The Barrel Bolts

 

The joint depends on barrel bolts, which are readily available at the local hardware store and very inexpensive. Two bolts are inserted from the inside of the body into holes drilled into the headblock. The bolts poke out at the bottom of the mortise where they enter corresponding holes drilled into the tenons. In the tenons, at right angles to the bolt holes, there are drilled holes which carry the barrels. As the photo shows, these barrels have threaded holes that accept the bolts at right angles to their long axis.

My local hardware store has a wide selection of barrel bolts. All of them have 10 mm diameter barrels, but they come in various barrel lengths. I use the 20 mm length. This allows you to make the tenons 13/16th inch wide, just marginally wider than the length of the barrels.  See the photo below for clarification.

There is also a choice of bolt lengths. The length you want depends on the depth of your headblock inside the guitar body. My headblock plus the thickness of the guitar skin is 1 13/16 inches. For this headblock a 1 and 5/8ths long bolt works well. As you can see in the photo, the bolt goes into the inside of the headblock and emerges at the bottom of the mortise. From that point it needs enough length to go into the tenon and reach inside the sideways barrel, and not much further. The hole in the tenon for the barrel needs to be as close as possible to the tenon shoulder (Cumpiano’s notes make this point) to allow for as much undrilled tenon as possible between the bolt and the body. Actually, if you study Cumpiano’s notes and my photographs you will see that you have a little flexibility in how deep to make your mortise and tenon. I make my mortise 7/8 of an inch deep (at the stage when the tenon shoulders are perpendicular, later they will be undercut - see below). The 10 mm barrel takes up about 3/8ths of this, leaving 1/2 inch of tenon beyond the barrel. You could increase this a bit (say by 1/8 inch) by making a deeper mortise and you could still use the same length bolts, assuming you didn’t change the total thickness of the headblock plus attached guitar “skin”.

Tightening the bolts holds the joint together. A light tightening with an Allen wrench is all you need. In fact, the mortise and tenon joint does not have to be painstakingly accurate. The sideways fit should be fairly tight, but the bottom of the tenon should not hit the bottom of the mortise; there actually needs to be a millimeter or so of clearance.

Interestingly, the bearing surfaces for this joint lie entirely along where the extreme edges of the tenon shoulders hit the body. Notice in the photo how these tenon shoulders are angled back from a right angle. In this case, the body is a 12-fret 000, which has a fairly pronounced curve at the top of the body, but you would still want to angle back the shoulders a bit on, for example, a 14 fret which is almost flat at the head. You want the ends of the tenon shoulders to be knife edges that hit the body along their sharp edge. Later on, you can carefully hand-trim these knife edges to achieve your desired neck back set angle with accuracy.

In addition, the bolt holes in the head block are drilled deliberately oversized. This allows the entire neck to slide up and down a bit before it is tightened and creates some forgiveness if you to make slight errors in drilling the holes. Ideally, two holes in the tenon drilled from perpendicular directions need to line up precisely in the middle of the wood and oversized holes gives you a bit of leeway.  Most crucial though, this ability to shift the entire neck straight up or down slightly is extremely valuable when it comes to eliminating the notorious 14th fret lump on the fretboard extension!

I strongly recommend that you read the last paragraph carefully several times and make an accurate diagram of how your joint will look. This will insure that you can visualize the entire joint and that you make accurate cuts.

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