S&S D Duplex

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: November 24, 2002 9:48 AM
  Subject: Re: S&S D Duplex


  Delwin D Fandrich wrote:=20
      .... 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.

    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
  Yes, but isnt there some initial cross grain compression in the panel =
and tension in the ribs fresh out of the caul (dished caul variant) ? I =
mean why else would the panel assume an initial crown before it takes on =
humidity if not because the ribs are trying to restraighten themselves =
but are prevented in doing so. It would seem if this is the case that =
both issues # 3 and # 4 would be true then, tho it would also seem that =
this would increase the likelyhood of exceeding what compression levels =
the wood can handle when downbearing is added into the picture... no ?=20

  In my experience most of those using the straight rib/curved caul =
method do not dry their panels as much in the mistaken belief that this =
will reduce the amount of compression stress on the wood fibers. It does =
not. If, once both ribbed soundboard assemblies are at the same EMC, the =
ultimate amount of crown is the same then the amount of internal =
compression must be (and is) the same. That is, assuming that the =
ribbing is also the same.

  If the moisture content of the wood in each panel is the same at press =
time then the assembly that was pressed into the curved caul(s) will end =
up with more crown (initially) because of the extra internal compression =
developed as the panels take on moisture. (The ribs want to straighten =
out and the panel wants to expand beyond its dried state.) This will =
create even more internal compression and, ultimately, an earlier onset =
of wood fiber compression and compression set. I.e., earlier =
catastrophic failure.
  Del

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