Instructions for binding the sound hole of the flattop acoustic guitar. Soundhole binding is a decorative technique that is not often used, but it is a nice touch for an instrument with an offset soundhole or any instrument where a more traditional rosette is not appropriate. The instructions are for binding the hole with wood, a somewhat tricky operation that will test the luthier's wood bending skills. The loop of binding is joined with a horizontal scarf joint.
Initially appeared: February 18, 2010
Last updated: Tuesday, May 08, 2012
Wood binding material suitable for binding the guitar body can also be used to bind the soundhole. The strips are usually 0.25'' high by 0.1'' thick. A piece about 16'' long will bind any guitar soundhole. Although it is possible to trim the height of the binding material to match that of the top before hand, I like to leave the material at its full height and then trim it after it has been glued to the top. This makes it easier to get a good fit between the top and the binding by the method I am outlining here. It is also a bit easier to bend the binding if it is left at its full height - the shorter it is, the easier it is to break it while bending.
The first step is to cut the hole in the top. It goes without saying (so I will say it and maybe spare you a screwup I used to do regularly) that the radius of the hole should be larger than the desired finished hole by the thickness of the binding material. To be sure it is really round, I do the hole cutting with a fly cutter on the drill press, or a small router and circle cutter attachment, or a purpose-built hole cutter. See the page on Building the Top Plate of the Flattop Guitar for more on this. On the Ipomoea Acoustic Guitar in the following pictures, the soundhole reinforcement ring goes right up to the edge of the soundhole so I cut the hole in both at the same time. Here's the hole cut:
While your drill press (or router or hole cutter) is set up for the size of the soundhole, you can make a simple mold for the binding out of a scrap of plywood. The mold consists of a hole of the same diameter as the soundhole cut through the plywood, and a plug made of the same material. Since the binding material is 0.1'' thick, the radius of the plug is 0.1'' less than that of the hole. Then a 0.25'' strip is sliced out of the middle of the plug so you now have two half circle pieces that you can tighten up by pushing wedges between them. Here's the mold:
Bending a single thin piece of binding on a hot pipe takes a certain amount of skill and practice. The piece is fragile and very easy to break. There are three things which you can do to increase the likelihood of success. The first is to have spare binding material around in case you break a piece. The second tip is to not try to bend the material near the ends - leave at least an inch or a bit more near each end un-bent. The last tip is to make the bend in sequential passes, bending the material just a little on each pass. I tend to do this in four or five passes for rosewood. After the first bending pass I end up with an arc of about 12'' radius. Each subsequent pass makes the radius a little tighter until finally the strip is bent to approximate the circumference of the soundhole.
The process starts by pre-heating the iron and soaking the binding material a bit in warm water. Make the first bending pass, then spray the wood with water before starting each successive pass. It helps make the bend uniform if you flip the piece over for each pass so you start at a different end each time. Even bending pressure and even speed over the iron will result in a nice uniform bend for each pass. When you are done making bending passes use the nippers or razor saw to cut the un-bent ends from the strip. Wet it one more time, then stuff it in the mold like this:
Put the plug halves inside, and then push a few wedges between them to tighten the binding strip to the walls of the mold.
Leave the strip in the mold to cool and dry overnight.
Remove the strip from the mold and trim one end at approximately a 45° angle, using the razor saw. If the binding strip has any runout, make the cut in the direction of the runout. This will make it easier to plane the strip down to the surface of the top later.
Now push the strip into the soundhole so that the cut end lays over the uncut end. Press the strip as best you can to the walls of the hole and then mark the uncut end to the surface of the cut end like this:
Remove the strip from the hole then make another mark on the currently uncut end, parallel to the mark made in the previous step and approximately 0.25'' closer to the end of the strip. What we are doing here is cutting the strip a bit longer than the mark, to compensate for the fact that we needed to overlap the ends to make that first mark. Cut the end at the second mark using the razor saw, then fit the strip back into the hole.
Chances are good that it won't quite fit without some additional trimming. But don't go for a perfect fit. One reason we left the binding material at its full height is so we can trim it down to the surface of the top after the strip is glued in. As you can see, the two angle-cut ends wedge against each other and force the strip nicely into the hole. What you want to end up with is the piece being slightly too long but so that it adequately covers the walls of the hole at the joint.
Dry fitting is always a good idea, so do that now before applying glue. First, put a spring clamp onto the joint in the strip, wedging the strip into tight contact with the walls of the hole.
Take a good look at the fit around the hole. If the binding is pulled away a little bit in places, you can force it into place by using a metal can and some wedges to wedge the binding tightly into place. Be careful - you can apply enough pressure with the wedges to split the top. If you can't bring things in with a bit of wedging it is better to touch up the problem spots on the bending iron again.
It is a good idea to orient the joint so it is in a place that is not too visible. Probably the best location is on the player's side of the hole. When the guitar is held in playing position the player isn't looking directly at this side of the edge of the hole.
When you are satisfied with the fit, it is time to glue. Remove the strip from the hole. Mix up a little bit of glue and sanding dust from the wood species that the binding is made of. Apply plain glue to the walls of the hole. Insert the binding strip, placing the joint where you want it to be. Put a dab of the colored glue on the ends of the strip before applying the spring clamp. Then insert the can and wedges as needed. Let dry.
After the glue is dry the wedges and clamp are removed. The binding can now be planed flush to the outside and inside surfaces of the plate. A small block plane works fine for this. Be very careful which direction you plane over the scarf joint. If you accidentally plane into the joint it is possible to tear out huge chunks of wood which may necessitate redoing all your hard work. Planing right down flush with the top risks gouging the top, so after you get pretty close with the plane, finish the job with the scraper. Both the outside and inside surfaces should be planed and scraped in this manner.