Bracing is one of my favourite elements of building acoustic guitars. It is the perfect balance of ensuring structural integrity and sculpting the sound of the resulting instrument. Brace too lightly and the soundboard could fold under the several hundred pounds of string tension. Brace too heavily and the soundboard will not be free to resonate, and the sound will be muddy and lifeless.
Building acoustic guitars is a constant calibration of many different factors (body size, body shape, timber selection) and bracing encapsulates this perfectly. The appropriate bracing pattern must be selected for the desired sound (pre-war ladder bracing? X bracing? A bracing?), then the size of the braces in terms of thickness and height of each brace determined for the stiffness and strength of the specific soundboard timber being used. That is a lot of juggling to do, and it is wonderful.
The bracing for the parlour guitar is a pretty standard X brace, with a single finger brace on each side of the main “X” brace, two transverse braces across the upper bout, and a single tone brace off the lower leg of the treble side of the “X”, along with three soundhole reinforcement braces and a bridge plate. The first step I always take is to draw a full size plan of the bracing, onto which I mark the curvature of the soundboard (there will be more on the curvature in my next post). I also then draw the position of all of the braces onto the inner face of the soundboard itself, using a very fine and light pencil. This helps ensure that they are in exactly the correct position come glue-up.
In the past I have glued all of the soundboard braces in one go. This can mean a glue-up session involving 11 or more braces, each with multiple go bars providing clamping pressure. Which tends to get pretty intense. So for this guitar I decided to split the glue-up into two sessions. The first session would be gluing the flat braces, and the second would fit the curved braces.
I prepared all of the brace stock in one go – cutting a 12mm wide, quartersawn, slice off a large block of spruce I keep for bracing. This was my first proper opportunity to give my new (to me) 1900 era Disston D8 a try, and it cut through the 3.5″ spruce like butter. There is a reason why the D8 is the king of rip saws! The spruce slice was then flattened with my No.8 jointer plane, and the slice split into smaller sections each containing two braces. These smaller elements were planed down to thickness and the flat braces then split off using a fret saw. The reason for thicknessing the braces in pairs is that this gives larger work pieces which are easier to hold in place whilst planing.
The three soundhole braces are each 8mm thick, while the two finger braces and the tone brace are 6mm thick. All braces were left at 15mm high for ease of gluing, and I left each of them 6mm overlength. The soundhole reinforcement braces each have angled ends where they meet other braces. Rather than use a sliding bevel to mark these angles, I showed each brace to the bracing plan, and marked the intersection with the adjoining braces using a small marking knife. The waste was then removed with my 12″ carcass saw and I snuck up on the final angles with a low angle block plane.
With the flat braces ready to be glued, I turned my attention to the bridge plate. This was made out of a straight grained piece of maple, which I planed flat and cut to shape using my carcass saw. The lower legs of the X brace pass under the rear corners of the bridge, and the bridge plate extends 8mm in front of the bridge and 15mm behind the bridge. Once cut to size and shape, I then bevelled the edges of the bridge plate with a block plane.
For the first stage of glue up I placed bracing plan on the deck of the go-bar station, with the soundboard facedown. The flat braces and bridge plate were then glued in place using original Titebond and secured with go bars, before being left overnight for the glue to cure.
In my next post I will describe the more involved processes behind the curved bracing – which is where the fun really begins!