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: