TM 1-414
AIRCRAFT WOODWORK G-8
during the latter part of the growing season. In some woods the distinction between these two parts of an annual ring is very clear, whereas in others the springwood merges into the summerwood with no distinct dividing line.
7. Grain. -a . The recommended usage of the term "grain" is that it be confined to describing the direction in which the fibers run, that is, straight, spiral, interlocked, wavy, or curly grain. However, the term is also loosely used to refer to annual rings as coarse, fine, even, edge, and flat grain ( see par. 11 for edge grain and flat grain) ; and to the relative size of pores and fibers as open grain and close grain.
h. Wood employed in aircraft construction is straight-grained with-in limits of deviation as set forth in the specification for the particular lumber used.
8. Common defects in wood.-Irregularities which occur in or on wood which may lower its strength are regarded as defects. Common defects in wood are described below.
a. Cross grain.- Cross grain may be regarded as any deviation of the grain from parallelism with the axis of a piece of wood. It may be classified as spiral, diagonal, or a combination of both.
(1) Spirl-(a) Spiral grain occurs when the fibers take a spiral course in the tree trunk, as if the tree had been twisted (fig. 40). On a truly tangential (plain-sawed or flat grain) surface (fig. 5) the oblique direction of the checks, resin ducts (brownish hairlike lines in softwoods), and pores (visible in many hardwoods) indicate the direction of the fibers. Normally, this direction is parallel to a line drawn through the apexes of the "flowers" formed by the annual rings on a plain-sawed surface; however, if spiral grain is present the fibers run at an angle to this line.
If sufficient pores, resin ducts, or checks are not visible, the direction of the grain can be determined by the direction in which free-flowing ink spreads or fibers pull up. The presence (but not the slope) of spiral grain is indicated on quarter-sawed faces when the planer tears out chips in such a way as to show that the grain runs into the piece instead of parallel to its axis.
(b) Spiral grain, when pronounced, is objectionable because it weakens the wood, tends to twist the lumber during seasoning, and usually produces a rough surface when quarter-sawed faces are planed against the grain.
(2) Diagonal.-(a) Diagonal grain in lumber is a defect produced entirely by sawing and results when the direction of sawing is not parallel to the bark. Spiral grain is frequently confused with diagonal grain ; however, there is no relationship between the two since spiral grain is a defect of growth. On a truly edge grain surface, diagonal grain is easily detected by the slope of the annual rings.