Rib Radii

[Andrew Anderson]

After having determined the length and depth of the curve (at center)  
we took the stock we were going to cut into and laid it out as follows:

At each end of the cut affix a vertical stop (usually a piece of  
plywood screwed into the side of the timber being cut inside the cut  
away area).  At the center install another stop (nail) at the depth of  
the cut.  Take two strait-edges for "rays".  Place one against the  
edge stop and the upper middle stop.  Place the other over the first  
at the upper middle stop and run it parallel to the edge of the  
timber.  Fix the two together so the angle they make together cannot  
change.  That angle is where the pencil must remain.  With the two  
fixed and pencil in-place draw that angle down to the two lower stops  
(one after the other) while keeping both rays tight against the upper  
and lower stops.  You will find the edge of a perfect circle drawn  
into the edge of the timber.  The length of the strait edge (we used  
strips of plywood) need only be just a little longer then the distance  
between the lower stop and the middle, upper stop.

This is essentially the analog version of a calculus method.   
Utilizing a string for large diameter circles is practically  
problematic.  The size of the room can be a problem.  The stretch- 
ability of the string usually increases with length and adds another  
variable.

I hope this is useful,
Andrew Anderson

On Aug 6, 2008, at 10:25 PM, Greg Newell wrote:

> David and list,
> 	I’m wondering, as I write this, if I’m gonna look like a fool yet
> again for posting what is probably obvious to everyone. If I  
> remember the
> question correctly you asked how one cuts the radii not how one  
> measures or
> makes it. My procedure was to take a sheet of hardboard (that brown
> compressed paper like stuff) and lay it out on the driveway. At  
> whatever
> radius I’m trying to make I lay out a string to that fixed point in  
> the
> driveway and attach a pencil to the string at the end which is now  
> above the
> hardboard. Naturally this makes a compass with which I make the  
> radius I
> want. After cutting the hardboard on the compass line I just made I  
> lay that
> over the wood or laminate that I want to make the rib out of and use  
> the
> router w/ guide sleeve to follow the hardboard radius I just made.  
> Simple
> enough but perhaps this isn’t what you were thinking about.
>
> Greg Newell
> Greg's Piano Forté
> www.gregspianoforte.com
> 216-226-3791 (office)
> 216-470-8634 (mobile)
>
>
>
>