Make a soundboard--Measurements

[Delwin D Fandrich]

----- Original Message ----- 
From: "David Love" <davidlovepianos@earthlink.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 31, 2003 9:20 PM
Subject: Re: Make a soundboard--Measurements


> Actually, that was my question, what did you use to make the pattern for
> the targeted radii?  My guess is that you didn't stretch a string out
25',
> 50', 60' etc. and scribe an arc, or did you?  My thought was to take a
> piece of wood of a predetermined length and secure it at one end to a
piece
> of wood that will become the caul.   Draw a line along its length.  Then
> calculate the amount of deflection off the straight line for an arc of
> given length on a circle of x radius.  Then push the ends together until
> you get that amount, of deflection.  Scribe a line.  Cut it out.
Question
> is, will a strip of wood when treated that way form into a perfect arc?
> Any other suggestions would be helpful.
>
> David Love
> davidlovepianos@earthlink.net
>


Ours are made of MDF. We have now "simplified" our rib sets to just five
radii. And to make them, I cheat. The curves (i.e., the caul pieces) are
drawn full-scale (i.e., 1:1) in my CAD system with vertical offsets taken
every 25 mm from a baseline. These offsets are then transferred to an
appropriately sized piece of MDF. When the dots are connected the thing is
cut out on a bandsaw and smoothed out on a horizontal belt sander. This is
then used as a pattern to route the rest of the required pieces. When
enough are cut they are edge-glued together to form a caul that can be as
wide as you want it to be. Our current cauls are 160 mm wide. We use top
and bottom cauls that are simply placed on a frame as needed and clamped
together. A have another somewhat more sophisticated system in the works,
but this works well enough that it is not a very high priority.

After experimenting with all kinds of shapes and contours I'm back to a
simple radius. When doing a bunch of different ribs for a variety of
different pianos it makes life much easier. You don't have to worry about
how to offset the rib so the high point of the curve is exactly under the
bridge load center. The advantages to the various different curve shapes is
99.5% theoretical and 0.5% audible. If that. The shape of the back side of
the rib is another matter.

Del