Soundboard Pre-Stressing/Downbearing, was: Baldwin accu-hitch pins

[Farrell]

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I have always been curious about the soundboard pre-stressing method of =
targeting downbearing, but when I ask how much to push the soundboard =
down, I get answers like: "enough", "not too much" and the like. The =
idea of getting a feel for how much the soundboard will deflect under =
load I should think would help target proper downbearing Now I see here =
we are using some numbers. This is a good thing. You say David Hughes =
deflects the board 3mm at the center strut. You state that you drive =
wedges under each strut. How much do you deflect the board at each =
strut?

Now the other question I have is that I assume the reason one would =
pre-stress the soundboard is to incorporate into your work how much the =
soundboard will deflect under load. But if you deflect the soundboard =
3mm or whatever, wherever, for any given soundboard, what does that do =
for you (i.e., you are pushing a stiff board 3mm down, and you would be =
pushing a highly flexible board 3mm down - under strung conditions, =
obviously the flexible board would deflect more under final strung =
downbearing)? You could simply calculate the needed angles without =
pre-stressing the board. Now if you could pre-stress the board with =
downward pressure approximately equal to that of the final string =
downbearing in the various regions of the scale, then we would be truly =
simulating the soundboard's reaction to the strung-up load and =
accurately calculate our desired string downbearing angles, etc. Of =
course to do this we would need some sort of pressure gauge/device.

What about a rig similar to a pneumatic rib clamp. Just use the half =
with the fire hose. Clamp it to the rim above the bridge, inflate, and =
there you go with measurable pressure applied all along the long bridge =
in much the same manner as downbearing would be once the piano is =
strung. You could also use four segments of fire hose to apply =
appropriate variable pressure to each of the four sections (and one on =
the bass if you want to do that). The result would be a board stressed =
in very much the same manner as it would be after stringing. Then you =
could set your downbearing exactly where you want it, armed with the =
knowledge of exactly (or there abouts) how much the soundboard will =
deflect!

So now, readers..........you are thinking one of two things: Either =
Terry Farrell needs to get his head screw on properly, i.e. it's a dumb =
idea, or it sounds like a great idea, now go try it Terry and write us =
back. So what do you think?

Maybe it is the case that with a given soundboard construction, the =
boards are consistent enough to yield predictable result when =
pre-stressed 3mm or whatever. Is this the case, and thus would yield a =
more accurate pre-stressing a waste of time?

I guess I'm just thinking and probing for information. Any input would =
be appreciated. I realize the thing to do is put in a couple dozen =
boards, try some different methods, and see what happens. But boy oh =
boy, that will be years, and I would fear, some tears. I'm just trying =
to arm myself with as much info a possible before I screw up too darn =
many pianos.

Terry Farrell
 =20
  ----- Original Message -----=20
  From: Erwinspiano@AOL.COM=20
  To: pianotech@ptg.org=20
  Sent: Sunday, January 20, 2002 1:07 AM
  Subject: Re: Baldwin accu-hitch pins


  In a message dated 1/19/2002 9:15:16 PM Pacific Standard Time, =
davidlovepianos@earthlink.net writes:



    Subj:Re: Baldwin accu-hitch pins=20
    Date:1/19/2002 9:15:16 PM Pacific Standard Time
    From:davidlovepianos@earthlink.net
    Reply-to:pianotech@ptg.org
    To:pianotech@ptg.org
    Sent from the Internet=20

             David


     David Hughes and I both drive wedges in uder each strut to compress =
the board along its entire span. We start at the central strut then =
first and last capo then bottom strut. Then the dime quarter nickel =
business. I use 2.5 mm top treble 2.25mm first treble an bout 2mmin =
tenor tapering to 1.5 or so down low on tenor. Bass is set with about =
1/2 degree at the top tapering down to 0 at the bottom.
  Pretty much the same.
     Dale



    Dale:

    My conversations and classes that I attended with David Hughes (if I =
remember correctly) led me to believe that he prestressed the board =
under the central plate strut by 3 mm and then set the bearing dime, =
nickel, quarter, in low tenor, mid-range, treble, respectively.  That =
seemed to pretty much be his method without additional measurement.  I'm =
sure he arrived at this method by much experience, but he didn't seem to =
sweat it beyond that.

    David Love


      Dale:








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