R,C&S question JD

[AlliedPianoCraft]

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
> 
> 
> 
> 
>
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