<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; =
charset=iso-8859-1">
<META content="MSHTML 5.50.4134.600" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face="Comic Sans MS" color=#000080></FONT> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=Richard.Brekne@grieg.uib.no
href="mailto:Richard.Brekne@grieg.uib.no">Richard Brekne</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">Pianotech</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> November 24, 2002 9:48 =
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: S&S D =
Duplex</DIV>
<DIV><BR></DIV>Delwin D Fandrich wrote:
<BLOCKQUOTE TYPE="CITE">
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">....
Where he notes differences are
<P>1) The dish cauled assembly will immediatly become stressed =
when
released from the call, where as the flat panel will not. <BR>2) =
An
asymetrical spherical curvature which is desirable can be easily =
achieved
with the dished cauled assembly and practically impossible with =
the flat
panel. <BR>3 The crown achieved with the dish cauled assembly will =
be
stronger and more stable then with the flat pannel. <BR>4 The dish =
cauled
assembly is more sensitive to being over stressed by too much
downbearing.</P></BLOCKQUOTE>
<DIV dir=ltr><FONT face="Comic Sans MS"><FONT color=#000080>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.</FONT></FONT></DIV><BR> </BLOCKQUOTE>
<DIV>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 ? </DIV>
<DIV> </DIV>
<DIV><FONT face="Comic Sans MS" color=#000080>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.</FONT></DIV>
<DIV><FONT face="Comic Sans MS" color=#000080></FONT> </DIV>
<DIV><FONT face="Comic Sans MS" color=#000080>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.</FONT></DIV><FONT face="Comic =
Sans MS"
color=#000080></FONT></BLOCKQUOTE>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">
<DIV><FONT face="Comic Sans MS"
color=#000080>Del</FONT></DIV></BLOCKQUOTE></BODY></HTML>