Recrowning/bridge wedging

[Jim]

Hi Danny again:

When a soundboard comes out of the press and into the ambient air
which in most cases has more moisture than the board when it came out
of the "hot box", the board takes on moisture and swells. This causes
the ribs to arch even more. The swelling board puts pressure, not tension
on the cells. When you add to this the downbearing, more compression
is added to the cells. When the piano goes to a more humid climate,
even more compression takes place as the cells fill with more water.
This added pressure is so great that the Tenor section of the piano
will push enough harder against the strings that the pitch may rise
perhaps as much as 20 cents.

In all of this, we never see the word tension. If the board had reverse
crown, then we could talk about tension. Then the analysis of the Golden
Gate suspension bridge would apply. Incidentally, you can build a much
longer bridge as a suspension bridge than you can as as arch with a
keystone. I was using that argument falsely when I was promoting my
suspension soundboard on the Conn C42 scale. It's funny what we do as
salesmen isn't it? Oh yes, we still had downbearing on those pianos,
but the sustain was lacking.

Now, back to the discussion of kerfing bridges every 6 inches or so. The
side grain wedges which were used 40 years ago did not work, because
maple is not strong in side grain compression. We even tried cutting
wedges across the grain in order to have end grain support for our kerfs.
This was some help, but not as good as filling the kerfs with epoxy or
polyester resin, which is very strong in compression and which carries
vibration better than maple. The idea of the multiple kerfs is to
provide many "keystones" to eliminate the sagging bridge effect and to
help the ribs to bear up against the down bearing of the strings.

For those purists who feel that this is destroying the soundboard system,
they, have probably never actually done it this way. Yes, it would be
nice to always put in new blocks, bridges and boards, but not everyone
is equipped to do this, nor do they have the skill. Then, the customer's
wallet is another consideration. One could use arguments that it should
never happen that a person only replaces bridle straps in reconditioning
an action, because, if the bridle straps are worn, so are the butt
leathers, so are the flange bushings, so are the catcher leathers, so are
the jack flange joints, so are the hammers, so are the dampers, so are
the damper lifter cloths etc. ad nausium.  Where do you stop? It would
be nice if we had such a reputation that we only did complete rebuilding
on every piano we took in, and that we had only clientele with deep
pockets. Judgment is needed from the technician as well as the customer
in deciding what level of work is appropriate. This may be one of the
toughest jobs we have, that is, of making sensible estimates. There is not
one rebuilder I know of who cannot say that there are jobs s/he had done
in the past that s/he wishes for personal reasons that it had been done
more completely. I hope that all rebuilders are able to have the clientele
that can support the type of rebuilding which makes them proud of every-
thing they turn out at the present time.

Jim Coleman, Sr.


On Tue, 3 Jun 1997, Danny Moore wrote:

> Jim wrote:
> 
> > The idea of the Golden Gate Supension bridge is that a thin bridge can
> > be suspended by cables mounted from tall towers. My idea for a
> > soundboard was that instead of having a crown, the soundboard would be
> > in tension which is a tiny bit more analogous to the tuning fork on
> > sprung spruce board.
> 
> Jim, and other interested parties,
> 
> Isn't the standard spruce board in effect a "tenstion" soundboard.  I'm
> sure you've more experience than I, but when rebuilding soundboards,
> I've noticed that when one removes the go-bars after the glue has set
> and the soundboard has had a chance to re-absorb the ambient humidity,
> the crown is really more apparant than after it is glued to the rim.
> I've always likened this to a bi-metallic bar as used in thermocouples.
> It seems to me that the unloaded soundboard is "crowned" as a result of
> tension, then controlled by the amount of downbearing.  In my opinion,
> too much downbearing is as detrimental as too little.  Also, it seems
> that, in rebuilding, the only control the rebuilder has over impedance
> is downbearing, as the rebuilder is working within the constraints set
> forth by the original designer/manufacturer.
> 
> Thanks in advance for your thoughts.
> 
> Danny Moore
> Houston Chapter
> 
>