Marking out: Bridge pin positions

In order to accurately locate the bridge on the soundboard's top surface and determine the positions of the hitch pins, I need to trace out the bridge curvature in pencil and locate the bridge pins.

The bridge curvature is a function of the Trasuntino's scaling parameters. In my version of the design, the majority of the instrument follows a true Pythagorean scale with c''=273 mm. This scale is accurate from the top down to c, an octave below middle c'. From that point downwards the strings shorten somewhat to keep the instrument from being inordinately long.

I made up a chart of Pythagorean string lengths derived from this scale, and, working from the treble downwards, I used a long wooden T-square with a tape measure to measure out each string length. I lined the square up successively with the tick marks made along the nut, and at the far end I put a little cross on the soundboard to mark the string length.

Back in September, when I was determining the curvature of the bentside, I mostly used Pythagorean C and F# string lengths to derive the curve and I did not make definitive conclusions about the bass string lengths. The time had come to be more precise, so, starting from c (below middle c'), I experimented with some foreshortened string lengths to see what the far end of the bridge curve would look like. Based on some foreshortening ratios from the original design, I found that the bridge would straighten out and also move leftwards well away from the bentside. I felt I didn't like this look so much. After looking through emails from my friendly harpsichord maker, I was reminded that Italian bridges were often made pretty much parallel to the bentside. In testing out this shape, the appearance was nicer and a side effect was that the bass strings were slightly lengthened, something which my contact had suggested to me could be beneficial to the overall design. The flip side was that the bass hook (the little straight section of bridge going off to the left) would now be longer and would carry about 10 strings instead of 4. This didn't seem to be a problem. I decided to put the hook as far in from the tail as the bridge was spaced from the bentside.

After finishing all this, I realized that I probably could just have set a compass to a fixed spacing and simply traced the whole bridge curve out without all these measurements. It really is quite consistently placed with respect to the bentside curve: no surprise, since the shape was derived from the same data. I have read that Italian bridges were often located by purely cosmetic means, with any ramifications to the scaling apparently secondary to a pleasing appearance. Live and learn, I guess.

I went over the space between each cross in pencil to make a solid curved line showing the bridge position. My plan was to then determine the hitch pin positions along the tail and bentside by going off the straight line of the string at a 12 degree angle and putting a cross right at the edge of the soundboard for the hitch pin.

But, in looking over my T-square, I grew concerned that perhaps it was flexing a bit, particularly at the far end when I was marking down string lengths of some 1800 mm. Any flexing might cause a lateral shift of the bridge pin positions, and my hitch pin positions would only be accurate if the bridge pins were well located. So I decided to double-check the bridge pin positions using the same register that had originally placed the tick marks along the front edge of the soundboard.

I made this jig to hold the register, with a stout oak board to keep the register from flexing and a guide to slide the whole thing along the edge of the spine. The register is clamped so that the edge of the first slot is 37 mm from the left edge, just as it was when the first markings were made.


All I had to do was slide the jig along and see if my original bridge pin marks were aligned with the edge of each slot, like this:


It turned out that most of pin positions were good; a few corrections were needed only in the bass region.

In the photo above you can see that I am working on the second register's pin positions. I only needed to clamp the register slightly to the right to align with the first right-facing string, and then I could mark off all the bridge pins for the second register, which would be spaced 3 mm to the left of the first register's strings (as seen from the nut).



Once the bridge is glued on it will obscure all my pencil marks, so I will actually have to mark out all the bridge pins a second time. I'll slide the same jig along the bridge, marking positions with a little awl, then drill each dimple with a small drill bit and push each bridge pin home.

Hitch pin positions are to follow.

Marking out: Nut and tuning pin positions

Some weeks ago I cut the soundboard to its final shape: this was as easy as clamping it to the frame of the harpsichord and routing all around it with a laminate trimmer.

With the final shape established and the board clamped in place again, the process of marking out the locations of the nut pins and tuning pins commenced. Once again, the registers that were made many months ago acted as the marking stick that determines the positions of key elements.

Before marking out, I had to calculate where to locate the leftmost string in order that the string band be centered on the instrument. This is determined by subtracting the width of the string band from the interior width of the instrument and dividing the answer in half. Back in the autumn, when determining the bentside curvature, I estimated this would be something like 35 mm; it turned out to be 37 mm, which is close enough.

The string band width comes directly from the distance between the first and last register slot, with one caution: a harpsichord with two registers has left-facing and right-facing jacks with strings on either side. Therefore the lowest string is left-facing but the highest string is right-facing. Marking out with the register gives the position of all strings belonging to one register only, not the other. The final right-facing string is located one more register slot to the right of the final left-facing string (a distance of 13.75 mm), minus the string spacing between narrow pairs (which I have decided will be 3 mm), giving a position of 10.75 mm to the right of the final left-facing string.

I clamped one of the registers in place along the back edge of the register gap, making sure the first slot was 37 mm from the left edge. With a pencil I made a little tick mark on the soundboard along the edge of each register slot:


I also put a tick mark for the final right-facing string based on the calculations above. Next, I drew a line on the wrestplank to mark the nut position, which has its centre at 46 mm from the front edge of the wrestplank. With a square, I transferred the first set of tick marks to this line. Then, with a compass set to a width of 3 mm, I marked the position of all the remaining right-facing strings:


Finally, I drew parallel lines 13 and 26 mm from the front of the wrestplank and, using a bevel gauge set to 10 degrees, I drew lines from each tick mark on the nut to intersect these two lines: each left-facing string the first line, each right-facing string the second. The intersection points mark the tuning pin positions, which are shifted 10 degrees to the right of each nut pin. I pricked these holes into the wrestplank surface, shifting pins that represented accidentals slightly forward to make visual identification of the pins easier:


The next step will be to use a large T-square and tape measure to mark out the string sounding lengths on the soundboard, which will locate the bridge, and to determine the hitch pin positions along the bentside.

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.