There are three different types of neck blanks from which to make a guitar neck. One is to use a 24 inch neck blank and attach a solid heel block to it. Another way is to make a neck with a stacked heel from a single 35 inch neck blank. The third way is to cut a one-piece neck from a billet. I will begin by showing how to do the scarf joint and this is the same for either the 24 inch or 35 inch neck blank.
The neck blank should be about 3 to 3 ¼” wide or about 7.5 cm to 9 cm wide. The length should be either 24 inches or about 61 cm or 35 inches about 89 centimeters. On my steel string guitars, I measure a skosh more than 3 inches or about 80 mm in from one end and draw a line. I then measure another 3 3/8” or about 87 mm from the first line and draw another line. On my classical guitars, I measure in from one end 5 ½” or 14 cm and draw a line. I then measure another 2 ½” or 6.5 cm from this line and draw another line. These line are then squared down the side of the neck blank. With a straight edge, I connect these two line diagonally across the side of the neck blank. This should be about a 15 degree angle. However, the thickness of your neck blank can alter this. I always check the angle with a protractor to make sure it’s not more than 15 degrees. The next step is to cut the scarf joint at the line you’ve just drawn. I usually use a bandsaw to do this but you could also do it by hand. You can even make a jig to cut it on a table saw. After cutting the diagonal line, I clean up the side that gets glues to the bottom of the neck blank using a block plane. I then use a sanding block to remove any plane marks. Check for fit to make sure it is a good fit before gluing it up.
There are several ways to glue the scarf joint. You can clamp the neck blank to a bench and then use clamping cauls and C clamps like I’m doing here. You can also clamp the neck in a vise and then use clamps to apply pressure to the joint while it glues. This method keeps the neck straight as you apply the pressure. Whichever method you use, remember that you only need a little bit of glue. Now I can thin the peghead to the correct thickness. Don’t forget to subtract the thickness of the veneers that would be placed on the peghead from your final measurement. For my guitars, I thin the classical peghead to 18 mm or 71 hundreds of an inch and my steel string pegheads get thinned to 11mm or 43 hundreds of an inch. My final thickness for the peghead after the veneers are applied is 22 mm or 87 hundredths for the classical peghead and 15mm or 59 hundredths for the steel string peghead. As you work, make sure the scarf joint stays square. Now we can lay out the scale.
You will notice that after that after thicknessing the peghead, the point of intersection with the face of the neck blank has moved back towards the heel block. This sometimes makes the neck blank too short and at this point, throw the neck blank away and order a new one. I am kidding. There is a little trick that can help stretch the board. I use a jointer to plane the face of the neck blank, this moves the scarf joint point of intersection forward and thus stretches the length of the board. It doesn’t really stretch the board but you get the idea. If you take complete passes along the face of the neck blank, you make it too thin on the heel block end. Especially if you want to do a stacked heel. I, therefore, make three passes on the peghead end to one pass on the heel block end just like I’m showing here. When using this board stretching trick, I never plane thinner than about 17mm or 67 hundredths of an inch on the nut end of the neck. You can join the face of the neck blank until you have enough length for the scale and tenon. Notice how the point of intersection is now much closer to the glue line of the scarf joint. Make sure you have enough length on the peghead for your peghead design.
You can now go ahead and layout the scale length. If you are using the one-piece block on a 24 inch neck blank, then just glue the heel on at this time. If you are doing the stacked heel, then cut the neck off at the end of the tenon or flush with the appropriate fret if using a butt joint. After cutting off the excess of the neck blank, align it with your 12th or 14th fret markings on the side of the neck blank and calculate how much you need for each section of the stacked heel. Don’t forget to add the measurement for your tenon if needed. Mark the pieces and after cutting it into sections, align them in the same order on the bottom of the neck blank, to glue the stack heel piece together, just place a scrap piece of wood in your vise and then align the piece and apply pressure by closing the device. After appropriate glue time, remove the neck from the vice and you now have a stacked heel.
To cut a one-piece neck from a billet, I start by marking the shape of the neck onto the billet. You can use an existing neck, or your plans to do this. Once the shape is on the billet, then use a bandsaw to remove everything that doesn’t look like a neck. You can cut two neck blanks from one piece of wood. You need to have a sharp blade to execute this smoothly and may need to make relief cuts during the process. Once the neck is cut out or you have made the neck with a one-piece heel block or stacked heel, go ahead and continue building your guitar.