Key Lever Weighting

The balance pins and key frame support the key levers, which move like a playground teeter-totter or see-saw. Without being weighted, the keys tend to be front heavy. So the back of the keys require extra weight to ensure that they fall back down after being pressed. How much weight is needed to ensure they fall back down just right? After some research, I found that about 2.5 grams placed on the front of the key where a finger would normally press down during playing was good enough. Lead is added to the back of the keys to counter-balance the 2.5 gram weight on the front. A penny minted in the U.S. after 1985, which weighs exactly 2.5 grams, was placed on the front of the key. Lead cut from 1/4-inch diameter wire was shifted in position on the back of the key lever until the key was balanced. The lead poses a potential health risk, so gloves are a good choice for personal protection.

After marking the position of the weights in their correct position, 1/4-inch holes were drilled into the key levers. The typical width of the key lever is about 1/2-inch. The holes were drilled to a depth of 3/8-inch using a drill press with a 1/4-inch brad point drill bit. 

The lead often comes as a coil. When being cut to length, the lead has a tendency to deform. The deformity can cause the weight to resist being inserted into the hole. If there is too much resistance, the lead has a tendency to mushroom out at the top when being tapped down with a hammer. (The lead also has a tendency to oxidize quickly; so I keep the lead in a sealed plastic bag). 

One way I found of reshaping the weights back to perfectly round was to roll the piece of lead between two blocks of wood. The lead is soft enough to become perfectly round with only modest pressure applied during the rolling.

After 51 balancing acts, the keys are now properly weighted, I hope! 

Adding Key Coverings - Part 4

After successfully scoring the lines, the edges of the natural keys were rounded with sand paper. A four-way nail file with grit varying from 240 to 3000 (often available in an inexpensive 12 pack) proved to be useful for the rounding. A triangular file proved useful for refining the notch where the rounded edge meets the front decorative line.

After completing the wood surfacing, the keyboard is ready for weighting and leveling!

Adding Key Coverings - Part 3

The boxwood key coverings were flipped over and glued onto the key levers, again with hide glue. 

The decorative lines on the keys were carefully scored using a straightedge clamped to the keys. The X-Acto knife made straight, but very thin lines as before. This time, the lines were widened slightly using the tip of a thin screw driver blade instead of the Japanese saw. The screw driver blade acted somewhat like a mid-19th century American farm plow, which tracked the X-Acto lines easily and widened the lines as desired without slipping.

Adding Key Coverings - Part 2

The boxwood coverings for the natural keys were scored with a knife as a decoration to distract the eye from the joint formed between the front of the key and the skinnier back of the key between the sharps. Later, the side edges will be rounded as shown on the José Calisto harpsichord (Portugal, 1780) residing in the National Music Museum. In the photo of the Calisto harpsichord, the decorative lines are just visible toward the back of the head of each natural key.

http://collections.nmmusd.org/Keyboards/CalistoHarpsichord/CalistoHarpsichord.html

A straightedge was clamped to the keyboard to guide an X-Acto knife that was used to score the lines by hand. The technique worked relatively well, except that the X-Acto blade produced a line that was too thin to be seen easily. Real trouble began when I tried to use a very fine tooth Japanese-style saw to enhance the line. The photo below shows my attempt to score multiple keys at once using the saw held against a straight-edge clamped to the keys. On the third line, the straightedge slipped, resulting in an unacceptably crooked line.

Fortunately, the key coverings, which were bonded with hide glue, were able to be removed. The first attempt to remove the key covers was to bake the keys in an oven to raise the wood temperature to the melting point of the glue (about 145℉). Setting the oven to 250℉ did not get the wood hot enough for the glue to melt. Baking at a higher temperature was considered too risky, as the wood might start to smolder. So a second attempt at melting the glue was made by applying steam to the joint directly from the spout of a teapot using a towel wrapped around the key lever as a kind of steam box. Again, the glue bond would not get hot enough to melt. Finally, the key levers were immersed in water that was first heated to a boil and then poured into a glass jar, which itself was immersed in a heated pot also filled with water to keep the temperature from cooling off too fast. Usually, two minutes of soaking was enough to melt the hide glue. In some cases, a little mechanical impact with the tip of a screwdriver was needed to break the bond. Hide glue is strong stuff!

The image below shows the keyboard after the boxwood key covers were removed. In some places, the decorate arcade caps, which were bonded to the front edge of the keys using Tightbond yellow glue, did not survive the differential expansion that resulted when the two wood grains running in different directions absorbed water from the soaking.

Hopefully, the key coverings can be flipped over, re-glued, and re-scored with much cleaner lines.

