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. =20 Appears to me you are pointing out a similarity rather than a = distinction. Yes, both designs are commonly made to be more flexible = around its perimeter. 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. =20 One could easily build a "truly compression crowned board" with either = tapered or untapered ribs. One could easily build a rib crowned board = with both types of ribs. Rib tapering does not determine soundboard = construction. If one's measure of rib crowning is a rib which has a variable = thickness along the unfeathered area and is taller than wide, =20 That is not a measure of rib crowning. Rib crowning is a product of = shaping the panel side of a rib into an arc. Rib taper does not = necessarily have nothing to do it, nor does rib cross section. 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. =20 I have read repeatedly on this list how pretty much all of what is being = done with rib crowned soundboard designs and rim modifications, etc. = have all been done before. These have been discussed ad-naseum. = Certainly some new thining is also going on here, but the nuts and bolts = of it are clearly recognised as having been used again and again in the = past. I think what might be novel here, at least considering the last 80 = years or so, is that anything different from what that famous NY = manufacturer does could possibly be worthy of consideration. This may, or, may not, be fortuitous but it is certainly useful as a = counterpoise to the "standard" approach typified by Steinway. =20 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. =20 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.=20 I would agree with this, however, I might suggest that whereas "rib = crowning has a long history and appears to have been well explored", it = has perhaps not been thoroughly explored in the past. =20 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. =20 Although I am not sure they qualify as RC&S boards, perhaps or = perhaps not, these boards certainly appear to be RC. =20 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. =20 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. =20 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. =20 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. =20 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. =20 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. = Build a few panel models and expose them to humidity extremes. Measure = changes in crown. Observe crown/shape recovery. Such an activity is a = good way to learn about "compression set". Soundboards can get squished = with enough compression and enough time and do not recover to their = origninal shape. Regards, Robin Hufford ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/54/52/0d/1e/attachment.htm ---------------------- multipart/alternative attachment--
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