FIGURE 3.
Make a left
and a right
motor,
placing the
wheels on
opposite
ends of
the motor
axle.
and drill slowly. Wipe off the oil when you’re
done drilling.
Secure the motors using four 4-40 machine
screws and nuts. The heads of the screws
should be on the side of the motors; the nuts
should be on the underside of the robot.
Because the holes are somewhat oversized, you
can use the slop to help align the motors. You
want to make sure the wheels are as parallel as
possible.
Finally, construct the skid by threading a 6-
32 hex nut onto a 6-32 x 1/2” machine screw.
See Figure 4 for the assembly detail. Place the
hex nut near the head of the screw. Insert the
screw into the hole and fasten in place with a 6-
32 acorn nut on the bottom side of the base.
Tighten the hex nut against the T to secure the
screw in place.
Only a few holes need to be drilled in the strapping T
to secure the motors. Use a 9/64” drill bit to make holes for
4-40 x 1/4” or 4-40 x 3/8” machine screws. The small
fasteners and the somewhat larger holes provide some
“slop” in mounting.
For each motor, position it near the end of the crossbar
where you want it to go. Use the mounting flanges of the
motors to mark the holes for drilling. There’s already some
holes drilled into the T, and you may be able to use one or
two as a starting point; you’ll need to drill the others to
match the motor mounting flange.
After marking, use a center punch (the spring loaded
kind is easiest) to indent a spot for drilling. The small
dimple created by the punch helps prevent the drill bit from
“skating” across the metal.
For best results, use a drill press and clamp the T in a
vise. (Avoid holding the T with your hands because if it’s
yanked loose during drilling, it could cause an injury).
Place a block of soft wood (such as pine) behind the
metal for support, and be sure to wear eye protection. Set
the drill to slow speed to avoid overheating the bit. Add a
drop of oil over the punched mark, and slowly drill through
the metal.
Drill a single hole — also using a 9/64” bit — near the
bottom of the T stem for the skid. Do the same for this
hole as above: mark with a center punch, add a drop of oil,
Upgrading the Gearbox Motors
The DC motors used in the Tamiya three-speed crank
axle gearbox are designed for three volt operation, and they
are relatively inefficient. They’re fine if you plan on
controlling your Mini T-Bot using switches or relays and a
three volt battery pack, but they’re not a good choice for
electronic motor control. Powering them at the usual 4. 5 to
6 volts will shorten their life, plus require motor bridge
circuitry capable of supporting three to four amps (yes,
amps!) of current if the motor becomes stalled (it stops and
won’t move).
The three-speed crank axle gearbox uses a Mabuchi FA-
130 motor. The 130 size is common (it’s popular in slot
cars), and you can get replacement motors that are not
only rated at higher voltages, but consume a lot less
current. One such option is the 130-size motor offered by
Pololu (their item #1117). It’s rated at six volts, and has a
stall current of only 800 mA. The price is under $2 per
motor.
The lower current motors not only consume less
battery power, but allow the motor to be used with low
cost H-bridge control circuitry. Suitable single chip solutions
include the SN754410, though I prefer using a L298 as it
provides a bit more functionality. (More about interfacing
the Mini T-Bot to an L298 below.)
FIGURE 4. Detail
of the non-rotating
skid, composed
of a 6-32 x 1/2”
machine screw,
hex nut, and
acorn (cap) nut.
44 SERVO 06.2011
Enhancing the Mini T-Bot
There’s no requirement that you design your Mini T-Bot
with the same Tamiya three-speed crank axle gearbox
motors and dollar store toy wheels. As an example, Figure
5 shows an alternative design, using a collection of readily
available Tamiya parts:
• Tamiya #72004 worm gearmotors. These provide
gear ratios of either 216:1 or 336:1, making the
robot slow but strong. I usually select the 216:1 ratio
when constructing small desktop bots. You need two
motors. (The mounting flange spacing is the same as
