One thing I learned when rebuilding this guitar is how much of a misnomer "flat top" really is. In fact, a guitar is made up of so many curves, I believe it is more difficult to reassemble one than it is to build it the first time. Although some of the tools and techniques used are the same, there are two very different mindsets and limitations involved. Like my fellow engineers and contractor brethren, my guess us some luthiers are better at building new, while others are better at repairs.
There were two types of back repairs to be made. The easier ones were what I call split ends, and at least one may have been a result of some of my amateurish dis-assembly tools and techniques. To repair the split ends, I broke them open just enough to make sure had good gluing surfaces. I cleaned the old glue off of them with a little more steam and some light sanding, being careful to keep the edges square and preserve the curvature where they meet the back panel.
In order to let the ends of the braces into the kerfing, I needed the braces to be a little bit longer so that the ends would reach the edges of the back panel. One option was to replace all of the braces, but I knew it was impossible to find the same wood, and I was concerned that introducing a different wood in all of the back braces might alter the tone. By shifting the brace locations, I could reuse all but one of the original braces. The solution was to replace the other brace with one made of a stronger but thinner piece of wood, so the difference in tone would be minimized. With a few clamps and some scrap wood, I started by gluing and clamping the two reused braces closest to her waist. Each of these needed to be shifted less than a half inch towards the waist to get their ends to meet the back panel edge.
I used hide glue, so I allowed the glue to dry for at least 24 hours before unclamping. The next brace was at the lower bout closets to the end block, since it also needed to be shifted less than a half inch towards the end block. With that one in place, I added the longest one, a new oak brace, at the full width of the lower bout.
I put the shortest brace aside, and used the original long brace to replace the one at the waist. With all of the braces attached, I masked off the back panel and used a sanding block to smoothen off just the rough edges of the braces. Their stoutness was not something I wanted to change, but they appeared as if they had never been sanded during original assembly. I took off just enough to expose fresh lumber. The aroma of 50-ish year old rosewood was an inspiration.
With all of the braces sanded, I gave the inside of the back panel one last light sanding, just enough to completely remove the patterning left from the old brace locations.
Next, I transferred the new brace locations onto the sides with pencil, to locate where the notches needed to be. Due to the curvature, I had to use some tape to help hold things in place. Then, with a straight edge, I transferred those marks onto the faces of the kerfings with a black marker.
Using my Dremel with a rotary cutting attachment followed by some hand sanding, I made notches in the kerfing and sides, and stopped when it seemed I had gotten the back and sides to mate up well.
There was also one issue on the front panel to be handled before gluing on the back panel. I turned my attention to the flimsy bridge plate I had removed and set aside. Like with the back braces, I did not want to make any drastic changes that might affect the tone. Applying a little structural engineering, I made a set of transverse braces that I attached to the two braces adjacent to the bridge. Slots in the transverse braces trap a very short and stiff oak bridge plate, converting the twisting from the bridge into a pair of forces that get transferred into the ladder bracing. Although this approach has something in common with the Gibson L-0 and L-1 models, played by delta blues legends such as Robert Johnson, my research indicates this is a unique solution as a repair to a ladder-framed guitar top. For sure, it is a whole lot simpler, and truer to the original design, than replacing all of the top bracing with a modern X-bracing design! The next picture shows my first pass at this with roughly shaped oak transverse braces and bridge plate.
The Gibson L-0 and L-1 top bracing was a two-way reinforcing system that combined design elements from both archtop and flat top bracing practices common in the early 1920's. This bracing design was abandoned after a few years of production and was replaced with X-bracing.
|Photo used with permission from: http://hamlettinstruments.com/repair-services/gibson-l-0|
After cutting and fitting the braces and bridge plate, I used some clamps to pre-assemble the new pieces and allowed the glue to dry.
After a final fitting, I made a jig and some cauls to clamp the assembly in place.
After the glue dried, I set the top aside and started in on making two dozen spool clamps from oak dowels, felt pads and some threaded hardware.
Before gluing the back on, I gave the sides and top a final clean-up sanding and made a few minor remaining repairs to the end block, kerfing and the loosened joint between the top plate and end block. Some scrap spruce with hide glue was adequate for replacing a few inches of kerfing in various locations. I using some epoxy to graft a piece of oak to the end block where I had done some damage during the removal of the back panel. A scrap piece of 2x4 attached to my neck separation jig came in handy for clamping up the repair to the top plate and end block joint.