SERVO 10.2017 29
we chose the fixed wedge instead.
For adaptability, we chose to have
three interchangeable weapon bars on
this build. We allocated about eight or
nine ounces to the weapon bar;
mostly so that we could overbuild the
frame, and hopefully not enter the
world of wild instability that many
high-KE bots find themselves in.
We also chose to spin at a
modest 6,000 RPM for the same
reason, and also because the
combination of Outrunners and
batteries that we had on hand gave
us that speed.
Because Phantom II was intended
to compete at the CIRC Bot Brawl
which limits bots to 12”x12”x12”, we
went for a spin diameter of 12” and
designed the robot accordingly. The
primary bar is a 12” long, 1/4” thick
7075 aluminum bar with hardened
4140 steel impactor teeth at the tips.
We went for maximum reach and KE
with this bar, making it a good
The secondary bar — for fighting
wedges and beaters/drums — is a
shorter 11” 1/8” thick hardened 4130
steel bar (for cost reasons) with
sharpened leading edges at the tips to
hopefully dig into wedges instead of
skipping off of the surface.
This bar was designed to be
shorter both for weight reasons and
to lessen the leverage of an uppercut
from a beater bar or drum, in an
effort to prevent the bar from
snapping or bending.
The last weapon is a sacrificial
solid 1/4” 7075 bar, mostly for the
purpose of warding off vertical disk
spinners with its 12” reach, but
without putting any impactor teeth in
danger of shearing off from a vertical
The lighter bar also gives a
quicker spin-up time for active
defense, and gives us the option of
beefing up the top armor or adding
some other unforeseen repair without
The bars were waterjet cut for us
by fellow builder, Brian Adamson and
Analytics Lounge (big thanks!).
The last components of the bot
were the weapon pulleys and belt. We
had the pulleys printed from
Markforged Onyx for a number of
Onyx pulleys had been tried and
proven on a number of Beetleweight
robots, and we chose them primarily
because printing is an easy way to
have a custom-shaped part.
We included a conical depression
on the motor pulley so that it would
sit lower on the Outrunner, and allow
us to reduce the weapon height by a
With an aluminum pulley this
would have required a lathe and/or a
CNC router — neither of which were
viable options for us. The cost and
weight savings of ordering printed
pulleys didn’t hurt, either.
The final bot weighed in at
exactly 3.00 pounds with the heaviest
bar. A full report on its performance
will have to wait, but we competed at
HORD in Ohio, went 2-2, and tied for
5th out of 16 Beetles!
We also didn’t kill any motors
(not even the Silver Sparks), so
although we have electrical issues to
work out and some modifications to
make, we are pleased with the results.
Figure 9: The robot with its primary bar installed. There are still
some structural and electrical issues to resolve, but we’re happy
with its performance.
Figure 8: Our three weapon bar choices. We ran out of time to
paint them, but they still looked sick!