for the Mark I were equipped with adapters that
reduced the pipe diameter from four inches to one
inch, and the adapters were cemented in place. To
make the Mark II, we would need to start from
scratch. So, we just took this as another opportunity
for optimization. We would be even more careful this
time to ensure that we didn’t have weak points that
could develop leaks.
Our theory on what must have happened with
the leaky joint on the Mark I is that we must not have
gotten complete coverage with cement in the joint
when we first assembled it. When cementing PVC
together, you should give it a good twist to ensure
that it spreads the cement around and creates a
thorough seal. For the Mark II, we pressed and
twisted with such determination that it felt like we
were performing Feats of Strength for Festivus.
Putting it all together did seem to go a bit quicker than
last time, mainly because the design was largely the same.
The finished product was even bigger than the Mark I
because the large valve and couplers made the barrel
protrude even farther, just like a proud Festivus pole. Much
like impeccably wrapped presents under the tree, we would
have to be patient and let the anticipation build before
testing the new Christmas cannon. We didn’t want all of
the Feats of Strength to go to waste by pressurizing the
cannon before the cement had fully cured.
Chunkin’ Around the
After carefully counting down the time on our advent
calendars, we were ready to test the new and improved
Christmas cannon. Upon mounting it on Protobot, we
discovered another opportunity for optimization. The pitch
control mechanism from last time (which used the lifting
mechanism we first crafted for the February 2015 article)
was just not tough enough to lift the heavier Mark II. So,
we reintegrated the pulley system that we first used in the
April 2015 issue, utilizing a double tackle arrangement to
give the mechanism four times the mechanical advantage.
The pulley mechanism helped the pitch control arm
raise and lower the Christmas cannon with ease, and the
finished product kind of seemed like the type of booby trap
that Kevin McCallister might set to entangle the Wet
Bandits in tinsel. Which probably isn’t too far off. The
finishing touch was to wrap the chambers and barrel with
red ribbon to give the cannon that holiday je ne sais quoi of
a shiny candy cane.
Our goal with the Christmas cannon was to take the
tedium out of tree decorating. Yes, of course, it’s a holiday
tradition, but the repetitive nature of retrieving ornaments,
We set up an unadorned tree in the backyard, down
range of our Christmas cannon. A little bit of finessing with
the radio joystick got us to an angle that looked like it
would be on target. For our initial tests, we used a solitary
foam ball, just to make sure we were aimed well. We
pressurized the chamber with the same compressor and
Schrader valve arrangement that we used for the Mark I,
starting with a low pressure of around 40 PSI. We twisted
the ball valve handle as quickly as possible and with a dull
roar, the compressed air whooshed out of the chambers,
through the ball valve, and into the barrel. The foam ball
bounced coyly off of the base of the tree target, signaling
that it might have stuck around in the branches had we
fired it with a bit more pressure and a higher angle.
For the next test, we stuck with the same pressure but
SERVO 12.2015 57
GIVING THE GIFT OF MECHANICAL ADVANTAGE.
MERRY CHRISTMAS YA FILTHY ANIMAL!