Having surfaced the Red Gum for the parlour guitar back, my next task was to cut it to shape. I cut acoustic guitar backs 6mm oversized, to account for the thickness of the sides (plus a litte spare for good luck) when gluing the body together, and so I marked out the shape of the back in two stages. The first stage was to trace around the plywood body template with a pencil, to establish the size and shape of the soundboard. I then traced around the template again, this time using dividers set to a 3mm spacing. This created a fine scratch line in the surface of the work 3mm outside the pencil line. I cut to this scratch line with a fretsaw, resulting in a back the same shape as the soundboard, but oversized as intended. I also left a large square tab at the end of the upper bout, so that the back forms an integral heel cap with continuous grain.
Thicknessing the back was very much the same process as it was for the soundboard. I used calipers to mark off the thickness of the work every inch or so, and then planed down the highspots using a combination of my No.8 jointer to remove the bulk of the material, and then a low angle block plane for fine tuning, until the back was a consistent thickness of 2.95mm.
It is common to reinforce the glue line of acoustic guitar backs by using an off cut of the soundboard, orientated with the grain running from side to side so that the grain acts as stitches across the joint. I shot the first edge of the yellow cedar reinforcement strip before I had cut the strip to size, as the large off-cut from the soundboard was easier to hold than a small strip would have been, particularly as a narrow strip is likely to deflect under pressure from the plane. This also gave me an opportunity to try out my new Lie-Nielsen No.51, which predictably excelled at shooting the end grain of the cedar (although it was ridiculously over specified for such a simple task – I’m looking forward to trying it out on some much more demanding furniture work soon). With one edge straight, I then cut the strip slightly over width, and shot the other edge straight until the strip was 20mm wide. To prevent the narrow strip from deflecting while shooting the second edge, I clamped the remaining soundboard off-cut to the shooting board and trapped the reinforcement strip between the larger off-cut and the plane.
The reinforcement strip will ultimately be planed down to 2mm in thickness, but for ease of workholding I glued it to the back while it was still at full thickness (around 5mm thick). The strengthening strip was glued in using go bars – I always love how for this operation the line of go bars look like the line of a sail!