The original cast iron skillet shell spinner robot that started it all.


It all began during MIT’s Independent Activities Period (IAP). My roommate and I decided it would be a lot of fun to try out combat robotics. Somehow we decided that a good first robot weight class would be the featherweight class (30lbs). We had no idea what we were getting ourselves into.

Being uber ambitious first time builders, we figured we would make what we called a full body flywheel flipper. The basic idea was that we would have flywheel clutched to a cam wheel and whenever anyone pushed on the sides of our robot, the flipper clutch would engage.

Some unfinished CAD models:



Surprise, surprise! We realized how difficult this would be to make, and decided to regroup and come up with a new plan.

Because we were planning on competing with this robot at Motorama’s Robot Conflict, we decided to look at the current 30lbers and go with something much simpler than the flipper.

If it wasn’t totally obvious from the intro to the post, we ended up deciding to build a shell spinner! We were definitely concerned about other robots flanking us so we wanted our weapon side to be every side. Also, the reigning Motorama champion was Triggo, a very perfected shell spinner.

Triggo in action:

Our first hurdle (of many) was to find the shell for our robot. Our initial thought was to go find the biggest wok we could, but somehow Boston’s China town doesn’t sell any woks??? Plan B was to go find any other round kitchenware and go with it. A full day of searching later and we finally found something possibly sturdy enough for a battlebot. A cast iron skillet; YOLO


It seemed to fit all our components “fairly well” (we were a little naive).

To drive the shell, we opted for using an outrunner with some sort of friction material (in this case a long board wheel) that spins against the top plate of the shell. The shell would be supported by massive angular contact bearings on a dead shaft in the middle of the robot.


This robot had a lot of fun machining especially because we were pretty much making the robot out of scrap lying around MITERS.

To machine the massive bearing blocks that support the entire shell, we first cut off a chunk of 3/4″ aluminum we got off of some old lab equipment, squared it up, bored out the center on the mill and drilled mounting holes. We then found that the square shape of  the bearing block kind of interfered with the shell friction motor so we had to turn it round. We did this by making a fixture which just consisted of some round stock with the appropriate bolt holes.

Originally we wanted our shaft to be made out of some stainless pressure vessel we found on campus, but unfortunately it was SUPER not machinable. We were able to get the rough shape with the carbide tools in the CNC shop next door to MITERS (left picture), but we had a lot of difficulty drilling the bolt holes and so we switched over to aluminum for the final shaft.


Luckily the shell didn’t need too much machining, just a bore for the shaft clearance, and bolt holes for the bearing blocks.


Voila! (potato picture)

The next nightmare was making the baseplate that would hold everything. Going along with our completely scrap made robot theme, we found a nice heatsink that was machined out of a single billet of aluminum. It still had about 1/4″ left at the bottom so we figured we would just mill off the fins of the heatsink! My ears still ring with the sound of all the chatter the aluminum made.

Some miscellaneous pictures of the rest of the parts we machined but I’m too lazy to write about.

And there it is! The only issues being our shell was a good 3 inches off the ground, was made out of cast iron (it’s pretty brittle), and it was the night before we were leaving for Motorama.

A short video compilation Fred Moore threw together of our initial testing:


Luckily our wizard of a friend, Alex Horne was able to somehow weld some 4130 scrap we found to the pot.

Full speed testing with blades:


Needless to say, the competition was a total disaster for us. We suffered from the same issue most shell spinners suffer from; “Help I’ve fallen and I can’t get up.”

In our first match, our blades dug into the ground as soon as we spun up and we flipped onto our head.

Our second match didn’t go much better and so with that amazing win-loss record of 0-2 PMTH was retired.

Nonetheless, it did it’s job of getting us excited for more combat robots!