This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment Re: Soundboard crown ----- Original Message -----=20 From: Phillip Ford=20 To: Pianotech=20 Sent: August 17, 2003 1:53 PM Subject: Re: Soundboard crown ----- Original Message ----- From: Erwinspiano@aol.com To: pianotech@ptg.org Sent: August 16, 2003 9:29 AM Subject: Re: Soundboard crown >>Phil-- My thought was that any wood will shrink over time = especially quartered wood because of it's expansion contraction = properties and actually that could take some time.=20 Wood does not shrink on its own--at least not so you would = notice--only as a result of changing environmental conditions, i.e., = with changes in the MC of the wood. Kept in an temperature/humidity = stable environment what would cause it to shrink? I was thinking that normal seasonal variations in humidity would cause = it to shrink. I was refering to the first comment, above. The reduction in panel width = that takes place because of compression set is not technically shrikage. = It is a reduction in size resulting from compression damage to the wood = cells.=20 I assume you're referring to the (so far, unsubstantiated) phenomena = of "oil-canning." I don't think I was. But I'm not really sure what that is. Can you = explain what you mean by this? The effect noted when depressing the bottom of an oil can. Initially it = is bowed out slightly. By pressing on it it pops in to approximately the = same radius in the reverse despensing a more-or-less precise amount of = oil in the process. I wasn't really referring to a compression crowned board. I think the = configuration that I have in mind would be uncrowned or reverse rib = crowned, if that makes sense. I wasn't thinking of a CC board that had = collapsed. I was thinking more of a board that was made deliberately = made to have a reverse crown, such as some of the pianos that Dale has = seen (assuming they were intended to have reverse crown). So, take an = undried panel, glue on some ribs, and load it up. It will reverse crown = and the board will be in tension I think. Or machine crown some ribs to = give the board a reverse crown when they are glued on. Load it up and = once again I think the board will be in tension. Now if this board is = subjected to a drier environment that that at which is was glued up what = will happen? I was assuming that the board would take on additional = tension. Enough to overload it? I don't know. But if not, then when = this board is subjected to more humid environments than that at which it = was glued up it would probably be less likely to sustain compression = damage than a CC board, or even a rib crowned board with positive crown. Yes, I see what you mean. I don't see how, in anything remotely = resembling average climate conditions, you would generate enough tension = to cause the thing to crack. Assuming the panel was at some MC in the 7% = to 9% range when it was ribbed. I still think, however, if the board were taken down to typical = compression-crowning MC levels before ribbing the effect of the = developing stress interface would work to force the board into a = positive crown.=20 The propensity of any soundboard to develop cracks depends on its MC = when glued to the ribs. If a panel is glued up at 3.8% to 4.0% MC even = in a reverse crown configuration it is going to develop significant = compression as it takes on moisture. How could it avoid doing so? It is = the extremely low initial MC and the act of gluing the thing to those = perpendicular-to-grain ribs that creates the problem. It will be interesting to read about the results of your experiment, = however. When will you be finished? I was also speculating about what would happen to this board if it was = subjected to a drier or more humid environment. My thought was that in = a drier environment the board would want to move up. My thought was = that the board is describing an arc. As it dried this arc would have to = get shorter, so the board would want to flatten out, which in this case = would mean moving up. You seem to think it would move down. Why do you = think that? Good question. You'll have two forces fighting each other, but they'll = not be equal forces. I think the stress interface will win. At least = until the panel self-destructs. I think the shrinking panel will crack before it creates enough lifting = force via tension to accomplish anything and the stress interface will = pull it down. As well, I think an expanding panel will work against = itself but the stress interface will create the enough force to push the = ribs up no matter how those ribs are initially crowned. If the ribs are = machined with a reverse crown and glued to a panel with very low MC, the = expanding panel will still want to force the assembly positive. It may = not make it but it will sure try. Will this be part of your experiment? With a conventional board having crown and with a downbearing load I = think the board is always in compression.... =20 I am uncertain what is meant by "a conventional board." Assuming = this refers to a compression-crowned soundboard system, the key words in = the above are then "having crown." However, a strong argument can be = made that this is no longer--if it ever was--the conventional soundboard = system.... I wasn't intending to comment on CC vs RC boards here. By = conventional, I meant a board that was intended to to have a positive = crown, which includes every piano I've ever seen (but not every piano = that Dale's ever seen). I therefore felt safe in calling this = 'conventional'. Ah, but that doesn't correlate to the first statement above which = states, "With a conventional board having crown and with a downbearing = load I think the board is always in compression." This is the comment I = was responding to and wondering just what was meant by "conventional." = Over the decades I suspect far more pianos have been built with either = hybrid systems (probably the most common) or rib-crowned system than = have been built using compression-crowned systems. And both the hybrid = and the pure, or nearly pure, rib crowned board will have crown even = without any compression in the panel. Our panels are ribbed at 6.5% MC. = When atmospheric conditions are such that the panel is at 6.5% MC there = is no internal compression within the panel. Yet there is design crown. = Over the years if these boards are subjected to very high levels of = humidity (and the corresponding high MC) they will also undergo some = amount of compression set. Still, there will be crown when they return = to equilibrium at whatever MC that may end up being. I was pondering a what if scenario. What if you built a board that = was intended to have a reverse crown. How would it behave? Would it = have any advantages over a 'conventional' board that is intended to have = positive crown? As well, it is quite possible to have a string downbearing load = without having positive crown. Yes, I know. But in this case it would be a deliberate design feature = rather than an unintended consequence. Well, there is always the Rippen--the only modern piano I know of built = deliberately to have reverse crown. Actually, the (laminated) boards = were ribbed flat and, when loaded with string bearing, were forced to a = reverse crown. They worked. At least within the limitations of their = less than desirable scaling. As to whether they might have worked better = with some form of positive crown, I have no idea. Nor do I have any idea = how they would have worked had their panels been made of solid stock. Del ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/9a/44/d0/ef/attachment.htm ---------------------- multipart/alternative attachment--
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