Belly rail crown - Why???

[Keith Roberts]

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I have more problems with clamping while gluing without having to worry =
about making two mismatched pieces of wood conform to each other during =
the gluing process.
Since the bevel doesn't matter much to the performance of the board, in =
my lame way of thinking, the bevel on the rim should match the board =
when it is in the unstressed position. A great glue joint here seems to =
be essential.
Of course Del probably has the pneumatic sound board, (push one button, =
come back tomorrow), clamping system that will flatten any piece of =
wood. For the ultimate in gluing systems.
Keith R
  ----- Original Message -----=20
  From: Delwin D Fandrich=20
  To: Pianotech=20
  Sent: Saturday, November 23, 2002 11:33 PM
  Subject: Re: Belly rail crown - Why???



    ----- Original Message -----=20
    From: Erwinspiano@aol.com=20
    To: pianotech@ptg.org=20
    Sent: November 23, 2002 10:14 PM
    Subject: Re: Belly rail crown - Why???


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

    Del
    ....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....=20
  They don't necessarily start out that way. While the rib-crowned =
soundboard certainly has the potential of greater stiffness -- there is =
always the option of more and taller ribs -- in practice I'm not sure =
they start out all that much different. A well done compression-crowned =
soundboard can be quite stiff indeed. But it may not keep that stiffness =
very long. It's in holding the long term stiffness that the rib-crowned =
soundboard comes out ahead.=20

        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.
  Well, I don't know that there is all that much wrong with the =
practice. I'm mostly just not convinced it does much good.=20

        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.=20
  Come on, Dale, you know better than that. Sure, gluing the board to =
the rim makes it act like a clamped-edge diaphragm, but wood is still =
awfully thin wood and it still bends under load. Especially so when the =
load is essentially across-grain and most of it is along the bent side =
or at roughly a 45=BA angle as most of it is along the bellyrail and the =
straight side. And along the bass curve, well, who cares?

       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.
  And, that's pretty much my point. =20

       Perhaps  petty, lame and faulty thinking but it's mine all mine.
  Yup. Lame it is. And you can have it. Unless it's already there in =
which case I'll have some too.

  Del

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