Soundboard Compression & Cracks

[Farrell]

>       i.e. it's the same as forcing a crown into a single rib with no
panel
> attached and gluing to the case.

If I am understanding you correctly, you are suggesting that a rib that has
some crown forced into it and then glued into a rim will keep its crown -
after loading with string pressure? I'm not 100% sure of how it would work,
but if I understand Del correctly, he would argue that the 1/8" of crown
initially in that rib will be reduced to 1/16" after string loading, and
then that 1/16" of crown will not be enough to support several hundred
pounds of string pressure. It's not like there is a foot of crown there and
the structure is engineered like a dome - in that case you likely could
place several hundred pounds on it, have it deflect a tad (maybe a bit more
than a 1/16" of an inch!?!?!?!?) but still support a lot of weight. The tiny
amount of crown forced into a flat rib won't do it - I guess the rib will
actually compress along the grain a tad, along with the case flexing out a
tiny, tad - and there you go, flat rib, or inverted, depending on how much
downbearing you had to start with.

To keep the crown in a board after loading, you either need a compression
board without extensive compression set, or a rib radiused/crowned board, or
a combination of the two. I'm sure the belly Gods will correct me if I am
misleading anyone here!

Terry Farrell

----- Original Message -----
From: "Erwinpiano" <Erwinpiano@email.msn.com>
To: <pianotech@ptg.org>
Sent: Monday, June 11, 2001 11:01 PM
Subject: Fw: Soundboard Compression & Cracks


>   Hi Terry
>
>    I like your questioning here on this topic.   It has been my experience
> that when the compression board (or any board)was first made the rib ends
> were free to bow in and out before glue in.  So here you have flat ribs
now
> significantly bowed because of compression.  Now glue that board in the
case
> and some of that crowning of the rib will stay there(or partially there)
for
> life even if the soundboard looks like a Venetian blind and has lost most
of
> its compression.
>       i.e. it's the same as forcing a crown into a single rib with no
panel
> attached and gluing to the case.  The rib with its forced bow stays that
way
> once the glue dries.  Once a board is removed from the case the ribs
return
> to flat because of no residual compression to keep them bowed.  I've seen
> this many times upon a boards removal from the case.
>    Really in this instance if most compression is gone in the panel but
yet
> some crown is left then hey it has become a rib crowned board( at least
> perhaps a little)
>
>      And yes it's soooo interesting an I 'm guessing your life isn't all
> that dull.
>       Best
>
>       Dale Erwin
>
>
>
> ----- Original Message -----
> From: "Farrell" <mfarrel2@tampabay.rr.com>
> To: <pianotech@ptg.org>
> Sent: Sunday, June 10, 2001 6:31 AM
> Subject: Soundboard Compression & Cracks
>
>
> > Take a compression-crowned soundboard that is not collapsed (still has
> some
> > crown) that also has a few cracks through which you can see daylight.
How
> > can this be? The only thing that gives the board crown is the lateral
> > compression across the grain as the wood gained moisture after its
initial
> > drying during construction. Now if you have daylight cracks in the
board,
> > does this not mean the board is not under compressional forces? Or is it
> > that the inter-crack areas are still under compression from being glued
to
> > the non-dimensionally changing ribs (relatively speaking)? But then
> why/how
> > is it that one area of the board can be under compression and an inch or
> two
> > away, the board is under tension????????
> >
> > What is it about the soundboard that is soooooooooo interesting? Or is
my
> > life just that dull?
> >
> > Terry Farrell
> >
>