Spurlock shimming method.

[Overs Pianos]

Roger, Ron N, Del and list,

Been following this as a reader only so far.

Ron N wrote:

>  > >I know what a buttress does, and the ribs are the constraining element.
>  > >Once again, the assembly is crowned and will support a considerable load
>>  >before it is ever put into the rim. The rim does not support crown - the
>  > >ribs and panel compression do.

Then Roger wrote:

>  >              Interesting hypothesis,  you trying to say that there will be
>>  no difference with the board supported at the perimeter, or not.
>>  Well I suggest you put a 100lb weight in the centre of of an unsupported
>>  board and measure the movement of crown, then do the same with the board
>>  glued into the piano, and let us know the results.
>  > Roger

Then Del wrote:

>I know, I know...I'm not Ron. But some time back when I was writing the
>original piece for the Journal I did just this. Actually, it was more like
>230 lbs -- don't ask -- but the results were the same. I put the board in
>the piano loose -- i.e., without it being glued to the inner rim and loaded
>it. Then put it in with glue and loaded it again. The results were similar.
>I won't say exactly the same because of the difficulty of measuring and my
>unwillingness to spend a week setting up a more precise experiment.
>
>There are two problems with applying the Roman arch concept to the piano
>soundboard structure. The first is that the arch radius (i.e., crown radius)
>is much too large, especially after the soundboard bridge is loaded by
>applying a fairly substantial amount of string downforce. The second is that
>the material we use (wood) is not rigid enough to support crown in this
>manner. Even if there were some initial stiffening effect from the arch it
>would soon dissipate as the wood goes through its cold flow process.

While I agree with you Ron N, I can see where Roger is coming from 
here. There must be some minor contribution made by the crowned 
('arched') sound board being attached to what we hope is a relatively 
rigid perimeter (rim). Del's results tell us that it must be quite 
minor, and while I agree with Del's comment about the large arc 
radius and the lack of rigidity in spruce, the arched panel must 
nonetheless help the strength of the board to some small degree once 
its glued to the inner rim.

It is interesting just how flexible seemingly solid objects can be. 
Try placing a magnetic base dial guage ball-end on the bridge with 
the base attached to the adjacent plate (we adjust plate set bolts 
using a dial gauge in this position). Then lean on the edge of the 
rim and watch the dial gauge indicator move. Even with a heavy 
concert grand rim the movement will amaze you.

Fazioli build their sound boards to a nominal crown specification of 
9mm per meter (these are [1994] minimum figures). They expect about 
half of the down bearing to be lost when the piano is strung and at 
pitch. Assuming then that the resultant crown at pitch is about 7 mm/ 
metre, it follows that there must remain some slight assistance to 
the strength of the board which derives from the remaining panel 
crown (Fazioli also shape the ribs before gluing them to board - a 
known RC sound board producer).

We've just fitted (last Friday) the sound board to the piano we're 
bringing to Reno. Interestingly, we lost over 1mm of sound board 
crown when it was glued into the piano. Yes the inner rim was angled 
to around 1 degree in the direction of the panel crown, but I didn't 
check the angle of the free sound board at its perimeter against the 
angle of the inner rim before gluing them together. I suspect that 
the inner rim angle may have been insufficient to match that of the 
panel edge. The board had crown in the mid sections of around 11 mm 
prior to installation. No answers here yet - just another question 
which may be answered a couple of boards down the track.

Ron O
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Website:  http://www.overspianos.com.au
Email:        mailto:ron@overspianos.com.au
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