Tools & Materials:
Woodworkers Marking Knife
Fine Dovetail Saw
Fine-Toothed Japanese Pull Saw
10″ Table Saw
24″ Stainless Steel Straightedge
Wood Chisel Set
Mortise & Tenon Neck Joint:
The Mortise and Tenon Joint is my joint of choice for an Acoustic Guitar neck or a Ukulele neck. I feel that I have more control of production with this joint, I can come up with a better fitting joint and I have more options for anchoring the neck to the guitar body. Upon finishing the mortise and tenon elements of the neck and head block, there are several alternatives for the actual attachment of the neck to the body.
Squaring Things Up:
I first square everything up on the neck. I make sure that the neck, headpiece and heel materials are all dead level on each side of the guitar and there is not a bit of variation. Make sure your neck is consistent in thickness as well (you should have done this prior to gluing it up.) Also make sure that the end of the heel is exactly square.
Marking the Joint:
The joint is relatively easy to make. Start off with cutting the tendon part of the joint on the heel of the guitar neck. You should carefully mark the lines on all three accessible sides of the neck so you can gauge your cutting progress.
These cuts are extremely important to make properly. If they are sloppy and not exactly equal on either side of the tenon, fitting your neck later on will be a nightmare.
Clearly mark the line of the guitar body horizontally across the face of the neck (the fretboard surface of the neck) with a sharp Marking knife, or a very sharp pencil. This distance will be exactly the distance from the back of the nut to the center of the 12th or 14th fret (depending on the instrument being made).
Next mark lines down the side of the heel block stack. Line them up precisely with the top mark. Make sure each line is exactly vertical by using the a small woodworkers square. Even be careful that you hold the knife or pencil precisely and consistently. Little variations can lead to quality problems later on in the building process.
Mark across the bottom of the heel and line up with the two side lines.
Cutting The Joint:
Since this joint should be cut before any rough neck shaping has taken place, you will be working with a neck plank can be easily cut on a Table Saw or a Band Saw. Just be sure to do a dry run on the neck cuts before you actually do any cutting.
You can also easily cut the tendon out with a fine backsaw or a dovetail saw or a Japanese Pull Saw. Check your progress often and make sure you cut absolutely vertically.
GLS Tip: If you have trouble cutting vertically you can clamp a wood block right next to the area to be saw-cut to better guide the saw. If you do this it is better to use a deeper saw like a miter saw for better control of your vertical cut.
Take a sharp chisel and clean up the joint. Sand smooth with sanding sticks and assure the joint is square and perfectly vertical.
Cut the Mortise Joint:
The next step is to cut the mortise (channel) into the Head-block. It is best to do this prior to gluing the Head-block into place on the guitar sides. This joint is easiest, by far, to cut on the Table saw. Make sure you have a nice tight fitting joint with no play. Dry fit the joint and check it for squareness. Match this mortise closely to the tenon that was just cut on the neck. The joint should be slightly snug, but not require force to mate the joint.
Additional Joint Anchorages:
I know of three additional measures that you can add to make this a more secure joint.
Method One: Use vertical dowels the full height of the Mortise and Tenon joint. Dry fit the joint, use shims to place the Head-block in the proper orientation to the neck and clamp this together. Drill two holes on either side of the tendon, using a drill press that will fit dowel stock that you intend to use.
I use (2) or (3) pre-made dowels on each side of the joint. I like them because they have the spiral glue slots that allow for glue squeeze-out. I leave the dowel heads down just a bit to allow for easy drill-out in case a neck reset is required.
Method Two: Use horizontal dowels that go through the sides of the Head-Block and pierce the Mortise and Tenon joint in two locations. This method works best if you use a tapered dowel pin in lieu of straight dowels. The idea here is that the pins are friction fit and eliminates the need for a glued joint for the neck. This method is too complex for this discussion and will be addressed in a separate article.
Method Three: Use mechanical fasteners such as Stainless Steel Bolts with Threaded Brass Inserts. The brass inserts are drilled into the tenon and the Head Block is drilled out to receive the Stainless Steel Bolts, which have an internal hex head for tightening with a allen wrench.
This is a very secure and easy way to fasten the neck to the instrument body. You can even do it without seeing any glue joint. This method will allow mounting and removal of the neck with easy, which is needed when aligning the neck both horizontally and laterally with the instrument body.
Gluing the Head Block to the Sides:
Glue the Head-block to the sides, making sure the joint is centered on the global center-line of the body. Once the glue is dry, trim the sides to the Mortise joint edges.
Put the instrument in the inside mold top side downward. slip in the neck and double check the fit of the Mortise and Tenon. Double check the center-line of the guitar body to the center-line of the neck and make sure there is no discrepancy.