The registers

Registers (or guides, according to some) are long strips of wood with a series of parallel slots. These slots hold the jacks that carry the plectra up to the strings. The term register is also used to describe the entire set of jacks and strings that make up a stop in the harpsichord: one speaks of "the 8-foot register", for example.

To make the registers, I first cut 5 strips of walnut to a length of 48" and a width of 20 mm. Next, at the router table, I set up to find the mid-point of the width of the strips, as shown below. I used a pointed centre-finding bit that can fit either a router collet or a drill press, and lined it up with a couple of scribe marks that showed the centre line:


Once the fence was properly positioned, I routed out most of the interior of each strip with a 14 mm router bit. The result was a channel with walls 3 mm thick on each side:


The Trasuntino has two registers (two stops), both playing at the same pitch. The plucking points of each are different, however, and this gives a distinct tone quality to each. Guitarists will be familiar with the concept of tone quality depending on where along the string they choose to pluck or strum. It's the same with the harpsichord: the closer the plucking point is to the player, the more nasal the tone, and the farther, the more "round", for lack of a better word.

A total of 4 strips is required because each register must be paired with a lower guide directly below, otherwise the jacks would tend to tip sideways. I cut one extra strip just to be on the safe side.

The original Trasuntino has 50 notes, and I decided to put one extra pair of strings in at the treble so the top c''' wouldn't be lost when shifting the keyboards to high pitch. So each register and lower guide needs 51 parallel slots. Cutting these slots is actually quite tricky, because the spacing must be very accurate. Any minuscule error in positioning adds up over 51 slots to cause a significant discrepancy. A mistake of just 0.1 mm—barely visible to the naked eye—would lead to a cumulative error of 5 mm (almost 1/4") by the time all the slots were cut.

Spacing is also important because the slots reflect the octave width of the keyboard, which in my instrument will be 6-1/2", slightly less than the 6-5/8" in the original: this is a bit much, I think, as it exceeds the octave span of the modern piano and could be a bit uncomfortable. To be precise, the spacing along the register is actually a whisker more than 6-1/2" because the register is tilted by 8 degrees due to the tapering of the wrestplank. If you know trigonometry, you can figure out how much more: it's about 1.6 mm per octave.

I thought a great deal about what kind of jig would make the slots. One way to do it would be to make something like a box joint jig for use at the table saw (box joints are often used to join together parts of a drawer). Stacked dado cutters are set up on the saw to cut slots of the desired width. The jig is fixed to the mitre gauge and has a peg the same thickness as the slots. One slot is cut, then the slot is fitted over the peg, which has been carefully located so that all subsequent slots will be cut at the correct spacing. The whole thing looks like this:


(The image above is courtesy of Dan at Song of the Great Lakes Woodworking Studio, and is used by permission. Check out Dan's page: he's a very skilled woodworker and banjo maker. Thanks, Dan!)

I'm showing you this approach only to tell you that I decided not to use it. My slots were cut at the router table and I ended up using a jig of my own design. One reason was because I wanted to make jacks about 3/16" thick and just under 14 mm wide, and I was able to obtain a 3/16" router bit without difficulty. The other reason was that it seemed that, with a little trigonometry, I might be able to set up the jig and avoid the lengthy trial-and-error process that these types of jigs inevitably entail.

Here is the jig:



An aluminum fence—a kind of "angle iron" extrusion with an L-shaped cross-section—4 feet long, 3/4" wide on each side and 1/8" thick is clamped to the router table at each end. Two strips of wood clamped to the table touch the fence right beside the router bit and keep it from flexing when the jig is used.

At one end, a 1/4-20 screw run through a threaded insert is used to provide fine adjustment capabilities:


The screw has 20 threads per inch, so it's easy to calculate how much it moves with each full turn or portion thereof. Since the screw is at one end of the fence and the router bit is pretty much exactly in the middle, adjusting the screw mechanism a given amount will result in half of that amount at the bit's location.

The registers are tilted by 8 degrees, so I had to make an angled sled that rides along the aluminum fence:


A strip of oak provides the surface that rides against the aluminum, and a piece of poplar is joined to this at a 98 degree angle. This angle is kept rigid with a brace and an added thick block in behind. The poplar has a step cut in it, the depth of which is a bit more than the thickness of the remaining material in the register blank. This is where the register sits while the slots are routed. For fine adjustment of the exact depth of cut, I stuck on layers of green masking tape.

