Routing the Purfling Grooves
purfling is the technical term for inlay that runs around the periphery of a musical instrument. purfling knots is the name for the fancy work we are discussing here.
The inlay begins with designing a pattern. A computer graphics program is used, to make it easy to edit the shape, vary its thickness to simulate the thickness of the inlay line, etc. Only one half of the pattern is drawn. The whole shape is created symmetrically by flipping the pattern. This pattern is adjusted for pleasing effect by flipping it on-screen, to generate the whole design. After the design is settled, it is printed and glued to a thin wooden plate (usually a rejected piece of walnut dulcimer back).
The router is actually a laminate trimmer, used by kitchen cabinet installers. It amounts to a miniature router with a very powerful motor. I tape a special light with a long, flexible optical fiber tube to the side of the router and bend its tube so I can get some light. For router bits I use carbide end-mills, such as found in machine shops. For the back purfling, I use a .125" bit. The special router base is a home-made template follower.
Here is a close-up of the template follower base in use. You can also see the little light I rig up so I don't have to work in the dark at this. The base is plexiglass, with a small brass tube. the cutter is .125". The cutter spins inside a miniature brass tube which is the template follower. The brass tube's walls are only .0135" thick, and clear the cutter by another .0135". The result is that the cutter moves along the template only 1/32" away. Very fine detail can be followed.
Here's another view of the base, so you can see better the little tube. This tube rides along the edge of the template, keeping the edge of the cutter 1/32" away. Making this arrangement was actually quite simple, but it does a really good job of allowing me to create intricate designs. Because the distance between the template and the cutter is small, it will only put a 1/32" radius on any corner. More than enough for this inlay work.
So, we've glued down the template to the back and begun to cut the groove on the area of the main bout. You can see that the router does not dwarf the work. It is important to use tools that are the right scale for the work. This enables you to have the right "feel" for the cutter, to be able to control the tool with the right sensitivity and good control. You aren't using large muscles to control small movements.
Next we pry the template off, flip it over, and carefully position it exactly symmetrical to the previous placement. (Carefully layout of all this work is critical, by the way, as you are carving the back before you know exactly where the ribs of the instrument are finally going to lie against the back.) Then we go over the template with the router again.
Now we pry off the template, and suddenly a rather simple curve, repeated in a mirror image, has become a beautiful, complex shape. We need to trim out to sharp corners, patch any boo-boos (I still make one once in a while, but I defy you to find my patches!), and do some light sanding to get rid of hairy fibers left by the cutter at edges here and there. Now the fun comes. Next, we start inlaying!
Not until much later, after the body has been completely assembled and just before fitting the top, can we complete the edge purfling. The edges must be defined in order for that to be done. Here are a couple of views of the groove being routed with the laminate trimmer.