The more moisture a board absorbs or releases, the more it swells or shrinks. However the surface of a board moves differently depending on the grain direction and type of grain. Wood movement along the grain is virtually negligible. From 0 to 28 percent moisture content, a typical board will move only 0.01 percent of its length. It will still move approximately 8 percent in flat grain and 4 per cent in quarter grain. Woodworkers believe quartersawn lumber is more stable because of this. This is also why boards with mixed grains (and mixed expansion rates), tend to cup.
So how do you predict how much a board will move and in what direction? That depends on the grain direction, type of grain and time of year. The relative humidity rises with warmer weather. This causes wood to expand. It is recommended that you allow for 1/10cm movement over 30 cm of plain grain, and 1/20c over the same amount of quarter grains. If youre 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.)
This pedestal table’s legs are made of wood that runs parallel to their longest dimension. It makes them as strong as they can be. Were the grain to run parallel or perpendicular to the pedestal, the legs would be weakest at their narrowest point, the ankles.
Wood is Strong along the Grain The wood cells are made of long, tough cellouse fibres that are bound together with a glue-like substance called lignin. The cellulose is much more durable than the lignin. Consequently, its 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 is a huge part of woodworking design. Can you imagine what might happen if you cut mortise-and-tenon joints in which the grain ran across the tenons? Theyd snap if you just looked at them sideways. But tenons that are cut parallel to the grain will outlast those who cut them.
But wait theres more – when strength is paramount, grain direction may not be your only consideration. Certain wood species are stronger than others. Windsor chairmakers, for example, typically use hard maple, birch and hickory for legs, rungs and spindles. These parts are quite thin so weaker woods will not work.
The wood’s density is a good indicator of its strength – it measures the weight of a particular volume of material. Wood density is measured by calculating its specific gravity – the weight of a volume of wood compared to the same volume of water. The wood will be denser and more strong if the ratio is higher.
Specific gravity, unfortunately, doesnt predict when a wooden board will break, sag or dent. For this, there are measurements of strength.
The wood species’ compressive strength is a measure of how much load it can support parallel to its grain. The legs of a chair with a large relative will buckle if they are occupied by a corpulent person.
Bending strength shows the load wood can withstand perpendicular to the grain. How many kids can stand on that chair rung before its firewood?
The wood’s stiffness is a measure of how much it will deflect if loaded perpendicularly to the grain. How far will your collection of cannonballs sag on those shelves?
The hardness reveals how resistant the surface is to abuse. What can you do to get rid of your frustrations 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 birdhouse. 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. Its the accumulated insights and inspirations of 5,000 years of craftsmanship. This craft has a mysterious material at its heart that is still revealing all its secrets. PW
Nick Engler is contributing editor for .
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