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The Ways of Solid Wood Woodworking Carpentry Cabinetmaking
Pro's Column
The Ways of Solid Wood:
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By Ian Kirby
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The standing tree has three parts: roots, trunk and crown. The roots take up moisture containing raw materials and anchor the tree, the trunk transports raw materials to, and supports, the crown; leaves in the crown, activated by sunlight, convert the raw materials Into nutrients - which are passed down the trunk to sustain the life and growth of the tree.
We convert the trunk into lumber- the most convenient of natural materials- but trees do not grow solely for the convenience of woodworkers. To our advantage, they grow long to hold the crown in sunlight and thick to support the crown against wind and the elements. The inherent beauty of color and grain, the smell, the accessibility to simple cutting tools are favorable to us as well. But trees also grow twisted, branched, damaged and diseased because of climate, location, geology, the depredations of man and animals and the nature of its species. All these factors, favorable and unfavorable, show up as characteristics in the wood we use for making furniture.
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 Dimensional Instability
Still to be considered is the most important characteristic of all: after wood has been dried and converted into useable parts from the tree, it changes size according to the amount of moisture in the air.
In other words, wood is a dimensionally unstable material. No other factor plays such an important role in the way we design furniture made of solid wood. The parts of a cabinet must hold together and at the same time be allowed to shrink and expand. The most outstanding example of a dimensionally stable unit composed of pieces that are dimensionally unstable is the frame and panel.
To better understand the nature of wood tissue, let's examine a small block of oak 1 in. x 1 in. x 4 in. If we magnified the block 100 times, we would see the elements or individual units which comprise -the wood. The block is largely composed of elongated fibers and vessels which support the tree and transport nutrients. Our block, in fact, looks somewhat like a box of drinking -straws. However, unlike straws, the vessels and -fibers are irregular. They vary in diameter, in length, in orientation and in, wall thickness.
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 Moisture Exchange
If the surrounding air contains more moisture than the block, it will take up moisture until the moisture content of air and block are the same. This moisture does not occupy the cavities but rather permeates the walls of the fibers and vessels. As the individual walls expand a minute amount, the block as a whole expands in its width and thickness but not in its length. This moisture exchange is reversible. If the walls contain more moisture than the surrounding air, they will give off moisture until the moisture equilibrium of air and tissue is reached. The net result is shrinkage of the oak block. A material which constantly balances its moisture content in this way is said to be hygroscopic.
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 The trunk is essentially an elongated cone shape, and a section through it looks like a series of concentric circles defined by the annual growth rate.
The tree grows larger each year by adding a layer of tissue to its outside. The growth cycle is both annual and seasonal, with periods of vigorous growth alternating with periods of no growth.
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A species of tree, say, red oak, may grow larger or smaller, faster or slower, due to its location and the climate. But regardless of such external differences, all red oaks contain the same characteristic elements arranged in the same order.
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Differential Shrinkage
A section through the tree, being essentially circular, has a radius and a tangent. If we cut our block of wood from the trunk so that one of its sides is a radius to the circle, then the adjacent side is a tangent to the circle. The sides are referred to as the radial and tangential faces. How the radial and tangential directions of the tissue are arranged in a piece of wood has a dramatic effect on its response to shrinkage and expansions.
Because of the anatomical structure of wood. tangential shrinkage is greater than radial shrinkage: this is called differential shrinkage. The average amount of tangential to radial shrinkage for most common species of wood has been recorded, and the difference between the two expressed as a ratio. The higher the number, the more we can expect wood of that species to distort. The average is 2, which means a typical block of wood shrinks twice as much tangentially as it does radically. If there were no differential shrinkage, there would be no distortion of boards as they are dried-the pieces would simply become uniformly smaller.
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By examining the end grain of a board we can determine where it was positioned in the log. If it was cut so that its faces are on the radius to the circle, then the growth rings go across its thickness, and the edges of the board are a tangent to the circle. The board will tend to remain flat as it shrinks and expands because the tissue is oriented radially and tangentially equally each side of a centerline through the board. The differential shrinkage does not distort the board because the radial shrinkage acts exclusively across its width and the tangential shrinkage acts exclusively across its thickness.
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 Cup, Twist, Spring and Bow
If the board were cut so that its faces are on the tangent to the circle, then the growth rings go across the width of the board. However, there is a distinct difference between the orientation of the growth rings on one face of the board to the other. The surface of the board furthest from the center of the tree has more tangentially oriented tissue than the surface nearer the center. This dissimilarity of the two surfaces, which we should really consider beginning at a plane passing through the center of the board, means that the differential shrinkage will not be balanced equally each side of the board. The face furthest from the center will shrink more since it has more tangentially oriented tissue in it. The result is a cupped board.
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The vessels and fibers which comprise the bulk of the tissue in this particular board may be very evenly distributed because the tree grew evenly over the years; they may be aligned straight with the edges of the board because the tree had a straight, cylindrical trunk; and the sawmill cut the board parallel to the outside of the tree. Yet, despite the most favorable of circumstances, the board cupped because of the differential shrinkage.
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 Now imagine the orientation of tissue in the board under less favorable circumstances: suppose the tree has grown crooked rather than straight; it has grown spirally as trees frequently do; it has large branches; and the saw mile further complicates this misaligned tissue by rnisaligned sawing. The end result is that the orientation of radial and tangential tissue could be any direction in relation to the faces and edges of a board cut from this tree. (Granted, all these changes in grain direction would produce a very attractive board, but the effect of differential shrinkage in such convoluted tissue inevitably produces distortions as the board dries.
These distortions, known collectively as warping, fall into four main types. Cup has already been noted; the other three are twist, spring and bow. They can occur singly or in combination in each workpiece and usually occur over a period of time during drying. However, if the wood has been kiln dried carelessly. There may be considerable unevenness of moisture con- tent between the outside and the inside. This, combined with a convoluted orientation of the wood tissue, may set up internal stresses which are suddenly released when the board is ripped down its length on a table saw. To avoid being at risk. the operator must always position the fence of a table saw to allow distortion to occur without hindrance.
This article appears as Chapter “Why Wood Distorts” In Ian Kirby’s Book: Woodworking Down to a Line. The entire book is available from Cambium Publishers at http://www.iankirby.com |
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