Last autumn I resawed some walnut I bought back on my very first lumber buying expedition in August 2007. The walnut was planed to a 4.9 mm thickness, with a projected final jack thickness of about 4.6 mm. After resawing and planing, I stickered everything in layers, under bricks to keep it all flat:
The top item is a sheet of holly 3 mm thick, from which the jack tongues will be made. The sheets of walnut are underneath.
Having sat around for about a year, all this material is quite stable, which is an essential basis for producing jacks that are to be well-behaved.
The first order of business was to slice the walnut sheets up into long strips slightly wider than the finished jacks:
These were stacked together on edge, a dozen at a time, taped together on the underside, and planed to establish the final width of the jacks (13.4 mm):
The jack slots in the register are a bit over 14 mm wide, so there is a clearance of about 0.6 mm. This is fine; in fact it could be a little more and still be OK: Skowroneck's book suggests that even 1 mm of clearance isn't problematic.
These bundles, still taped together, were cut down into individual jack lengths on the bandsaw. I'm aiming for a final length of 9.7 cm, so I cut to 9.8 cm to give me a little room to sand the ends and eliminate the roughness left by the bandsaw. The required jack length is actually 10.4 cm: the extra length will be provided by an end screw that will allow the jack height to be adjusted. I know that historical harpsichords didn't have this little convenience; it's the one place where I feel a modern screw could possibly be useful. A generation ago, horrible modern jacks were made that had far too many screws all over the place: see this web page for photos.
Here's a box full of jack blanks:
The most critical part of the jack body is its thickness; the clearance in this dimension is about 0.2 mm at most. Too little and the jacks might rub in the register slots during the dry winter months; too much and the plucking of the strings will be inconsistent as the jacks wobble around.
I suppose one could thickness jacks by machine until the required dimension is reached, as I did with the edges. However, a machine-planed surface isn't completely smooth; under raking light a washboard-like series of ripples can easily be seen. It's best to hand-plane the jack faces, since the hand plane gives a completely smooth surface without ripples. An alternative might be to thickness-sand instead, but sanding tears wood fibres and mats them down instead of cutting them cleanly like the plane does, and these fibres might decide to stand up again sometime later, compromising the smooth surface. Since the jacks have a more generous clearance in the direction of their width, I don't think there will be any trouble leaving the edges machine-planed. They feel smooth, even if they aren't on a microscopic level.
Here's the setup for hand-planing jacks to a controlled final thickness:
Two hardwood rails are screwed to a plywood board. Each rail has a groove with its base exactly 5.0 mm above the plywood. The hand plane seen at left slides in these grooves. A jack is held in place between the rails as shown below:
Scrap wood pieces keep the jack from shifting sideways or backward as the plane rides over it. These scraps must obviously be thinner than the finished jack so as not to interfere with the plane. Note the white paper shims inserted under the jack: these are used to raise the blank up each time the plane cuts away the top surface.
The hand plane is a Veritas low-angle smooth plane with a 38-degree bevel-up blade. The blade is bedded at 12 degrees, yielding a cutting angle of 50 degrees (York pitch, for the plane experts out there). This yields a smoother surface than the usual 45 degree cutting angle, at the expense of more physical effort to push the plane.
A well-adjusted plane should be able to take off a shaving just one thousandth of an inch thick:
To thickness a jack with this setup, a jack blank is put in place and is planed until no more shavings come off. Then a paper shim 0.07 mm thick is put underneath and the jack is planed again. Next, the jack is turned end-over-end to keep the grain angle at the surface consistent, and the other face is planed and shimmed a few times until the correct thickness is reached:
The final jack thickness is about 4.6 mm. The register slots are about 4.76 mm, and the wiggle of the planed jacks within the registers seems right to me: there's just a little bit of play.
Two registers full of jacks:
The first order of business was to slice the walnut sheets up into long strips slightly wider than the finished jacks:
These were stacked together on edge, a dozen at a time, taped together on the underside, and planed to establish the final width of the jacks (13.4 mm):
The jack slots in the register are a bit over 14 mm wide, so there is a clearance of about 0.6 mm. This is fine; in fact it could be a little more and still be OK: Skowroneck's book suggests that even 1 mm of clearance isn't problematic.
These bundles, still taped together, were cut down into individual jack lengths on the bandsaw. I'm aiming for a final length of 9.7 cm, so I cut to 9.8 cm to give me a little room to sand the ends and eliminate the roughness left by the bandsaw. The required jack length is actually 10.4 cm: the extra length will be provided by an end screw that will allow the jack height to be adjusted. I know that historical harpsichords didn't have this little convenience; it's the one place where I feel a modern screw could possibly be useful. A generation ago, horrible modern jacks were made that had far too many screws all over the place: see this web page for photos.
Here's a box full of jack blanks:
The most critical part of the jack body is its thickness; the clearance in this dimension is about 0.2 mm at most. Too little and the jacks might rub in the register slots during the dry winter months; too much and the plucking of the strings will be inconsistent as the jacks wobble around.
I suppose one could thickness jacks by machine until the required dimension is reached, as I did with the edges. However, a machine-planed surface isn't completely smooth; under raking light a washboard-like series of ripples can easily be seen. It's best to hand-plane the jack faces, since the hand plane gives a completely smooth surface without ripples. An alternative might be to thickness-sand instead, but sanding tears wood fibres and mats them down instead of cutting them cleanly like the plane does, and these fibres might decide to stand up again sometime later, compromising the smooth surface. Since the jacks have a more generous clearance in the direction of their width, I don't think there will be any trouble leaving the edges machine-planed. They feel smooth, even if they aren't on a microscopic level.
Here's the setup for hand-planing jacks to a controlled final thickness:
Two hardwood rails are screwed to a plywood board. Each rail has a groove with its base exactly 5.0 mm above the plywood. The hand plane seen at left slides in these grooves. A jack is held in place between the rails as shown below:
Scrap wood pieces keep the jack from shifting sideways or backward as the plane rides over it. These scraps must obviously be thinner than the finished jack so as not to interfere with the plane. Note the white paper shims inserted under the jack: these are used to raise the blank up each time the plane cuts away the top surface.
The hand plane is a Veritas low-angle smooth plane with a 38-degree bevel-up blade. The blade is bedded at 12 degrees, yielding a cutting angle of 50 degrees (York pitch, for the plane experts out there). This yields a smoother surface than the usual 45 degree cutting angle, at the expense of more physical effort to push the plane.
A well-adjusted plane should be able to take off a shaving just one thousandth of an inch thick:
To thickness a jack with this setup, a jack blank is put in place and is planed until no more shavings come off. Then a paper shim 0.07 mm thick is put underneath and the jack is planed again. Next, the jack is turned end-over-end to keep the grain angle at the surface consistent, and the other face is planed and shimmed a few times until the correct thickness is reached:
The final jack thickness is about 4.6 mm. The register slots are about 4.76 mm, and the wiggle of the planed jacks within the registers seems right to me: there's just a little bit of play.
Two registers full of jacks:
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