Servo - June 2008

Loki Crosses the Pond — Part 1

Loki front.

Loki and Shelob on display at Yuri's Night, Moffett Field, CA, April 12, 2008.

failure? I neglected to have the feet overlap so that the CG of the bot was always over a foot. I had made the body wider, but not the feet longer. After a few tests, I determined a good length for the feet and made another pair. I may still use the first pair of feet by making a narrower body with a smaller controller board. The Don Tronics DT106 board holds some promise, or I may just design a new dedicated board from scratch.

Loki back.

Loki Gets Legs

Mechanical Construction

Depending on your resources, you can build the body parts of Loki in several ways. As mentioned, I drew the parts in CAD, and milled them out with my CNC’d Sherline mill. Not everyone has CNC of course, so the usual procedure is to lay out the parts with a blue dye and a scriber, and then cut them out. Drill all the holes first. PCB stock is easy to cut; a bandsaw makes simple work of the exterior contours. Dremel has a new scroll saw I’d love to have. Of course, a hand tool would also work. Mind the notches! Although exact location is not critical, they should be similarly located on matching parts. None of the dimensions are critical, and the decorative holes can be left out at the builder’s discretion. I like all the curves and holes, especially in the body and upper “decks.” The curves could be left out, which would also simplify making the parts. For the large holes in the body, you’ll want to drill holes near the corners, and then saw between them to remove the material. Finish up with a small jeweler’s file, and they should be good. “Nibblers” are also useful for this job. Make two sets of “LokiFootPartsV2” and “LokiSonarPlate,” and one “LokiBody2.” You’ll also need one “LokiIRbracket” of one sort or another.

An alternate way to lay the parts out is to print out full scale templates on a laser-jet printer, then cut them out and Scotch-tape them onto the PCB stock (follow the templates to cut out the stock). One could even lay out a PCB, etch it, and then cut out the parts. I don’t know if it would be any more useful for the home PCB maker, or if a PCB house would accept the job and route them out for you. Just a thought.

To put the legs together, trial-fit the pieces, then clean them up with fine sandpaper and de-grease. Assemble a foot, heat the copper foil up using the tip of the iron in contact with both pieces of the joint, and quickly tin. Tack solder the joint in a place or two. Keep testing the foot for a good flat stance on a flat surface as you tin and tack solder the remainder of the joints. Then go back and form good small fillets between the pieces. No harm in practicing on some scrap first!

Notice that the two brackets the knee servos bolt into are made of aluminum. I would have used aluminum for all the pieces, but I can’t solder aluminum! A simple bend of these pieces in a bench brake makes quick work of these parts. The aluminum may be a little harder to work with. In light of this, I also have drawings of this bracket for making them from PCB stock.