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<DIV><FONT size=2><FONT face=Arial size=3><EM>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. </EM></FONT></FONT></DIV>
<DIV><FONT size=2><FONT face=Arial =
size=3><EM></EM></FONT></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT size=2><FONT face=Arial =
size=3><EM></EM></FONT></FONT> </DIV>
<DIV><FONT size=2><FONT face=Arial size=3><EM>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. </EM></FONT></FONT></DIV>
<DIV><FONT size=2><FONT face=Arial =
size=3><EM></EM></FONT></FONT> </DIV>
<DIV><FONT size=2><FONT face=Arial size=3>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.</DIV>
<DIV><BR><EM> If one's measure of rib crowning =
is a rib
which has a variable thickness along the unfeathered area and is taller =
than
wide, </EM></DIV>
<DIV><EM></EM> </DIV>
<DIV>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.</DIV>
<DIV><EM></EM> </DIV>
<DIV><EM>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.
</EM></DIV>
<DIV><EM></EM> </DIV>
<DIV>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.</DIV>
<DIV><EM></EM> </DIV>
<DIV><EM>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. </EM></DIV>
<DIV><EM></EM> </DIV>
<DIV>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.</DIV>
<DIV> <BR><EM> 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. </EM></DIV>
<DIV><EM></EM> </DIV>
<DIV>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.</DIV>
<DIV><BR><EM>Regards, Robin Hufford</EM> =
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