Soundboardcrown

[Delwin D Fandrich]

----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: "Pianotech" <pianotech@ptg.org>
Sent: December 17, 2002 10:17 AM
Subject: Re: Soundboardcrown


> Ron. I think what Peter is doing here is inducing a small amount of crown
by gluing a flat rib to a flat panel in a curved caul at ambient RH. And of
course, this will produce a small amount of crown. He then dries the board
down and glues in into the piano. It would appear that he is relying on the
rigid-rim-supporting-the-crown theory to keep the board crowned after it
expands a bit with the increased RH. Obviously, there are a few of us that
do not feel that theory holds much water, let alone crown.
--------------------------------------------


No, Terry, the rim doesn't really enter into it. If you take a
compression-crowned soundboard assembly back to its condition at the point
of ribbing the compression stress that holds crown disappears. In other
words, if you belly a board at 4% MC using flat ribs and a flat press it
crowns up in a normal room atmosphere. If you then take it back to 4% MC the
crown disappears and the whole thing is more-or-less flat again.
"More-or-less" because there has probably been enough compression set within
the panel that, back at 4%, it would end up under some tension and go into
reverse crown.

What Peter is doing is simply taking the compression out of the panel
temporarily, letting the assembly flatten out, and then gluing it to the
rim. As moisture goes back into the panel it once again becomes a
compression-crowned soundboard assembly.

Del