Making the bridge and nut

The bridge and nut define the sounding lengths of the strings in a stringed instrument. On a harpsichord, both usually have a similar if not identical appearance, and their position on the instrument determines the name: the bridge is glued to the soundboard, while the nut is glued to the wrestplank's top surface. A bridge, therefore, conducts string vibrations into the soundboard while a nut, together with the heavy pinblock, reflects vibrations back towards the bridge.

There is occasionally an exception to this: some harpsichords have their nut placed on a small piece of free soundboard, meaning that the wrestplank ends in front of the nut instead of continuing all the way to the register gap. This is called a "hollow wrestplank", but is not a concern in the Trasuntino design, which places the nut on the wrestplank.

The shape of the bridge reflects the scaling choices made by a harpsichord's designer, and the exact curvature is a function of how closely the design adheres to or departs from a theoretically just scale. I have already indicated that the Trasuntino follows a just scale through much of its range, so all but the lowest octave of the bridge follows a Pythagorean scaling curve. The shape of the nut cannot be too radical as it must fit within the confines of the wrestplank. Nuts range from perfectly straight to slightly curved; typically they are closer to the gap in the treble. The Trasuntino's nut is unusual in being parallel to the front of the wrestplank.

Unlike most other stringed instruments, the harpsichord's strings do not touch the wooden bridge and nut surfaces without first bending around a metal bridge/nut pin. If a harpsichord string were to touch wood first, the tone would be feeble and choked. Therefore the cross-section of the bridge and nut must have a little slope or hollow on the edge facing the sounding portion of the string. This provides a little clearance for the string and prevents it from touching a wooden surface before it reaches the pin.

Hardwoods are the material of choice for bridges and nuts, and walnut is appropriate for Italian and certain other harpsichords (although, interestingly enough, the original Trasuntino actually has a cypress bridge). I started by obtaining a walnut offcut a bit longer than 6 feet. Since this is nearly the length of the finished bridge and I still need a nut of about half that length, I made sure my walnut was of a width that allowed me to work on both its edges, thereby producing two identical pieces. All operations were carried out at the router table.

The first step was to establish the height and width of the bridge: 13 x 8 mm, respectively. I did this by cutting out a centred groove with a straight-cutting bit, making the groove 13 mm deep and adjusting its position until the remaining material was 8 mm thick:


Next I used a portion of a router bit designed for decorating small items like jewellery boxes to cut a profile that included a cove and groove:


The cove will provide the string clearance I discussed earlier, while the groove will receive the brass pins. I deepened this groove slightly with a 1.6 mm straight-cutting bit, and then chamfered the back edge with a 25-degree chamfering bit:


This chamfer makes it easier for the strings to slope downwards on their way to the tuning pins and hitch pins.

All that remains is to cut these parts free at the table saw.