Neck (Pt. 1)
Now that the body is done, my attention turns to the neck.
There are a lot of woods that make good guitar necks, including maple, rosewood, walnut and many others, but for acoustic guitars, mahogany seems to be the standard choice. It is strong, light weight and very stable. Weight is a concern because players don’t want their acoustic guitars to be “neck heavy,” and the stability is a concern because nobody would want to worry about their guitar’s neck distorting every time there was a change in temperature or humidity. The majority of a guitar’s tone is determined by the body, but the neck adds something, too. Although there are exceptions, acoustic guitars with mahogany necks age generally felt to be a bit more resonant than those with other types of necks. This guitar will have a neck made from a single piece of Honduran mahogany. Here I have drawn 2 necks onto a piece of mahogany that is 3″ X 4 ” X 28″
I cut the necks from the block with a bandsaw. The extra pieces are used for head and tail blocks of other instruments (the wedge-shaped pieces make great doorstops), so there isn’t much waste.
I next want to make the joint where the neck will fit into the body. This is one of the most critical parts of guitar construction, because if the neck doesn’t fit properly the guitar won’t play right and nothing else about it will matter. First I have to square off the peghead on a belt sander.
After I do that, I can determine exactly where the neck-to-body joint will be located along the neck shaft. I am using a 26 1/2″ scale length for this guitar and the neck joins the body at the 12th fret, so that joint will be 13 1/2″ from the edge of the peghead (13 1/4″ for the first 12 frets of the fretboard, plus 1/4″ space for the nut).
One more preparatory step is to cut the channel for the truss rod into the neck. There are several different designs for truss rods, but they are all intended to do the same thing, which is to offset the upward bowing of the neck brought on be the constant pull of the strings. The truss rod on this guitar will be adjustable through the soundhole and the commercially available rod I am using works quite well. Here is the rod sitting in the channel that was cut on a table saw.
There are many acceptable ways of attaching the neck to the body. A dovetail joint is probably the most common. Bolt-on necks have become popular recently. I prefer to use a mortise and tennon joint. Each system works fine when properly executed and each can be a nightmare if not done properly. I have been using the mortise and tennon approach for almost my entire guitar building career and have had very good results. I have gotten good at aligning the necks and getting the proper angle to the body of the guitar and I have not had any problems with necks coming loose over time. I have done some dovetail joints but never felt as satisfied with them. The mortise and tennon joint involves both the neck and the body. First, the mortise is cut into the headblock of the body using saws and chisels. The primary cuts in the headblock were made before the body was assembled, but now I have to open up the mortise. Here are a couple pictures:
The next step is the trickiest and most critical: cutting the tennon into the neck. This has to be done in such a way that the center line of the neck lines up with the center line of the body. More importantly, the cut of this joint establishes the angle that the neck forms to the body, which is absolutely critical. If the angle is too steep, the plane of the fretboard will point up from the guitar’s soundboard and the strings will be too low on the finished guitar. Conversely, if the angle is too shallow, the plane of the fretboard will point down and the strings will be too high. Minor adjustments can be made by altering the height of the bridge and saddle, but these don’t allow for much compensation. My goal is to make the neck angle such that when the neck and fretboard are attach to the guitar, a straight edge across the top of the fretboard will leave a 3/8″ gap at the bridge line. That is when I know the guitar will play the way it should.
Unfortunately for this blog, I was so involved with cutting the tennon that I didn’t think to take any pictures, so here is a photo of the tennon at the end of the neck. I have cut away some of the extra wood on the neck; the lines are where the fretboard will be:
Here is one more photo of the mortise and tennon together. The neck won’t be glued to the body until the peghead is built.