Curve on Bridge Bottom

[Erwinspiano@AOL.COM]

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In a message dated 5/1/2002 9:49:06 PM Pacific Standard Time, 
RNossaman@KSCABLE.com writes:


> Curve on Bridge Bottom 
> Date:5/1/2002 9:49:06 PM Pacific Standard Time
> From:<A HREF="mailto:RNossaman@KSCABLE.com">RNossaman@KSCABLE.com</A>
> Reply-to:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> To:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> Sent from the Internet 
> 
> 
> 
> 
> >
> >       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.

>>>>>>>>>>>At this point I'm not convinced thats all there is to it. In a 
free state the board is straight along the grain and in the example above I 
stated that. I have glued boards  into rims with no bevel in the rasten and 
no curve in bridge bottom (Hardman & Weber) or in the rim and these did not 
form but a smidge of this crown along the bridge line. How do I explain this? 

> 
> the rib crown is higher in the middle of the board where the ribs are 
> longer  than
> it is at the ends where the ribs are short will guarantee this, whether a
> bridge is attached to the board or not. 

>>>>Don't foget though that the treble and bass ends usually need some 
coaxing down at the ends which is also allowing the longer ribs to have more 
of the  effect you stated above. In this case it seemed even more so. If the 
rim were to be simply cut slightly different, asending a bit so to match 
boards perimeter at that tail then this effect would not occur and there 
would be no or nomminal crown along the bridge line.
.
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
> >>>>>>>> The rim certainly could be cut to enhace this natural feature 
> though and that may be the case here though niether of us could find a way 
> to prove it. At any rate the board does have this shape intended or not and 
> I'd prefer to have the bridge conformto it as opposed to fight it or force 
> it out, what would be the point. Being that this was the first S&S L I've 
> bellied (done a few Os)  the effect here was considerably more noticeable, 
> No doubt in part to a different board and rim shape.

 >>>>>>>> What I think would be of interest would be to attempt to measure 
any residual crown along the grain after the bearing goes on as Terry 
suggested. I think a thread could be pulled across the top of the board under 
the strings and then make some allowance for the top being thinned. Perhaps a 
comparison of that dimension with the rib compression too would be of 
interest.  What would that tells us? Hey I don't know I'm not the expert just 
a casual observer.
                                 Back to the salt mines
> >>>>>>>>>>>>>>>>>Dale>>>>>>>>>>>>>>>>>>>>>>>>>>


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