Servo - December 2015

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 Christmas Tree

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.