Slip the tire over the hub cap as shown in Figure 5. Place
the geared hub on top of the assembly and secure it with
four #4-40 x 3/8” machine screws as shown in Figure 6. The
easiest way to clean up the tire and make it run true is to
mount the tire to a piece of scrap and slowly rotate it against
a sander like the one shown in Figure 7. You will need six
wheels for the PBOD. Once complete, set them aside.
There are four gears used to put the six wheels in
motion: two idler gears and two drive gears. One each is
used on each side of the PBOD.
The idler gear shown in Figure 8 is positioned between
two wheels on each side of the PBOD as in Figure 9. You
will need to print two idler gears.
The drive gear (Figure 10) is attached to the motor
and is used to drive two wheels. To attach the gear to the
motor shaft, you will need two #2-56 hex nuts and two
#2-56 x 1/4” machine screws. The hex nuts are inserted
into the two slots in the drive gear and secured with the
two screws as shown in Figure 11.
You will need two drive gears — one for each motor.
The PBOD chassis is made up of the six components
shown in Figure 12. These components are the sides,
bottom, and ends. In addition, there are two motor
mounts. Printing the parts can be problematic when printed
in ABS as they may want to warp. They can also be difficult
to remove from the bed. If you have problems printing
them in ABS, try switching to PLA on heated glass.
Basic Chassis Assembly
Construction of the assembly is relatively simple. One
part is slipped over the small cogs on another part as
shown in Figure 13. If the fit is too tight, you can use a
razor knife or file to reduce the size of the cogs.
Once attached, a #6 hex nut is inserted into the slot as
in Figure 14. A 6-32 x 3/8” machine screw is used to
secure the nut and hold the parts in place. Figure 15
shows a complete exploded view of the chassis. While I
recommend nylon hardware for this portion of the
assembly, you can use steel or aluminum hardware. You
can also add a small #6 lock washer to the machine screw
to keep them from working loose. This works for both
metal and nylon hardware.
You can find the STL files for the various parts on the
PBOD web page at www.kronosrobotics.com/pbod.
One thing you need to know, however, is I had placed
four small holes in the corner of the chassis bottom so I
could install a top to the chassis that would hold the
weapon. The problem is the hardware for the wheels
would not allow me to place either a standoff or spacer in
the corners. For this reason, I decided to add some small
tabs at the top of the sides and end parts to secure the
Next month, I will complete the chassis assembly and
add the wheels, gears, motors, controller, and radio. Once
complete, you will be able to take the PBOD for a spin,
literally. Please be sure to post questions and suggestions in
the SERVO Magazine forums at http://forum.servo
I am still looking at machining the parts on a CNC mill
for you folks that don’t have 3D printers. The chassis shown
at the beginning of the article was actually machined with
my CNC mini mill. SV
SERVO 12.2013 57