Thursday, January 31, 2008

smoking transformers

Having just received a new transformer in the mail, I was able to piece together a first attempt at a new power supply tonight. This allowed me to properly test the hot wire cutter for the first time. I am happy to report that it sliced through a block of ice quite easily. I had it up to about half of (theoretical) maximum power, and it cut as fast or faster than any hot wire I have used to date. This was the first time I have ever seen 19 gauge nichrome get red hot. I was afraid that it might become too brittle when hot and snap under the tension, but it remained intact. Unfortunately I had to cut my test short because the new transformer started making ominous crackling noises and then started to smoke a little bit!

The unit is rated for 2000 Watts, but, in hindsight, I think that rating is only for one of its secondary windings. It has a primary winding of heavy gauge wire rated at 100V, a secondary winding of the same wire at 95V, and various lower voltage secondaries from 15V to 30V. I plugged in a variac with its variable tap connected to the new transformer's primary winding and attempted to use one of the lower voltage taps to power the hot wire at 0-30V. It started to crackle at about one quarter power and smoke at half.

I think my problem was that the winding I used does not have thick enough wire for my power level. Instead of using the new transformer to step down from high to low voltage, I'll just use a lower voltage tap on the variac and use the 95/100 tap on the new transformer for isolation.

hot wire cutter

I got the new hot wire ice cutter mostly finished and in testable condition the other night. Unfortunately I am still missing one part needed for the power supply, but I was able to perform an initial test with a much weaker power supply from last year. Since the new device uses a very short length of low gauge wire (compared to the gear cutters of last year), it draws much more current. I hope to take advantage of this with a new power supply in the neighborhood of 1500 Watts for faster cutting, but for now I have to use my old power supply at very low voltage to prevent overheating of the transformer. It works, very slowly.

The device is made mostly of plywood with a 24 inch square piece of HDPE for the work surface, 19 AWG nichrome for the cutting wire, and assorted hardware. It should support the wire with enough tension to keep it reasonably straight while ice is pushed against it. So far I've cranked it up to 20 pounds of tension, but I think it will handle twice that. The cutting angle is adjustable to 45 degrees to either side. I still need to add an adjustable fence and rulers for the fence position and cutting angle.

Thursday, January 24, 2008

my plastic isn't very plastic

My sheet of UHMW was nicely planar, but it had small grooves in concentric arcs left by some sawing or milling covering both sides. I thought it would be a good idea to make the working surface as smooth as possible, so I tried sanding it. No dice! One of the properties of UHMW that makes it attractive for its many uses is extreme abrasion resistance - it out-wears steel 10 to 1! It should be planable, but I only have a mini hand plane, so that didn't work very well. Then I thought I might be able to smooth out the grooves with a propane torch, like finishing ice. Bad idea! The process did work somewhat as intended, but it also slightly discolored the surface and, worse, caused major warpage! It turns out that UHMW has some pretty strong internal stresses originating from uneven cooling at production, and those stresses result in warping when it is heated near the melting point.

I tried to flatten the piece by heating the whole thing up and then letting it cool slowly while being held flat by heavy things. Unfortunately, it didn't work. In hindsight, I should have just left the grooves. I doubt that they would have been much of a problem in the first place.

I decided that having a flat piece of plastic was important enough to warrant buying a replacement, so I took another trip to Colorado Plastic Products to rummage through the scrap bins. I couldn't find any UHMW scraps of the correct size this time, so I settled for some HDPE. I'm pretty sure it will work just as well for my application, and it is quite inexpensive.

Wednesday, January 23, 2008

make your own working paper clock

The very cool Automata / Automaton Blog points out a neat book, Make Your Own Working Paper Clock. "Cut this book into 160 pieces, glue them together, and have a paper clock operated by weights that keeps perfect time and can be rewound and regulated."


One of our guiding principles this year is simplicity. We want to make a working mechanical ice sculpture, but we want the design to be simple enough that we can actually build it without compromising the design during construction. One way to gauge complexity is by counting mechanical parts such as axles and gears (I'm not counting bushings).

In our first year, we attempted to build a mechanical ice sculpture with 25 mechanical parts (9 independently moving assemblies). We failed but managed to assemble 6 mechanical parts (2 independently moving) almost functionally. In our second year, we attempted 22 mechanical parts (11 independently moving) and succeeded in building a functional mechanical sculpture of 5 mechanical parts (2 independently moving).

This year we hope to construct a sculpture consisting of only 8 mechanical parts (3 independently moving). I think this is an attainable goal. With the experience we now have under our belts, I believe we can produce a few more working parts than last year.

Tuesday, January 22, 2008


Although Lars and I are very excited about Ice Art this year, we somehow have failed to find time for as much preparation as we had the last couple years. We even forgot to register for the event until a friend reminded us! Fortunately we were able to get one of the last spots in the single block.

Both of us are working on tool preparation, of course. I'm primarily working on a new hot wire cutter while Lars is attempting to resurrect the ill-fated bandsaw of 2006. We are both working on improving our arsenal of chisels.

I guess it's time to wake up and start blogging.