Ron Nossaman wrote: > In purely compression crowned boards, the overall compression is the > problem, not the starting MC. Similar panel compression levels can be > gotten by either severe drying, or pressing the assembly in a deeper > dished caul. If the resulting crown is the same, the panel compression > is the same no matter how it was arrived at. I agree with Ron on this one except that the starting point is important. It determines the range of variations in humidity the soundboard can safely live in. If your soundboard is going to live in New Orleans it needs to be ribbed at a higher point than if it is going to live in Nevada. > I also note that my original request for an explanation still hasn't > been approached. John said that the ribs of a compression crowned > soundboard supported most of the crown, and the panel supported very > little. I'd still like to know how that can be when it is panel > compression that is bending the rib in the first place, thus supporting > both the rib and the bearing to retain crown. O.K. I didn't say or mean to say "that the ribs of a compression crowned soundboard supported most of the crown". What I was saying is that once crowned and installed the panel of does not support the bearing load in any significant way. I agree that the compression of the panel forms the crown but this does not mean that the panel supports the bearing. I know this is hard to grasp but I suggest looking at the evidence. The panel is not in a position to support bearing and we see that the panel does not compress very much more when the bearing is applied. Knowing the basics of elastic theory we can see that indeed the panel is not doing much in response to bearing. Let's see what happens when a strip of of spruce is compressed across the gain. If we apply a force of 200 psi the panel will reduce in length about .345%. If we see that a similar strip of wood has been compressed by .345% we can assume that there is about 200 psi of compression. The principle works both ways. Lets now glue a rib on the strip of spruce while it is compressed a bit more than .345% (we do this because there will be a little spring back). The structure crowns and the strip is compressed with 200 psi of force. Now let's step on the assembly and straiten it out. We measure the length of the spruce strip now and find that it has been further compressed .001% (or something like this). To find out how much compression is now in the strip of spruce we multiply .001 by 580. The increase is about .5 psi. This is the evidence that shows that the panel does not do much of anything to support bearing in in either a panel crowned or a rib crowned soundboard. John Hartman RPT John Hartman Pianos [link redacted at request of site owner - Jul 25, 2015] Rebuilding Steinway and Mason & Hamlin Grand Pianos Since 1979 Piano Technicians Journal Journal Illustrator/Contributing Editor [link redacted at request of site owner - Jul 25, 2015]
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