Weight of Timbers
The weight of piece of wood depends on several factors. It will obviously vary with the amount of water it contains. For this reason it is important that, when the weight of timber is stated, the moisture content at which the weight determination was made should be cited.
The weights of timber when seasoned refer to a moisture content of 12%. The weight of a timber at any other moisture content within the range of say, 5 and 25% can be estimated with fair accuracy by adding or subtracting 0.5% of given weight for each 1% moisture content above or below 12%.
In all species a considerable variation in weight is found to occur apart from that arising from differences in amount of contained moisture.
It is necessary to stress here that shrinkage and movement are not directly related to one other. For example, it is possible that a wood may shrink quite appreciably in drying from the green to 12% moisture content yet it may undergo comparatively small dimensional changes when subjected to a given range of atmospheric conditions in service. The reason is that the so called ‘fibre saturation point’, or moisture content value at which appreciable shrinkage begins to take place, varies considerably with the different species. Furthermore, the moisture content change of one timber corresponding to any given range of atmospheric conditions often differs considerably from that of another.
Shrinkage values therefore are useful only in estimating roughly the dimensional allowances necessary in converting green material. It must be pointed out, however, that a further allowance for possible loses owing to distortion must be added.
Movement values, on the other hand, give some indication of how the dried timber will tend to behave when subjected to atmospheric changes in service. A so-called ‘stable’ timber is the one that exhibits comparatively small dimensional changes in passing from the 90% to the 60% humidity conditions.
Finally, it may be remarked that the moisture content values in the 60% humidity conditions correspond roughly to the average values likely to be attained by wood exposed to normal indoor conditions. For most timbers, this average will be of order of 12-13% moisture content but for some, e.g. afzelia, it will be more nearly 9-10% moisture content.
The following classification of the steam bending properties of woods has been adopted by the Laboratory and for purpose of comparison, it may be noted that a very good bending species such as home grown beech (Fagus Sylvatica), specimens 1 in. thick may safely be bent to a radius as small as 1.5 in. or a specimen 2 in. thick to a radius of about 3.0 in.
Radius of curvature (in inches) at which breakages during bending should not exceed 5%
Classification of bending properties (material supported and 1 in. thick)
Shrinkage and Movement
Shrinkage measurements in the tangential and radial directions, which were obtained in kiln drying (from green state to a moisture content of 12%) of material sawn through and through are expressed in fractions to the nearest 1/16 in./ft., but, to increase their usefulness, they are also given as percentages.
The term movement is used in referring to the dimensional changes that take place when seasoned timber is subjected to changes in atmospheric conditions.
To determine the movement values quoted, test samples were conditioned first in air at 90% humidity, and then in air at 60% humidity, the temperature being 77 degree Fahrenheit in both cases. The moisture content values of samples when in equilibrium at two humidities are given, as are also the movements corresponding to the particular moisture content range.
Wood Bending Properties
In accessing the bending properties of timber, the most important factor is the minimum radius of curvature at which a reasonable percentage of faultless bends can be made for a given thickness of clear material. This radius varies according to weather the timber is bent with or without a supporting strap after a suitable softening treatment, or is bent cold in form of thin laminae at a moisture content of about 12%. The ratio R/S where R is the radius and S is the thickness of wood, at which the breakages during bending do not exceed 5% is determined by test.
Other assessments are obtained from observations of end pressures, bending movements and general behavior of selected pieces bent to a standard radius of curvature.
Classification of timbers according to their steam-bending properties is however; based mainly on minimum bending radius of sound, clear specimens 1 in. thick at moisture content of about 25%. The specimens are subjected to saturated steam at atmospheric pressure for a period of not less than 45 minutes before bending.
|Radius of curvature (in inches) at which breakages during bending should not exceed 5%||Classification of bending properties (material supported and 1 in. thick)|
Less than 6
Bends of smaller radii than those given may be obtained and utilized if, for example, certain bending defects that may then occur can be removed in the final machining and finishing operations. On the other hand,, it must be stressed that faultless bends of the radii listed for various species can be produced only by using selected material and efficient bending methods.
The term durability generally refers to resistance of a timber of fungal decay and it is used in this sense. Durability is of importance only where a timber is liable to become damp, as for example, where it is used out of doors. It is of no consequence where a timer is used for purposes such as furniture where it can always be kept dry, because, under these conditions, wood-destroying fungi are not active.
The durability of most timbers varies a great deal and even pieces cut from the same tree will often show wide differences, so it is only possible to speak of durability in appropriate terms.
Except otherwise stated, the durability refers to heartwood, the sapwood of almost all timbers is either perishable of non-durable. It is essential to remember this when dealing with timbers such as oak, which may sometimes contain a high proportion of sapwood and therefore, will then be much less durable than their grading suggests.
