measuring grain slope of wood
by Victor A Boyce, CSN

Here's a handy tip that all woodworkers should know. This knowledge is most important when choosing woods to be used in aircraft building that will allow you to get the best pieces available

The best wood to use when building an aircraft is wood that has a perfectly straight grain. i.e. wood where the fibres of the grain are parallel to the edges; it is easier to work with, less likely to split, can be laminated and bent into many shapes. Stronger structures can be made from this class of wood.

But is not always possible to obtain this quality with a straight grain that runs the entire length of the piece without some kind of grain runoff. So an allowance must be made for some deviated or sloping grain however, if the grain slope is too steep, the piece must be rejected. So the answer is to measure the slope to determine if the piece can be used.

The grain slope must be determined and carefully measured on both the flat and edge grain surfaces. These two measurements being at right angles to each other must then be combined to obtain the total grain slope. This will be steeper than either of the individual slopes.

In cases where one of these faces has straight grain with no run-out, the true slope of the grain can be seen on the other face, just measure this slope, no calculations needed.

For general aircraft work the grain slope must not be steeper than 1 in 15 and for wing spars the slope should not be steeper than 1 in 20. As previously stated, wood with no slope at all is best.

If the annual rings can be seen on the edge-grained face, simply measure the slope of the annual rings. If the annual rings cannot be distinguished, consider this face to be a flat grain surface.
On a flat grain surface a little more effort is required to determine grain direction. This can be accomplished in several ways. Look closely at the surface and note the following items that will indicate grain direction.

1. Small resin ducts which will be in the same direction as the fibres, small checks (separation of the fibres), streaks of different coloured wood in the grain. Then draw a line parallel with the grain and measure for any grain deviation or slope as it is called.

2. Small drops of ink placed on the surface will spread along the fibres and indicate grain direction. A line can be drawn through these small indicators, from which grain direction can be determined and measured.

3. If a small surface scratch can be tolerated, use a scribe with a long handle about 2' long fitted with a stout pin or needle to make a light scratch indicating grain direction. The operator holds the scribe by the end of the long handle and pressing the point of the pin into the grain, pulls the scribe in the direction of the longitudinal axis or what appears to be the grain direction. The long handle prevents the operator from steering the scribe and allows the pin to follow the grain. If this is done carefully the pin scratch will follow the direction of the fibres and clearly indicate grain direction; several parallel scratches may be made if desired.

The total combined slope of the grain is determined by taking the square root of the sum of the squares of the two slopes.

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