Woodworker's Journal 2008-32-6, страница 40
Shrinkage Flatsawn board The next drying stage is the removal of bound water from the vessel walls, and it's here that changes begin. As drying continues, the board shrinks in width and thickness but not in length. If shrinkage were to occur equally on both faces and both edges, the board would remain as flat as it came from the saw. However, shrinkage is unequal: it shrinks about twice as much in the tangential plane as it does in the radial plane. This is called differential shrinkage. Differential shrinkage is the sole reason that wood distorts. A flat board can distort in four ways: cup, bow, spring and twist. Various splits or shakes may also occur. These distortions are further complicated by the fact that trees grow in a spiral fashion. The amount of spiralling varies from hardly perceptible to the highly visible, such as may be seen in the splits or cracks in a wooden utility pole.
Tangent Tangent Flatsawn Board. There is more tangential exposure on the top half than the bottom half of this board. Therefore, the tissue 011 the top half will shrink more than the bottom half and the board will cup. In this flatsawn board example, the outcome of shrinkage differential is a cupped board. Tangential face Differential Shrinkage. A section through a tree is essentially circular: it has a radius and a tangent. If a board 1s cut from the log so the faces are tangential, the adjacent edges are radial. The shrinkage In the tangential direction is on average twice that in the radial direction. Center tine What is a "warped" board? The term "warped" is a nonspecific term that denotes a misshapen board. The glossary from The Collins Complete Woodworker reads: "Any defect in lumber that deforms a board's shape." And while that may be an accurate definition, it is the term itself that is not useful. By using the terms cup. bow, spring and twist, a woodworker can accurately describe the specific distortion of a board. This is much more useful information than "warped " "Spring" or "sprung" board has local variations, the most common being a "crooked" board. Spring: the board remains flat in width but curves in length, like a river going around a bend. Twist: the board curves in length and width like a propeller. Twist Woodworker's Journal December 2008 39 |
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