JIMRPT@AOL.COM wrote: > > Tom; > I'm going to take selective excerpts from your post and ask questions about > them rather than give my opinion...OK? > (tension = stretching........compression = compacting, truisms both....except > when piano techs are talking and then we can mean either :) > > <<"I've seen how drying a board makes the > cracks open up, and returning the board to normal humidity closes the > cracks again, more or less.">> > What is causing the cracks to appear and disappear? Is it change in tension? > Compression? > > <<" But I don't see how increasing the humidity > would increase tension and cause cracks.">> > > This question still stands, i.e., > <<<<"Does a board that > has been 'dried' exert enough 'tension so that even a dried shim will have a > tendency to split/crack along the glue line when it (the board) regains its > moisture > equilibrium?">>>> > Think about a ballon with a line drawn on it while it is deflated ...now > inflate the ballon ...what happens to the line? Does the same thing happen to > the shim/glue line as the board returns to its normal moisture content? I would equate the rubber part of the balloon to the lacquer on the surface of the board and the air inside to the wood fibers of the board. The rubber is being stretched by the air and the air is being compressed by the rubber. If you expose the board to moisture, the wood expands and tries to lengthen the ribs, thereby putting the ribs in tension. Looking at it the other way, the ribs are resisting the expansion of the soundboard panel, and so the board is put in compression. > > This is kinda out of context since we are speaking of 'shims' installed after > a board is in place but still it reflects the same general principle.... > <<"The rehumidifying of the > dried out panel glued to the ribs generates a compressive force which, > constrained by the ribs glued on the underside, can only widen on the > side away from the ribs (the top), and therefore a curved surface > develops. So even the top surface is under compression, ....">> > > If the "top surface is under compression" where do cracks come from? > >From compression failed wood cells? Is this the cause of the cracks or is > this the fault that allows the crack to show? If the top surface of the panel > is "under compression" is it possible for a crack to show? Not as long as there is compression. If the board expands beyond it's elastic limit and then is dried, a crack will develop at the point of compression failure. I'd be curious to know why the board failed at that particular point. Is it at a glue joint where there is a sudden change in the grain angle? The grain of quarter sawn lumber can lean 45° either way. If you had absolutely vertical grain orientation in the entire board, would it ever crack? Do shims fail because you still have a mismatch of grain orientation? > > <<"That's why I have this fantasy that > drying the board and shims, before routing the board and gluing the > shims, would tend to support the crown,">> > Well I am not 'sure' that it is a "fantasy" but...........is a shim, or even > a small panel replacement, on a compression crowned board going to exert > enough force to compete against the rest of the preformed panels? Will the > curve of the crown still be intact while you are replacing this > shim/panel?...if so what is going to happen to that shim/panel when you allow > the board to regain moisture? The point I was trying to make was that every inch of the board contributes to the forming of crown and if you remove part of the panel (say, an inch), it seems logical that if you dried the shim, and the board while you were at it, glued the shim on to the ribs and the exposed edges of the panel, then when the board was returned to normal EMC, there would be some theoretical, and maybe actual, restoration of crown, assuming that the shim would do what the rest of the board did when it was first made. > > In a compression crowned board is the top surface under compression? If so > what allows the board to curve/develop crown? Is it the difference between > the ability of the ribs to hold the board flat versus the ability of the > board to curve the ribs when both are held captive by the rim?...or is it the > ribs curving and forcing the board to crown?...or a combination of the two? The rim is not part of the crowning mechanism; the crown is in there before it gets glued to the rim. So it has to be the interplay between the expansion of the superdried board and the restriction by the ribs of one side of the board. It's difficult to think of an analogy but consider a bimetallic strip, used in thermostats, circuit breakers, etc. Two metals of different coefficients of expansion are bonded together. If you heat the strip, one of the two metals will expand more which causes the strip to bend. The metal that expands the most will be on the "convex" side. Similarly, the boards and ribs of a soundboard panel exploit the fact that wood expands readily across the grain and much less so with the grain. > If the entire board is under compression why would it curve/crown at all? > Why is the measurement across the top of the crown greater in 'normal' state > of equilibrium than when it is at a 'dry' state? Where does that extra > measurement come from?....... because the board is under compression?...or > because.................................................? ...you've pumped more air into the balloon. > > Nuff questions......... > IMO a shim fitted while in a dried condition to a board in a dried condition > is gonna fail...don't matter what kind of glue you use...'if' fitted/glued to > an unsupported board. > But then what does I know? Mr. Nossaman ain't tole me what I think yet! :-) > Jim Bryant (FL) The foregoing is all just what I've believed in the past and probably don't amount to more than just theoretical postulations. I've read what Del has said about all this and have no argument with him. Ain't nothin' like a new board. But there are situations where, for example, a piano is breaking strings and the owners want to extend the life of their 100-year-old hand-me-down but don't want to pay for a new board because it sounds just fine to them. In that case, I would most likely shim the cracks because it would be the technicianly thing to do, first of all. Also, I'm with you in believing that a crack could eventually lead to rib separation and now is a good time to do some repairs. I think that it would be possible to shim successfully (no cracks developing for 20 or 30 years or ever) if we knew precisely what causes the cracks to form. It's getting too sleepy out to start designing experiments but I'd like to know what causes cracking, whether it's insufficient curing time of the wood, changes in grain angle at a glue joint, different grains per inch from one board to the next, or ? G'nite Tom Cole
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