"Morris Dovey" wrote

You /are/ missing something. The (surface and matching plywood rib) curve
is a parabola with a curve length of exactly eight feet with the focus at
the point midway between the edges. There aren't any circular arcs other
than the ends of the small tubes, which haven't been 'extruded' yet.

I tried making a cone and sectioning to produce a parabolic curve, but
still had the problem of making the length of the curve come out right.
'Taint as easy as it looks. :-p

Is this for a parabolic reflecter? I have made a bunch of those for a solar
powered pool company many years ago. We used metal coated mylar and set the
panels into a frame to support them. We thermoformed them in a vacuum
forming machine. The machine was home built.

We would cut the shape we wanted in metal and use that to form the plaster
mold. Mount that mold on the thermoforming bed and heat the plastic. Turn
on the vacuum and the panels were instantly formed. Trim them and collect
eight of them to make on parabolic reflector.

Ahhhh....., the wild visionary days of a mispent youth. Dreaming of riches
in the solar heating biz. But I got it out of my system a long time ago. I
am much better now. :-)