to the 6 mm hole size with a 15/64” or letter ‘B’ drill bit.
This hole carries the ball bearing race on the bottom
side of each hip servo. A .0935” (#42) drill bit is used to
drill the four little servo horn hole patterns. Of course, you
should center punch and/or then center bit drill the holes
for greatest accuracy of placement.
Next, we need a pair of leg brackets to hold the two
rear wheels. These should probably be bent up from sheet
aluminum of 1/16” thickness or so. The exact dimensions
(length and hole spacing) for this part will depend upon
the size of wheels you select and — to some extent — the
length of the legs. I used an inexpensive sheet metal brake
that’s available from Harbor Freight. Great to have around
Also desirable are the two chassis spacers to be used
on the rear of the chassis. Again, they can be bent up from
1/16” aluminum. These spacers are a little more work to
make, but open up extra room for the battery. Standard
1/4” aluminum hex spacers (standoffs) cut to the
appropriate length can be used instead. Hint: Start with
2” spacers and cut 1/16” off the end (file smooth).
On the topic of wheels, you should be able to use most
anything on this bot. I had a pair of 2.5” diameter by 1/4”
thick HDPE (high density Polyethylene) that I designed for a
Micromouse laying around, so I went with them. You can
also pick a pair of wheels designed for use on R/C servos.
The R/C servo wheels won’t be bolted to a servo, but
they’re a nice size. (Who knows, you might want to build a
wheeled bot later!) Small 2-56 screws through the leg
brackets secure the wheels, or you can make up a custom
axle (basically a threaded 1/4” hex spacer). I made up a
pair of fancy hubs to mount my custom wheels, but most
anything will do that will allow the wheels to turn freely. I
even considered a pair of peanut butter lids for wheels.
A 6V battery (Ni-MH, 1,600 mAh battery pack) for the
servos, a 9V battery for the electronics, and a switch or two
constitutes the electrical system. Make sure the wire you
use is heavy enough for the servo power. You might want
to add a second switch for the 9V battery, or you can
simply unplug the battery when not in use. Mount the
battery pack well back on the chassis as shown to keep the
majority of the weight on the rear wheels. There’s a
drawing for a simple battery clamp, or use Velcro™! Having
the battery in the extreme rear means less weight on the
legs and less strain on the servos. After experimenting, I
found out that the rear end was so light that it needed the
weight of the battery pack to keep it on the floor.
Now we’re down to the processor boards — the brains
of the bot. We have several choices here. If you intend to
grow into a full hexapod or quadruped, then you may want
View of lower chassis on
wheels. Battery pack
w/clamp, leg spacer and
Cutting chassis plate.
SERVO 09.2009 53
Cutting second side of chassis plate.