My 3/16" router bit has a fairly short cutting edge, and it also has a 1/4" shank, so I needed to provide a clearance slot for the bit to pass all the way through the width of the register. Because the shank of the bit has no cutting edge, I couldn't just fire up the router and let the bit cut its own slot into the jig. Instead, I nibbled away a good-sized slot at the bandsaw Next, I found a piece of scrap from which I could cut small sections that would fit in back of the slot and act as sacrificial backer blocks to minimize tearout as the router bit exited the register blank. You can see the slot and the sacrificial piece stuck in it below:


You may be scratching your head about all these details, so let me explain the overall idea. The problem in cutting all the register slots is to very accurately set the gap between the fence and the router bit. I determined that my slot spacing had to be 13.89 mm, in order to match the changed octave spacing I wanted to use. Now, there's no way anyone can measure such a small distance properly in this type of setup: the router bit is round, the cutting edges have to be exactly located, the measurement has to be made along a line exactly perpendicular to the fence, and so on. There's virtually no room for error, and even with vernier calipers I doubt if anyone could do this accurately. So this jig takes advantage of the fact that any particular movement of one end of the fence is effectively halved when measured at the router bit. That means you can make a larger and easier movement at the end of the fence and have a 50% smaller movement at the router bit. The screw adjustment (1 turn gives 1/20", or 1.27 mm) allows even finer control: I can easily make 1/8" of a turn, which moves the end of the fence 0.16 mm, which translates into a movement of just 0.08 mm at the bit.

Enough said! Now for the jig in action.

The process starts with one slot already cut in the register blank. The blank is set in the jig, which is pressed against the fence, and the slot is hooked over the fence with the slot's left edge against it. When the jig is pushed forward along the fence, the next slot will be cut at a controlled location. Then that new slot is hooked over the fence, the third slot is cut, and so on. For the register blank to sit securely in the sled, the slots must be cut at least as high as the distance between the green tape and the top of the aluminum fence. The router bit must be raised as high as the fence (3/4").

Why does one slot already have to be cut? The first slot is a fair distance from the edge of the register and the fence would interfere in trying to cut this slot. So each register blank has to get the first slot cut before the jig is set up. I did this with the mitre gauge tilted to 8 degrees. A backer strip supports the register blank, which is kept from shifting around with a bit of double-sided tape:



A pencil mark shows where the first slot will be cut:



Slotted:


The next picture makes all of the preceding verbiage clear. It shows the jig in action: the sled carrying the register blank is pressed against the fence, the previously cut slot is hooked over the fence and the router bit is ready to cut the next slot.


Note that the fence is actually thinner than the width of the slots, which is unlike the setup used in the box-joint jig discussed earlier. This discrepancy is unimportant because all I needed to do in compensation was to press the edge of the slot firmly against the fence as I pushed the sled forward.

Now I must admit that, despite all the clever thinking that went into my jig, I had to do a surprising amount of trial-and-error slotting on scrap pieces before I finally nailed the spacing I wanted. Initially I used a bit of trigonometry to calculate the position of the far end of the fence, which I located with respect to a straight line drawn along the entire router table through the centre of the bit. This was not as accurate as I hoped it would be, so I used it only as an initial position and dealt with the screw adjustment thereafter. In retrospect I think the fence, being of aluminum, is too flexible and that must have thrown off all the careful adjusting I tried to do. Even clamping those two wooden strips behind the fence didn't help much. In fact, I had to watch how I clamped them in place: I could inadvertently bend the fence simply by pushing the strips up against it. So getting the job done was a lot more frustrating than I expected. But, eventually, I ended up with 4 registers that had a first-to-last slot total spacing of 693 mm. This is only a whisker off from my theoretical target of 694.5 mm, and is within reasonable limits of accuracy.

The end product, needing only a bit of careful sanding to remove fuzz and fibers left from the routing:


Eventually these will be cut to final length—they're much too long right now—and assembled in pairs. In some instruments the lower guides are installed underneath the wrestplank, but I'm going to approximate a box guide construction, in which a single rather thick piece of wood has slots punched through it. I'll take pairs of registers and join them together, with spacer blocks inside the long interior groove to keep the two pieces properly aligned and spaced.

In designing this jig, I had to watch out for a couple of potential traps. First, the fact that the slots are routed with the register upside-down, coupled with the 8 degree tilt of the whole assembly, renders everything non-symmetrical. The 8 degree tilt is such that when the registers are in the instrument, the left end of the register is farther away than the right. But I had to make the sled with an 8 degree tilt the other way, because otherwise when the registers got turned right side up, the slots would not be parallel to the sides of the instrument as they had to be. I had to play with a paper mockup of the register with a dozen slots cut in it just to make sure of this. Secondly, I realized I had to adjust my spacing calculations to account for the diameter of the router bit. The slot spacing is left edge to left edge, whereas my original calculations were from left edge to the centre of the router bit. Luckily I realized this not long after starting to design the jig.

The registers are a critical part of the instrument because they determine the shape of things to come. Using the registers and the known lengths of the strings, the curvature of the bentside of the harpsichord can be determined. When the registers are laid across the panel that will become the keyboard, the spacing of the backs of the keys can be properly marked. The positions of the bridge and nut pins are derived from the registers as well. If there are any minor flaws in the placement of the slots, those flaws can be transferred to these other parts, thereby cancelling out any misalignment.