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
58 SERVO 12.2016
A FLAPPER STICK.
BEFORE FLAPPER STICK TREATMENT. AFTER FLAPPER STICK TREATMENT.