This weekend I spent some more time on the Dreadnought Guitar project. In the last post, I had assembled the sides, created the inside mold for the guitar body and installed the kerfed linings. This weekend it was time to profile the kerfed linings to match the eventual contour of the guitar top and back.
As I mentioned previously, both the top and back of the guitar incorporate a radius. These components are radiused from neck to tail and side to side creating somewhat of a domed shape to each. These domes on the top and back plates of the guitar serve the purpose of making th components stronger and help them to resist changes in humidity more than corresponding flat components would.
The top has a more gentle radius angle of 1.5 degrees whereas the back is more severe at about 5 degrees. As a result, in order to adequately attach the top and back plates to the sides, the appropriate angles must be made to the edges of the guitar sides to create a good gluing surface. You’ll recall from my last post that I installed the kerfed linings 1/32″ proud of the edges of the sides so that they could be contoured appropriately in order to effect this assembly step.
Building the Sanding Stick
While there are many potential ways of profiling or contouring the kerfed linings and edges of the guitar sides (i.e. creating a set of radiused dishes with abrasive paper adhered to them to sand the edges), the method I used was simpler. I created a sanding stick to be used to sand the edges at the two different angles for the top and back. This jig was fairly straightforward to build – the only tedious part was getting the angles correct.
I started with a piece of straight stock about 24 ” long by 3 1/2″ wide and some scraps to create wedges of the appropriate angles. The two wedged sections are on opposite sides of the stock and are about 8″ long. One wedge is at 1.5 degrees and the other is at 5 degrees when measured to a point about 4″ from the other end of the stock. 60 Grit sandpaper is adhered to both faces of the stock at end opposite the wedges so that the midpoint of the paper is located at the 4″ mark that I designed the angles to intersect.
Using the Sanding Stick
To use the sanding stick, I mounted the guitar body into my bench vise using the waist clamp as a mounting mechanism. I took the time to mark the edge of the sides with a yellow grease pencil so that while sanding the kerfed linings, I would be able to tell when I had reached edges of the sides.
With the guitar back facing upward I placed the 1.5 degree wedge of the sanding stick at the tail block as a pivot point and sanded the neck area, moving the abrasive end of the stick across the upper bout of the guitar. I continued this process, moving the pivot point along the edge of the sides until I made my way around the perimeter of the guitar. Once I had sanded away the grease pencil lines I made sure that both the neck and tail blocks were sanded across their width to the same angle as the kerfed linings.
Once the sanding of the top edges was completed, I reversed the guitar body in the waist clamp and repeated the process on the back. This time with the 5 degree wedge starting at the tail block to match the more severe angle of the back. In the picture, you can see a closeup of the neck block area with the kerfed linings and edges of the sides sanded to the correct angles.
Installing the Rosette
Because the top and back will be radiused, the braces that will be eventually applied to them for support are also radiused. In the case of the top, there is a decorative rosette to apply around the sound hole. It was necessary to install the rosette now while the top was still flat.
The sound hole and rosette channels were pre-cut into the top. The materials for the rosette consist of three pre-bent, ringed segments. A herringbone center pattern and two narrow segments (black/white/black rings) for the inner and outer edges. As you can see in the photo, the rosette rings were cut and needed to be fit to the routed channels in the guitar top.
Because of the way the fingerboard will overlap the top, only the inner ring of the rosette will show so, the joint on it needs to look good. The other two rings will not be seen, but they afforded me the opportunity to practice the technique for fitting and installing them. I started with the outer ring by putting glue into the channel and spreading it with a toothpick. I then lightly pushed the rosette material into the channel following it along around with the rounded back of my chisel handle. There was not an opportunity to practice fitting and cutting on this ring because there was not enough material to make it around the entire sound hole.
However, on the center ring there was enough material to make it around. So, I began by stabbing an angled cut on one end of the material with my chisel. I then dry fit this along the channel and marked with a pencil at each edge of the cut on the trailing end of rosette material. I removed the material, cut the opposite end with a chisel and then installed the same way as the first ring. The result was close, but not perfect. On the last ring (which will show) I carefully followed the same procedure being more careful to track to the channel in the dry fit before cutting. The result was a much fit after gluing. The last step in the rosette installation was to apply some clamping pressure using two MDF cauls and some wax paper in between to keep things from sticking together.
Scraping the Rosette
After about 3 hours of drying time, I removed the clamps and cauls from the rosette. There was some glue squeeze-out as expected so, some judicious scraping of the rosette was in order. I sharpend the card scraper, put on a fresh burr and went to work.
The scraping was going along well until I heard the crack! I looked down and saw that the top had split along the previously joined glue line just above the sound-hole. Not good! I did not take a picture of this but, it was obvious that the book-matched top had split along the center glue line because I could see dried glue along the split edge – luckily there was no real wood tearout.
I was not sure if I could fix this or not – the top was already profiled in the shape of the guitar and the lower bout of the top was still glued together so, gluing and clamping this was very problematic. I first scraped the edge joint area as best I could with my card scraper to get down to bare wood. However, I did not want to remove any material that would leave a gap, just dried glue. Eventually, I settled on a method to clamp the pieces together by adhering pieces of scrap wood to the top of the guitar adjacent to each side of the glue-line with double stick tape. I then carefully clamped these together while also clamping the top to my workbench to keep it perfectly flat. I let the glued joint dry over night. When I took it out of the clamps and completed the scraping of the rosette area and glue-line, I was pleased to see that the repair had worked! In the picture, you can see the completed and scraped rosette area.
Because of the way this split looked, I think that it was the result of a poor original glue joint and not any mistreatment from me. However, in the process of repairing it I learned some things that will be useful to me in the future. We have to remember that wood is not a perfect material and sometimes making repairs or fixing mistakes that we make is part of the woodworking process. This is especially true when we venture into new areas but, working through these pitfalls is also one way that we grow as woodworkers.