I started work on the back on Tueday. I had already planed some of the saw marks out of it when I did the resawing, but not all. It was getting on the thin side, about 3mm, and I wanted to have some stock to correct any misalignment errors that might occur during the gluing.
First the two halfs were clamped onto the shooting board and I ran the Jack plane over the edge a few times until it produced long curly shavings along the full length of the boards. Then plane was adjusted to a fine setting and shot it a few more strokes and held the two halfs butted together up against my 100W lights to examine the fit from top to bottom. But, like the soundboard, I ended up with a little belly in middle and light shining through at both ends.
I went back to the shooting board, clamped the back in again and started shaving off the middle only, slowly working my way out to the ends making the strokes longer and longer each time the blade stopped producing shavings. The light test was done again, and the whole procedure was repeated until no light came through the seam at all and a perfect fit was reached.
I don't know whether it was the hardness of the bubinga or the edge off the blade that wasn't as keen, but I had considerable more difficulties getting the desired fit on the back seam that I did with on the spruce top. I must have repeated the cycle at least ten times before a satisfactory result was reached.
The center strip was made from the fifth binding and purfling. I was somewhat confused trying to figure out how to go about the process of gluing it in. All of my books suggested to include it when gluing the back, some all at once, some in stages, but none really elaborated on how to get the strip dead straight so it wouldn't ruin the center seam I'd just made such an effort to joint with utmost precision.
I ended up wedging the binding between the two unfinished sides and planed it freehand to final dimension, about 5.3 mm including the purfling already glued to it, checking the progress regularly with my calipers. It was very hard to get it dead even along the entire lenght, but I got it to be within ±0.05mm. I used both my block plane and my jack on very fine settings; The block plane with short strokes for the bigger irregularities and the Jack to even it all out with full strokes. I tried to keep the planes flat and steady by holding them so my fingers would function as guides against the bench (sides).
The Gluing was pretty straight forward. I used an MDF clamping caul wax paper to prevent the whole back getting glued to the jig and caul.
In retrospect I was wondering if it wouldn't have allowed for more accuracy if the binding had been glued on in two stages; First to one side of the jointed back and then to the other with the purfling added inbetween. This would have facilitated an opportunity to 'candle' the glued-on binding with the other back piece and making final adjustments if needed. However, I'm hoping this will all be of an academic nature as the back will receive the center joint reinforcement strip on the inside.
JOINTING THE BACK SEAM