John, this is one of the best descriptions I have read here on "Traditional (CC) board construction. Very clear and concise. I have on question with the RC&S board construction. Are the ribs curved or flat when they are glued up? Al Al Guecia Allied PianoCraft PO Box 1549 High Point, NC 27261 (336) 454-2000 PianoTech at alliedpianocraft.com www.alliedpianocraft.com ----- Original Message ----- From: "John Delacour" <JD at Pianomaker.co.uk> To: "Pianotech List" <pianotech at ptg.org> Sent: Saturday, January 26, 2008 7:58 PM Subject: Re: R,C&S question JD > At 23:17 +0100 26/1/08, Richard Brekne wrote: > >>Hi JD... What I see here is that in an RC & S board under >>downbearing load at anything near glue up MC, the ribs will be in >>the opposite condition with respect to which half of the rib is >>under compression and which half is under tension then traditional >>boards. > > Well, the "traditional" board never experiences downbearing load when > the moisture content is anywhere near what it was when the ribs were > glued on because it relies on moisture uptake to form the crown by > internal compression. At the time the ribs are glued on the > un-ribbed side of the board is slightly stretched by being forced > into a dished table by the curved ribs, but once the glue is set and > the assembly is removed from the press this tension is at first > reduced and then replaced by compression as the board takes on > moisture, vainly tries to expand along the ribs and somewhat less > vainly tries to expand on the unribbed side, so that by the time the > board has acclimatized the various forces have increased the > convexity, or upward curve, of the whole structure, the greatest > compression being at the glue line, where no expansion is possible. > > Once the board is installed and the piano strung, the crown is > pressed down by the force at the bridge and the compression at the > top of the board is further increased. > > >> The [RC&S] panel's probably reasonably significant compression >>will be due string load forcing it (and the ribs) down. So the >>panel will be in somewhat similar condition to compression reliant >>assemblies... while the ribs will be in opposite orientation. > > As I understand it the RC&S board is subjected on glue-up to a > greater tensile force on the unribbed surface but (a) since it has > been less dehydrated and (b) because it has been forced round a > tighter radius against much less flexible ribs, some of this tension > will either remain or be reduced to a point while the assembly is > free. Compression at the glue line will exist, of course. Once the > piano is strung, the downbearing will press down the soundboard less, > because the beams (ribs) are more solid, and hence there will be less > reduction in the curvature and less increase in compression (or > decrease in tension) at the surface of the board. Now those who > practice this art will be able to give actual rough values to these > phenomena, but this method of construction seems to have as one of > its aims the avoidance of anything close to the degree of compression > to which a high-class traditional board is subjected. The > specialists will correct me if I am wrong. > > As you say, a lot of things are unclear, and these discussions often > start off with some hope of providing enlightenment, facts and > figures, and a statement of principles but all too often, almost > always, deteriorate into a rather vague mish-mash, a bit of dogma and > bye-bye. I try to limit myself to facts and experience and as much > science as I can muster, which is not always much! Opinion is > worthless -- and in my view excellent piano tone is far less a matter > of taste and opinion than you have recently suggested. There are a > number of measurable qualities in the sound of a good piano. It's > nice to have a fairly good string scale but I could name several > pianos that won gold medal after gold medal in the old days with > quite outrageous stringing scales and took the prizes because the > work they did on the belly was good. The belly itself produces > nothing, but what it does with what it's given is what makes the > difference between a "satisfactory" piano and something that sends > shivers up your spine. > > JD > > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/20080127/559988c6/attachment-0001.html
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