system was taken from my recently
retired 12 lb robot Apollyon, which
used 18 volt Dewalt drills for
locomotion. Using these motors with
a six-cell LiPo battery would mean
approximately 3 HP in the drive
system, and when geared for a travel
speed of 10. 5 ft/s, it would reach that
speed in as little as one second.
With the drive system figured out,
the next major design element
was weight balancing. With the
small chassis size and rigid
limitations as to how thick the
armor would be, the first
decision was to use steel as the
primary structural material.
Using steel had several benefits.
The relatively high density meant
it would need smaller design
changes to get the same shift in
the center of gravity, and using
steel meant that the chassis
could be welded into several
major sections which would
result in a very durable chassis.
The main body of the robot
is a mix of 1/4”, 1/8”, and
1/16” 4130 steel plate with
1/4” steel plate making up the
back plate, optional axle
wedges, inner side rails, and
outer side rails. The 1/8” steel
plate was used for the front
plate and the weapon
mounting bracket. The 1/16”
steel plate was used for the top
and bottom armor. All three
plates were also used for
portions of the various weapon
With the chassis cut and welded,
the next step was the assembly of the
drive system, which also led to the
discovery of a small but nearly
disastrous design flaw. When the drive
system assembly was planned, the
decision to weld the majority of the
chassis together had not yet been
made. When it came time to assemble
SERVO 04.2014 29
FIGURE 1. Model of the main chassis showing a generic attachment arm.
FIGURE 2. Waterjet cut steel plate used in the assembly of Spanky.
FIGURE 3. The back plate and outer side plate assembled prior to welding.