Mahogany is one of the most popular cabinet woods because of its easy cutting quality, durability, colour and a wide range of its artistic figure.
Ultrasound detects wood defects.
Ultrasound is the name given to the sound waves having frequencies much higher than those to which human ear can respond. These sound waves were discovered in the last century.
One of the sources of the ultrasound is a quartz crystal. When electric current is passed through a quartz crystal, it alters in length and gives off these high pitch sounds.
Ultrasound is known to have found various uses in industry, science and medicine. At some enterprises it is used for welding together aluminium and steel, gold and germanium which cannot be welded by other means. The diamond, being the hardest of all minerals breaks up under the action of ultrasound. Ultrasound can also be used to perform the most precision work. For example, it can bore 1/10 millimetre hole through a diamond.
In fractions of a second ultrasound can wash impurities from the surface of any metal. Variouse machine parts can be washed cleaner than they could have been washed by all the known methods before. Besides the washing is done at a lower cost. Ultrasonic cleaning proved to be especially successful in treating metal parts of irregular shape.
Ultrasound was also found to be very useful in the detection of various internal defects in a metal or wood.
Defects in wood, such as knots, rot, discoloration influence the strength properties of wood and lower its value as a building material. The various defects found in wood differ considerably in their shape, composition and origin. In many cases, and especially with thick lumber, it is difficult to detect such defects visually.
A lot of experiments having been made, scientists came to the conclusion that ultrasound is one of the most effective methods for detecting such defects.
When ultrasound is sent into the wood tested, the reflected sound waves are recorded on the screen of an oscillograph in the form of an impulse. Any sudden change in the pattern of the impulse on the screen indicates the position of the defect. The accurate determination of these defects is very important for the woodworking industry, especially in the production of furniture and export lumber.
Wood as a raw material.
Wood always was and still remains one of the most important natural raw materials necessary to man.
Wood is known to have wide variety of grain pattern, colours and densities. It is easily worked with tools and machines. Unlike many materials, wood is resistant to mild chemicals and insulates against electricity and heat. Indeed, the air inside its cellular structure makes timber the best thermal insulator of all known building materials. That is why wood is widely used in construction and furniture making, in making refrigerators and cooking utensiles.
On the other hand, wood can burn. It can be attacked by insects and fungi. It shrinks and swells with changes in humidity. The strength of wood is not the same in all directions. Therefore, concrete, metals and plastics are considered to supersede wood in many cases.
There are many ways of avoiding deterioration of wood. It can be impregnated with preservatives to guard against biological attack. Flame retardants can protect wood from fire. Chemicals can also be applied to avoid shrinking and swelling. Sometimes it is desirable that wood should first be subjected to mechanical or chemical treatment. The product received will be of much higher strength then the wood itself. Fibre boards and particle boards are good examples of such products.
The amount of waste in wood - working industry is several times less that in mining or metallurgy. Energy requirements for processing wood are also lower. It takes about 1500 kilowatts hours of electricity to produce a ton of sawn timber while the production of the same weight of aluminium requires energy 45 times as much.
Nowadays wood is still often used as fuel. When wood is burnt we lose 90 per cent of its value. Therefore much research effort is now being spent to make this useful material accessible for other purposes. New methods of processing wood are being investigated, new ways of getting valuable products out of it are being introduced.
Over 5000 different types of wood products are manufactured every year. Certain of these have been known for a long time.
Storage of logs in sawmill yards.
At the log yard of the sawmill the logs have to be unloaded, stacked, bucked, sorted and finally moved to the sawmill shop.
As the logs are usually very heavy, suitable mechanical equipment should be used wherever possible. The choice of the equipment depends on the conditions varying from mill to mill.
The layout of any log yard depends upon local conditions. The division of the yard into sections is determined by the work to be carried out in the yard.
If the logs are carried to the sawmill building on rails, the yard should slope slightly down towards the mill building. It is an advantage if the yard is rectangular.
It is difficult to make efficient use of mechanical equipment on square yards. The size of a sawmill yard depends on the volume of timber to be stored and on the method of storage. If several species of timber are to be stored separately the yard has to be substantially larger.
The timber is brought to the yard by road vehicles or by a logging railway. To simplify the unloading of the lighter round logs the road bed should be raised to allow the timber to roll down by its own weight.
For heavy thick logs lifting gear or travelling crane has to be used.
The timber should not rest directly on the ground, otherwise damage from dirt and moisture may result. It should be laid on frames consisting of transverse logs or cants.
Light timber may be stacked on concrete blocks but this is only possible if the weight is not too great. To avoid the risk of the pile of logs collapsing, fewer logs have to be placed in each succeeding tier.
The storage volume can be increased if the sides of the stacks are proved with sufficiently strong braces of wood or iron. Heavier logs of equal thickness may also be stacked in crossed tiers.
The amount of timber which can be stacked will also depends on the length and diametre of the logs. The longer the timber, the more logs may be placed in one tier.
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