Although you cut hearts in the soundboard, the real heart of the instrument is the fretboard. It must have both strength and precision. When you press a string down onto a fret, the free-vibrating length must not buzz against the next fret down. The most precisely made fretboard can become unplayable if it warps. Use all the precautions available against warping: straight, even grain; dry wood; and second seasoning. Second seasoning means shaping the wood very close to its final dimensions and then setting it aside to dry some more before you make that final pass with the jointer plane.
As I mentioned earlier, many dulcimer makers hollow the underside of the fretboard. This lightens the fretboard while allowing it to retain much of its stiffness. Although you will find it a simple task to hollow the back, do it first and let the fretboard sit for a few days before the final planing.
The face of the fretboard receives the frets and the strum hollow. The strum hollow is a simple cutaway place near the tail end of the fretboard that gives clearance for picking and strumming when you play. The frets need to be laid out in the precise spacing given in the table or copied from your model. The frets themselves can be made either with T-shaped fretwire (which you buy from an instrument repair shop) driven into a fine saw kerf or with wire staples driven into predrilled holes. If you buy fretwire, buy enough to practice with.
The saw kerf must be well matched to the size of the wire and the hardness of the wood. The wire needs to be a tight fit, but not so tight that it exerts a wedging action on the fretboard and bows it back when all the wires have been driven in. Softer wood will give more and can take fretwire in a narrower kerf than a hardwood like ebony. Make sure you have the right cobination before you proceed. Wet the kerf with a single drop of water and tap the fretwire in at one end with a smooth-faced hammer. Continue tapping down the fretwire until it is seated across the whole face of the board and snip off the end. When all frets are in place, bevel their ends with a file. As a final assurance that the frets are all level, dress them with a large, flat, smooth file, sliding it up and down the frets until they are all brightened by its passage. This will leave sharp corners that you will need to round with files or abrasives.
The bridge and the nut hold the strings above the fretboard. They need to be made of very hard stuff to keep the wire from biting in. They also offer a chance to make some color contrast among the woods. Boxwood or bone each look sharp inset against a walnut fretboard. The bridge at the foot of the instrument can be either free-moving or inset. Although insetting the bridge will keep it from slipping, don't glue it into place. This will enable you to make height and depth adjustments should you need them. The nut fits between the fretboard and the head in the same way.
"The Woodwright's Eclectic Workshop" By Roy Underhill
© 2012 The University of North Carolina Press