evaluating sdbd. crown... Del

[Brian Trout]

Hi Del,

Thank you for clarifying for me.  Sometimes it's hard for me to get these
pictures in my head into words.  I can picture it on a blackboard as clear
as day!

And I also appreciate the thoughts on individual rib clamps.  I originally
made the ones I'm using now because I didn't have any, and it was either
make something, or use every single box of tuning pins I could muster up for
weights... well, lets just say I didn't want to go there.  They're really
quite low tech.  Just a cam action on each end of two boards that squeeze
the rib against the soundboard.  The one on the top side of the soundboard
is not as heavy as the one on the bottom, so that when it bends, it's
putting in a nice crown... where's that blackboard when you need it... and
since the bottom sides of my ribs are flat, I have a pretty good idea of
where I'm at crown wise.  Like I said, low tech.  But they work.

Thanks again, Del.

Brian Trout
Quarryville, PA
btrout@desupernet.net

----- Original Message -----
From: Delwin D Fandrich <pianobuilders@olynet.com>
To: <pianotech@ptg.org>
Sent: Tuesday, September 21, 1999 10:53 AM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano


> It is fairly clear that Brian knows what he is talking about here, but I
> would like to clarify one or two points.
>
>     1)  In the first illustration below, he states "...it puts the
> soundboard and the ribs at odds with each other, and provides way too much
> internal tension."  The word should be compression, not tension.
Somewhere
> along the line of piano evolution the idea came along that tension was
> somehow desirable in the soundboard panel.  Several writers have (and
still
> are) mis-using the word.  Compression-crowned soundboard assemblies place
> the panel under compression, not tension.  On the top, the bottom, in the
> middle --
> wherever you care to measure them, they are under compression.  This is
true
> until either internal cell compression has exceeded the fiber stress
> proportional limit and the cells have been permanently damaged or until
> long-term creep has permanently altered the shape of the wood cells
(without
> necessarily damaging them.  The top of the rib will be under tension, but
> not the soundboard panel.
>
>     2)  Illustration #2 is also pure compression crowning.  Regardless of
> how it is obtained, if the soundboard assembly depends on compressed wood
to
> either form or maintain crown, it is compression-crowning.
>
>     3)  Even in this case, the soundboard panel will be under some
> compression most of the time.  As the soundboard panel is being glued to
the
> ribs, the top surface of the panel might be under some slight tension, but
> as it is loaded by the downforce of the strings, the entire panel will be
> under some slight compression.  This will generally be true except when
the
> soundboard panel is exposed to very dry climactic conditions.  During more
> humid periods, the panel will be under slight compression.
>
> (And Brian, why build a "press?"  In our shop we still use the same
> individual air clamps that I built some 15+ years ago by choice.  They are
> far more versatile than any fixed press would be and they do an excellent
> job of applying even and uniform pressure to the rib while gluing.  What
> more could you ask?)
>
> Del
>
>