Published on 8. May 2013 by Johannes Huefken

Today we want to glue small boards to a board - the so-called piano turret board.

Since keyboards can be frighteningly deformed if some rules are not observed during their production (keys are stuck together and partly twisted), I would like to introduce two rules to you.

To make these comprehensible to you, the first issue is a fundamental phenomenon with wood.

  1. Wood is in motion
  2. The rules
  3. Gluing the board


1. wood is in motion

The following technical percentages of shrinkage refer to freshly cut wood that is completely dried. Freshly cut wood has a water content of approx. 35%.

In addition to the shrinkage data, the picture shows which shapes the wood takes during drying. This effect is the result of the different degrees of shrinkage in radial and tangential direction (sketch of FH.-Eberswalde).

Now we do not use freshly cut wood in instrument making. In addition, the wood is never completely dried even in a dry room. The values show how much wood can change, especially if it is not well dried (7-11%).

Wood can also swell when the ambient humidity increases. Let me give you a small example: If the room has a relative humidity of 30%, the wood will assume approx. 6% wood moisture. If the relative humidity in the room rises to 70%, the wood humidity rises to 13%. That's 7% difference. These 7% are already 1/5 of the shrinkage figures on the sketch, which are based on a moisture loss in the wood of 35% water content to 0% water content in the wood. A board that is 200mm wide and 20mm thick would swell to 204mm x 20.2mm if the moisture content rose by 7% (assuming a board with the annual rings running like in the next photo).

Brettchen mit liegenden Jahrringen. Dabei bezeichnet man die zum Baumkern gewandte Seite als rechte und die zur Borke gewandte Seite als linke Seite. Also befindet sich die rechte Seite des Brettes auf diesem Bild unten.
Fichtenholz mit stehenden Jahrringen – dieses ist ungeeignet für die Klaviatur!

2. The rules

Spruce wood with standing annual rings - this is unsuitable for the keyboard - the spruce boards for the piano table must have horizontal annual rings as in the upper picture. The drill always tries to slip into the soft growth rings of summer growth. Thus, it is necessary to drill through the yearings in the widest possible right angle.

Adaptation of the boards to the future saw joints

Joining the wooden panels:

In the picture you can see the spruce boards that are later glued together. They are on the keyboard drawing. The drawing shows the back end of the keyboard. The width of the boards was dressed in such a way that the glue joints of the boards lie on the future saw cut to match the future saw joints. You may be surprised that I think about cutting out the keys before gluing them together. But a glue joint within a key also means that two different woods are glued together. Each wooden board has its own inner tension. If different woods are forced to get along with each other, it can happen that the key becomes crooked. As a result, the spatius can become uneven, even to the point of clamping keys. In keyboard construction, spatien are the small gaps between the keys.

The picture shows how wood would develop during post-drying. (Sketch from Holzhandel.de)


3. the gluing:

Before gluing, the boards are turned in such a way that the right and left sides always alternate. This prevents the keyboard from becoming crooked. The picture shows how wood would develop during post-drying. (sketch by Holzhandel.de) Glued with little pressure. The glue can be a common wood glue like Bindan or Ponal. Today we had a somewhat complicated section with the wood moisture. If you have become dizzy with so many percentages and my explanations have confused you, it would be a pity. You are welcome to send me questions by e-mail or comment. In the next article we will continue with the keyboard panel. It should be about the keyboard frame in which the keys are guided. 

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