Curve on Bridge Bottom

[Farrell]

> The bridge doesn't form this long-way crown, the ribs do when the board is
> glued in.

Yes, of course, absolutely. But the essence of my original question is: Is there not an advantage to preserving this rib-induced pre-bridge-installation soundboard crown with a bridge bottom machined to match the board crown (or perhaps you want to machine the curve you think you will have after board loading - whatever). It seems to me it's like installing 16 of 17 machine-crowned ribs, but putting the last one in flat - you know it will bend because there are already 16 on the board with a bend in them - probably won't make much difference.

Again, I think I am really asking two questions here: First, from a practical standpoint - you have already said there doesn't seem to be much difference - I'll take you at your word. Second, from a theoretical standpoint - I should think there would be at least a theoretical advantage (and if so, perhaps a small, barely perceptible practical advantage - perhaps the practical advantage might only show up after thirty years, as maintaining original crown better?).

Terry Farrell
  
----- Original Message ----- 
From: "Ron Nossaman" <RNossaman@KSCABLE.com>
To: <pianotech@ptg.org>
Sent: Thursday, May 02, 2002 12:29 AM
Subject: Re: Curve on Bridge Bottom


> 
> >
> >       While dry fitting a 1927 S&S L board today I was thinking about your
> > question as to bridge curvature in the bottom of bridges so I thought I'd
> > take a few measurements. 
> >    Prior to fitting the board in the case there is no evidence of crown along
> > th bridge line.Non can be seen or measured. After dry fitting in the case
> > with a few clamps it took on a great deal of crown along the bridge line. I
> > was so take by this I took a number of digital photos with a long straight
> > edge along the bridge line.There was probably a good 5/8ths of an inch  gap
> > at each end of the straight edge. This crown was nice and hemispheical . Even
> > taking into account the modest diaphramizing this is alot and obviously
> > intentional. The rasten/inner rim is cut this way intentionally.
> >      The bridge ,not glued on yet was laid on the board in its location.
> > Though it has some crown cut into it , it is not as much as the dry fit
> > boards crown induced by simply clamping to the rim. Very interesting ehhh??
> >     Never the less to my mind the board will flatten out less with the bridge
> > in a crowned configuration than a flat cut bridge, providing the bridge is
> > glued to the board on a crowned suface as I described previously. However
> > nominal this bridge support may be it has to resist bearing at least to some
> > degree. I'll take whatever I can get.
> > >>>>>>>>>>>>>Dale Erwin>>>>>>>>>>>>>>
> 
> 
> Hi Dale,
> Look at unloaded crown heights for "standard" 18M (60') radius ribs in a
> generic board. 
> Rib #1, 2.6mm 
> #3, 7.2
> #5, 9.4
> #7, 6.3
> #9, 1.8
> #11, 0.5
> 
> The crown height being, obviously, dependant on the length of the rib. The
> numbers are a little different with graduated radii ribs, but the effect is
> similar. In a free state, not clamped to a rim, the board will be flat along
> the grain because the stiff long grain holds it that way. The perimeter of the
> board, however, is anything but flat. If this board perimeter is clamped down
> to a flat surface, a crown will form along the long grain. The very fact that
> the rib crown is higher in the middle of the board where the ribs are long than
> it is at the ends where the ribs are short will guarantee this, whether a
> bridge is attached to the board or not. The rim would have to be cut wildly off
> plane for the board to not form this crown, so I'd have to say this isn't the
> result of any intentional sculpting of the rim at all. Nor is it the result of
> any special bridge crowning or attachment method of the bridge to the board.
> The bridge doesn't form this long-way crown, the ribs do when the board is
> glued in.
> 
> Ron N