out. However, this also means that replacing any blades (especially in the middle) will be very difficult.

Drum with longer pins

Making the armor was probably the biggest pain of the whole robot. I had tweaked and prototyped the bent sides with thin plastic and was almost sure that would work when translating it to the metal. However, after several hours of attempting to bend the annealed 1075, I found that it was almost impossible to get the shape that I wanted. However, using the sheet metal tool with more precise bends turned out to be a more successful method.

The armor was then taken to the ovens to be hardened and tempered. One of the sides warped a little during quenching, but I still managed to get it onto the frame. The convenient part of this design is that once the armor is attached to the side-frame, it doesn’t ever need to be removed. Each side comes off the robot as one piece for convenience.


I managed to finish Deathcap a little ahead of time, so I took it out for some testing. Here’s one of the videos.

Hitting the concrete dulled the blades a little, but it was nothing compared to the damage the concrete floor took! There are now several sets of 5-tooth bite marks all over the floor…


Here is also a hilarious spontaneous rumble with Deathcap, Jamison Go’s “Silent Spring”, and Charles Guan’s “Stance Stance Revolution” and “Colsonbot.”

From testing, I realized that the front wheels were so soft that each time I hit anything with the drum, the down-force would cause the blades to hit the ground, and Deathcap would be stuck in a loop of endless backflips (which, on the concrete was not so great for the blades). I needed more solid wheels than Lite Flite foam wheels. Turns out, old Dominant Mode wheels were just about the right size for Deathcap still and at least two were still usable. I had to make new hubs and pulleys to fit my ratio, but otherwise they worked beautifully.

DragonCon 2015 Micro Battles

Deathcap’s debut at DragonCon was largely successful. It played in 4 matches (3 wins, 1 loss). Sadly, Deathcap met its downfall (literally) when I accidentally drove into the knock-out pit in the arena. I need to practice driving… Anyway, the four fights are below :).

I was very happy with the S7 blade performance (it hits hard!), even though one blade broke in half at one of the weak spots during a match with Project Terminus.

Damage to Flippy
Damage to Flippy
Deathcap fighting Naked Singularity missing the middle blade
Deathcap’s missing blade while fighting Naked Singularity

Several pins sheared (some in multiple places!) but the pins still continued to do their job of rotationally constraining the blades. The pins were, as predicted, very difficult to replace.



During frame assembly, I realized that Deathcap has far more parts than it probably should. It makes it significantly more difficult to assemble, and sometimes taking out one part means taking apart almost the whole thing. Perhaps in a future iteration, I’ll move to something simpler. I also decided that it’s time to incorporate wedgelets into the design to better deal with wedges.

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