Bending wood sides by placing them in boiling hot water is a method that has been used for many years. When my father and I began guitar building this is the method that we utilized exclusively. If you want to get into guitar building at a minimal cost, this is certainly one way that you can do it. The only expense you are out is the cost of the boiling vat, the used gas burners and some plywood and hardware for the forms.
Bending sides by the boiling method has several advantages and disadvantages. Listed below is the procedure that you would use to get your sides boiled and clamped to the molds.
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 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.
Take a sharp marking knife and cut a fine line along the center of the waist curve. This will be the alignment mark when the wood is first inserted the side into the bending jig. Do this for both sides in book-matched fashion. No not cut this mark too deeply as you will need to sand the mark out prior to your finishing operation. If you feel uncomfortable with the marking knife a heavy pencil line will work as well.
Prepare Vat For Bending:
The boiling vat can be constructed from galvanized steel. It should be a minimum of 6″ wide x 36″ long x 4″ deep. Place about 3″ of water in the vat. Light the burners and wait for the water to come to a boil. To help facilitate the boiling process, place a lid over the top of the vat. When the water is actively boiling, slip the side in the water. There will be a tendency for the wood to float to the top, so a hold down will be required for the wood. The easiest way is to place wood spreaders inside the vat so that they hold the wood beneath the water level about an inch. Do NOT place any metal in the vat to weight down the wood as this will stain the wood and you will not be able to remove the stain. Wood can be used and that is what we recommend with the wood spreaders.
Let the side boil for at least 90 to 120 minutes. At this point the wood should be plastic enough to place on the guitar molds without much concern for cracking. If exotic woods are being boiled the water will turn very muddy, which is good because the resins are being boiled out of the wood. There is a bit of concern with this because this also removes some of the color of the wood and there could be a mis-match in coloration between the back and side wood. Only trial and error will solve this issue with different pieces of wood.
Inserting Sides Into the Mold:
Prepare the Mold(s) by having them open and ready to receive the wood. It is essential this is completed as swiftly as possible as the window for bending is rather short. It is best to do this with (2) people. With experience you will be able to do it alone though.
Take out the first side, using a pair of tongs, being careful not to burn yourself. It is recommended that a heavy pair of cotton gloves are worn for this operation as the wood will be too hot to handle with bare hands. Check and double-check the registration mark. Insert the wood into the Mold and center the mark for the waist beneath the waist tension block.
Aligned the wood parallel with one side of the Mold. Now begin the tightening process on the waist block. Bring it down so that the side is within about 1/2″of contacting the Mold. Now gently bend the upper bout around the Mold and slip on the Upper Bout Tension Block and tighten it so that the side does contact the form, but to absolutely tight.
Do the same thing with the lower bout. Tighten the waist block down tight. Next tighten each of the bout tension blocks. Repeat the process with the other side. Set the molds aside in a warm room for about a week to facilitate drying.
Tips and Techniques:
- The most critical bend is the waist of a classical guitar. Being so sharp a curve, wood such as Rosewood or other hard wood may begin to crack when tightening the waist tension block. Any time that can saved before tightening this block will make this bend easier. Another critical bend is the upper bout of the dreadnought guitar.
- Having the bending operation in a very warm room will minimize the dry-out of the wood prior to bending the waist curve.
- You will find that if the Mold is secured to a vice or other work surface, you will be able to save time.
- If after removing the sides from the Mold there is considerable spring-back, you will need to fit the sides in a squeeze jig. This allows you to over-bend the sides so they will form more closely to the inside mold.