Damper kerfs

The jacks need a slot for the felt damper that mutes the string. This is a simple matter to take care of at the bandsaw:


A stop block controls the length of the kerf, which reaches just below the position of the plectrum mortise. Instead of centring the kerf I positioned it closer to the edge of the jack. This keeps the jack body from bending inwards into the tongue slot when the damper is inserted: instead, it will bend outwards slightly.

The kerf must be tight enough to hold the damper securely; otherwise the damper will work loose over time as it thumps down onto the string.

I tested the inexpensive 1/4" bandsaw blade I had and found its kerf was too wide to provide a good grip. So I used a little trick I read about to reduce the set of the blade teeth: I ran the saw while pressing lightly on the teeth from both sides with a pair of grindstones. After making lots of sparks and several test cuts, I succeeded in getting a kerf of about 0.85 mm, down from the original 1.37 mm. This is tight enough to hold the damper without shifting.

Here is the first prototype jack to be complete in all respects (tongue, axle, spring, end screw, damper and plectrum):


Note that the top of the slot was eased with a triangular file to help insert the damper more easily.

Now it's just a question of completing the jacks by tapping the bottom holes and installing the end screws. I won't cut any more dampers at this time because the damper lengths depend on having all the plectra installed and voiced first, so that the register on and off positions can be properly determined. Voicing must wait until the project is basically done: it's the last thing to do before playing and enjoying the finished instrument.

I am indebted to the renowned organologist and harpsichord maker Grant O'Brien for publishing, on his web site, a detailed description of the construction of several Neapolitan harpsichords, including drawings and photos of his jacks, which I have shamelessly borrowed from. I adjusted his jack dimensions to suit the cutters and router bits I could easily get, but my final product is not significantly different from his except in the choice of wood and my decision to use only one damper, instead of a pair.