With the top and back glued to the sides, binding can be applied to the guitar body edges. The binding is wooden stripping that wraps around the edges of the body, providing both decoration and protection. Purfling consists of thin bands of wood adjacent to the binding, used primarily for decoration (though some have suggested that it “loosens” the joint between the top and sides, promoting vibration of the top).
The binding strips are made with purfling glued to the edges by creating a “sandwich” of the binding material and purfling veneers and then slicing it into the binding strips. These strips are then taped together and pre-bent in the side bending jig so they’ll easily conform to the profile of the guitar body. Shown below are the binding strips for two guitars. I glue the purfling material to both edges of the binding so I don’t have to worry about left- and right-handedness. The unneeded purfling strips will be scraped away after gluing.
Before cutting the binding and purfling grooves, however, a flat is planed at the top of the guitar body where the neck will attach. The body is clamped to the workbench as shown (I also support it from below on a chair). (Note that in the photo below I began routing the binding groove before remembering to plane the flat…)
The flat is adjusted such that the centerline of the neck aligns with the centerline of the guitar body. This is measured using a square and a straightedge, with the square resting on the planed flat and the straightedge placed along the guitar body centerline.
The flat is then adjusted with the plane until the edges of the straightedge and square line up perfectly.
The binding process starts by cutting stepped grooves around the edge of the body to receive the binding and purfling. Because all of the surfaces are curved – the sides, and arched top and back – it’s hard to find a way to cut the desired square-edged groove. I use the jig pictured below, available from Elmer Guitar, which mounts a router vertically in a carriage that is free to slide up and down on a post bolted to the workbench. The router has a rabbet bit installed with a guide bearing that allows it to cut a groove of specified depth, which should be equal to the thickness of the binding. I use an Amana Tool 49342 bit, which provides a series of bearings allowing for cut depths in increments of 1/32″, but there are bits available from lutherie supply houses that allow finer gradations in depth. You can also wrap tape around the guide bearings to adjust the depth.
The guitar is mounted in a carriage with brackets whose height can be adjusted so that the guitar sides are vertical. The guitar is then slid on the carriage so that the router bit cuts the desired groove. (Note that the instrument shown below is a ukulele – I don’t have giant hands!)
The routing jig is first set to cut the binding groove. A piece of scrap wood is used to test the groove depth and width to ensure they are a little less than the binding height and thickness (the binding is left slightly proud, and then scraped flush).
The guitar body is then slid along under the router in the jig, with the router bit cutting the groove. Below, the binding groove is shown routed around the perimeter of both the top and back.
The depth of the cut is now increased to the depth of the binding plus the glued-on purfling by changing the router bit guide bearing. The scrap wood from before is used to adjust and check the depth, as shown below.
The top and back perimeters are then re-routed, but stopping shy of the end binding strip that was previously glued in. The remaining part of the groove will be cut by hand, with a chisel.
The top and back purfling grooves can now be routed, setting the depth of cut appropriately by replacing the guide bearing. As before, the groove is stopped shy of the back inlay strip.
The purfling grooves can now be chiselled up to the back and end binding strips.
The purfling is now mitered at the ends of the grooves. This is easily done with a sharp chisel, using the back of the chisel as a mirror – when the angle is exactly 45 degrees, the reflection of the purfling on the back of the chisel will form a right angle with the purfling.
The first pre-bent binding strip (with side purfling attached) is marked at the butt of the guitar and cut to length. The attached side purfling is then cut and mitered to meet perfectly with the purfling on the end strip.
The loose back purfling strips are also mitered, and we’re ready for gluing. Glue is applied to the back purfling strips and the binding, and these are then placed into the groove and secured with fiberglass reinforced packing tape. Glue is only applied to about 6″ at a time. It’s a bit tricky at the start, getting the binding secured and the back purfling strips in the groove and both mated tight with the mitered purfling on the end and back strips. But once the first few inches are glued and taped, the rest goes pretty easily. Make sure to press tightly when taping so as not to leave any gaps.
Though the fiberglass reinforced tape provides good holding power, it’s sometime not quite enough to pull the binding tightly into the groove, which can leave gaps in the joint. To address this, clamps can be carefully used to pull the binding strip in where there are gaps. I’ve also begun using a jig that can provide additional pressure using elastic cord wrapped around the body as shown.
The edge of the binding strip is then rounded slightly with 150 grit sandpaper. The bound guitar body is shown below.