when measurements beat tracing
Wednesday, October 17 2012
In the fourteen-inch-wide concave transition between the south-facing glass of the greenhouse downstairs and the south-facing glass of the new upstairs, I've decided to paint the inside of the concavity black and then span corrugated polycarbonate over it to make a system for producing hot air using solar energy. Because the polycarbonate is corrugated, I would need something at the studs to support the ridges of the corrugations, especially if I later attach a massive hinged lid to the studs (something I've been seriously considering for years). I'd bought a set of plastic space-fillers designed to fit beneath such corrugations, but I'd bought the corrugated polycarbonate at Lowes and the space fillers at Home Depot and it later turned out that the two were not compatible. So I had to make a space filler out of wood. Yesterday I'd done this by tracing the pattern of the corrugations onto a piece of wood and then cutting along the pattern with a scroll saw (a process that produced a pair of equally-usable space fillers). I'd installed two of these space fillers, but by today I realized that the tracing process had been too inaccurate. For example: a slight bow in the polycarbonate while tracing would produce one set of space fillers that was concave and another that was convex. So today I took a different approach: I took careful measures of polycarbonate and then, using those measurements, I transferred the pattern to a piece of wood. The things I noted in my measurements was that the waveform was symmetrically trapezoidal, had an amplitude of plus or minus one quarter inch (or a half inch from peak to trough), had a frequency of three cycles every eight inches, and that the top of every peak and the bottom of every trough measured five eighths inches in length. From that description, anyone reading this could have produced the space fillers that I produced today using a scroll saw. Though these space fillers were not absolutely perfectly, they were a big improvement over what I'd produced by trying to trace the corrugations. Sometimes tracing is definitely the way to go, particularly when shapes include curves. But in this case I could distill the essence of the pattern down to a very small collection of numbers representing straight line segments.
By this evening, I'd finished the concavity, the studs, the supports for the studs, the corrugation space fillers, and painting everything black. The only thing left to do was to cut the polycarbonate down to size (from 144 by 24 inches to 144 by 14 inches) and then to attach it. I used tin snips to cut the polycarbonate and attached it to the studs using short wide-headed screws with rubber washers. Since the studs were four feet apart, I added four additional aluminum clips along the top edge to hold it reasonably flat against the carpentry. There were still a number of gaps that needed to be filled (some nearly a half inch in width), and for this I used silicone caulk. To finish the solar hot air box, I'll still need to close off the east and west ends, which are still open. But at least for now this transitional space is no longer a place where rain, snow, and leaves can collect.
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