considered using the Agent 390 platform we had equipped
with an air cannon in the April 2016 issue, but it was not
equipped to provide the 7.4V we needed for the servo
motors. We opted for the trusty VEX platform since it was
already set up to meet the power requirements for the
servo motors. The bot was last used as the platform for a
Mighty Zap linear servo lifter in the October 2017 issue.
We removed the linear servo and arm assembly, and
just kept on disassembling from there. We’ve used the VEX
base as a testing platform for so long that it was assembled
in a way more focused on facilitating rapid design changes
instead of strength. It was a bit flimsy, in other words, and
not well-equipped to crush things without being more likely
to suffer self-inflicted damage than to dole out compaction.
We added additional structural braces, and rearranged the
placement of the receiver and battery to leave a clean area
in the front of the bot for mounting our mechanism.
For the chomping mechanism itself, we envisioned a
stationary bottom jaw and a top jaw actuated by two
gearboxes. The ample mounting holes on the large gears
meant that we could mount struts for the top jaw directly
to the gears, and complete the jaw with a crossbar
across the struts. The clearance holes in the large gear
accommodate 6-32 screws, and we were able to
scrounge up a few in Robot Central that were not
only long enough to pass through the scrap pieces of
U-channel that we found for the struts, but even
longer than that. We planned to fasten the assembly
together with Nylocs, and we were not about to
waste time tightening down the nut over an overlong
The scrounged 6-32 screws were brass, so were
easy to cut down to size using the handy dandy bolt
cutting feature on our trusty wire strippers. The ends
of the screws were as easily bitten off as Frodo’s ring finger
as he overlooked the fires of Mount Doom.
Devising the mount for attaching the gearbox to our
base plate was a bit more challenging. One of our chief
(and only) grievances with the square tube style gearbox is
the uncharacteristic dearth of mounting holes. There are
some mounting holes in the familiar hub pattern on the
narrower side of the rectangle, perpendicular to the output
shaft. Other than that, there are only pairs of mounting
holes in the corners on the wide side of the rectangle,
tucked far into the corner of the tube.
We wanted to mount the gearboxes so that the gear
train and output shaft (and wide side of the rectangle)
were vertical, so that meant the only mounting holes were
in the vertical plane on the back end of the gearbox. Four
mounting holes per gearbox — plenty, we thought. Maybe
Steven Spielberg was similarly confident that Bruce would
work flawlessly to bite and crush everything from boars to
unsuspecting people during the filming of Jaws.
We had a length of U-channel that had the same major
dimension as the wider side of the gearbox. We cut two
SERVO 03.2018 57
DENTAL WORK WITH PNEUMATICS.
FASHIONING THE BASE PLATE.
BEGINNING TO LOOK BITEY.