Popular Woodworking 2000-06 № 115, страница 29
The Way Wood WorksEverything you wanted to know about wood but didn't know what to ask. Wd is a cantankerous substance; there's no two ways about it. Its virtues, of course, are legendary. It's attractive, abundant and easy to work. Pound for pound, it's stronger than steel. If properly finished and cared for it will last indefinitely. But none of that makes up for the fact that it's a complex and often perplexing building material. Unlike metals and plastics, whose properties are fairly consistent, wood is wholly inconsistent. It expands and contracts in all directions, but not at the same rate. It's stronger in one direction than it is in another. Its appearance changes not only from species to species, but from log to log — sometimes board to board. That being so, how can you possibly use this stuff to make a fine piece of furniture? Or a fine birdhouse, for that matter? To work wood — and have it work for you — you must understand three of its unique properties: • Wood has grain. • Wood moves more across the grain than along it. • Wood has more strength along the grain than across it. Sounds trite, I know. These are "everyone-knows-that" garden- variety facts. But there is more grist here for your woodworking mill than might first appear. Wood Has Grain As a tree grows, most of the wood cells align themselves with the axis of the trunk, limb or root. These cells are composed of long thin bundles of fibers, about 100 times longer than they are wide. This is what gives wood its grain direction. Additionally, a tree grows in concentric layers, producing annual rings. You must pay close attention to these two characteristics — grain direction and annual rings — the way a sailor watches the wind. Ignore them, and they'll bite you big time. Sawyers commonly use two methods to cut trees into boards, each revealing a different type of grain. by Nick Engler Nick Engler has been writing about woodworking for a quarter of a century, arid is the author of more than 50 books on the subject.His workbench, which is made from hard rock maple and white oak, is rated to withstand a five-alarm tantrum. ASTOUNDYOUR FRIENDS Want to predict exactly how much a given board will move in a year's time? Look up the specific movement rate for the wood species in the Wood Handbook.Then call the National Oceanic and Atmospheric Administration to find how much the relative humidity changes from summer to winter. Multiply the change in humidity (as a decimal) times the movement rate (another decimal) times the width of the board. Of course,this level of precision is completely unnecessary, but the chicks digit. |
