The pop culture puns aren’t getting any better, but they are definitely here to stay. I am so terribly sorry, gentle reader.
The final stage of work for the parlour guitar back was the bracing. This guitar has a 4.5mm curve across the length, which works out to a little over 16 foot radius. As with the soundboard, I don’t curve each brace by the full extent of the curvature, or even by the same amount. Instead, I mark the full curve on the bracing plan for the back, along with a flat centreline. The point at which the top edge of each brace intersects with the centreline and curve line indicates the level of curve needed for that brace.
The parlour guitar is something of a departure for me, as previously all of the back braces would be the same dimensions for a given guitar. However, the 19th centure parlour guitar on which this current instrument is based had two different sized sets of back braces. So (counting from the heel of the guitar) the first two braces were 8mm wide, while the third and fourth braces were 20mm wide. It is always good to experiment, so I decided to stay true to the original and follow those dimensions rather than do what I have always done before.
I use cedrella for all of my back bracing needs, and once I had milled the stock to the required dimensions it was then a simple job of shooting the correct amount of curve using my No.5 bench plane and the “magic point” method I wrote about previously.
Before I could fit the braces it was necessary to bring the cedar reinforcement strip on the back joint down to height, and also cut the slots for the braces. A low angle block plane made short work of planing the reinforcement strip down to 2mm heigh, and also introduced a gentle camber across the width of the strip which was a pleasing visual touch. To slot the reinforcement strip for the braces, I measured off the front edge of each brace from the plan, and kerfed this line into the reinforcement strip with my marking knife. Where a snug fit is required, I prefer not to rely on measurements of workpieces, and instead let the work itself determine the width of the slot. To do this, I hold the marking knife in its kerf and place the brace against the flat of the knife blade. The marking knife can then register against the back edge of the brace to mark out the correct slot width. The two knife kerfs are deepend with a 1″ chisel, and the waste removed with a sharp narrow chisel. This method allows for any minor variatons in the width of the braces and ensures that I get a good tight fit everytime (it also means I don’t have to worry about too many numbers, which is always a relief).
Because the braces were of differing heights, I decided to do the glue-up in two stages. It is possible to shim lower pieces on the go bar deck, but it adds another variable not to mention unwelcome faff. So I took the easy route of doing a two stage glue up. A bracing cradle was made out of ply shims, as I did for the soundboard last month, and the thin protective ply mat was placed between the cradle and the back to prevent the cradle denting the show surface of the back. The first set of braces were then glued in and left overnight, before adjusting the ceiling height of the go bar deck and gluing in the second set of braces.
Once the glue had cured for all the braces, I bought them down to height using my Lie-Neilsen low angle block plane. The narrower braces offered a good balance of stiffness verses resonance at 13mm heigh, while the two wider braces were reduced to 7mm in height. Assessing the correct height was (as always) a case of flexing the back gently and listening to the tap tone change as the bracing height reduced. My new Sterling Tool Works depth gauge is rapidly becoming indespensible for checking brace height accurately and swiftly. I then profiles the braces using a combination of block plane and the small (25mm) thumb plane. The narrow braces were given a domed profile, while the two widers braces have a gentle camber. Each end of the bracs was then scalloped over 35mm of length, bringing them all down to a height of 1.5mm.
Comparing the tap tone of the back to that of the soundboard suggested that some fine tuning of the soundboard would be beneficial. I increased the amount of curve on each of the lower X brace legs, by 1.5mm, and reprofiled both legs. The scallop in the middle of the tone bar was also deepened by around 0.5mm, and this minor work livened up the soundboard sufficiently.
The back is now complete and can be put aside until the guitar is assembled in the new year.