evaluating sdbd. crown & bridge downbearings in a new piano

[Brian Trout]

Hi Jim,

I'll have a crack at the questions, although I'm certainly no authority.

1. Is it not increasingly difficult to bring wood from, say, 6.0% to 4.0%
> MC, than it is to get it to the initial 6.0%? (Recognizing that the lower
> the MC, the more unstable the material becomes, until one ends up working
with dust).

I would hope that no one ends up with a pile of dust.  That would be a
pretty expensive pile!  Is it more difficult?  I would suppose it's all
relative.  If I recall correctly, the big boys in N.Y. (S&S) have several
stages of hot boxes that they put their boards through.  I don't know all
the particulars, but I was told that they progress in temperature so as to
provide drier and drier environments for those boards.  I don't know that
it's more difficult to move from 6% to 4% in a 20%RH hot box than it is to
move from 8% to 6% in a 30%RH hot box.  (My figures are probably off a
little, but the concept is what I'm hinting at.

> 2. Does component handling and assembly timing not become even more
critical at lower MC's?

Absolutely.  But again, it's somewhat relative.  It'll be more critical if
the area you're doing the gluing in is 85%RH than if it's 25%RH.  Another
factor that could have an effect on the whole thing as well is the glue
you're using, and whether it has water in it.  (Another can of worms.)

> 3. If a panel is pressed, by whatever means, to a radius of 60', would the
"net" radius be more on the order of 56' or so? Regardless of the net value,
I must assume it can be calculated somehow to be repeatable.

I believe you've pretty much got the gist of it.  If the panel is dried out
to a lower MC, you'll have more of a change as the board soaks up moisture.
If it's dried less, you'll have less change.  I believe that a major factor
in repeatability is repeating the process.  I don't mean to be smart, but
many of the factors that contribute to the crowning process, by whatever
means you practice will become at least somewhat predictable.  As you do,
you learn.

> 4. What happens if, for whatever reason, the ribs and panel are not in
equilibrium with each other at the time of gluing/pressing? IOW, consider
that they came from two different hot boxes.

The board to be most worried about as to it's MC is the soundboard panel.  I
think you'll find that it will make very little difference whether the ribs
are at 4%MC or 8%MC.  It's the grain direction that makes the difference.
The length of a rib will not change significantly with changes in MC.  The
width of a soundboard panel will change dramatically with changes in MC.

> 5. How does a heat table relate to a hot box, and the respective virtues
of either?

You've got me on this one.  I've never used a heat table.  I don't think
I've ever seen one.  I would suspect that it could add some work time to the
gluing operation, much like what S&S does when it heats up all the parts of
the rim before it all gets glued together and pressed.

Just my two cents.  You know me... I just can't help myself.  :-)

Brian Trout
Quarryville, PA
btrout@desupernet.net

----- Original Message -----
From: harvey <harvey@greenwood.net>
To: <pianotech@ptg.org>
Sent: Sunday, September 26, 1999 7:31 PM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano


> Just a few tangential questions for someone...
>
> 1. Is it not increasingly difficult to bring wood from, say, 6.0% to 4.0%
> MC, than it is to get it to the initial 6.0%? (Recognizing that the lower
> the MC, the more unstable the material becomes, until one ends up working
> with dust).
>
> 2. Does component handling and assembly timing not become even more
> critical at lower MC's?
>
> 3. If a panel is pressed, by whatever means, to a radius of 60', would the
> "net" radius be more on the order of 56' or so? Regardless of the net
> value, I must assume it can be calculated somehow to be repeatable.
>
> 4. What happens if, for whatever reason, the ribs and panel are not in
> equilibrium with each other at the time of gluing/pressing? IOW, consider
> that they came from two different hot boxes.
>
> 5. How does a heat table relate to a hot box, and the respective virtues
of
> either?
>
> If I can't participate in the answers, at least I can stir the pot of
> confusion! <g>
>
>
>
> Jim Harvey, RPT
> Greenwood, SC
> harvey@greenwood.net
> ________________________
>  -- someone who's been in the field too long.
>
>
>