Servo - March 2014

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