Soundboardcrown

[Delwin D Fandrich]

----- Original Message -----
From: "Peter Kestens" <peter.kestens2@pandora.be>
To: "'Pianotech'" <pianotech@ptg.org>
Sent: December 17, 2002 4:48 AM
Subject: RE: Soundboardcrown


>
> > If you put no crown in the ribs, where did your initial crown
> > come from? What was the moisture content of the soundboard
> > panel when you glued the ribs on? Is your soundboard press
> > curved? If so, what is the radius of the curve?
>
> Dell, the initial crown came from the press itself.  Blowing up the
> hoses that'll press the ribs on the board, will bend the wood of the
> press for about half an inch.  Really, you can see raising the wood of
> the press.  You also see bending the board for at least that amount.
> Moisture content of the wood I can't tell you.  I can't measure it.  But
> environment humidity is about 60%, a normal amount in Belgium.
>
> As you can think already, the press is not curved, but if I decide to
> curve it, what's the amount you suggest?

I understand. But this is not enough to ensure that you will have adequate
crown in the finished soundboard. Most of the crown you do end up with will
dissipate rather quickly as the wood fibers in the soundboard panel take on
their initial compression set.

Also, what happens when the relative humidity goes down just a little?
Assuming a normal room temperature and a relative humidity of 60% the
moisture content of the wood panel will be around 10.5% to 11.5%. That is
very high to be pressing a soundboard and ribset.


> >
> > Typically, if you are going to depend on just the curve of
> > the rib-to-soundboard press to form your crown you would want
> > to end up with a whole lot more than 3/8" crown (on the
> > longest ribs) when it comes out of the press.
>
> Does this also means that one looses a certain amount of crown a few
> days, weeks after the board came out the press?

Yes. Most of what little crown you do have will be lost in the first few
days and weeks after coming out of the press.

If you are going to depend on compression-crowning techniques you will have
to plan on reducing the moisture content of the wood soundboard panel to
about 4% or so. This will require a special heated and carefully controlled
room in which to dry out the soundboard panel.

A much better way is to cut some crown into the side of the rib that is
glued to the soundboard panel. A good basic starting point is to use an 18 m
crown radius for the longest ribs and 15 m for the shorter treble ribs. (No,
this is not what I use but it is a good, safe starting point.)

With this crown cut into your ribs, and using the same air hose press that
you are currently using, this will ensure that you do have adequate crown
even in times of low relative humidity. And it (the crown) will last.

Even with this technique you should try to dry your soundboard panel down to
approximately 6% to 7% moisture content. This will still require a special
heated room, or box, but it can be quite simple. A plywood box large enough
to hold your soundboard panel and with room for a small space heater will
do. You can estimate the moisture content of the soundboard panel by
monitering the temperature and relative humidity inside the box and
comparing those to charts printed up for the purpose. The US Department of
Agriculture has placed the entire book, "Wood Handbook: Wood as an
Engineering Material" online. (I'm sure someone can give you their web
address -- I don't have it handy.) A chart such as this is printed In that
book (albeit in Imperial units of measure -- sorry).

Good luck.

Del