Of all the processes of building a guitar, how the sides are bent is most certainly the one drawing the most curiosity. Even for people who have some knowledge of cabinet making, bending wood remains quite a mysterious feat!
Essentially, this process calls upon a combination of heat and humidity to soften the wood fibers and allow a straight board to get some curves.
Let’s detail the process some more.
NOTE : Click on any picture to enlarge it.
There are several methods to bend wood. Only two are used by most luthiers. The first and most traditional is the bending iron.
To bend a side with such an iron, it must be heated first until it reaches a temperature high enough to sizzle droplets of water on contact. Some even let the temperature rise enough to blacken wood pressed against the iron! The board to be bent is then wetted, by submerging it entirely in water for a few minutes or in small sections as bending progresses. Indeed, only a few millimeters are bent at once with a bending iron. The board is pressed against the iron — where you want to create a curve— with a delicate rocking motion, by pushing against the board on both sides of the iron.
Depending on the thickness and specie of the wood being bent, it may take from a few seconds to a few minutes to bend a single centimeter. But, little by little, the straight board will slowly take on the shape of a guitar side.
This method has the advantage of being flexible (no pun intended!), i.e. a single iron can be used for any guitar shape, even for other instruments. However, the technique can be difficult to master (resulting in irregular curves, in broken sides when too much pressure is applied) and is time consuming. Although some luthiers might be faster, it takes me anywhere from 30 to 45 minutes to bend a single side. When you build guitars for fun, once in a while, a bending iron will do nicely. But when it is your livelihood, efficiency becomes a priority! That’s why I use a different method: a Fox-type side bending machine.
The picture above shows the Fox-type side bender, named after its designer, Charles Fox, renowned luthier and founder of the American School of Lutherie.
This type of machine has the disadvantage of requiring a different mold for each guitar shape, even two if you want to offer cutaway versions of each guitar as I do.
Here are the two molds necessary for my classical guitar shape, one standard and one with cutaway.
If the Fox-type bender takes up more space than a simple bending iron, it does produce good and predictable results, and much faster. Bending a side only takes about 5 minutes.
When bending a side with a Fox-type machine, the piece of wood is first soaked in water (I simply shower it for a few seconds) and is sandwiched between two flexible stainless steel slats (slightly wider than the side itself) and a heating blanket. The heating blanket, which reaches a temperature of about 300° F (150° C) distributed evenly along its length, makes the bending process much easier.
In the picture above, you can see the various layers of the sandwich and their order.
Once the heating blanket reaches its operating temperature, a press will first pinch the side at the waist. Then, both ends are slowly brought down against the mold.
In the following video, you will see a side being bent in a Fox-type bender. Please note that the heating blanket was turned on about two minutes before the recording started to allow it to reach its operating temperature. At the beginning of the video, you will see me bend down as if to ‘listen’ to the mold. And that’s exactly what I’m doing! The sound of the water boiling out tells me the temperature is high enough to start the bending process.
Once the side is bent, I let it cool down for about 5 minutes before heating the blanket again for another 5 minutes. This heating-cooling cycle is repeated twice, otherwise the side would not keep the desired shape once it is taken out of the mold. Once the two sides are bent, they are transferred to an outside mold where they will be left to settle for 24 hours before being worked on.
The picture above shows the two sides of a Dreadnought resting for the night in the outside mold.
Voilà! I hope you have a better idea of the techniques used to bend guitar sides. Other techniques are used outside of lutherie (such as steam ovens, for instance), but all rely on the same combination of heat and humidity.
Acoustics,
