Down-Bearing for Old Board /AND YET MORE!

[Farrell]

Forgive me. One more question. This one encompases the whole jist of the
matter: Why do this (pre-compress the board)? Is it strictly to "get a feel"
for how the board is going to react to loading via string pressure? It seems
apparent that we are making some pretty big assumptions here regarding how
much pressure we are in fact putting on the board with our wedging
technique. But I suppose this is just something that one gets the "feel for"
and after a few times learns that tapping "just so much" with "just
such-and-such a wedge", yadda, yadda, yadda, it becomes a useful tool in
determining how much downbearing to put on a piano.

I think I understand now why Andre Bolduc says: "Oh, no, no, no, don't
pre-compress the board. Use these numbers with my board and you will have
good downbearing." He builds his boards the same way each time with the same
materials, and just has the experience to know that certain downbearing
amounts at various locations across the board will result in proper
downbearing on his boards. But I can see that when dealing with unknown
aspects of a board, new rib design, new/variable wood source, or an old
board, the pre-stressing evaluation can provide some good insight.

When I first heard Andre say that I thought - yea, yea, yea, everyone does
seems to do the same task differently in this business - sometimes without
apparent rhyme or reason. But I think what we have here is a case where the
more unknowns you have, the more you may want to look at it from more than
one angle. And John Hartman may do the pre-stress thing each time on each
board he installs (I'm not sure whether he does or not), but if he finds
that he gets good results from doing it that way, then he is obviously doing
it the right way - for him!

Oh, and like Ron N., I can't let a lie go on. I went out to my shop, took
out the plate and counted only eight full length shims - most being about
3/8" wide - and a few shorter ones (maybe it seemed like 83 because it was
the second time I shimmed the board - the first time was two years ago by
hand in my Florida garage and I did not dry the board down enough and all of
it cracked again). A lot for a board that is only about three feet long!
When I blocked/wedged the board up I guess (without exaggeration) I likely
had about ten wedgeing locations between the framing and the ribs.

Thanks so much for all the info. I think I have all the brain tools I will
have before doing this job.......so here goes!

Terry Farrell
Piano Tuning & Service
Tampa, Florida
mfarrel2@tampabay.rr.com

----- Original Message -----
From: "Erwinpiano" <Erwinpiano@email.msn.com>
To: <pianotech@ptg.org>
Sent: Saturday, February 03, 2001 12:48 AM
Subject: Re: Down-Bearing for Old Board /AND YET MORE!


> Hi Terry
>         EIGHTY THREE SHIMS !   Wasn't that about half the soundboard area?
> And you know there is no such thing as normal just bigger and not so big.
> And so your not new or young( define that please.!  )
>      The answer to your musing of the Big question is that you have
deduced
> correctly. Less crown/stiff less bearing more crown /& stiffness more
> bearing.  By the way this antique method of bearing setting has been
> handed down to us from the halls of the famous  piano factories .   It is
> quite reliable especially for the guys who are building there own boards
and
> know what to expect using there rib stiffness requirements.
>    While  pounding on the board with my right hand I'm holding the strut
> with  the fingers of my left hand and pressing the wedge with thumb of
left
> hand . Generally  if it's a rib crowned board , after about three thumps
on
> the board  at the central strut will drive the wedge to a point where
> further pounding   will  only feel like your pounding on a wood floor.
This
> is an aquired feel.  If it's a compression crowned board (most older ones
> are and I haven't removed a single board yet that showed any rib
crowning.)
> the crown will usually have shrunk a great deal and you could reverse
> pound(crown) it with to many blows. It will have a different feel!
>      At this point you have taken the slack out of the board and it's
> stiffening up quite nicely as John hartman says.  I believe  this
simmulates
> about 6 to 8 hundred pounds of string pressure by pre- compressing the
board
> ,which is probably only half the final amount of overall pressure at
a -440.
> So you can see that the extra 2 to 3 mm. gap measured at the string rest
in
> the pre-loaded state  will add extra pressure but not overload it your old
> board.  In fact considering all the wood damage I would not pre stress it
as
> much .
>     As Ron N said you can also do some weight loading test and get the
same
> or similar deflections . Take a deep breath it's not rocket science
> fortunately (way tooo much stress in that field.
>        Remember to pet the dog and hug the kids and control the our
> collective obsessive compulsive tendencies.  We must after all feed the
soul
> as well as our Faces!
>    Best Dale Erwin
>
>