S&S D Duplex

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: November 23, 2002 12:14 PM
  Subject: Re: S&S D Duplex


    I'm not at all sure what you mean by "natural" crown. If it is some=20
    factory's doublespeak for compression-crowning, the process is =
anything but=20
    natural.=20
    =20

  Natural crown is an older English (British) term which seems to rougly =
equate your term Compresson Crowning. Junghanns text discerns between =
straight ribs on a flat panel, vs a panel pressed into a dished caul. In =
both cases we are talking about a thoroughly dried board, ribs across =
the grain. And in both cases crown is achieved by allowing the finished =
glued assembly to take on moisture. Where he notes differences are=20
  1) The dish cauled assembly will immediatly become stressed when =
released from the call, where as the flat panel will not.=20
  2) An asymetrical spherical curvature which is desirable can be easily =
achieved with the dished cauled assembly and practically impossible with =
the flat panel.=20
  3 The crown achieved with the dish cauled assembly will be stronger =
and more stable then with the flat pannel.=20
  4 The dish cauled assembly is more sensitive to being over stressed by =
too much downbearing.=20

I would take issue with #3 and #4. Assuming the ribs start out flat and =
are not machine crowned in any way, ultimately both are =
compression-crowned and depend on the ability of the wood fibers to =
resist the force of compression. It doesn't matter if the force comes =
from having been pressed in a dished caul or from taking on moisture =
after having been dried to some very low moisture. It is still =
compression and the wood cell structure reacts the same way.=20



  He (Junghann) also states that both these, plus the forementioned will =
expererience an increase in cross grain pressure and crown as the panel =
takes on moisture. And with the flat ribs / flat panel all crown is =
entirely a result of taking on moisture.=20

Well, to some extent all soundboards experience an increase in =
across-grain pressure as the panel -- the wood cells -- takes on =
moisture regardless of how they are crowned. The question is how much =
tolerance does the wood have to resist structural failure because of =
that pressure . If the fibers are already stressed -- compressed -- to a =
point close to their maximum fsl (fiber stress limit) due to the =
compression-crowning process there will be little tolerance for any =
further stress and the fibers will begin to crush. If the soundboard =
panel is essentially neutral and it begins to take on moisture the =
fibers -- not being close to their fsl -- still have a lot of resilience =
and will remain intact.
 =20

    > ....Steinway is about as careful as anyone could be with this =
process. Their controls are really quite good.

  Well, I have to take your word on that I suppose. Tho there has been =
quite a bit of fun poked at the NY plant for total lack of humidity =
contol on this very list recently. Another common comment going around =
is that the pitfalls of this method are not as well understood by =
craftsmen today as earlier.... because specialization in such "arts" is =
dissapearing. I have absolutely no idea as to the validity of such =
claims myself.=20

To the first point -- that would be in the factory itself. Not in the =
soundboard conditioning room. As far as I know the soundboard ribbing =
process is quite well controlled.=20

To the second -- I have no specific knowledge on which to make a =
judgment other than by looking at the finished result.=20

I do know it is not possible, in today's labor market, to come up with =
workers who understand wood and woodworking as the result either of =
their upbringing or as the result of their education in our school =
system. I am nearly brought to tears interviewing people coming out of =
what passes for a wood shop education these days.



  "As todays manufacturing is hardly able to use such changed of =
humidity deliberatly, this methods seems no to promise success. However, =
the authors (Junghanns (I guess)) seems to be different as he says the =
following.=20

    "An equal radius of the rounding of all ribs is desirable. If this =
is not achieved then the finished assembly will be under unequal =
stresses, a condition that should be avoided at all costs."
And all that trouble I go through to create all those unequal stresses =
... what shall I do? But I do understand the theory. It is based, I =
think, on the notion that the response characteristic of the soundboard =
should be uniform across its span, or surface. At the risk of starting =
the Great Soundboard War all over again, if you will consider the =
soundboard as a series of overlapping loudspeaker drivers you will see =
why this is not desirable. You need a tweeter in the treble and a woofer =
in the bass. Their response characteristics are not -- should not be -- =
similar.

  Fenner goes on to say then that Junghann further states that for this =
same reason the bridge had to be fitted to the crown of the soundboard, =
and that this is contadictory to what is found in older texts, and he =
(Fenner, I believe) agrees with the older texts.=20
And my bridges will still go on flat.

Del


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