Bookmatching the Top Sets
Bookmatching begins with making an accurate straight edge along each board, so the joint will be strong and reliable.
Here is a picture of my woodrack, showing some spruce and walnut that has been resawn into thin top and back sets waiting for bookmatching. We select a bookmatch set from our stock. Each of the slices have already been sorted into pairs, and the pairs taped together. Notice the diagonal stripes that help us keep track of which slices of the board were neighbors. It is important that we choose neighbors and not just random pairs, since we want both sides of the instrument to have similar acoustics.
After choosing a spruce topset from the stack, we clamp it into the bench vice, braced between two stiff boards and lined up so the top edge is at the same height. We clamp at each end also, to increase stiffness. Then plane the top edge smooth with a 22" joining plane. You can judge from the "bite" of the plane whether you are cutting along the whole length. The object is to get a perfectly straight edge.
Well, practically perfectly straight. In actuality, it helps the clamping a lot if there is the tiniest bit of concavity in the edge, so the pieces mate at the ends but have to be pushed together about 1/64" in the center (easily done with a little clamping pressure, even on large boards). Here is a view of the joined edge being tested with a 4-foot straight-edge ruler. You want t see a little wiggle in the center, with the ends of the ruler supported on the very ends of the two boards.
Next we mount the spruce topset boards in the bookmatching clamp board, for gluing. The clamp board is simply a good, straight board with two straight wooden rails firmly fixed along either edge. The clamping pressure is applied with wedges, which bear against a small "spiling" or light wooden rail that applies the pressure evenly to the edges.
At the top of the page you see the two spruce tops in their clamp set-ups. Note the steel weights used to keep the two boards from buckling under the clamping pressure. These weights also act to keep a good alignment between the edges, by firmly pressing them against the flat bottom of the clamping board. The glue used is hide glue, and you can see the glue pot up in the upper right corner. Once the glue is applied, the two pieces of the topset are placed carefully on the board. (The clamping board has a finish that resists the glue.) The joined edges are aligned, with a little offset of 1/8" or so (so we can still see the centerline even after the boards have been finished to final thickness). Then the boards are lightly clamped with wedge pressure, and the edges of both boards tapped lightly with a mallet, to make sure nothing has cocked up out of alignment. Then a sheet of wax paper is laid over the glued joint (prevents iron stains on the wood), the weights placed, and the wedges firmly set.
After glueing and clamping, the bookmatched sets are stacked in the woodrack to relax for a while, and to do a little more drying. The next step is the thicknessing step. Currently, I use a home-made thickness sander that I inherited from Walt Martin, of Sunhearth. I've also been toying with a tool called a Safe-T-Planer, which is a rotary planer mounted in a drill press.