Pneumatic Press Bending

[Farrell]

To those souls that delve into soundboard building with a pneumatic press:

David Hughes runs 35 psi hose pressure in his press. Others have indicated that they run 40 or 50 psi. Clair Davies indicates he runs 100 psi in his cauls. Mr. Davies indicates that Steinway uses 100 psi (end of discussion, right?). He cites the Bruce Hoadley book "Understanding Wood", which I believe is generally a well respected resource. It reportedly indicates that 100 psi is a minimum pressure needed for a superior glue joint on softwoods. Is there some justified thinking that suggests this guidance should not apply to the rib-to-soundboard glue joint?

I am nearly ready to build my first soundboard-to-rib caul. I just need to figure out how tall a piece of yellow pine do I need to sufficiently resist caul bending. Clair Davies uses 2 x 6 boards on his rig (top and bottom), but he uses 100 psi pressure and he says he gets about 3/8" concave bend in the bottom board on his long cauls. David Hughes uses 2 x 6 yellow pine on the bottom caul and 2 x 4 on the top, cuts a 60-foot radius curve in his bottom caul, and runs 35 psi hose pressure. He does not address caul bending in his article, so I would guess its either negligible, not enough to worry about, or I suppose it bends a bit but he finds this advantageous (from his stated goal of putting a 60-foot radius on his boards, I suspect the latter is not the case). Any thoughts on this before I build? Otherwise, I guess the thing to do is build one caul and load it up and see what happens. I suppose one could always epoxy some wood onto the sides of the caul to give it more stiffness if needed - and obviously build the next one with larger dimensions.

In the 1997 articles by Clair Davies with his upgraded press, he uses four angle irons to keep the top and bottom cauls aligned. Others simply through-bolt the two together. The angle irons looks to me like a slick idea. Anyone aware of any disadvantages to this approach?

Thanks for any ideas.

Terry Farrell