Under the red guard there is a cylinder with three cutting blades that spins thousands of times per minute. On each side of the cutter is a flat table, with the left table being even with the cutter blades. The right table is just slightly lower than the left table and the blades--I adjusted it to be 1/32" lower. As you pass a board from the right table to the left, the blades remove 1/32" of material. On the first pass of an uneven face, only the high points are cut. Imagine that the face of the board looks like a series of hills and valleys. If you turn that upside down, the board would rest on the hills, and the valleys would not be touching the table. The cutter takes 1/32" off the top of the hills with every pass until the hills are even with the valleys and you have a perfectly flat surface.
That flat surface then rests against the verticle fence which is a perfect 90 degrees to the table. Running the board over the cutters in this way flattens one side perfectly square to the flat fence. Flip it over and do it for the second side, and you get a flat surface with two sides square to it:
If you look at the top of a guitar neck, you will see that the headstock is at an angle to the fretboard in order to provide downward string tension on the nut.
The quickest way to achieve this is to simply cut the neck shape out of a larger piece of wood, but that not only wastes wood, it results in a weak area that is subject to breaking. This is because the grain of the wood "runs out" at the headstock area.
Maybe you've heard of the strong-man trick of tearing a phone book in half. They way they do this is to slightly fan the pages so that the end of each page is slightly exposed. When they begin they are only ripping a page at a time, and once the rip gets started on a page, the rest is much easier. It is much harder (impossible?) to rip a phone book when each edge remains perpendicular to the others.
The same situation applies to wood, which is all paper is anyway. If the wood grain runs the length of the board, is perpendicular to the surface or nearly enough so that it exits or "runs out" at each end rather than at the surface, that board is very difficult to break. Necks cut out in a single piece have grain that exits on the surface of the headstock:
It's not unusual for a peghead shaped this way to snap if the guitar is dropped on it. The traditional way to handle this is with a scarf joint in which the board is cut at an angle, flipped over and glued to the underside, creating the necessary angle. Not only does this conserve wood, but it avoids the grain runout. A peghead made in this fashion has grain that exits out the end, not the surface.On my last guitar I had to cut this 15 degree angle on my small bandsaw, and it took a lot of work with a plane, scraper and sand paper to bring the cut surfaces completely flat, which is necessary--critical, actually--for a strong glue joint. There's a 170 pounds of string tension trying to pull this joint apart.
It took me hours (and a lot of frustration) on the last build to achieve those perfectly flat surfaces. Last year, however, I found online some plans for a jig that would cut the joint on a table saw. It was claimed that very little clean up was needed. I was excited, but also a little skeptical.
Nothing works that easy for me, it seems.
It was a moot point, however, because I didn't have a table saw. Until I did. Now I do. So one of the first things I did in starting Clark's guitar was to build this jig. It holds the neck at a 15 degree angle to the blade. I had to be careful that the fence holding the neck was perfectly perpendicular to the table and that it was flat on the table, otherwise the cut would be off. I tested it on some scrap plywood until I got a clean cut with a square edge, and then it was time to do it for real. Here's the jig with the neck blank clamped to it.
I squared a line seven inches from the end and lined up the neck so the blade would cut on this line, made the cut, and checked it out.
Amazing. Not only was the cut perpendicular, but it was perfectly flat. Here's the two pieces stacked on top of each other. See, no gaps under the engineer's square. That means it's flat. You have to look real hard to see where the peghead piece is laid on top of the neck piece.
Sweet. All I needed to do was clean up the saw marks with a scaper and some sandpaper, and it was ready to be glued.
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