Rib Structures Part 2: Bending Ribs


Posted by Michael Doran on Thursday, July 2nd, 2009

After the blocks have been glued to the mold I shape them to the right curve to match the model I am making. The shape of the blocks defines the shape of the corners on the top and back, so I am careful to get the shape that will give me the look that I want.

Ribs are the only part of the instrument that are bent. The top and back plates, for instance, are carved out of thick pieces of wood. In the cellular structure of all trees there is a component called lignin, which adds strength and plays a crucial part in conducting water through the tree. When the lignin is heated it will become pliable and easy to bend. When the lignin cools it will retain the form it was bent into. Usually wood to be bent is heated with direct heat or hot steam or water. I use a bending iron to bend wood. It is a large piece of metal, usually non-ferrous so there is no issue with rust, shaped to roughly the curve of the ribs and heated from the inside with heating elements. The heating elements are controlled by an adjustable thermostat. In 16th century Italy the bending irons were heated on an open fire, I can imagine bending ribs that way would be exciting. In fact many classic instruments have scorch marks on them from bending irons left to long in the fire. If the iron is not hot enough the ribs are more likely to crack as they are bent.

When the c-bout ribs are bent to match the mold and the blocks I glue them in with a counter part to apply even pressure.

When the glue has dried I carve the outer curve of the corner blocks and the surface of the neck and tail blocks to their final shape. The ends of the c-bout ribs taper to almost nothing at the tip to hide the glue joint on the rib corners.

Usually the upper rib is one piece (though when the neck is attached to the body the upper rib is cut through), sometimes the lower rib is two pieces with a joint in the middle of the tail block. This is because wood that is cut for violins and cellos is usually just a bit to short to cut the entire lower bout off of the back. When I can I ┬álike to use a one-piece lower rib because it is just one less joint to fit. It’s also traditional. When I find a nice piece of rib stock that is long enough I may use the wood on several instruments.

When I have the upper and lower ribs bent to shape I glue them to the blocks again with counter-forms.

The main part of the rib structure is now done. The ribs at this point are still fragile, next I will glue on the linings to add strength and a good gluing surface for the top and back.