After getting the body together, the next step in the process was to trim the overhanging material from the back and top to be flush with the top and then to rout the channels for the binding and purfling. The binding will be placed along the seams of both the back and top where they join the sides of the guitar in order to hide the end grain that would otherwise show. The purfling is a strip that is placed adjacent to the binding all along the perimeter of the top of the guitar for decorative purposes. In my case, the binding is Ivoroid and the purfling is a black and white Herringbone pattern.
I have to say that this was the most nerve-racking part of the build so far. After all of the work that has gone into building the box up to this point, the idea of taking a router to it was a bit daunting! As I will explain, this operation had the added complication of dealing with the arch of the top and back while routing to maintain the router in perfect position so that the channels were at the appropriate depth and width to receive the binding and purfling.
I didn’t take any pictures of the flush trimming operation, but I will walk through the binding/purfling operation because the same procedure and jig were used to first trim the top and back flush with the sides. Of course, this was done with a flush-trimming bit in the router.
What you realize very quickly when examining into how to perform this operation is that because of the arches in the top (1/5 degrees) and especially the back (5 degrees), the router base – even on a small laminate trimmer – will not allow the cutter to remain parallel with the sides while moving around the guitar. Similarly, because of the varying contour of the back and top the cutter may also have a tendency to move up and down during the cut leaving you with channels that vary in size.
There are varying ways to deal with the challenges posed by this situation. Everything from scoring and cutting the channels by hand with a specialized tool to creating an elaborate telescoping and/or vertically movable fixture for the router to mount on while moving around the guitar body. In the end, I opted for a compromise and created a simple jig for my Bosch Colt router. In the pictures you can see the jig clamped into position on the router base. The main section is designed to align with the bearing of a rabateing bit that cuts the correct depth channel. If If this section is held vertical and in contact with the side of the guitar while routing the channel will have a uniform depth. To help with this there is a small piece double-stick taped to the router base, effectively moving the base contact point very close to the edge of the guitar where the arch will not affect the width of the channel being routed. The last picture shows the router in position to rout the channel for the binding.
The operation started with trimming the back and top to be flush with the sides. There are some other issues to deal with here concerning grain direction. Because of the rotation of the bit and moving with and against the grain there was a danger of tearing out pieces of the back and top during the routing operation. As a result, I first made a series of clockwise climb cuts in select places and then followed up with a complete pass around the guitar in the standard counter-clockwise direction. The picture shows the path of the clockwise climb cuts in red and the following counter-clockwise pass in blue.
Routing the channels for the binding was next. Even though the back had more arch to deal with, I started there thinking that if I made a mistake, any fix would be less noticeable on the back. The four climb cuts came first and then the standard cut. This was done with a bearing on the rabateing bit which cut a channel that was bout .60″ deep to match the thickness of the bindings. Things went OK on this, with a couple of bobbles of the router that will require some hand cleanup. So, then it was on to the top following an analogous procedure. Again, things went OK but some hand cleanup will be required. Next, I switched bearings on the bit, reset the cutter depth and adjusted the jig to allow routing of the purfling channel along the top – same sequence of cuts with lots of trepidation! Again, a bit of hand cleanup will be required but, the stepped channels for the top binding and purfling are now in place.
In the pictures you can see the results of the operations along with the dry fit of the binding and purfling. The jig worked well. Probably the only thing I would change the next time is to use a slightly smaller piece for the contact point so it is affected even less by the contour of the back and top. Also, I found that keeping the body stationary while routing was a challenge. It turned out that keeping the router in position was not that hard with the help of the jig but, keeping the body from moving during the operation was an issue. The next time I think I will create a fixture that clamps and raises the body into a stable position for the operation.