Speaking of special gauges to measure the crown radius, you don’t need anything that special. Attached are a couple of pictures of how to use a simple 15” straight edge and a simple formula to figure out the crown radius on a soundboard. A bit easier than stretching strings but beware of uneven crown or undulations along the width of the board. Using a set of calipers (the one pictured is ordinary but you can use a digital too) measure the width of your straight edge and then tare the scale on the calipers so that when you set the straight edge against the panel and measure the distance from the top of the straight edge to the panel itself you will get a reading of only the difference. That’s how much crown there is in that segment along the length of the straight edge. Using simple circle math you can then figure out the crown radius given the length of your straight edge and the measured displacement. The formula is: r = ((a^2 – d^2)/2d) + d Where r is your radius, a is equal to ½ of the length of your straight edge and d is the measured displacement or crown along the length of the straight edge. You’ll have to be consistent with units of course. If you want radius in feet then you’ll have to convert your d measurement to feet. If you want the measurement in meters then you’ll have to convert your d measure in mm’s (probably) to meters. The table below gives you an idea though. On my straight edge measuring 15” long or 380 mm (381 actually but who’s counting). The right hand columns are the measurements taken and the crown radius is then calculated. A displacement of .01 inches means a radius of 233 feet or 71 meters. The board I was measuring in the picture had about this much. I’d say it had flattened out a bit. BTW there’s a good article on this particular subject in the February 1984 PTJ written by Bob Hohf. On an unloaded board, what would be the minimum amount of crown you’d be willing to work with and why? To measure crown radius Ruler Length mm (15") Length x .5 mm Radius (m) Radius (ft) Measured Crown (mm) Measured Crown (in) 380 190 71 233 0.25 0.01 380 190 36 117 0.51 0.02 380 190 24 78 0.76 0.03 380 190 18 58 1.02 0.04 380 190 14 47 1.27 0.05 380 190 12 39 1.52 0.06 380 190 10 33 1.78 0.07 380 190 9 29 2.03 0.08 380 190 8 26 2.29 0.09 380 190 7 23 2.54 0.10 David Love www.davidlovepianos.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://www.moypiano.com/ptg/pianotech.php/attachments/20110712/bde7572e/attachment-0001.htm> -------------- next part -------------- A non-text attachment was scrubbed... Name: IMG00267-20110711-2314.jpg Type: image/jpeg Size: 56090 bytes Desc: not available URL: <https://www.moypiano.com/ptg/pianotech.php/attachments/20110712/bde7572e/attachment-0002.jpg> -------------- next part -------------- A non-text attachment was scrubbed... Name: IMG00270-20110711-2319.jpg Type: image/jpeg Size: 56335 bytes Desc: not available URL: <https://www.moypiano.com/ptg/pianotech.php/attachments/20110712/bde7572e/attachment-0003.jpg>
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