Tuesday, March 13, 2007

the great satellite disaster of 2007

I traveled to Fairbanks with a small hot wire gear cutter and parts to make a large one. At Belfair I spent the better part of two days making the large gear cutter and an axle guide prior to the start of the event. The structures were made primarily of balsa wood, fragile but lightweight enough to allow for gradual, gravity powered gear cutting without bending the resistance wire. Lars had the bright idea that I could use a single axle guide with the small and large gear cutter mounted at either end. We decided that this configuration looked somewhat like a satellite.

I didn't try to make a gear until the third day of the competition. By that time my power supply had apparently fallen off of one of our work benches and sustained some damage from the low temperature impact. The case was broken, and the Kapton tape holding the transformer to its mount had broken (They use the stuff to make space suits, so you'd think it would hold up in cold temperatures! I guess I must have had it against too sharp of an edge.) Fortunately, the wiring was intact, and current was delivered to the gear cutter as I attempted to make our pinion.

When I tested the hot wire gear cutting technique at home, I ended up with a nice gear and a puddle of water. Unfortunately, the colder temperature (-20°F or so) during the competition didn't allow for a similar result. No liquid water escaped at all, and I helplessly watched the device pass slowly through the entire cylinder of ice while all of the melted water re-froze immediately behind the hot wires.

I tried adjusting the voltage. More power made it work faster but also caused cracks to appear in the block of ice; no setting prevented the horrifying re-freezing. With only a half an inch to go, the power supply failed for an unknown reason (temperature affecting the semiconductor in the TRIAC dimmer switch?), so I began using the backup power supply (same design, smaller transformer). It finished the job, but I was left with a solid cylinder of ice trapped within the gear cutter assembly. Only a hint of a gear shape was visible.

I was almost ready to give up on making gears at all, but I figured that there was a chance that the technique might work if the individual resistance wires were bent slightly upward so that gravity could pull the liquid water along the hot wire toward the outer edge of the ice block. I decided to flip the whole contraption over, tweak the wires, and run it upside-down, passing the wires through the gear blank in the opposite direction. Even if it didn't work, I would at least free the ice from the gear cutter.

Here's a fun exercise to try at home: Place all your fine china or other fragile valuables in a large box with a bowling ball in the center. Don't use any packing material. Take the box for a walk around the block, flip it upside-down at least once, bring it home, open it, and witness the destruction. This is pretty much what it was like turning the balsa wood frame around with a block of ice trapped inside. I thought I could turn it over (holding the ice, not the wood) and pass it up from beneath the sawhorse workbench, but it was too big, or I was too clumsy, and it was, well, a disaster.

I did finally manage to get the small gear cutter in place, upside-down, over the gear blank resting on temporary supports, but the large gear cutter was badly damaged and completely separated from the axle guide in the process. I bent the resistance wires, attached the backup power supply, turned it on, and nothing happened. The backup power supply had failed! Thus our quest for gears ended.

Perhaps last year's technique would have been better, but the speed advantage of hot wire cutting seemed tremendous. After this experience and our first field use of The Boiler (Lars's hot tube cutting machine - effective but slow and with some cracking), we've determined that no heat cutting technique should be used in sub-zero weather. If we decide to use heat cutting in the future (and the speed advantage may well lead to that decision), then we will have to perform the operation on warmer ice with a warmer ambient temperature, perhaps in a tent.

No comments: