Given the awesome dangerous power of the 6,000 psi
canister, the Action Gas guy was curious about what we
planned to use it for. So we told him, and he thought it
was awesome. We hoped it would be — assuming we could
get the ballistic missile of a canister back to Robot Central
safely. It was a very cautious ride back from Lake Elsinore.
Back at Robot Central, we strapped the canister to the
leg of the built-in workbench to keep it secure in case the
“Big One” hit. If that ballistic missile hit the ground the
wrong way, we would need more than The Rock in a
helicopter to save us.
Once the canister was secure, we still had a few more
errands to run. Specifically, we needed to find a solution to
a problem we didn’t have with low pressure testing. For the
PVC prototypes and for the low pressure baseball tests,
shooting at our plywood target was fine. It was starting to
crack a bit, but the baseball would bounce fairly harmlessly
off of the target and roll lazily down the hill in the backyard
— our makeshift shooting range. This time, however, we
were hoping to shoot a steel slug at up to 300 miles per
hour. The plywood target wouldn’t stand a chance, and we
were worried that after ripping through the target that the
slug would keep on going and either ricochet back off the
hillside and hit a defenseless car sitting in the driveway or
keep traveling up the hill and into the wild blue yonder.
We wanted something that could absorb the force of
the projectile; something that our slug could burrow into
instead of ricochet off of. So, we hopped back in the truck
and headed to the local feed store for some bales of straw.
The yard guys at the
feed store were a little
incredulous. The quality of
the straw bales, they
warned us, was subpar. It
was the last of their
current stock, and had
gotten a bit damp and
muddy in the unusually
wet weather Southern
California had experienced
in the last few weeks. The
yard guys asked what we
planned to use the bales
for — maybe bedding or
something like that. Our
answer of “target practice”
was truthful, though we
think more likely conjured
images of an archer with a
bow and arrow rather than
a high pressure robotic air
cannon.
We hauled the straw
bales back to Robot
Central and stored them in
a horse stable in the backyard. The bales were not nearly as
heavy as the nitrogen canister, but were certainly over 100
pounds each. Everything about this project was turning out
to be heavy.
A Clockwork Test with Orange
Straps
With our nitrogen strapped down and our straw bale
target set up, we were almost ready for testing. The last
step was to prep our slugs. McMaster had a nice selection
of large diameter steel rods, so we ordered some to have a
few options for potential projectiles. One potential slug was
a 2-7/8” diameter 6” long rod of chromoly (numerically
known as 4140). We were concerned that diameter could
be too small for our 3” diameter cannon barrel, so we also
ordered a few 3” diameter rods ( 3” long), thinking that the
tolerance on the diameter could end up giving us the
perfect fit.
We had a few alloys to choose from among the 3”
long slugs, too — 1018, 4140, 1045, and 8620. The softest
alloy was the 1018, and that rod had a few dents from
shipping. That wasn’t ideal because it would probably
deform so much upon impact that we wouldn’t be able to
reuse it. As it turned out, we didn’t need to hem and haw
about the virtues of different alloys because the 2-7/8”
diameter chromoly rod ended up being a perfect fit for our
cannon barrel. Plus, it weighed about 10 lbs, which had a
nice round appeal.
56 SERVO 04.2017
READY TO PRESSURE UP WITH THE
6,000 PSI CANISTER (EVAN FOR
SCALE). OUR 10 LB SLUG, READY FOR GLORY.
SHAPING UP THE 10 LB SLUG.