Soundboard drydown for installation

[Richard Brekne]

Hi Stéphane

Well... just got done re-reading Rons article and as I understand it 
turns out that Terry's answer really is wrong on two counts and perhaps 
(probably right) on the third.  As to the conclusion as to whether its a 
good idea to dry the ribbed assembly down before gluing into the rim... 
lets just say whatever reason there is for that doesn't seem to have 
much to do with creating or supporting crown.... tho I still have a bit 
of reserve head scratching to do on that matter.

Ron does two things in his experiment.  The first more or less proves 
the end grain is substantially more tough then this idea that it will 
just crush under pressure idea that has been thrown around. He 
constructs a small single ribbed panel, dries it down and wedges it into 
small base and buttress jig made from old pinblock material. The sides 
of this panel would not be able to move outwards period.  Upon taking on 
humidity the thing even after a few weeks took on no crown.  The reason 
given has to do with the geometry of buttressed arches... and if 
correctly assessed (and I see no reason to doubt his assessment) a 
direct consequence is that the edge wood of the top half of this little 
panel did not crush despite the pressure it was under. The only thing I 
can see possibly going wrong in this part of his experiment is that it 
appears in the foto that the underside of the ribs were flush with the 
base of the jig... which may have influenced the lack of appearance of 
crown... but I kinda doubt it.  Still... I suppose I'd like to repeat 
his experiment without that "constraint".   In any case all this  kinda 
shoots the sh#t out of the crushed edge argument.

The next part of Rons experiment was simply to demonstrate that there is 
no outwards pressure on the rim... or in the case of his experiment his 
base and buttress jig when a ribbed and crowned panel was placed into 
it.. and wedged tightly as before.  What happens when downbearing is 
applied.... is that the edges actually pull away from the <<rim>>. 

So Terrys comment below was wrong on both the first two counts. The top 
edge wont crush to begin with, and the top edge doesn't expand outwards 
to begin with.  At least not in the face of an increase in down 
bearing... and all of this is well justified by the same geometry 
relating to buttressed arches as above.

Anyone re-reading the article will be able to see this readily.

As for the rim in a real piano being resistive enough... well... Rons 
experiment doesnt really get into that.  And in my mind there are a 
couple questions left open about what happens to an already crowned 
panel that actually does expand outwards against a resistive ridge...  
Perhaps it will behave exactly the same as the flat panel in his 
experiment... I just haven't thought it through enough yet.

In anycase... whatever reasons there are for drying out the panel... and 
fitting it tightly to the rim before letting it take on humidity do not 
seem to me to have much to do with the crowning issue. 

Then there was JD's post which I am still in my mind trying to fit into 
this picture.  I'm not sure what he was saying there... but it seemed 
like he was stating that a rib crowned panel glued into place would if 
anything move in the direction of reverse crown upon taking on humidity. 
Much of the middle part of the rib was above his two <<force lines>>... 
and a panel taking on humidity along with that part of the rib would all 
want to expand... Putting the rest of the rib under some tension.... How 
that could result in a kind of reverse crown strain I dont see at this 
point...and perhaps thats not what he was saying at all.  But if so.. 
then how that in turn would affect whether the edge grain of the panel 
would be exerting outwards pressure on the rim or pulling away as in 
Rons demonstration... well all this has my head aching again :)

Actually... I love it.... wish I knew more and wish this medium of ours 
allowed for better communications among folks who clearly share an awful 
lot of the same kinds of interests but dont seem to manage to get across 
to each other.

To end... I'd say the most obvious relavant thing I've seen in these 
posts that have tried to address your question is that a tighter fit 
probably results in less movement.. or wiggle someone said in the 
position of the soundboard... and hence bridges with changes in 
humidity.. which gets into all kinds of other stuff... your glue 
questions for one...

Hope this all serves to yield a nice evenings sleep :)

Cheers
RicB



        Hi Terry (and Mike, and Ric).

        If I get you all right, there is no need for drying the ribbed
        panel again
        before gluing it in the piano.

        But then, if I understand well, there would be no problem either
        gluing the
        ribbed board in the piano in a 60 % humidity environment, and
        let the piano
        undergo severe winters at 28 % either.  Is this what you think ?
        If so, I have to admit I must update my own thoughts (which
        happens often
        these last days), for example about soundboard cracks causes.
        So all the climate matters are between the ribs and the board
        (and the
        method of assembling), right ?

        By the way, my miserable board did come back to normal shape
        (after 48
        hours) and is glorious again.  I see no reason anymore to commit
        suicide
        today.

        Best regards.

        Stéphane Collin.


    'Course, none of this actually works - the ribs won't shrink much,
    the panel
    top edge will just crush when it expands the 1/10 mm that it might
    move and
    the rim has enough flex to negate any "arch" support.

    Terry Farrell