You will hear me stating over and over again that we shouldn’t introduce stress into the guitar at any point in the construction process. This includes the clamping procedures for the top plates, back plates, guitar necks, purfling, binding, braces etc. It is essential that all part of the instrument be fitted together with precision. This theory applies to the Side Spring-Back that we will often see immediately after taking the acoustic instrument sides from the Heated Side Bending Jig.
How Do You Know if You Have Spring-Back?
It’s very easy to tell. Upon removal of the sides from the Side Bending Jig, check the side against the Plans or Template. You will see that the waist bend is probably relaxed a bit and the ends of the sides angle outward from their intended shape. Since this will introduce a stress point to the instrument, this needs to addressed. There are several methods that I have used over the years to solve this problem and these methods are covered in this post.
Analyze The Sides:
When matching the sides against the Plan or the Template, determine where the spring-back has occurred and how much of an issue it will be. If the waist has lifted a bit, that will throw the upper and lower bouts outward even more. So there may only be issue with the waist being under-bent. The best bet is to attack one area at a time.
Methods to Solve Wood Side Spring-Back:
Use a Heated Bending Iron:
The first method, in my estimation is the best method and it will yield the best results. Wet the areas where there is spring-back with a spray bottle and water. Heat up the bending iron sufficiently, gently roll the side on the iron until steam rises and you can feel the wood relax a bit. Do this over a large area so as not to disturb the natural curvature of the side.
All that is needed is touch it up the bend just a little bit. Do not concentrate the bending in one area too much as humps can develop in the sides. The waist bend is a bit tighter so as indicated above, adjust that area first. It is often to “over-bend” each area a little bit to allow for some spring-back from this method as well. This is by far the most difficult method to master. It takes experience, a good working knowledge of wood and how it reacts to the bending iron. The best way to start out is by bending some scrap sides to get a feeling for the bending process. Once you feel comfortable with the procedure, try it on to the actual instrument sides.
Upon a satisfactory over-bend, clamp the side in an Inside Form to dry completely.
Reheat The Side on the Heated Bender:
The easiest method that I have used is to leave the sides cool on the bending jig. Remove the sides the next day, spray them down with a misting of water from a spray bottle and place them into the Heated Siding Bending Jig Once again. This time leave the Jig on for about 10 minutes. Let the sides cool down for about 2 days and take them off. You will find, again depending on the wood, that the sides very little or no spring back and that the sides have taken a “set” for the form shape.
Place The Sides In A Squeeze Jig:
If after removing the sides and you have a spring-back issue, immediately place the sides in a squeeze jig. The Inside Form works great for this job as well and that is my form of choice. Just place the sides in the form and apply the Spreader Jacks to the sides. Slip in some shims to hold the top and bottom of the sides out from the form about 1 to 1-1/2″, depending on the amount of springback that there is in the wood. If the waist has spring back, place a fitted shim between the Inside Form and the sides. Keep the side in the form for several days to a week. If there is still some spring-back it may be necessary to resort to the Bending Iron for help.
Build an Over-bend Into Your Form Inserts:
When you build the forms for either the Heating Bending Jig or the Side Boiling Method, build in some over-bend into these forms. Lay out the instrument shape on the plywood being used for the form insert. Next “pull” the two outside edges inward a bit (about 3/4″ ) for each edge. Keep the curve for the upper and lower bout nice and smooth for the layout. Do the same with the bend at the waist. Build-In a slight over-bend by dipping the waist curve about 1/4″ toward the center of the instrument.