Soundboard Clamping for Downbearing

[Erwinspiano@aol.com]

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. Peter writes> >> 
>> 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.

         Peter. 
       This is NOT  the only sure way to keep some crown. Its not at all  
Even if you over bend the ribs it isn't going to help all that much In 
creating or maintaining any crown. With the / your compression crowned method 
that uses flat ribs, 98% of the total amount of crown achieved will be caused 
by the moisture absorption of the panel after the ribs are glued on in the 
dried state and then reabsorbing  ambient moisture again. 
     If you're really hung up using the method then cut a 60 ft. radius crown 
on your press bars and go for it. But since your starting over why not make a 
fixture to cut some crown in the ribs themselves before they are glued on to 
the board. It sound like you're  interested in having  a stable reliable 
crown and resultant good tonal experience but I'm not sure you're listening 
or if there is a language gap going on here. For sure you need to grasp the 
bigger picture before you proceed and from your above post you have started 
to grasp the important points of what all of us have been saying but you need 
to go a step further down the road before you will get to your final 
destination.
   For the record let me say again, because I don't think you heard it, that 
a Pure compression crowned soundboard method is far more destructive to the 
wood cells and an extensive  loss of crown will happen Way sooner. So if this 
is what you want be advised. 
      Rib crowning methods require less drying of the panel and a  crowned 
rib of appropriate thickness and radius  insures longer term crown with much 
slower crown loss and wood cell destruction.
       Regards and good luck
        Dale Erwin

> >>  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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