Adding the Key Lever Coverings - Part 1

The natural key levers are covered in boxwood, in keeping with the Iberian harpsichord tradition of light covered naturals and dark colored sharps. The boxwood was ordered in strips 1/8-inch thick, which were edged to the appropriate width (one width for the natural heads and another width for the space between the sharps. The sharps were covered with Gaboon ebony, cut from a blank 1/2-inch thick. The front edge faces of the sharps were sloped and the edges and corners were slightly rounded. The boxwood and ebony were glued to the key levers using hot hide glue (Brooklyn Tool and Craft, 192 gram strength). A straightedge was used to evenly align the ebony sharps relative to the front edge of the keys.

The Case

The design for the case has a steeply curved bentside due to the strict Pythagorean scaling.  The choice of method for bending depends on the radius of the curve needed and the thickness of the piece.  If unconstrained, a bend will cause the outer fibers to be pulled apart (tensile strain) and the inner fibers to be squeezed together (compressive strain).  Poplar in a dry climate can withstand a tensile strain of about 1.5%.  For the small Iberian case the value of the strain, approximately equal to the thickness of the lumber divided by the radius of curvature,  is about 1 cm (0.5 in) divided by 45 cm (18 in) = 0.022 (or 2.2%).  A special fixture that keeps the length of the outer part of the curve fixed, such as a steel band clamped at the ends, can avoid any tensile stress by subjecting the lumber through the whole thickness to compression.  Wood can tolerate a much greater strain in compression, such as up to 30%.  However, a machine to apply the necessary compressive force would require significant time and expense to construct.

A practical alternative is to build the side from a number of thinner layers laminated together.  Using three layers, each layer would be subjected to a strain one-third that of the full thickness piece, or about 0.7%.  Each layer can be bent more easily and with much less chance of failing.  Closely spaced wood bars that are clamped across the laminations apply the bending force needed to follow the required curve.  The hitch pin rail that is glued to the inside of the benstside is constructed in the same way.

The image below shows the jig holding the bent hitch-pin rail.  The benside itself, already constructed, rests on top of the jig.

The next image shows a view of the same from above.

The next steps are to trim the bentside and hitch-pin rail to the proper width and length.

Cutting Apart the Key Levers

The keyboard plank is prepared for cutting apart by making a 3/4 inch wide by 5/16 inch deep rabbet along the rear of the keyboard plank.  The thinner section helps prevent the rear of the key from binding as it slides up and down on the rear guide pin

The keys are cut apart by starting with the lines between the B and C keys.  The lines between the B and C keys and the E and F keys extend the full length of the key plank.  A band saw is often used to make the cuts freehand.  As an alternative, a Bosch jig saw with a 0.03 inch thick blade was guided using two pieces of straight edged hardwood to guide the saw.  A scrap piece of lumber placed underneath the keyboard plank keeps the two halves in the same plane after the cut is finished.

The Bosch jig saw makes a thin, precise cut as seen in the following two images.

After cutting from front to rear on all the lines between B-C and E-F, the remaining cuts could not be made by clamping the straight-edged hardwood guides on both sides of the key.  I had finished making an adapter for the jig saw to allow using the E-Z Smart guide system designed for circular saws.  The following image shows the aluminum guide rail and the jig saw slipped into the base-plate.  One virtue of the E-Z Smart guide is that a fraction of a key width is sufficient to provide enough clamping area to hold the work to the bottom of the guide.  The threaded posts used in the clamps also serve as legs that hold the rail above the work table with the work clamped to the underside of the rail and enough clearance for the blade of the jig saw.

The next image show a close-up view of the blade cutting between G and A keys and the anti-chip edge of the guide.

After all the cuts are made the keys are still in groups of five and seven as shown in the following image.  A line 3/8 inch beyond the top of the naturals is drawn to mark waste area.  A hand scroll saw is used to cut the keys apart on a diagonal line between the existing cuts.  The hand jig saw is used to square off the final cuts.

The following image shows the keys after being cut apart and loosely laid out on the workbench.  The cut out between the sharps and the naturals will eventually be covered with the tops of the sharps.

A key lever mortise punch, a specially designed tool available at the Instrument Workshop, is used to enlarge the hole for the balance rail pins to allow the key to rock.  The tool is basically a triangular wedge of hardened steel with a cylindrical tip the diameter of the balance rail pin.  A little Caress bar soap rubbed on the punch makes it easy to pull out after being hammered into the wood.  If this is your first time make sure you practice on some scrap stock.  Also, be careful to align the punch to make a slot closely parallel to the long dimension of the key.  Here are the keys after placing on the balance rail pins showing the slot made with the punch.

The hole drilled at the back of the keys is used as a guide for making a slot of the same diameter.  The slot must be only a few thousands wider than the rear guide pin to prevent excessive sideways movement of the key without binding.  

The final images show the keys after placing them on their pins in the key frame.  A piece of scrap lumber is placed on the back of the keys to show them in their resting position.  (The short focal length camera lens added some distortion to what are actually evenly spaced keys).

Next step is to add tops to the sharps and decorate the top surfaces with a veneer.  I'm going to use solid Gaboon ebony for the tops of the sharps and boxwood to cover the naturals.