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Wood is an important and precious raw material. It can be used for many purposes, such as for the manufacture of
- furniture,
- window frames,
- doors,
- interior furnishings,
- vehicle and ship components.
Depending on the intended purpose of use, the logs cut down in the woods are
- processed in veneer mills into veneer by chip-forming and chip less cutting,- sawn in saw mills by cutting with the grain. Cutting-off of the rough edges is called edging.
Figure 1 Cutting-off of the rough
edges (1 unedged-sawn, 2 edged)
The sawn wood is called sawn timber which, depending on the different size and shape, is classified in the types of sawn timber as per table 1:
Table 1: Types of Sawn Timber
|
Sectional shape | |
Designation |
Features |
Thickness in mm |
Width in mm |
|
1/1 |
|
squared timber |
edge |
100 |
100 |
|
1/2 |
|
frame timber |
edged |
38 up to 100 |
2 × thickness |
|
1/3 |
|
board |
unedged-sawn |
16 |
2 × thickness |
| |
| |
edged |
16 up to 100 |
75 |
|
1/4 |
|
lath |
edge |
16 up to 100 |
75 |
|
1/5 |
|
ply |
unedged- sawn or edged |
6 up to 15 |
75 |
|
1/6 |
|
strip |
edged |
6 up to 15 |
75 |
After cutting-down and sawing the logs, the wood contains a
considerable quantity of water.
The water contained in the wood (wood moisture content) influences the physicotechnical properties of the wood.
Wet wood is difficult to work and changes its shape.
Any change of the relative air humidity, at a certain temperature, results in a change of the wood moisture content. This law applies to any type of wood and is the basis for maintenance, storage and drying of wood as well as for processing and working/machining of wood.
The moisture content in the wood is measured by means of the following methods and measuring instruments:
- Electrical measuring methods with resistance measuring instruments ace. to the principle of D.C. resistance measurement.- Kiln method (weighing method), where the water quantity contained in wood samples (small pieces of wood) is evaporated at 105°C in special drying kilns and determined by comparison weighing of the wood samples before and after evaporation.
The measure of the wood moisture content is the moisture expressed as a percentage (in %) of dry wood substance (weight in g). It can be calculated ace. to the following formula:
Task:
Calculate the moisture content of wood (in g and per cent) when
- the wet weight of a wood sample is 350 g,
- the dry weight of the sample is 200 g!
Depending on the purpose of use, the wood must have a certain moisture content when being worked/machined to retain
- its usability,
- its shape,
- its size.
Therefore, the excessive water is to be removed. This is done by controlled drying of the sawn timber.
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