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The process for joining the boards (or plates) is fairly simple, but requires extreme precision in creating an absolutely perfect straight edge for gluing. The last thing we want is to be able to see the joint running up the middle of the guitar.
I use two tools for creating the perfect edge for joining the plates. The first is a Lie-Nielsen low angle jack plane. This a super sharp long plane that can be adjusted to take of the thinnest of shavings while leaving a very smooth surface. The process is to clamp the boards down on top of each other and the run the plan along the edge using the plane on its side.
To check that the edge is perfect, the two plates are then held together along the join line in front of a bright light or window to see if there are any gaps. This is called candling.
Sometimes, despite my best efforts with the plane, the joint isn't perfect, in which case I will resort a special right angle sanding block. This is run up and down the joint edge until the line is perfect.
Once we've got a perfect join edge, we can go ahead and glue them up.
Gluing plates together is tricky because we are trying to squeeze two quite bendy large 5mm thick peices of wood together along a very narrow edge. Apply any pressure to the sides and they'll pop up.
For this reason, luthiers use a range of plate joining jigs. As a side note, when you get started building guitars, you can spend a lot fo time building jigs.
I used to join plates using this jig - basically you cut the plates to make them a wide wedge shape and then jam them between two peices of plywood, all the time using a cinder block to stop them buckling up. Note also the wax paper - this is used to stop the glue leaking out and gluing the top to the jig. This approach is somewhat unelegant.
Now I use this plate joinging jig that I built. It's based on a commonly used design. The idea is that the two plates are glued, joined and then sandwhiched between the two cross frames. Ropes are tied around them, and to increase the clamping pressure, triangle wedges are hammered under the ropes to increase their tension. This jig is pretty much fool proof and well worth the afternoon I spent creating it.
Once the boards are joined, we have to thickness them. I used to do this with a hand plane, and while many people do it this way, their skill level is greater than mine. Planing guitar tops and backs requires an exceptionally sharp plane that is adjusted to just take a minimal amount of material off. It's a long, difficult process, especially for guitar backs that have a lot of "figure" or patterning in the wood.
This method requires constant measuring to ensure you've got even thickness.
The "cheat" is to thicknessing is to get a drum sander. I finally broke down and got a supermax 16-32. It's life changing. I use it for thicknessing tops, backs, sides, fretboards, bridges, braces...
I'll have a separate post about tools.
To thickness the back, top and sides with a drum sander, you gradually creep on your desired thickness.
For the back, I target about 3mm. For the sides, 2.5mm as much thicker and they will crack when bending. For the top, I start at 3mm and then go from there.
The thicknessing of the top is a bit of a dark art. You know it is done when you can hold the two edges of the glued top and flick it with your wrists to try to get a metallic whop sound from it - like you were trying to make thunder noises from a sheet of metal. When you get a nice satisfying sound, you're done. You can always sand it a bit more later, but you should stop with the drum sander. On my most recent build the sweet spot was about 2.7mm
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