When choosing a timber fir a structure which is to be exposed to conditions conductive to decay, it is of utmost importance to decide before the timber is acquired, how the required durability is going to be obtained. Usually there is a choice between using a naturally durable wood and a less desirable to select one that can be easily treated.
This is particularly important where a very long life is required. Where a timber has to be selected for other properties, or where the choice is limited by price or availability, the question of durability is often neglected and there is then a risk that timber chosen may not be durable enough in its natural state nor permeable enough to allow adequate preservative treatment.
Amenability of Preventive Treatment
The ease with which a timber can be impregnated with preservatives is important when it is to be used under conditions favorable to decay or to attack by insects or marine borers, as, for example, for poles, sleepers, or piling. Only a few timbers are durable enough in their natural state to give long service when used for such purposes and it is now common practice to employ preserved timber, which is not only cheaper but if preservative treatment is properly done, will usually outlast a durable wood.
It is not possible to preserve all timbers equally well. Some are virtually impenetrable and cannot be given a satisfactory treatment, whereas others are permeable and can be heavily impregnated with preservatives. Where a long life is required under exposed conditions it is essential therefore to chose a timber which can be well impregnated.
In this connection it is important to remember that the sapwood of a timber, although nearly always perishable, is usually much more permeable than the heartwood. Consequently, round timbers containing an outer band of sapwood can generally be impregnated much more satisfactorily, and can be made to last longer than sawn material of the same species.
The terms used to describe the extent to which a timer can be impregnated with preservatives are defined as follows:
These timbers can be penetrated completely under pressure without difficulty, and can usually be heavily impregnated by open tank process.
- Moderately Resistant
These timbers are fairly easy to treat, and it is usually possible to obtain a lateral penetration of the order ¼ - ¾ in about 2-3 hours under pressure or the penetration of a large proportion of the vessels.
These timbers are difficult to impregnate under pressure and require a long period of treatment. Incising is often used to obtain a better treatment.
- Extremely Resistant
These timbers absorb only a small amount of preservative even under long pressure treatments. They cannot be penetrated to an appreciable depth laterally and only to a very small extent longitudinally.
It should be realized that particularly in the case of denser timber, drier material will have greater resistance to cutting, increased dulling effect on tools and more tendency to produce tool vibration. The increased brittleness, with lower moisture content, is however of assistance in planning timber having wavy or interlocked grain, as the chips break more easily and such picking up as occurs has less depth.
For continuous production with some of the refractory timbers, reduction of spindle speed may be needed, particularly when charring or undue dulling occurs. Except where the saw or the cutter spindle is directly connected to the driving motor, the required speed can, of course, be obtained by altering pulley ratios, when practicable. It is seldom possible to change the angle at which a cutter is mounted in a machine and alteration of cutting angle has to be made by honing or grinding a bevel on the leading face of the cutter.
The blunting effect of abrasive timbers is most troublesome in sawing. An increase in the output of ordinary saws cutting this class of timber is obtained if the teeth make a definite cut and do not merely scrape their way through the wood.
In practice, this is achieved by the use of:
(a) saws having fewer teeth (i.e. longer pitch)
(b) higher rates of feed, and
(c) lower saw speeds, than normally employed.
Method (a) is useful in dealing with hardwoods of medium density and, as saws having fewer teeth cut more freely, an increase in feed speed is often possible. Dense hardwoods may tend to overload teeth of long pitch and the most suitable means of minimizing the difficulty is often a reduction in saw speed.
Abrasive timbers are cut best on power-fed machines and pauses in the feed are detrimental to the performance of the saw. The blunting effect of an abrasive timber increases as the moisture content of the timber is reduced. The methods described above may therefore produce only a small improvement in saw performance if the timber is dry. The use of teeth tipped with hard metal such as tungsten carbide is often the only way of dealing satisfactorily with such material.
Veneer and Plywood
Figured veneers for decorative purposes are usually cut from selected fitches in slicer, a process that is feasible with almost all species.
For economic plywood manufacture, however, veneer must be produced by rotary cutting in a lathe, and a continuous supply of high grade logs is essential.
A species that peels well may still prove unsuitable on other counts, such as excessive weight, low yield of clear material for faces, poor drying qualities, color variation, lack of smoothness or a tendency to wrap in the finished board.
Nely on April 29, 2015:
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Kim on June 20, 2012:
Properties of timber
Chandra Haekal on July 02, 2011:
It's Nice info guys..
Can I get your more info?
FuzzyCookie (author) on September 02, 2010:
Hi LeanMan, thank you for stopping by and appreciating this information :) cheers!
Tony from At the Gemba on August 23, 2010:
I love working in timber and have built many structures over the years to extend my home and garden. Great informative hub, well done.