Popular Woodworking 2000-06 № 115, страница 30

• Plain-sawn boards are cut tangent to the annual rings. The sawyer "cuts around" the log, turning it for each series of cuts so the faces of the boards will show mostly flat grain (also called tangential or plain grain).

• Quartersawn boards are cut through the radius of the growth rings. The sawyer cuts the logs into quarters or bolts, and then saws each bolt so the boards show quarter grain (or radial grain) on their faces.

Lumber doesn't always show a single type of grain on its face. Plain-sawn boards in particular may show mixed grain — flat grain in one area and quarter grain in another. The grain between the two, where the surface is cut at a 30- to 60-degree angle to the annual rings, displays rift grain.

Each type of grain has a distinct pattern, depending on the wood species. You can use these grain patterns to enhance the design of your furniture or your bird-houses. More importantly, if you know how to "read" the patterns, you can predict which way the wood will move and how much.

Wood Moves Across the Grain

Because of its unique structure, wood is constantly expanding and contracting. And you must cope with this movement in everything you build.

Wood moves as its moisture content changes. After the tree is felled and the sap has evaporated, the wood fibers continue to absorb and release water like a blotter. How much water they hold depends on the relative humidity of the surrounding environment. The more humid it is, the more moisture the fibers soak up. This moisture content is the ratio of water to wood. In extremely humid conditions,

GLUE STEPS: AT LAST, A PRACTICAL SOLUTION TO AN AGE-OLD DILEMMA!

Want to see a practical application of this information? With what you now know about wood grain and movement you can solve a persistent problem that has dogged too many woodworkers for too long: glue steps.These are tiny changes in the surface level from one board to another at glue joints.They are especially unattractive in table tops where boards are joined edge to edge.

The common misconception is that these are caused by improper gluing technique. A talented and experienced craftsman once spent hours trying to convince me that glue steps are caused by the adhesive out-gassing. (In my shops the adhe-sives are better behaved.) Despite his ardor, the glue steps are the result of uneven wood movement.

12% 10% Moisture Content Moisture Content

When two boards of uneven moisture content are formed edge to

8% 8% Moisture Content Moisture Content

...the board with the higher moisture content shrinks more and a glue step develops.

Quartersawn Flat-sawn

A similar thing happens when you joins boards

with a different grain...

Quartersawn Flat-sawn

...The plain-sawn boards change thickness less than the quartersawn and a glue step appears.

Sometimes a woodworker fails to shop-dry his lumber — let it rest in the shop long enough for all the boards to reach the same moisture content.When parts with dissimilar moisture contents are joined, the moister part moves more than the drier one. Or a craftsman glues flat grain to quarter grain, joining two surfaces that move at different rates, even when they have the same moisture content. In both cases a step results.

To avoid glue steps, shop-dry your lumber for a week or more before using it so the moisture content of the wood has a chance to reach an equilibrium with the relative humidity in your shop.When gluing boards edge to edge, always glue flat grain to flat grain and quarter grain to quarter grain.

as much as 28 percent of the total weight of a board may be water — 28 parts water, 72 parts wood. The rule of thumb is that the moisture content of wood changes 1 percent for every 4 to 5 percent change in the relative humidity.

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 almost negligible. From 0 to28 percent moisture content, a typical board will move only 0.01 percent of its length. However it will move about 8 percent across flat grain and 4 percent across quarter grain. This is why woodworkers consider quartersawn lumber more stable. It's also why boards with mixed grain (and mixed expansion rates) tend to cup.

So how do you predict how much a board will move and in what direction?

Ankle weak

Wrong

Wrong

The wood grain in the legs of this pedestal table runs parallel to the longest dimension, making the legs as strong as possible.Were the grain to run parallel or perpendicular to the pedestal, the legs would be weakest at their narrowest point, the ankles.

10 Popular Woodworking June 2000