S&S D Duplex

[Richard Brekne]

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Hi Del...

I think I understand what you mean here. Basically you are
saying that if you use flat ribs, whatever crown you end up
with is more or less a measure of how much internal
compression the panel is under. Otherwise there would have
to be a difference between flat ribbed / flat cauled and
flat ribbed / curved cauled.... or what ?

So please indulge me for a bit so'st I can perhaps get this
right.. all this goes to the flat rib / dished caul variant
of Compression crowning. Follow along and tell me where / if
I hop off the wagon.


     --If you start off with just a panel in a dished
     caul and push it down... then you have a panel
     thats tensioned on its outside and compressed on
     its inside.

     --Push in a few ribs and the sides of the ribs
     that meet the panel are tensioned.

     --If panel and ribs are glued thus and allowed to
     dry, all stays unchanged as long as the thing is
     in the caul. But when you take it out then the
     whole thing wants to try and return to its flat
     state but cant. The compressed underside of the
     panel and the tensed sides of the ribs work
     against each other being glued tight as it were.

     --What does happen tho is that it returns about
     half way and you have some crown already before
     taking on any humidity, and the inside of the
     panel is less compressed then it was in the caul,
     the outside less tense, and the ribs less less
     tense.

Let the panel take on humidity..

     --and the compression on the underside of the
     panel increases, the tension on the ribs
     increases...

     --and the top side of the panel... hmmm.. its
     still under some degree of tension if I am correct
     up to this point. So at some point enough humdity
     will put the top side under compression.

Ok... so clear this one up for me please.

Thanks.. RicB


Delwin D Fandrich wrote:

>      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 ? 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
>




--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html


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