Making Curves for Ribs/Cauls

[Farrell]

Thanks Ron for your comment about bending a rib. You indicated that if you clamp the ends of a rib and bend it out in the middle, you will not get an arc of a circle. I got myself a very straight 1 x 1 inch by 8-foot aluminum pipe yesterday. Since getting back today from the Sing-A-Long Sound of Music (what a hoot!) I have been experimenting with bending it and seeing what kind of curve I get.

I chose aluminum because I figure it should be pretty consistent resistence to bending along its length. It seems to bend into some kind of parabola - the curve is tightest in the center of the bar and decreases out toward the ends. I found for, let's say a four-foot rib, if you support the aluminum bar at the points two feet out from the center each way, like you would for a rib, the outermost six inches to a foot is pretty flat - all the curve is in the center two to three feet. But, if you have the fixed points at the ends of the 8-foot length, you get a nice curve throughout the center four feet of the bar - again, tighter in the center and less toward the ends of the four-foot section - but still a nice curve throughout the rib.

I measured a series of two-foot long cords along the curve described above. The distance from the center of the cord to the curve are indicated below. The first two numbers are the placement of the cord ends (like a number line, with the center of the bar at zero) and the last number is the distance from the center of the cord to the closest point on the curve. This was on what would have been a 60-foot radius circle measured on a five-foot arc.

-12" and 12" = 2.5 mm
  -6" and 18" = 2.2 mm
   0" and 24" = 2.0 mm
   6" and 30" = 1.8 mm
 12" and 36" = 1.5 mm

That spread, center to center measurement will cover a four-foot long rib. Pretty fun making a nice curve like that. Now all I gotta do is figure out what kind of curve I need/want!

I guess the next step is to make some cauls from the various shapes I can generate and make some ribs and hang some weights from them and see what happens. Cool. What fun!

Terry Farrell