How to calculate changes in the size of a furniture board depending on humidity

Greetings.

The calculation can be applied to planks, beams and any other elements of timber structures.

This question: "How will a wood product behave in the future?" usually interested in both beginners and experienced craftsmen.

To make such a calculation, you need to understand that ideal conditions do not exist, that different types of wood lead itself in different ways, that the change in size depends on what kind of cut the board has - radial or tangential. Changes along the fibers are very insignificant and can be safely ignored.

You also need to understand that such a calculation will be approximate. In practice, changes and deformations of wood may differ in one direction or another from the calculated one.

Finished wood furniture can be used in different humidity conditions. There are also seasonal variations in humidity. Accordingly, parts of wooden furniture can either dry out or expand.

The range of moisture changes in wood can be allowed in the range from 12-14% to 6-8%. That is, we can assume that on average the moisture content of wood can vary by 7-8%.

There are tables of shrinkage coefficients for different types of wood. In the reference books where these tables can be found, it is sometimes difficult for a non-specialist to understand, especially the calculations, which are given as examples, relate to either the moisture content of the wood or the change in dimensions in percent.

In practice, I want to understand how much in millimeters the size of a board or furniture board of a certain width can change.

For this, we can assume that the coefficients of the table show how many millimeters a board with a width of 100 mm changes. when the moisture content of the wood changes by 1%.

For example, a 150 mm pine board was purchased. width in the construction market. The moisture content of such wood is usually about 20%. With natural drying in half a year - a year, its moisture content will be about 12%.

The moisture change is 8%. Multiply 8x0.31 for a tangential cut, we get 2.48 mm per 100 mm board. We multiply by another 1.5 (board 150 mm) we get 3.72 mm. That is, the board will shrink by almost 4 mm.

In practice, I have checked this many times - purchased boards of 150 mm. after drying, they have a size of about 145 mm.

For a furniture board, you can take the average coefficient for a rough estimate. So for pine it will be (0.31 + 0.18) / 2 = 0.25mm. For oak (0.29 + 0.19) / 2 = 0.24mm

That is, pine furniture board 300 mm wide with an initial moisture content of 14% can dry up to 6%. Change 8%. Let's calculate the size change - 0.25x8 = 2 mm. is 100 mm wide. Multiply by 3 (300 mm) and get 6 mm. That is, under the given conditions, the furniture board can dry up to 294 mm.

And thus, the wider the part, the greater the change can be.

And when the magnitude of the changes is known, it is no longer difficult to take into account when designing: the permissible width of the parts, their mating, the method of fastening, etc.

When wooden furniture is made according to ready-made, checked drawings, then in the design of such furniture you can see how changes in the moisture content of the wood are taken into account.

And when you design your furniture yourself, knowing how to calculate the change in dimensions when the moisture content of the wood changes will help make your furniture more durable and reliable.

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Alexander.

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