Ron, I can't agree with your conclusions. Let's step back and examine this from a different perspective. Compare machined crowned ribs with laminated ribs for example. Laminated ribs are made from flat strips of wood and glued in a form to create the crown. There is stress in the rib because each strip is trying to return to its flat shape. The laminated rib is in equilibrium while it is crowned. You would think that such a rib would be easy to flatten out since each laminate is trying to return to its original shape. But as many of us know this laminated rib will have pretty much the same stiffness and have no more tendency to go flat than a similarly crowned solid rib. If you cut a rib from a Panel crowned soundboard (parallel to the rib but half way between the adjacent ribs) you have a laminate made from a rib and a strip of cross grain spruce. The two pieces of wood form their crown just the same as the above laminated rib. Once formed this structure is going to behave just the same as if the crown were formed from a crowned rib. I admit that it took force to form the crown in a panel crowned Soundboard but once formed it will have just the same stiffness as a Rib crowned soundboard. The method of crowning has no effect on its stiffness. Furthermore, the idea that the small additional compression of the panel due to bearing will stress the panel of a PC soundboard more than a RC board is just not right. .002" compression (strain) of a cross grain strip of spruce say 48" long will indicate the same stress whether it started in a stressed state or not. Stress/strain remains proportional if measured in inches per inch (percent) and if kept below the plastic limit (the compression of the panel of a PC board is below this limit). Richard brought up the idea that maybe a PC board would be stiffer because the panel was compressed. I think it is a mistake to think that the panel or any other wood structure would become stiffer by by simply subjecting it to compression, tension or any other force (if this were true it would turn physics on its head, things would get stronger as they are stressed). It is easy to get this idea from hearing what's been said (me included at one point) about how bearing works to stiffens the soundboard. The soundboard appears stiffer to a vibrating string (has higher impedance) because the soundboard is working together with the elastic force of the bearing as applied by the string plane. The soundboard does not gain stiffness within its self, it simply joins forces with bearing pressure. The conclusions I draw from this are: 1)The method of crowning has no influence on the final stiffness of the soundboard. 2)Soundboard panels do not become stressed to any great degree from bearing if the bearing is within the usual tolerances. 3)Soundboards do not become stiffer because of the stresses from crowning. John Hartman RPT Ron Nossaman wrote: > In a CC board, panel compression is not only supporting all of the > bearing load, but bending the flat ribs into a crown as well. The ribs > in a CC board are supporting none of the bearing load. They are, in > fact, adding to the load on the panel. How much down load do you have to > apply to the top of a flat rib to get it to bend up in a crown? If you > try it, you will find that the answer is "none", you have to apply lift > from beneath to bend the rib up into a crown, just like you have to > apply lift from beneath to support string bearing. Ribs don't support > bearing at all in a CC board until the crown is concave. > 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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