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.

Assembling the jacks

So far, I've assembled a couple of prototype jacks in testing out the jack design. Now that the process is being done for real with actual jacks, here's the procedure.

First, any burrs in the jack body caused by drilling the axle hole are trimmed with a small chisel:


Next, the axle hole in the tongue is enlarged with a miniature reamer:


The looseness of the reamed hole is checked with an axle pin held in a pin vise. The tongue should hang down freely under its own weight:


The jack is temporarily assembled to check the fit of the tongue within its slot:


If necessary, the tongue is sanded a little to narrow it for a looser fit:


Once again, the tongue should hang down freely under its own weight:


3 mm strips are cut from a sheet of 0.005" brass shim stock to make leaf springs:


Individual 19 mm long springs are cut from the strip and slid into the spring slot:


The 3 mm width was chosen to make the spring fit snugly within the slot.

Each spring is pre-tensioned by bending it forward:


Finally, the jack is reassembled:



Each assembled jack is tested to make sure the spring returns the tongue fully to the forward position. At present the springs are stronger than necessary, but they can be adjusted later by bending them back slightly.

Jacks update

Another hiatus from the project just wrapped up and I'm back at work. I ended up modifying my jack design slightly, as I was not able to control the backwards pivoting of the tongue to my satisfaction on the prototypes I made to date. In the bass, the tongue would bounce off the string behind it as the jack descended, due to the large amplitude of the vibrating string.

I now have the tongue flush with the front of the jack, instead of set into the middle of the jack's thickness. With this configuration, the base of the tongue tilts outward and touches the side of the register slot in front of the jack. This stops the tongue from moving too far back. All the other properties of the jack have been preserved, so I'm satisfied.

As of today, I have drilled the axle holes through all the jacks and tongues and routed the grooves for the springs. I'll continue with reaming the tongue holes and cutting brass shim stock for the springs.

Drilling for the axle

The tongue is held in its place within the jack slot by a little axle, which acts as the pivot point for the tongue.

Here's the setup for drilling the axle hole:


The axle hole is made by a #61 drill bit. A tongue is placed within the jack slot, then both are laid face-down on the board and slid to the right until the tongue touches the little nail shaft. At that moment the tongue has also bottomed out in the jack slot. To make sure the tongue is centred within the thickness of the jack, a few slips of index card fit inside the jack slot and shim the tongue upwards. Then the jack and tongue are drilled simultaneously all the way through.

Next, the tongue is removed and its hole is enlarged with a little reamer to enlarge it slightly. This ensures the tongue pivots freely on the axle.

Italian jacks often use a flat brass spring to push the tongue forward once the plectrum slips under the string. The bottom end of this spring rests in a slot of some kind. On some jacks I've seen, a groove was cut in the back of the jack and a slightly thinner piece of wood glued in, leaving behind a little slit. I thought a simpler approach would be to use a diamond burr to route a tiny dovetail slot down through the jack slot for a distance of about 10 mm. Since the slot is narrower in front, the spring can't fall out. A piece of 0.005" brass shim stock is cut to fit this slot, and is made long enough to come up to just below the plectrum mortise. The discrepancy between the bevel angles of the tongue base and the bottom of the slot in the jack gives a little extra clearance to the spring as it comes up behind the tongue from the dovetail slot.

Finally, the tongue is replaced in the jack slot and a 1.0 mm nickel-plated axle pin is pushed through the jack and tongue. The pin is a little shorter than the jack width so there is no excess to trim when it's pushed home.

The business end of a test jack, using a scrap tongue that is punched and holds a plectrum:


Finding the correct position of the axle is critically important. A low axle lets the tongue tilt back quite easily: too far back, in many cases. Then the tongue bounces off the string behind it, creating unwanted noise. There is also the problem of a low axle allowing the tongue to kick backwards just as the plectrum plucks the string. This represents wasted energy that should have gone into the string. In such a scenario, the exact moment the string is released is governed—wrongly—by the jack design, not by the characteristics of the string and plectrum. Worst of all is the lack of fast repetition of notes when the jack has an excessively low axle position.

A higher axle is the way to go, even though it makes the tongue less willing to tilt back. In this case, the spring must be carefully adjusted by experimenting with the spring strength and the position at which it contacts the back of the tongue.

I put together a few trial jack tongues and bodies and drilled the axle in several locations, starting with a position twice as far down as the plectrum mortise (19 mm, in other words). Then I put each trial jack into the harpsichord, installed a plectrum and played notes in various parts of the compass, observing the tongue behaviour. After moving the axle up slightly with each new attempt, I found a position at which the tongue kickback stopped. I also found I had to drill the axle closer to the front face of the jack:

Punching the mortises

Each tongue has a little mortise which holds the plectrum that plucks the string. This mortise is made by punching a slot through the tongue from back to front.

The punching setup at the drill press is a reasonably simple affair:


Using the drill press gives me a handle to press the punch down with, and also maintains a consistent punching angle. Jacks tend to work better if the plectra angle upwards slightly, so I tilted the table 5 degrees.

The punch itself is a 2.0 × 0.4 mm micro-screwdriver blade held in a small drill chuck. The tip passes through the slot in the maple block, which acts as a hold-down for the tongue and helps to pry the punch off as it is withdrawn. Underneath the tongue a slice of end-grain maple acts as a firm surface to work on. I set up the jig to punch the slot 9.5 mm from the top of the tongue, exactly at the point where the tongues were previously grooved across the back.

