Wednesday, January 5, 2011
Wood
Defination of wood:
Wood is an organic material, a natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists compression. In the strict sense wood is produced as secondary xylem in the stems of trees (and other woody plants),or The secondary xylem of trees and shrubs, lying beneath the bark and consisting largely of cellulose and lignin. the hard fibrous lignified substance under the bark of trees ,
Types of Wood:
There are two types of wood.
1. Soft wood
2. Hard wood
Characteristics Of Soft wood:
Softwood is from an evergreen or coniferous (cone-bearing) tree. Common varieties are pine, fir, spruce, hemlock, cedar and redwood. These woods are mostly used in the home construction industry. Cedar and redwood are excellent choices for outdoor projects, while pine is often used for "Early American Country Style" furniture.
Pine and most other softwoods will absorb and lose moisture more than hardwoods so are not as stable. Purchase the lumber at least two weeks before starting your project and keep it indoors. Place stickers (small pieces of scrap wood) between the boards to permit good air circulation around each piece of stock. This will allow the wood to reach an equilibrium with the indoor environment which reduces the likelihood of dramatic wood movement after a project is complete.
You will generally find only two grades at most suppliers: select and No. 2 common. The common grade allows tight, solid knots in the face of the board. For furniture applications the select grade is the better choice as it is free from most knots, though tight pin knots and small resin pockets are allowed.
You can achieve a more uniform stained surface by applying a wood conditioner to pine first, it's a good idea to run a test first on an inconspicuous part of your project to make sure you're happy with the results.
You will generally find only two grades at most suppliers: select and No. 2 common. The common grade allows tight, solid knots in the face of the board. For furniture applications the select grade is the better choice as it is free from most knots, though tight pin knots and small resin pockets are allowed.
You can achieve a more uniform stained surface by applying a wood conditioner to pine first, it's a good idea to run a test first on an inconspicuous part of your project to make sure you're happy with the results.
You will find that softwoods are sold in standard thickness and widths, for example a 1 X 4 will be 3/4" thick and 3 1/2" wide similar to construction materials. The material will usually be priced per lineal foot and the price will increase accordingly for the wider boards.
Panels made from strips glued together and sold as shelving are also available, these range from 12 to 24 inches wide and 2 to 8 feet in length.
Characteristics Of Hard wood:
Hardwood lumber comes from deciduous trees, the ones that shed their leaves annually. Popular domestic species are oak, maple, cherry, birch, walnut, ash and poplar. Of these common native hardwoods, only red oak and poplar are usually stocked in home centers and lumberyards, the others have to be obtained from speciality stores. The material stocked at home centers and lumberyards is usually sold in similar dimensions to softwood and by the lineal foot as well.
At speciality stores the thickness of hardwood lumber is specified in quarters of an inch, measured when the wood is in a rough state. The thinnest stock is 4/4, representing 1 in., and the thickest usually available is 16/4, representing 4 in. Rather than being milled to specified dimensions, like pine, hardwoods are sold in random widths and lengths.
Hardwoods are sold by the board foot, which is defined as a square foot of rough lumber that is 1 in. thick. If a board is thicker than 1 in., the dealer multiplies the square footage by the thickness to arrive at the sale price. An 8/4 board will therefore cost twice as much as a 4/4 board of the same size.
Working with hardwoods is quite different from working with pine, you cannot drive a screw through hardwood lumber without first boring a pilot hole. Cutting and planing hardwOak and ash, are known as open-grain woods. These species have alternating areas of relatively porous and dense wood, when stained the open-grain areas absorb the color readily while the harder areas are more resistant. This accentuates the grain patterns, creating a dramatic effect.
Cherry, maple and birch are closed-grain woods, demonstrating a more uniform texture throughout a board. Poplar is also a closed-grain wood, but its color ranges from a beige to olive green, and often has purple highlights thrown into the mix. Because of this unusual coloration, it is rarely used if a furniture piece is going to have a clear finish. This wood is best when stained or even painted. Poplar, being less expensive, is also a good choice for framing hardwood projects.oods requires extremely sharp tools.
Cherry, maple and birch are closed-grain woods, demonstrating a more uniform texture throughout a board. Poplar is also a closed-grain wood, but its color ranges from a beige to olive green, and often has purple highlights thrown into the mix. Because of this unusual coloration, it is rarely used if a furniture piece is going to have a clear finish. This wood is best when stained or even painted. Poplar, being less expensive, is also a good choice for framing hardwood projects.oods requires extremely sharp tools.
Characteristics Of Good Timber:
The quality of timber depends greatly upon the treatment the tree has received, the time of felling, and, above all, on the nature of the soil in which it has grown.
These branches of the subject do not fall within the province of the engineer or builder, and will not here be entered upon; it will be sufficient to point out some of the characteristics by which good timber may be known.
Good timber should be from the heart of a sound tree - the sap being entirely removed, the wood uniform in substance, straight in fibre, free from large or dead knots, flaws, shakes, or blemishes of any kind.
If freshly cut it should smell sweet; "the surface should not be woolly, or clog the teeth of the saw," but should be firm and bright, with a silky lustre when planed; a disagreeable smell betokens decay, and "a dull chalky appearance is a sign of bad timber."
The annual rings should be regular in form; sudden swells are caused by rind-galls; closeness and narrowness of the layers indicate slowness of growth, and are generally signs of strength. When the rings are porous and open, the wood is weak, and often decayed.
The colour of good timber should be uniform throughout; when it is blotchy, or varies much in colour from the heart outwards or becomes pale suddenly towards the limit of the sapwood, it is probably diseased.
Among coloured timbers darkness of colour is said by Rankine to be in general a sign of strength and durability.
Good timber is sonorous when struck. A dull heavy sound betokens decay within. Among specimens of the same timber, the heavier are generally the stronger.
Timber intended for use in important work should of course be free from the defects mentioned in page 388. The knots should not be large or numerous, and on no account should they be loose.
The worst position for large knots is when they are near the centre of the balk required, and more especially when they are so situated as to form a ring round the balk at one or more points.
The sap should be entirely removed. According to Mr. Laslett, however, the heart of trees having the most sapwood is generally stronger and better in quality than the heart of trees of the same species that have but little sapwood.
The strongest part of the tree is generally that which contains the last-formed rings of heartwood, so that the strongest scantlings are obtained by removing no more rings than those containing the sap.
Timber that is thoroughly dry weighs less than when it was green. it is also harder, and consequently more difficult to work.
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