Popular Woodworking 2000-06 № 115, страница 31
wood properties This chart shows some important properties for 18 common species of wood. Specific gravity is the ratio of the mass or weight of the wood to that of an equal amount of water.Tangential and radial movement are given as a percentage (%) of a board's measurement across the grain as it dries.To find compressive strength, engineers load a block parallel to the grain until it breaks.They find bending strength by loading a block perpendicular to the grain. Both are measures in pounds per square inch (psi). Stiffness is determined by applying a load perpendicular to a beam until it deflects a certain distance.This is measured millions of pounds per square inch (Mpsi). For hardness, a metal ball is driven halfway into a wood surface.The force required to do this is recorded in pounds (lbs).
That depends on the grain direction, type of grain and time of year. In most areas, the relative humidity climbs as the weather turns warmer. This causes the wood to expand. The rule is to allow for V4" of movement across 12" of plain grain and 1/8" across the same amount of quarter grain. If you're working in the summer, the wood shrinks as winter approaches. When building in the winter, count on the wood expanding when summer comes. (Thus the old saw, "Work tight in summer, loose in winter.") Wood is Strong Along the Grain The wood cells are made from long, tough cellouse fibers, bound together by a gluelike substance, lignin. The cellulose is a lot tougher than the lignin. Consequently, it's much easier to split a board along the grain (separating the lignin) than it is to break it across the grain (snapping the cellulose). This botanical trivia plays an enormous role in woodworking design. Can you imagine what might happen if you cut mortise-and-tenon joints in which the grain ran across the tenons? They'd snap if you just looked at them sideways. Yet tenons cut parallel to the grain will far outlast the woodworkers who cut them. But wait there's more — when strength is paramount, grain direction may not be your only consideration. Some species of woods are naturally stronger than others. Windsor chairmakers, for example, typically use hard maple, birch and hickory for legs, rungs and spindles. Because these parts are fairly slender, weaker woods won't do. A good indicator of a wood's strength is its density — the weight of a given volume of substance. Wood density is measured by calculating its specific gravity — the weight of a volume of wood compared to the same volume of water. Generally, the higher the ratio, the denser — and stronger — the wood. Specific gravity, unfortunately, doesn't predict when a wooden board will break, sag or dent. For this, there are measurements of strength. • Compressive strength tells you how much load a wood species will support parallel to the grain. If a corpulent relative sits in the chair, will the legs buckle? • Bending strength shows the load wood can withstand perpendicular to the grain. How many kids can stand on that chair rung before it's firewood? • The stiffness indicates how much the wood will deflect when loaded perpendicular to the grain. How far will those shelves sag when you display your col- GOTTA HAvE IT All this information is in the Wood Handbook:Wood As An Engineering Material.This woodworking classic was written by the Forest Products Laboratory, an arm of the U.S. Department of Agriculture. Much of it reads like an income tax form, but you won't find a more complete reference. Write the Government Printing Office, Superintendent of Documents, Washington, DC 20402-9325. lection of cannonballs? • The hardness reveals how resistant the surface is to abuse. How hard can you pound when taking your frustrations out on the workbench? A Parting Thought Too often we approach our craft as if it were a collection of recipes. Take two boards, chop them up on a table saw, add a dash of glue and — presto! — a bird-house. Or a Chippendale highboy, depending on how many boards and how finely you chop. But woodworking is more than knowing how to use a tool or follow a plan. It's the accumulated insights and inspirations of 5,000 years of craftsmanship. And at the heart of this craft is a surprising material that has yet to reveal all of its mysteries. PW |
