Soundboard Clamping for Downbearing

[Peter Kestens]

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-----Oorspronkelijk bericht-----
Van: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org] Namens
Erwinspiano@aol.com
Verzonden: vrijdag 20 december 2002 17:08
Aan: pianotech@ptg.org
Onderwerp: Re: Soundboard Clamping for Downbearing


In a message dated 12/20/2002 5:36:27 AM Pacific Standard Time,
mcpiano@globetrotter.net writes:



.
>Marcel wrote:
>
>>To me, after the crowning process, the board should not
>be dried again. I
>>think that high temperature MIGHT affect the glue
>hardness and cause crown
>>collapse (a little at least).
>
>Please explain what you mean by "high temperature". And why
>would it be used?
>
>



                         Marcel
                         I often keep the boards in the hotbox after
they are pressed at what ever temp needed to keep the rh between30 and
40% which is often 70 to 80  degrees in a high humidity sesaon. However
it doesn't hurt any thing if the board is dried down to it's pre-ribbed
state of say 90 degrees and 25 to 30% R.H. Even if you glued the board
to the case in that dry it will still regain whatever crown was lost due
to being redried when it gains moisture content.Besides whatever crown
there is due to the ribs being crowned is more than ample, even in the
driest conditions. Purely 
             Compression crowned panels will suffer greater losses of
crown under the conditons described above and will have very little or
none if they were'nt carefully dried to about 4.5% EMC before ribbing
instead of what Peter posted earlier describing basically no drying at
all. Peter, what were you thinking? 
 
The question is if I think ?:-)
 
1)I have experienced now that compression crowned boards loose very much
crown by unstressing them; so if you want to work this method you should
carefully dry the panel so the wood will swell and taking crown by the
change of humidity.  And you also have to "overbend" the ribs to keep
some crown after a few days, weeks.  That's the only way to be shure to
keep some crown.
2)Using compression crown and drying the (in my case not dried before
gluing on the ribs)board before installing it in the piano will present
the risc of having negative crown (which is the case for the moment with
me) and cracks (in extreme conditions).  Problems then are : how can one
attach the bridges to the board? By puching more you also bend the board
in the direction it may not bend.  If you have crown and you puch with
more force, the board is going flatter which will allow to have the
bridge full contact with its complete surface.  (Of coarse you can puch
that much so you will puch through the board, but I think no-one will
take that risc)
 
So if you want to know, I'm going to restart, ie taking of the ribs
whith a plane and placing new crowned ones, drying the board and the
ribs carefully to a MC as low as possible and then gluing and clamping
them together.
The "only" problem I still have: how much crown should I give to the
beams of my press: none, and arc of +/-18m for the longest ones and
+/-15 for the shortest ones, (like the ribs are precrowned),  +  for
having  more  crown at the end?  (How much should one have with a new
board anyway?) Can somebody me tell that please?  I only know that the
uncrowned beams of my press give about one cm of crown when I'm blowing
the hoses. 
 
 
I also want to thank all of you who have discussed this item.  Do not
think I feel blamed but it was hard at some points.  I have learned my
lesson and made my conclusions that you can read above.  But I think
this was the money I have to pay for.  And after all, in doing 15 years
of restoration of piano's, I have only to restart 2 times a job.  I
think that's not bad.  And also, making soundboards, I love it more and
more.
 
Peter
 



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