Neat Trick for Radius was Re: Rib dimensions

[Andrew & Rebeca Anderson]

As a timber-framer I learned how to match curves without having to use a 
60' radius line with a pencil tied in at one end. ;-)  This methodology is 
useful if you know with a given straight line how much you want your circle 
to bow out beyond that line.  In other words, if you can determine how much 
you want your curve to rise in a given distance you can draw it on the 
material you want to mark with the following methodology:
Determine each end of your curve and place a pin/nail/barrier.
Mark the center of the curve and place a pin at the desired height off of 
the line through the two end pins.
Place a straight edge between one of the end-pins and the pin at the top 
center.
Place another straight edge over it and beyond at the pin at top center and 
set it parallel to the line through the two lower end-pins.
Fix the two straight edges to maintain the resulting angle.
Set your pen/pencil in that angle and drag the assembled straight edges 
across the lower end-pin and the center pin.
The resulting line will resolve to an even circle segment when you have 
done both sides.

This is a calculus methodology that they don't teach in high-school 
geometry.  It makes small shops workable. ;-)
Your straight edges to have to be long enough to reach from pin to pin and 
of a nature that is easy to afix at the desired angle.

Hope this was useful for someone.
Andrew Anderson
of the Ft. Stinkin' Desert Chapter 799