In the past two months, I've continued along the lines suggested by the previous post, partly restringing the back 8' register in the lower regions and voicing the whole instrument down a bit. As was the case for the front 8', this has changed things for the better. Both rows of jacks now play well and repeat reliably.
With the overall strength of voicing altered, I tweaked the separation between both rows of jacks by revisiting the jack end screws. I ended up giving each another half-turn, so the back 8' now sits beneath the strings by the equivalent of 4 half-turns, and the front 8' by 8 half-turns. This gives a tiny bit more slack to the mechanism.
It's important to recognize that the primary means of controlling the interaction of the two registers when both are turned on is by means of consistent voicing, and not indiscriminate cranking of these end screws. The screws are meant to adjust how long it takes for the plectrum to rise up and contact the string from beneath. There may be small differences here and there due to the quill angle in the tongue, which are a result of the quill mortise punching process, and an extra half-turn or two of the end screws can correct for this, but that's the extent of their usefulness.
Showing posts with label jack. Show all posts
Showing posts with label jack. Show all posts
Thursday, December 31, 2009
Thursday, September 10, 2009
Debugging phase
Construction of the harpsichord is essentially done. Only the music desk remains to be made at some point.
The harpsichord has been playing since the end of August. I'm now in a kind of debugging phase in which I play the instrument and track down things that need improvement. So far I have
The harpsichord has been playing since the end of August. I'm now in a kind of debugging phase in which I play the instrument and track down things that need improvement. So far I have
- increased the depth of touch about 1/32" by substituting a thinner cloth for one of the two layers under the jackrail
- shimmed up the keys with more punchings at the balance pin to help increase the depth of touch
- moved both of my gap spacers which, despite my best efforts, had not been located exactly under the strings and were slightly rubbing the nearest jacks
- tweaked the off positions of the registers to make sure the plectra cleanly miss the strings
- glued little cloth squares between the registers so they don't touch themselves or the edges of the gap
- done a little remedial voicing to quills that feel stiffer or sound louder than their neighbours
The main outstanding issue to be resolved is the troublesome repetition of notes in the bass. In this region the amplitude of the plucked strings is large. Damping can be problematic as a result, and the tongue is often flung backwards quite energetically when the jack descends and the plectrum touches the string. So I am experimenting with damper shapes, stiffer springs and various other factors in an effort to resolve this problem.
Sunday, June 28, 2009
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.
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.
Friday, June 26, 2009
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.
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.
Tuesday, June 2, 2009
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:
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:
Saturday, May 30, 2009
Jack bodies, part 1
Now for a very critical part of the instrument: the jacks.
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.
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:
Subscribe to:
Posts (Atom)
