We could potentially use the chromoly slug without
modification, but that would be a bit sloppy. Using a
tabletop grinder, we chamfered the top and bottom edges
a bit to ensure there wasn’t any imperfection to hang up
the slug on its speedy trip through the barrel of the
cannon. We also drilled a shallow hole into the top of the
slug and cut a few taps into it. We had a long rod of all-thread that we could use to screw into the slug in case it
ever became stuck in the barrel. With our slug prepped, we
were finally ready to start cooking with high pressure
nitrogen. We wheeled out the nitrogen canister to the
backyard and strapped it to a fencepost. We carefully
removed the cap that covered the canister valve, working
with the delicate care of a bomb diffuser. We attached our
regulator to the canister valve with similar care.
Instead of setting Protobot up on the even ground of
the backyard, we used a panel of particle board to
hopefully provide some levelness and stability. We hooked a
strap under the platform because we didn’t intend to let
Protobot free wheel about when it would be shooting a 10
lb slug at up to 3,000 psi. Cognizant of Newton’s Third
Law, we wanted to prepare for the equal and opposite
reaction by strapping Protobot down and weighing it down
for good measure. We wheeled Protobot onto the platform,
then carried out the 120 lb cannon and placed it on
Protobot’s uprights. We strapped the cannon down to
Protobot and the particle board, and used cinderblocks as
wheel stops behind each of the bot’s six wheels. As a final
touch, we weighed down the front of the robot with two
50 lb bags of gravel. Everything about this project really
We placed Protobot close enough to the nitrogen
canister so that we could fill the cannon while the canister
remained strapped to the fencepost like Hannibal Lecter.
We put our panel meter on a chair next to the bot so we
could keep an eye on the pressure. It was finally time to
Safety, of course, was a chief concern. We weren’t
entirely sure how our stack of straw bales would handle our
cannon’s fastball from hell, so we started with only a few
hundred psi and worked up from there.
Our process of pressurizing the cannon was detailed
and methodical, and we would call out each step as we
SERVO 04.2017 57
AND READY TO ROCK!
MEASURING ACTUATION TORQUE.
LETTING OUT A
MAKING HAY AT 1,000 PSI.
THAT’S 1,963 PSI!