<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> <body bgcolor="#FFFFFF">   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">So far, so good. A stress interface develops between the soundboard panel and the ribs at the glueline.  RicB: <blockquote>-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.</blockquote> <blockquote> </blockquote> And now we're on the edge of the wagon getting ready to hop. Actually, we're probably already on the bunny trail. I would question that the outside of the soundboard panel would be just 'less tense.' Even at this point it is almost certainly under some amount of compression.  RicB: <blockquote dir=ltr style="MARGIN-RIGHT: 0px">Let the panel take on humidity..</blockquote> <blockquote>--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.</blockquote> Ah, now we're definitely hopping on down the bunny trail. And we're not alone out here. For some reason this seems to be a point of confusion with many.</blockquote> </blockquote> Richard Brekne writes: And I just KNEW this was exactly where things were going to happen. I guess thats a positive sign anyways.. :) It seems obvious that all the confusion, or dissagreement, or whatever it is, is centered right at this exact bit here... the presumed tension in the outside of the panel. The confusion starts exactly the moment we release the assembly from the caul then. Until that point, everyone seems to be in aggreement that there in tension in ribs (panel side) and outside of the panel itself, and compression on the inside of the panel.  What is not clear is WHY, (i.e. what the mechanisim is and how it works)  the tension in the ouside of the panel somehow reverses to compression when the panel is released from the caul. One is quite liable to think that whatever tension was there would simply be halved (assuming the assembly returns aproximately half way to its origional flat state). Actually, one could be tempted to think that this was the whole point of the process... to pre-tense as it were, the outside of the panel so it could take on crown without so much compression.  And if one DID assume that, then the rest of the logic that follows would seem to make perfect sense. Likewise if, as you say, the panels outside actually comes under compression then the rest of what you wrote below makes perfect sense. Indeed, unless I am mistaken,  if your claim that (paraphrased) << for equal amounts of crown, there are equal amounts of compression in both the flat ribbed / flat caul, and flat ribbed / dished caul methods) then there would have to be exactly the same amount of compression in the panel at this point as there would be for a flat rib / flat caul assembly that had the same amount of crown. One is left then with nothing gained whatsoever by employing  the dished caul at all. Or rather.. about the only thing you COULD accomplish would be to manipulate  the curvature of the caul in order to create a different shape to the crown then what one would accomplish with the flat caul. It's certainly not within my knowledge base to declare whats what here... but at least I believe I see where the different views I've seen brought up diverge and why they do so. Thanks muchly for the detailed and thorough reply Del. If you care to go a step forward and explain how the tension changes to compression on the outside of the panel as a result of being released from the caul, I am sure you would go a long ways towards clearing the entire issue up. Cheers.. RicB   btw... I've always been in the wagon... I just dont like getting jogged around much. I do like to hop off once in a while to chase down bunnies tho... I'll freely admit that :)     <blockquote TYPE=CITE> <blockquote dir=ltr style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">As the panel becomes compressed to the point it is able to develop a glueline stress interface sufficient to form or even maintain crown the whole panel is going to be under compression</blockquote> </blockquote> This (above) would seem to be the exact moment of confusion then... It happens rather quickly.. or what..?  just pop the assembly out of the caul and in the time it takes to settle into its initial crown the tension that was in the outside of the panel turns to compression, despite the fact that the panel never flattened out to release its tension that way. Clear this up, and you've cleared the whole issue up me thinks. <blockquote TYPE=CITE> <blockquote dir=ltr style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">. And, as it takes on moisture sufficient to increase that crown the amount of compression is going to increase. And this increase is evident through the entire panel, not just the surface nearest the ribs. Keep in mind that amount of wood cell compression required to generate the amount of force -- the stress interface -- necessary to bend a set of flat ribs into a crown radius of 18 m (approx. 60') is considerable. There are a lot of variables but it's going to be something over 1% to 2%. Now it doesn't matter to the wood if this compression comes as a result of being placed in a vice and squeezed or if it comes as a result of developed internal compression through the mechanism of being dried (shrunk), restrained (ribbed and brought back to some higher MC (expanded). Hence if a soundboard panel starting out at, say, 1,000 mm across grain, is put in a vice and squeezed -- compressed -- by 1% it is going to end up being only 990 mm wide. If by 2% it will be 980 mm wide. Or if a panel is stabilized at 4% MC and cut to a width of 1,000 mm and then taken back up to 12% it is going to expand to somewhere around 1,015 mm to 1,020 mm (yes, I've done the experiment). That's an expansion of about 1.5% to 2.0%. Now, if you dry this panel back down to 4% (shrink it), put it into a fixture that will not allow it to physically expand, and then take it back up to 12% MC, it will still be 1,000 mm wide (the fixture sees to that) but it will now have a considerable amount of internal compression. Something on the order of 1.5% to 2.0%. At least it will for a while. Since wood cells were only designed to tolerate compression levels up to about 1.0% they will quickly begin to fail. So, what happens if this panel is dried to a MC of 4% and bent in to a set of realistic curved cauls (let's stick with our 18 m radius). Yes, the outside surface will stretch by some small amount and place this surface in tension. Probably by some fraction of 1% -- I'm guessing here, but (assuming a soundboard of 8 mm thickness) I'd be surprised if the stretch (and tension) was much more than 0.01% . And the bottom surface will compress by about the same small amount. In other words, the top surface might want to stretch by about 0.1 mm and the bottom might want to compress by about 0.1 mm. Now with a set of flat ribs glued to the back of this panel restricting its expansion as the panel takes on moisture all of this would-be expansion turns into compression. And here is where we hopped out of the wagon: The amount of compression developed will be substantially more than is needed to neutralize that slight amount of tension and turn it into compression. What you will end up with is a panel with, for example (and using my admittedly hypothetical numbers), 1.49% compression on its top (outside) surface and 1.51% compression on its inside surface. But the whole panel will definitely be under compression. And when the compression ridges develop they will develop all the way through the panel, top to bottom and bottom to top. Can we get back in the wagon now? Del</blockquote> </blockquote>   -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway <A HREF="mailto:rbrekne@broadpark.no">mailto:rbrekne@broadpark.no</A> <A HREF="http://home.broadpark.no/~rbrekne/ricmain.html">http://home.broadpark.no/~rbrekne/ricmain.html</A>   </body> </html>