Before and after pictures showing how the tongue is punched:



This setup worked quite well in punching the mortises. The trick is to make sure the punch is not significantly wedge-shaped in profile, or it will split the tongue. I'm pleased to say that not a single tongue out of more than 130 was lost this way.

The one minor flaw I discovered is that the punching operation produces a bubble of compressed wood on the front face of the tongue that I had to slice off with a chisel to reopen the mortise. My end-grain maple wasn't a hard enough surface to prevent this from happening. Perhaps it would have been better to use a piece of metal with a small slot in it to allow the screwdriver tip to pass right through the front of the tongue. Otherwise, this was a creditable first attempt.

Jack tongues

The tongue fits in the slot routed into the jack body and is kept in place by a small axle pin. A mortise is punched completely through to receive the plectrum.

Holly, a fine-grained wood, was historically one of the commonly used materials for the tongues. I'm sticking with tradition and will be using the sheet of holly seen in the photo that also shows the resawed walnut the jack bodies came from.

First I cross-cut strips 28 mm wide from the sheet. Each strip can be imagined as a bunch of tongues attached together side-by-side, hence the need to cut across the grain. My tongue slot is 30 mm, but I feel the tongues should stop slightly short of the top of the slot so that the tongue doesn't absorb any of the repeated impacts of the jack hitting the jackrail over and over. That might bend the axle or break the tongue. The tongues will be slightly inset from the front face of the jack, instead of being flush.

Once the strips were made, I chamfered one edge at 25 degrees with a chamfering router bit. This edge will rest against the angled bottom of the tongue slot; the difference in angles between the two (25 degrees versus about 40 degrees) gives me some clearance that will be explained shortly.

Next, I needed to mark where the plectrum mortise would be. On some historical jacks, a small groove across the back of the tongue shows the location, and also thins the tongue to facilitate punching the mortise successfully.

I'm placing my mortise 9.5 mm from the top. That location was marked by making a 1/32" groove with a special small router bit. The groove is about 1 mm deep, reducing the thickness that needs to be punched through from 3 mm to 2 mm:


Individual tongues were sawn from these strips at the bandsaw. With a few strokes of sandpaper, the sawn edges were cleaned up.

A finished tongue:


I punched a test mortise on this one, and luckily it worked without splitting. More on making the mortises in the next post.

Jack bodies, part 2

Once all the jacks were planed to the correct thickness, I took them over to the strip sander and sanded the top and bottom ends clean, shortening each jack at the same time to a final length of 9.7 mm.

Next, I measured the thickness of the top and bottom ends. The process of hand-planing something small and short tends to pull the object up into the blade slightly, creating a microscopic taper. I found that most jacks differed by approximately 0.03-0.04 mm from top to bottom. I marked the narrower end with a black marker dot: this end now becomes the bottom of the jack, since it's marginally easier to slip it into the register slot.

Because I plan to use end screws to provide a little adjustability in the jack heights, I drilled pilot holes for 1/2" long #2-56 steel screws in the jack bottoms. This was done using a horizontal boring setup and a wooden rail to keep the jack parallel to the #54 drill bit:


Before the screws go in (once the jacks are completely finished), I'll tap the upper portion of the hole to help the screw get started, but I won't tap it all the way. This means the screw will tap part of the hole itself, which will keep it tight enough that it won't unscrew as the instrument is played.

After drilling, I chamfered all four edges on the bottom of each jack at the strip sander. This makes it still easier to slip the jacks into their register slots.

Now it's time to cut a slot for the jack tongue. The slot goes all the way through the thickness of the jack and terminates with an angled bottom so that the tongue's angled base can stop against it. This will allow the tongue to tilt backward, but will prevent it from tilting forward past the vertical.

Harpsichord makers usually use some type of circular saw blade to make this slot. I'm using a 3-wing slot cutter in a horizontal router table setup to cut a slot 3/16" wide:


To keep the jack from getting chipped, I made a zero-clearance table surface and plunged the cutter up through it. This supports the face grain of the jack and minimizes tearout at the end of the cut:


The walnut strip is a stop block that establishes the 30 mm length of the slot.

Before plunging the cutter through the table, I had to decide exactly where to locate the slot within the width of the jack. Each jack is 13.4 mm wide and the slot is about 4.8 mm (3/16"). I needed to leave room for the damper that will mute the string as the jack settles back down. The damper will slide into a thin kerf parallel to the tongue slot, which means the tongue slot should be a bit off-centre to leave room for this kerf. The simplest thing to do, I decided, was to subtract the tongue slot width from the jack width and divide the remaining width in thirds, with 2/3 assigned to the damper kerf position and 1/3 left over. These jacks will have the damper on the left, so a width of 5.8 mm is reserved for that. Next is the tongue slot at 4.8 mm, and 2.9 mm remains on the right.

The angled base of the tongue slot is made by setting the cutter height to terminate the cut at an angle of about 45 degrees from the jack face. As you can imagine, the higher the cutter goes, the more the cut angle approaches 90 degrees, so it has to be set relatively low. The angle is produced on the underside of the jack, as this photo makes clear:


Once everything was up and running, I found it necessary to use a push block to press the jacks firmly against the table. This minimized the vibration and chattering that the jacks had experienced when fed freehand into the cutter:


The finished slot: