This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment Personally, I think Terry is right when he makes the point that rib crowning and compression crowning are just two extremes on a continuum of possibilities. Although it may indeed be a trivial distinction, the feathering at the ends of a rib on a compression crowned board, where such exists, means, functionally at least, an element of similarity to a crowned rib. As far as I can see, the only truly compression crowned boards would be those with unfeathered ribs, which I have seen on just a few pianos, one of which was a small Mehlin, if memory serves. If one's measure of rib crowning is a rib which has a variable thickness along the unfeathered area and is taller than wide, then there are any number of older pianos out there with this characteristic. Particularly noteworthy, are the pianos of Chickering from about 1900 where, in their production they explored the use of various radii, altered rib scaling, shorter or longer ribs, larger or small average cross sections, larger or smaller treble sections, etc. The late 123 has one rib, again if my memory is correct, which varies in thickness by almost a quarter of an inch. Much of what is represented here as novel, again with all due respect to those who think so, appears to have been anticipated a century or so earlier through the efforts of this firm and the present "new" school of design now clamorously present as "modern" seems, in amazingly similar ways, to recapitulate the technical evolution of this firm. This may, or, may not, be fortuitous but it is certainly useful as a counterpoise to the "standard" approach typified by Steinway. The Boston school, as it were, of piano manufacturing, versus, say, that of New York, as a whole displays a greater similarity in this regard to English pianos while that of New York appears somewhat more similar to those of German ones, although, of course, Mason & Hamlin, in some aspects, at least in their independent production, is an exception to this generalization. Broadwood's ribbing, at least on the four or five mid 19th century pianos I have observed appears very similar to that of Chickering, even though I did not measure the rib heights looking for variations there. Nevertheless, I feel fairly comfortable asserting that rib crowning has a long history, predating even 1900 and would be more associated with English pianos in this regard; German pianos may well have favored the thinner, wide rib used in compression crowning. It may well be that, in the United States, the collapse of the piano industry in the third and fourth decade of the last century and the subsequent unchallenged preeminence of the New York school has led to the illustion that there has been no alternative to the crowning methods used there, which I think is markedly incorrect. Rib crowning has a long history and appears to have been well explored. There is no doubt that Chickering pianos exhibit substantially less cracking than those of Steinway. This appears to be consistent with the claims of the RC advocates. It is interesting to note that cracks in Steinways, in general, are significantly wider at the top surface of the board, as most know, than they are at the bottom which seems in keeping with the compression crowning view of tension on the top of the board and compression on the bottom; cracks found on Chickerings show dramatically less widening through the section of the board. Although I am not sure they qualify as RC&S boards, perhaps or perhaps not, these boards certainly appear to be RC. I have urged the view before that it is inappropriate to apply simple elastic moduli indiscriminately as is done here when using them to make a few modest calculations which are then generalized to the conclusion that damaging levels of compression are inevitable, much less, even achievable in ordinary soundboards, be they RC or CC. First and foremost as the intrinsic condition of every soundboard is one of a very complicated, idiosyncratic, triaxial stress distribution and not one of uniaxial or plane stress, plane stress moduli are inappropriate to the facts, yet these very moduli are used by those urging the validity of compression damage. What is the Fiber stress at proportional limit which is what is quoted for disabling cross grain compression? It is simply the point at which elastic action ceases and the material will be incapable of achieving, when freed of compression across the grain, its previous dimensions. This by no means is a point of "cellular crushing" or destruction which is so often claimed here. The net result of such an effect, should it occur, as Richard Breckne has just pointed out in a recent post is, simply an increasing density and increasing strength which, no doubt, increases the acoustic velocity somewhat, and, again, may allow an actual improvement in the perceptual aspect of tone quality or have other beneficial effects absent other structural failures in the soundboard panel. As I urged last year those interested in such things should take the various manuals which, at one time, were suggested to contain the material upon which such a claim of damage could be based and familiarize themselves with them. One will find, should one do so, for example, that the coefficient of variation were this simple property, that of cross grain compression strenth, applicable, according to The Encyclopedia of Wood is 28% which means that attempting to make any kind of precise extrapolation from the results of such a calculation is unreliable and inappropriate. As for an empircal indicator, I have never seen, in thousands of pianos a significant change in the circularity of nosebolt holes found here and there in soundboards, which, were such a level of damaging "compression set" as is frequently assumed here, underway, would be likely to develop a significant eccentricity and become elliptical with a major axis parallel to the grain. In my opinion, crack formation in older, high quality soundboards is, in most cases, the result of shear failure and stress concentration, and not an indication of previous compression ridging, followed by drying and the opening of a crack. Compression failure, per se, in my opinion, is more likely to be found in more modern boards that have been ribbed at too low a moisture content. Loading of a crowned, or even uncrowned, soundboard along both sides the bridges asymmetrically by the downbearing pressure, and their interactions, inevitably introduces additional significant shear on both sides of the bridges, in every case, which ranges with moisture fluctuations and downbearing load. Stress relief over time results in the very commonly found long crack or two or three or four, etc. running sometimes as a chord between the ends of the bridge or, sometimes, parallel to it a few inches away. Where the shear is greater, for example boards with a tenor bridge, a characteristic field of cracks may well develop. According to the selfsame Encyclopedia indicated above shear parallel to the grain is virtually the weakest property of wood. Combine the high shear levels in this area of the board with the weakness of the material in this regard, high moisture levels, the possiblity of detached ribs and throw in an additional factor: the stress concentrating effect of nosebolt holes that are found commonly in this area and you have an efficient mechanism for crack formation. It is this shear field that accounts for the non-random placement of cracks in boards and their association with nosebolt holes as it is extremely common to find a crack originating in a nosebolt hole and, sometimes, even the screw holes used for screws attaching the guide rail . These are not random events. A second mechanism for crack formation exists in the classic interaction of the differences of radial and tangential shrinkage and expansion along with the effects of grain angle which produces warping in free boards. Even though the flitches are laid up with grain angles aligned to minimize this, these efforts are only partially effective and differential stresses, unique to each individual flitch are still present in the completed panel which work their effects over time. These effects are to separate the individual flitches along the joint. I would argue that most cracks fall into either of these two categories and are not, themselves, the results of "cellular crushing or compression set", although a lot of newish pianos nowadays very plainly show compression ridges. I don't mean to imply that soundboards cannot undergo significant functional failures; of course, they can and do, but these are not failures of the wood material itself but, rather, structural failures of aspects of the panel assembly: the most important of which, in my opinion, is detachment of the soundboard from the ribs, bridges or rim. Regards, Robin Hufford Joe Garrett wrote: > Terry said: "You still don't know for sure because you don't know how > the rib was = > originally shaped. I assume you mean the "face" to be the surface glued = > to the panel and the spine is the opposite surface. What if the "face" = > was straight and the "spine" was contoured (feathered) originally. Dry = > down a panel and glue on the ribs. Let the board develop its compression = > crowning and now you have a curved straight "spine" and a curved "face"." > > Terry, > I think you are giving most piano manufacturers too much credit for > possibly doing it the way many do today. Let's face it, this was a > mass production setting. They took a "straight" piece of wood and > slapped it on another "flat" piece(s) of wood. If the R&D people said > that the the "Face" had to be curved before gluing, then the workers > found the quickest/easiest way to do that. They, in no way, fussed and > labored over these pianos like we do. It was a job, pure and simple. > So, if you find a rib that is curved on one side, (the face, > convex), and flat or concave on the other, then you can bet your sweet > bippy that the R&D specified a curved rib, BEFORE glue up!<G> > Joe Garrett, R.P.T. > Captain, Tool Police > Squares R I ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/78/05/b1/9b/attachment.htm ---------------------- multipart/alternative attachment--
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