Servo - December 2016

machinery? The answer sounds like something transported directly from the Roaring Twenties: a flapper stick.

A flapper stick is a strip of sandpaper at the end of a long rod mounted on a rotary tool. The sandpaper flaps around and smoothes the inside surface of the pipe. When using a flapper stick, it’s important not to activate it outside of the pipe because unless the rod is absolutely perfectly straight (and it won’t be), it will wobble around and wreak havoc. We conscientiously kept the flapper stick inside the pipes, and after only a few runs, the inner surface was as smooth and shiny as the undisturbed snow-covered expanse of a winter wonderland.

The only other components that we needed to modify in advance of the final assembly were the end caps. We needed to add quarter inch threaded holes to accommodate our input valve assembly, transducer, and purge valve. Prepping the schedule 80 steel end caps was a bit more involved than their PVC counterparts from the earlier cannons.

There were two ways we could do it: one would be to use a 7/16 drill bit for the hole and then tap it, but just using the drill bit for the hole would not give us the best threads. We would achieve better threads by first drilling a

27/64 hole, then using a taper pipe reamer. The tapered hole makes for better threads than the straight through hole, and given that these would be the only holes we would drill in our pressure vessel, we wanted to do it right. So, on the last day of Bot-mas before our big build day, McMaster brought to us a new drill bit, taper reamer, and tap. The end caps were very thick material, but were no match for the new bits and a steady hand on the drill press. Now that we had drilled some holes in our pressure vessel, we were ready to seal it all up.

We Wish You a Merry Bot-mas

The very final step before actually assembling the cannon was to prime the inside of the fittings and pipe. We cleaned out the threads with brake cleaner, and let them dry out thoroughly before priming them with self-etching primer. The primer would be a safeguard against any moisture build-up and resulting corrosion. We planned to use nitrogen for our big chunks, which would be dry and wouldn’t risk moisture build-up in the cannon. For some initial testing, we figured we would be using garden variety compressed air (which could be as moist as delicious cookies left out for Santa on Christmas Eve), so priming would be a good idea. Once the parts dried (quickly, in the warmth of the distinctly un-Christmas-like SoCal sunshine), we really were ready for the final assembly.

Putting the cannon together was a bit more challenging than the inevitable type of large toy assembly that goes on in households across the land on Christmas morning. Mainly because it was so crazy heavy. The fittings and valve all together (minus the pipes) weighed in at about 60 pounds — the weight of our lightweight combat robot, Troublemaker (all Troublemaker wants for Christmas is a stronger weapon motor; we’ll see what Santa brings