Ensure the brass shaft rotates smoothly,
reassemble the geartrain, and replace the
top case.
Remove the lower case. Remove three
wires between the printed circuit board
(PCB) and potentiometer, and solder two
small 2.7K series resistors to the PCB as a
voltage divider to replace the potentiometer.
Use a servo tester to verify proper function
and reassemble the servo. Repeat for the
second servo.
Making two hexagonal drive shafts is
the next mechanical challenge in
constructing this bot. The stock 8 cm steel
hex shaft will become two shorter shafts —
each needs 2-56 threads on one end to
thread directly into the servo output shaft
screw hole. If you have a lathe, it's a trivial
matter to cut the shafts and drill one end
out to accept a 2-56 screw or threaded rod.
Otherwise, you need to round up a 2-56
die to cut threads on the hex shaft. Use a
bench grinder to reduce the ends of the full
size hex shafts to a smaller circular diameter
which starts easily into the die. Work
carefully so that the threads are square and
properly aligned with the hex shaft. Don't despair if you
mess up once or twice; there is plenty of extra material
since each shaft only needs to be about 0.750" long with
0.200" threaded.
I recommend threading both ends of the stock hex
shaft, and cutting to length only after verifying the threads
are good and straight. Screw the finished
driveshafts into the servos. Hard "finger
tight" plus an extra half turn with pliers is
plenty tight for this application.
Fabricate some 5/16" spacers to glue
your servos to the leg gearboxes. Almost
any sturdy glueable material can work. I
used sheet styrene from a hobby shop,
glued together in layers with thick CA glue
and accelerator built up to 5/16 thickness.
These spacers get CAed to the flat
rectangular tops of the servos, and
ultimately to the outer case of each leg
gearbox.
Alignment is critical when final gluing
takes place. I recommend running the
servos with a servo tester during assembly
to make sure that no binding takes place
and your legs move smoothly as shown
at www.youtube.com/watch?v=
aSnp58Uhhug. After both of your
servo/leg gearbox assemblies are done,
CA your servo bottoms together, plug
both servos into the same tester, and take your chassis for a
spin. (Another video you can check out is www.you
tube.com/watch?v=zHmrUZENFhI.)
A 5 cm x 7 cm perfboard fits perfectly on top of the
spider — which is now about 1/4" wider than its stock
configuration. A single AA sized 14500 Li-Ion cell provides
SERVO 03.2014 69
Servo mod.
Spacer and shaft.