Acoustic Guitar Construction

I've seen molds that have a metal bar tangent to the innermost curve at the waist. And other do not. Is there some rule being followed as to when to insert the bar and when not?

Thanks very much.

The original Fox bender design had a spring loaded waist reinforcement bar that moved with the waist caul.

http://www.acousticguitarconstructionfo ... f=8&t=1371

Thanks, Ken. Seeing as my bender isn't being built from Fox's drawings, I suspect I can ignore the metal bar.

I built my bender with the bar design but I've never used it. Per Ken's suggestion I've always pre bent the waist on my pipe prior to it being placed in the bender.