Belly rail crown - Why???

[Erwinspiano@aol.com]

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In a message dated 11/23/2002 10:00:34 AM Pacific Standard Time, 
pianobuilders@olynet.com writes:

    Del writes
> It is my practice to set the rim and bellyrail, including the soundboard 
> cutoff rail, flat. Since the soundboard will be stressed -- i.e., have 
> considerably less crown -- once the strings go on I prefer to have a flat, 
> consistent mounting surface for the parameter of the board. Besides, my 
> cutoff bars tend to be so shaped that very little of the soundboard is 
> actually glued to the bellyrail.
>  
> And, while we're on the subject, for the same reasons I fail to see any 
> advantage to beveling the inner rim to (supposedly) accommodate the shape 
> of the soundboard. It is claimed that this somehow aids the maintenance of 
> crown but I surely don't see how. To illustrate simply clamp a piece of 
> spruce to a bench and see how easily it is bent.
>  
>            

             Del
             Heresy and poppy cock. No just kidding . I just laid a straight 
edge across the belly rail of an Stwy L and it has the stated approx. 1/4'' 
crown. While I can't see why this is necessary at least here it seems less 
desireable,at least to me, to encourage any more  undue stresses in the 
system than it already has by introducing flat rastens as practice.
   I.E. although you say the beveled rasten has no merit you certainly 
wouldn't want it beveled in the opposite direction as the current industry 
standard. To my way of thinking the beveled rasten is an engineering idea 
that aids the board, albeit slightly, to resist the flattening of the board 
under load. Recognizing that by in large most on the list are building rib 
crowned boards that retain more of there crown when under load than the 
former compression stlye boards. Rib crowned boards being much stiffer by 
design,  thanks to higher rib radii and stiffer spruce rib material (formerly 
sugar pine) and deeper ribs. I have installed boards on flat rims and the 
residual crown in the 2 I'm thinking of did turn out with less net crown than 
usual.
    I see nothing wrong with the beveled rim personally, if my boards are 
retaining a good amount crown even when strung I see no reason to induce 
another unnecessary possible strain at the edge of the board by changing all 
the beveled rastens that come through the shop to a flat edge. No, I'm not 
saying you were advocating that either.
    Simply clamping a piece of spruce to a bench and bending it isn't exactly 
like the continuos rim of board being glued to a curving rim. Gluing the 
board to the rim gives the whole structure a cohesive stiffness it doesn't 
have before that. Also the board is often thinned down to as little as .250 
at the edges so undue stress once again not desired. The 1 and 1/2 degerees 
typically found in the rim joint could as practicallity or compromise be 
reduced to half that and this angle would more likely conform to the residual 
crown angle after stringing.
         Perhaps  petty, lame and faulty thinking but it's mine all mine.
         Dale Erwin
          

>  
> 


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