S&S D Duplex

[Richard Brekne]

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Delwin D Fandrich wrote:

> --
>
> Is anyone actually doing this? I can't imagine why. It's the only crowning
> process I can think of that would actually place the soundboard panel under
> tension from the start.

I have no idea if this is in practice still or not, tho perhaps it was thought
if the panel was first under tension, then compressed with string downbearing
that there was some benifit to be had. This was one of 4 basic approaches to
achieving crown given by Junghann in what Fenner calls  todays standard text on
the subject.  Nowhere in the Junghann work is there any qualitative evaluation
of the different approaches.

>
> >
> > and that both the so called natural crown
> > and rib crown had pretty much identical characteristics thus.
>
> I'm not at all sure what you mean by "natural" crown. If it is some
> factory's doublespeak for compression-crowning, the process is anything but
> natural.
>

Natural crown is an older English (British) term which seems to rougly equate
your term Compresson Crowning. Junghanns text discerns between straight ribs on
a flat panel, vs a panel pressed into a dished caul. In both cases we are
talking about a thoroughly dried board, ribs across the grain. And in both cases
crown is achieved by allowing the finished glued assembly to take on moisture.
Where he notes differences are

1) The dish cauled assembly will immediatly become stressed when released from
the call, where as the flat panel will not.
2) An asymetrical spherical curvature which is desirable can be easily achieved
with the dished cauled assembly and practically impossible with the flat panel.
3 The crown achieved with the dish cauled assembly will be stronger and more
stable then with the flat pannel.
4 The dish cauled assembly is more sensitive to being over stressed by too much
downbearing.

He (Junghann) also states that both these, plus the forementioned will
expererience an increase in cross grain pressure and crown as the panel takes on
moisture. And with the flat ribs / flat panel all crown is entirely a result of
taking on moisture.


>
> If the ribs are flat before being glued to the soundboard panel it doesn't
> matter if the cauls are flat or curved. The end result is a
> compression-crowned soundboard assembly. Again, it is anything but a
> 'natural' process.
>

Well, its just a term, and an old one at that, and it would seem these two
straight ribs methods both have a deal in common. Tho it is my understanding
that Steinway chooses the dish cauled variant.

> >
> > Isnt this mostly indicative of either poor assembly conditions, poor
> > handling,
> > or lack of understanding of the vunerabilities in the process, and less
> > indicative of that the process itself is fundementally flawed ?
>
> Not really. Steinway is about as careful as anyone could be with this
> process. Their controls are really quite good.

Well, I have to take your word on that I suppose. Tho there has been quite a bit
of fun poked at the NY plant for total lack of humidity contol on this very list
recently. Another common comment going around is that the pitfalls of this
method are not as well understood by craftsmen today as earlier.... because
specialization in such "arts" is dissapearing. I have absolutely no idea as to
the validity of such claims myself.


> I have explained how this system places stress on the wood that exceeds it
> mechanical strength capabilities in my Journal articles and, to a lesser
> degree, on this list in the past. It's a lengthy explaination but it is
> fairly basic wood technology.

Yes. Just for your edification, the 4th method our Mr. Junghann mentions is
useing machined curved ribs which curvature corresponds to the curvature of the
caul ( the word hollow seems to be used for the caul... but I am not sure)  As
in the flat panel approach, no tension of compression is on the finished
assembly until it starts taking on moisture. --- Then follows a couple sentences
I am not sure who to attibute to... but I believe Fenner himself says :

"As todays manufacturing is hardly able to use such changed of humidity
deliberatly, this methods seems no to promise success. However, the authors
(Junghanns (I guess)) seems to be different as he says the following.

     "An equal radius of the rounding of all ribs is desirable. If this is
     not achieved then the finished assembly will be under unequal
     stresses, a condition that should be avoided at all costs."

Fenner goes on to say then that Junghann further states that for this same
reason the bridge had to be fitted to the crown of the soundboard, and that this
is contadictory to what is found in older texts, and he (Fenner, I believe)
agrees with the older texts.



> Del

Anyways... this is the best I can do so far. Hard to be sure exactly of the text
as my german is very poor, and the Norwegian translation from the German is
hopelessly flawed. So I have to combine what little I know of these things on my
own with German and This Norwegian translation. I have gotten a bit of
assistance from Jan Grossbach on the 4 paragraphs about the 4 crowning methods.
But there are some unclear segments in that text as well and Jan is a busy guy
so I hesitate to intrude on his private email too much.

Would be very interested in your further comments.

Cheers
RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html


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