Wiseacre Dave
Well-Known Member
I’m learning how to select and properly grade wood for use in aircraft construction. I’ve read the various specs, and threads on this site regarding wood grading, and my research has led me to the following observations and questions.
Here’s my key question, which I’ve not seen addressed anywhere. In small boards, or even any size board, must you measure the board’s end-grain angle relative to one face of the board (i.e. to either the top or bottom board surface) or can it instead be measured relative to the side face of the board (i.e. the edge face of the board)?
Let’s say you have a board that is ¾” x ¾” by 12-ft that you’ll use as a spar cap. This board was milled from a larger board that you have. Refer to the sketch from Forest Products Laboratory on various grain orientations and to my sketch showing the two boards.
In the larger board from which the spar cap is milled, you measure the grain to be 40 degrees relative to one face, on average. That means the grain direction is 50 degrees relative to the other perpendicular faces, on average. The acute grain angle measured relative to one face is geometrically NECESSARILY the COMPLEMENT of the acute angle of that grain measured relative to a perpendicular face. You choose to consider that larger board to be acceptable for use, with respect to grain direction, because the grain is between 45 degrees and 90 degrees relative to ANY ONE of the faces, REGARDLESS of whether the face is considered to be the top, bottom, or side of the board. Of course, that the board would also have to pass the grain slope criteria for the faces, as well as the knot and pitch pocket criteria, strength, etc.
The above scenario for presumed acceptability of the board begs a few questions.
1) Couldn’t I use a board that “fails” the grain angle test when the angle is measured with respect to the board’s width face (i.e. the board’s “top” surface) and yet “passes” with respect to the adjacent, perpendicular, thickness face (i.e. the board’s edge face)?
2) Given such a situation, and subject to other design considerations, in general does it matter which orientation the grain is placed in a structure, absent any specific requirements? For example, for a ¾” x ¾” board that has a correct grain angle relative to one face but not to a perpendicular face, I presume it wouldn’t matter which way the grain is oriented in the structure because both orientations would be perpendicular to the long axis of the spar.
I note that I have received a few boards in a factory-supplied kit that may be in this situation. That is, the grain passes the angle test with respect to the shorter edge face of the board but fails with respect to the wider perpendicular face. These milled boards were deemed acceptable by the factory.
I also note that a 1” x 4” x 7-ft Eastern White Pine board that I recently purchased, fails the vertical grain test with respect to the board’s “top” wide face but it may pass with respect to the edge thickness face, and I therefore may be able to use boards cut from this larger board.
What are your thoughts?
Here’s my key question, which I’ve not seen addressed anywhere. In small boards, or even any size board, must you measure the board’s end-grain angle relative to one face of the board (i.e. to either the top or bottom board surface) or can it instead be measured relative to the side face of the board (i.e. the edge face of the board)?
Let’s say you have a board that is ¾” x ¾” by 12-ft that you’ll use as a spar cap. This board was milled from a larger board that you have. Refer to the sketch from Forest Products Laboratory on various grain orientations and to my sketch showing the two boards.
In the larger board from which the spar cap is milled, you measure the grain to be 40 degrees relative to one face, on average. That means the grain direction is 50 degrees relative to the other perpendicular faces, on average. The acute grain angle measured relative to one face is geometrically NECESSARILY the COMPLEMENT of the acute angle of that grain measured relative to a perpendicular face. You choose to consider that larger board to be acceptable for use, with respect to grain direction, because the grain is between 45 degrees and 90 degrees relative to ANY ONE of the faces, REGARDLESS of whether the face is considered to be the top, bottom, or side of the board. Of course, that the board would also have to pass the grain slope criteria for the faces, as well as the knot and pitch pocket criteria, strength, etc.
The above scenario for presumed acceptability of the board begs a few questions.
1) Couldn’t I use a board that “fails” the grain angle test when the angle is measured with respect to the board’s width face (i.e. the board’s “top” surface) and yet “passes” with respect to the adjacent, perpendicular, thickness face (i.e. the board’s edge face)?
2) Given such a situation, and subject to other design considerations, in general does it matter which orientation the grain is placed in a structure, absent any specific requirements? For example, for a ¾” x ¾” board that has a correct grain angle relative to one face but not to a perpendicular face, I presume it wouldn’t matter which way the grain is oriented in the structure because both orientations would be perpendicular to the long axis of the spar.
I note that I have received a few boards in a factory-supplied kit that may be in this situation. That is, the grain passes the angle test with respect to the shorter edge face of the board but fails with respect to the wider perpendicular face. These milled boards were deemed acceptable by the factory.
I also note that a 1” x 4” x 7-ft Eastern White Pine board that I recently purchased, fails the vertical grain test with respect to the board’s “top” wide face but it may pass with respect to the edge thickness face, and I therefore may be able to use boards cut from this larger board.
What are your thoughts?
