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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. <br>
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. <br>
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. <br>
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. <br>
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. <br>
Although I am not sure they qualify as RC&S boards, perhaps or
perhaps not, these boards certainly appear to be RC. <br>
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. <br>
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. <br>
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. <br>
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.<br>
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. <br>
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. <br>
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. <br>
Regards, Robin Hufford <br>
<br>
<br>
Joe Garrett wrote:<br>
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<div><font face="Arial" size="2">Terry said: "<font
face="Times New Roman" size="3">You still don't know for sure because
you don't know how the rib was =<br>
originally shaped. I assume you mean the "face" to be the surface glued
=<br>
to the panel and the spine is the opposite surface. What if the "face" =<br>
was straight and the "spine" was contoured (feathered) originally. Dry =<br>
down a panel and glue on the ribs. Let the board develop its
compression =<br>
crowning and now you have a curved straight "spine" and a curved
"face"."<br>
<br>
Terry,</font></font></div>
<div>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.</div>
<div>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></div>
<div><font face="Arial" size="2">Joe Garrett, R.P.T.<br>
Captain, Tool Police<br>
Squares R I</font></div>
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