A hot iron used for bending wood sides is the most traditional and claimed by some luthiers as the best method to bend sides. The wood is heat up and when water is added, it turns to steam and the wood becomes much more pliable. Everyone who builds instruments should build at least one instrument using a hot iron. It is a lot of fun and you learn a lot about the binding characteristics of different woods. Also a bending iron can be a fine addition to a shop for touching up sides that have been bent on a heated side bender, such as correcting side sprint-back issues.
Hot Irons are available commercially as a tear-dropped shape iron mounted on a fire-resistant base and an internal heating element is controlled by a rheostat to control the amount of heat supplied to the iron.
Additionally, some builders construct their own iron by simply clamping a pipe into a vice and add heat via a blow torch. This is the way it was done in the days before we had electricity and many luthiers still use this method. This of course is the most dangerous method as it poses the greatest fire risk and is strongly discouraged.
Using the hot pipe for bending will be one of the most challenging procedures you can tackle in building. Be very alert when bending sides using this method. Use all of your sensory systems. Listen for popping water on the hot pipe, smell the sweet aroma of heated exotic woods and of burning wood. Listen for straining of wood fibers, which indicates a cracked side is imminently possible.
Here is the complete procedure:
Examine The Wood Sides:
Look for any end checking. This may ruin the whole piece if not found. Thickness sand the wood sides to just a whisker over their final thickness. Try to use straight grained, quarter sawn wood for the sides.
Mark Out the Sides:
Lay the sides on a workbench in book-matched fashion. This is of utmost importance. Mark the high points of the upper and lower bout from the Instrument Plans with a pencil. Mark the center point of the waist with a pencil.
Prepare Sides For Bending:
Soak the side in hot water for between 10 and 15 minutes. Use a 36″ long container such as a plastic window box. Weight down the side so it does not float on the top of the water. A bathtub or other vessel will suffice as well.
Also, if you will be clamping the side in an inside or outside mold, have those close by for checking wood bending progress.
The Bending Process:
Check that the bending iron is hot enough by splattering a little water on the pipe. The water should dance and sizzle. You are ready to go.
Use one hand for feeding the wood over the bending iron and the other hand for bending. Use the palms of your hands. The slower you feed your stock over the iron and the greater pressure you apply to the wood will vary the amount of bend you can get from the wood.
One method is to roll the side over the pipe to heat up the wood, then applying pressure, moving again, roll the side let it heat up and apply presssure and bend. Slowly advance the side to the next area to be bent and repeat.
It is best to start with the upper bout bent, over bend slightly to allow for some spring-back. Check often against your template or form and go on to the next area, which is the waist. This is the most difficult bend on a classical and some of the smaller bodied guitars and the ukulele, so pay careful attention and let your senses tell you what is happening.
Finally end up with the lower bout and again slightly over-bend.
Do a final check the side against the template or form, and also check it with your hands. If you sense a flat spot, gently rock that spot over the hot iron. If you need to reverse the bend, turn the side over and do so on the inside.
As a final step, clamp the side on an outside mold or form and into an inside form and allow the side to “set” as it cools off. Leave the side here for several days, if possible. This will assure you of minimum spring-back.
Tips and Techniques:
- The closer you are to the actual bending area, the more control you will have over the piece.
- If the pieces get too hot for you to handle on the bending hand, wear a cotton glove on that hand.
- Always be aware of fibers beginning to tear. You can hear the split starting. Immediately stop and inspect the work. Proceed very cautiously as this area may have been compromised and is likely to break if too much pressure is applied.
- For especially difficult areas or extra-sharp bends, keep a bottle of water with a spray nozzle handy to re-wet the that area of wood to create additional steam to aid in the bend.
- As pressure is applied to the heated side you will begin to feel the wood relax, if it is going to that is. It is at this time that provide the desired amount of bending pressure to bend the wood in the desired shape.
- Practice bending slats of wood of similar cross-section before trying it on the wood sides. You will quickly get a feeling for how much and where to apply pressure