evaluating sdbd. crown & bridge downbearings in a new piano

[Brian Trout]

<Roger writes:...which of the above processes matches our method?

It's a little hard to tell with just the information you've given.

If you will allow me, I'd like to share some thoughts.

Soundboard crown can be achieved in at least 3 different ways.

1)  Gluing a flat rib onto a very dry soundboard, clamping them flat while
the glue dries,and allowing the added moisture content that the board will
soak up to swell the soundboard, and provide the crown in the board.   In my
opinion, it puts the soundboard and the ribs at odds with each other, and
provides way too much internal tension.  But that's compression crowning,
design feature or not...

2) Gluing a flat rib onto a dry soundboard, and clamping them with a crown
while the glue dries, allowing both compression crowning and perhaps a
hybrid variety of rib crowning.  I have a problem thinking that those glue
joints are going to hold up over the next 50 - 70 years.?  But again,
there's a lot of internal forces working against each other within that
soundboard / rib set unit.

3) Gluing a machine crowned rib onto a fairly dry soundboard and clamping
them with the desired crown while the glue dries.  In this setup, there
isn't nearly the tension between the ribs and the soundboard.  There is
crown.  You've built it in.  The major tension on the board will arrive when
there is a load (downbearing) placed upon it.

There are a number of other factors which will affect the overall crown of
the board in the piano such as whether the board was dried down again before
it was glued to the rim, and whether the downbearing is very light or very
heavy... the list could go on...

There may be other variations I haven't thought of.  But I've been very
happy with #3.  That's the only way I've ever done them.  From time to time,
I tweak a bit here and there, but that's the basic idea.

Actually, I need to build a press.  I've been using individual rib clamps
which resemble the clamps in the presses I've seen, only smaller.  Another
project for my to do list...(there's another list that has no end...)

Good luck,

Brian Trout
Quarryville, PA
btrout@desupernet.net


----- Original Message -----
From: Roger C Hayden <rchayden1@juno.com>
To: <pianotech@ptg.org>
Sent: Monday, September 20, 1999 10:11 PM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano


> Please explain:
>
> "compression crowned, rather than rib crowned, soundboards. "
>
> We made a pneumatic soundboard press, it works well but which of the
> above processes matches our method?
>
> Roger Hayden, RPT
> Clarks Summit, PA
>
>