Thickness

Back guide of the clavichord key levers

Key levers of the clavichords are required to be noise free and less sensitive to environmental changes, such as humidity and temperature. Vast majority of the historically made clavichords have so called rack/tongue mechanisms, which use small piece of wood, metal or bone, sliding in slots cut on a diapason rail underneath the hitch pin rail. It is extremely difficult to reduce the noise below the acceptable leve, due to the friction between the tongue and the slot. From the noise stand point, C. G. Hubert's silent action is superior. However, as two pins at the both sides support the back end of the key levers, it has a disadvantage to be directly affected from the environmental change.
The guide shown on the left is developed from both low noise level and low sensitivity to the environmental changes. The metal pin does not contact to the wood, but only with the leather attached on the top and bottom of the key lever. Any changes of the key lever width, due to the environmental changes, do not directly affect to the width of the slot cut on the leather. At the higher notes, key lever width may not be enough to put the pin on its center. In this case, the leather pieces on the top and the bottom are attached overhanging from the key lever width and pins at the both sides support the key lever. Environmental changes of the key lever width are always below the acceptable level as any changes is proportional to the small key width.

　　

Put a bending fixture on an outdoor camping stove, then one end of the bentside board is clamped on the fixture and the other end is pulled down to give a proper curvature. To bend the board for a large area, the clamping position can be repeatedly moved as needed. Bending compliance of the board cannot be homogeneous and the board can be slightly twisted after removing the force and the temperature. If it happened, the pulling force at the both sides of the board can be independently adjusted. Using an industrial heat-gun, locally applying additional heat is sometimes effective for correcting the twist. Careful management of temperature is necessary not to burn the board.

The photo on the left is showing the bending process for a double bentside of a Germann harpsichord.

Over-spun wire　

When a higher wire tension is required, either longer speaking length, thicker wire or heavier materials can be employed. Brass strings, having heavier specific gravity than iron, need to be stretched with higher tension to achieve the same pitch. If materials heavier than brass is used, instruments can have strings with even higher tension. However, from the practical point of view , no materials other than brass or other copper alloy can be found. It is difficult to have harmonics rich sound with too thick strings. Then, an over-spun wire, is used. It provides heavier effective density by winding a soft and heavy wire over the core string.

String tension can be calculated with pitch(frequency), speaking length and weight per unit length of the wire. The weight per unit length is calculated with diameter and bulk density of the wire. It is convenient to use an effective weight per unit length calculated with an apparent bulk density of the over-spun wire. The formula to give the apparent density can be found here.

Detailed description of the winding machine can be seen by clicking below photos.

A hand drill is used as a power source. The lever in the front is a power switch. Two individual gear boxes, located at right and left ends, are connected with a hexagonal bar.

The left gear box, connected with a hexagonal bar, is fixed on a workbench with a clamp. The left gear box can be clamped at any position according to the wire length. The clamp is also convenient to apply tension to the core wire.

A magnetic device to measure sound board thickness

　Assuming a detailed explanations of how the device works is not widely available, though sometimes it is referred as "magnetic method", a schematic drawing, showing the principle of the measurement, is provided below. Measurement accuracy is not as good as those with dial gauges. However, the accuracy can be statistically improved by measuring several times and average them.

The schematic drawing shows only the principle. The blue arm in the left is slowly pulled-up. As soon as the magnet, attached to he coil spring, separates from the board, read the thickness of the board, indicated on the scale (red colored). Another magnet below the board still stays there. The blue arm needs to be pulled more, if the board is thinner.

When the outer case, which has a scale, is pulled up, the coil spring is streched and upword force is applied to the magnet, above the soundboard material. When the upword force becomes greater than the magnetic attraction force with another magnet under the board, the upper magnet comes apart from the board. At this moment, the scale indicate the thickness of the board.

Wire Tension

Wire tension can be calculated with following formulas.

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