Facts and nots : was Recommend Rebuilder?

[RicB]

Hi John,


    What I was thinking was, that if boards are glued together, then the
    compression takes place. If some of those wood fibers compress, and
    form the
    ridge, then the measurement, is less than if that portion had not
    compressed
    more than the rest, so the crown would be less. Or is the crown the
    same,
    because of the ribs, and the board just came in from the edges?

There is no calculation that can perfectly describe the amount of crown 
and how evenly it will develop over the entire surface of the panel. Tho 
no doubt one could calculate this assuming that wood was perfectly even 
in its strength characteristics. Wood isn't tho, so in that sense of the 
word there is the unpredictability that so much is made of by opponents 
of the method.  And indeed... producers of CC soundboards don't use 
those kinds of formulas to predict the degree of crown. They use 
empirical methods established a long time ago and I for one find these 
every bit as valid a method as using beam bearing formulas to calculate 
load bearing capacities that are to be matched against downbearing 
amounts. Its simply a different perspective offering different results, 
both structurally and acoustically.

I'm reminded of a discussion about comparing the Lowel downbearing gauge 
with Hartmans.  Much do to about nothing when it came down to it.

Crown however will be insignificantly changed by the presence of a few 
ridges.  The width of a ridge compared as a percentage of the width of 
the panel itself more or less says this in itself.  I'd agree tho, they 
don't look particularly nice... and they as such subject the method to 
lots of that same kind of hype (only in reverse) we hear so much about.  
But the fact remains that the major players on the field use this method 
successfully and have done so for a very long time. One simply can not 
escape this point, nor that this fact would be impossible even if the 
postulated flaws of the method were even a 10th of a percent as serious 
as claimed.

That said... other methods of crowning also have been used successfully 
for a very long time.  And if the RC&S concept is really anything new... 
it certainly has proved itself as well. As has the laminated panel, at 
least by one rebuilder of note.

Cheers
RicB

    I hope someone, knows what I am trying to say, as I am having a problem
    expressing myself. Obviously a field, I have not studied.
    John M. Ross
    Windsor, Nova Scotia, Canada
    jrpiano at win.eastlink.ca


         > Hi John
         >
         > The whole reason for compression boards is an involved, to
        say the least,
         > discussion. Compression ridges in themselves do little or
        nothing to the
         > sound of the instrument... nor do the cracks that eventually
        show
         > themselves because of these or because of a lack of enough
        compression
         > when the board was assembled.
         >
         > What compression ridges show are lengths of the soundboard
        where the wood
         > has been compressed beyond its tolerance.. the cells are
        destroyed in the
         > sense that their ability to swell or grow if you will with RH
        increases
         > to. They are simply crushed.  That in itself is not a big
        problem when it
         > comes right down to it either... at least acoustically.  A
        board with
         > severe compression set could be reused as an RC & S panel quite
         > successfully... which makes one ponder a bit on what is
        actually implied
         > by compression damage.
         >
         > A board fails to respond acoustically because of other
        developments in the
         > panel... that can very well be related to compression
        damage... or because
         > the support against crown may not be sufficient in a given
        area... or
         > because of  other reasons.
         >
         > As such... a compression ridge simply shows a weakness in the
        soundboard
         > that may (and probably will) develop eventually into a
        crack... and may or
         > may not turn into an acoustic problem given enough time and
        large enough
         > climatic instability.
         >
         > At least... thats what I hear from most holds, and it seems
        to match well
         > with my experience through some 30 odd plus years of working
        with pianos.
         >
         > Cheers
         > RicB