Experimenter

March 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/113663

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H in t s F or H o m e b uil d e r s Scanning the Bearhawk Plans A time-saving tip By Ken Scott Recently, three neighbors and I formed a consortium to scratch-build a Bearhawk LSA. Papers were signed, a bank account established, and soon we received the plans from designer Bob Barrows. Excited, we spread them across the living room floor. There were 29 hand-drawn sheets. Every one of them had the same sentence across the bottom: "Scale drawing for dimensions not shown." We quickly discovered that quite a lot of dimensions were "not shown." And when dimensions were spelled out, the copying process wasn't perfect. Variations of a 1/16 inch or more were common; sometimes just the line width was enough to make a very distinct dimensional difference. In a world where precise computer-aided design (CAD) drawings have become the norm, these seemed, frankly, a little crude. out by hand. Instead, we bought an assortment of aluminum remnants from a local aircraft manufacturer. We took the material and .dxf files of several parts to a local business that makes aluminum parts for computer displays. In 15 minutes, its computer-driven mills and punch presses made enough spar fittings for three airplanes. Wing rib blanks were cut from sheet aluminum, complete with lightening holes, in about two minutes per rib, and accurate to about two-thousandths of an inch. Scanning and reworking the plans in the computer have enabled us to quickly produce inexpensive and accurate parts. If this seems like cheating…well, airplane builders have always used the best tools they could, and we're happy to continue the tradition. We found a simple solution that not only gave us fully dimensioned plans but also provided a means of producing parts far more quickly and accurately than chopping them out by hand, one by one. We scanned the plans sheets at a local copy shop, and they did all 29 sheets for $30. The scans were saved as Adobe .pdf files, a file format that is almost universal in the computer world. Adam, one of the CAD hobbits who inhabit the far nooks of my workplace, opened the files in his drafting program (SolidWorks in this case, although almost any simple mechanical drawing program would work), and in an hour of simple tracing and dimensioning, produced very accurate CAD files for several parts. Using called-out dimensions, the program could instantly deduce the rest—"If that's given as 0.625 inch, then this has to be 0.500 inch." The CAD drawings traced from the .pdf files were saved as .dxf files. I don't know what .dxf means, but computer-operated machine tools do. The 5-foot airfoil profile (provided in the plans), scanned and traced in CAD, was laser-cut from a sheet of 3/16-inch steel for $50. We used the steel master as a router template to make form blocks for the main, nose, and rear wing ribs. The total time for scanning, redrawing, laser-cutting the master, and routing the finished set of form blocks out of high-density particle board was about five hours. Things got better from there. The usual method of producing flat metal parts from Bearhawk plans is to cut out the paper depiction, glue it to the metal with spray adhesive, then cut it EAA Experimenter 29

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