An Enhancement to the Outside Mold

by R.M. Mottola

Originally published in American Lutherie #79, 2004 and Big Red Book of American Lutherie Volume Seven, 2015



The outside mold is a useful tool for side bending. If the side slat is bent to roughly the ultimate shape of the side and then clamped into the mold while still hot and damp, the slat will conform to the shape of the mold while it cools and dries. This works pretty well for the following reason. If you make a bend in wood, as it cools and dries it tends to revert to its original shape. This springback phenomenon causes all manner of grief with some approaches to side bending, but it can be used to advantage when using a mold. Sides can generally be over bent slightly and then clamped into the mold, where they will relax into the shape of the mold as they cool and dry. So for the most part, when using an outside mold you can just bend the wood to only the roughest approximation of the final shape and let the mold and springback take care of the rest.

This technique works great for all of the areas where the mold is on the outside of the curve, but doesn’t work at all where the mold is on the inside of the curve, such as at the waist bends in an outside mold. That’s because springback makes the bent sides pull away from the mold at the waist as the wood cools and dries.