Cutting rib radii

[Will Truitt]

David, thanks for your observation here that the arc will not bend evenly,
it will bend more in the middle and less out towards the ends, more of a
parabola.  So let me ask this question:  Might that not be a Good thing -
the tighter the radius, the stiffer it will be, and the less tight, the more
flexible; all else being equal.  The greatest need for stiffness along the
rib would be under the bridge, where the strings are pressing down.  And we
want the panel and ribs to be be more flexible out towards the rim - which
is why we thin them. I have long wondered if the responsiveness of the
soundboard system might be improved by designing a rib in a parabolic shape
to meet these dual needs.  I don't know if it would be a meaningful
improvement, one that improves it but trivially so, or does nothing at all.
Nor do I know how we would assign real values to the shaping of such a rib.

Food for thought, maybe it's junk food....

Will Truitt  

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of David Love
Sent: Thursday, August 07, 2008 8:08 AM
To: 'Pianotech List'
Subject: RE: Cutting rib radii

I know it is done that way but when you bend wood I don't think it really
bends in a uniform radius, it tends to bend more in the middle and less out
toward the ends, I guess the shape is more parabolic.  Also, each piece of
wood will probably bend somewhat differently.  Since some of the curves I'm
using are fairly tight (especially on the short ribs) I'd prefer to use a
preshaped caul of some type.  The jig set up to translate the shape of that
caul to the rib with a router (rather than using a bandsaw and sandpaper) is
really the question.   

David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: Thursday, August 07, 2008 12:11 AM
To: pianotech at ptg.org
Subject: Cutting rib radii

Ribs are bendable enough that you can bend them to the exact radii you 
want, clamp them in place and remove material from the concave side so 
as to make that side flat. Unclamping will then leave that flattened 
convex line at exactly the radii you want. You can put in some fairly 
complex curves this way as well as long as your bends are not over too 
short a span to hold adequately.

Cheers
RicB