On Mon, 08 Jun 2009 19:29:39 -0500, Morris Dovey
wrote:

Afterward I wondered if a catenary with that same 1/8" chord height
might apply more even pressure...

Assuming the intent is to apply uniform pressure over the length of
the caul, use the deflection curve of a uniform beam, simply
supported, and uniformly loaded along the length. Since the beam
equations are linear - or at least approximated by linear equations -
the deflection curve would be a good approximation of the shape of the
caul that would give a uniform clamping pressure.

The assumptions aren't rigorously met since the caul doesn't have a
uniform cross section if one side is straight and the other is curved,
nor is it likely that the material's modulus of elasticity will be
perfectly consistent at every cross section along the length of the
caul.

Haven't run any numbers but I'd guess the difference in results from
using a circular or parabolic arc and the results of the beam
deflection equations wouldn't be worth worrying about.

Tom Veatch
Wichita, KS
USA