Make a soundboard--Measurements

[Ron Nossaman]

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>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.


No, it won't. It will be curved more in the middle and less at the ends. I 
used two straight edges. Starting with a line representing the anticipated 
maximum length of a rib of that particular radius, I computed the height of 
the arc segment represented by that chord at that radius. Then I drove pins 
in the points at the ends of the chord, and the center of the arc segment 
at the computed height above the chord. Placing the two straight edges one 
each on the pins at the ends of the chord, and overlapping above, but both 
touching, the pin in the center, I tacked them together. Placing a pencil 
in the shallow V produced at the intersection of the straightedges, I slid 
the straightedge assembly on the three pins so that the pencil went from 
one end of the chord to the other - describing the arc segment.

Realistically, the exact shape of the described arc isn't anything like 
critical to the ultimate performance of the assembly. The chosen radii and 
rib dimensions, balanced against the anticipated bearing load and resulting 
deflection are significantly more important.

Ron N

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