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John and Howard: If I can intervene on this debate as a scientist (not as a builder or woodworker), it is probably not advisable to be too pedantic on the meaning of either "grain" or "runout".

To a wood technologist, the term "grain" refers in a general sense to "arrangement and direction of alignment of wood elements when considered en masse " (Panshin and de Zeeuw 1970 - "Textbook of Wood Technology"), but as emphasized by Bruce Hoadley in his book "Understanding Wood" (Hoadley 2000), "grain" can have several other meanings to a woodworker, including also figure, plane of a cut surface, growth-ring placement, growth rate, relative cell size, etc., and of course is often used in many related compound words. Its intended meaning should be made clear by context or associated adjectives.

"Runout" is even more problematic. It is not a term recognized either by wood scientists or lumbermen, and the only references I've seen in woodworking have related to music instrument making. Its technical meaning comes from mechanical engineering, where it refers to the deviation of a rotating point from the axis of rotation, either as axial runout (e.g., cutting edge on a spinning router bit) or radial runout (e.g., side of a turning circular saw blade). My observation is that, in lutherie, it most often refers to the deviation of the longitudinal axis of wood tracheids or vessels from the surface plane of a piece. Visible medullary rays reveal that the piece is well quartered, but they don't say much about the angle of wood elements with respect to the surface. Wood tends to split along the direction of wood elements. If it splits ALSO along the axis of rays, it's because the splitting tool was oriented close to this axis, and that the parenchymal tissue (and sometimes resin canals) that make up most of the ray elements are weaker than longitudinal tracheid or vessel elements. On cut surfaces, such as a guitar top or bottom surface of a brace, one can sometime see tremendous "silking" (ray figure) while at the same time having considerable surface "runout", which will become apparent when you put a plane to the surface.

If you really want to use either the terms "grain" or "runout" of wood in a very specific way, you're pretty much free, provided you clarify your usage. No one can claim that every workworker has the same definition, nor should that be expected.

Excellent post Tim.

As a person that understands wood, what is your take on the original question regarding the need for quarter sawn brace material? And does the same apply to all commonly used brace stock? (as in spruce, WRC, etc.)

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Joe Beaver
Maker of Sawdust

I've had to think about this a bit, and do a bit of research, so pardon the delay in reply. It's also related more to engineering, which is on the periphery of my background.

Trevor Gore's recommendations (Gore & Gilet 2011: page 4-47 Contemporary Acoustic Guitar - Vol. 1 Design) are typical of those I've read in lutherie references:
"It is important to observe the grain orientation of the timber. The conventional wisdom is to have the grain in the braces perpendicular to the panel (vertical)." (He does explain an exception for low-strength timbers, such as balsa.)

Indeed, that's the way I was taught and I still do it that way; it makes intuitive sense to me, and I'm unlikely to change the practice without some data to support another orientation. Having said that, a quick bit of research in the literature and standard wood technology textbooks didn't come up with any discussion of the effect of grain orientation on bending strength -- surprised me actually. I also didn't find any supporting data or theory in Trevor's book -- that also surprised me -- if they're there, I just didn't spot 'em, but I'm hope Trevor will chime in to correct me. I have a (vague) memory that Alan Carruth has gathered data on this aspect of brace strength -- not sure where to find them or if they're even posted on OLF -- so hopefully he also will chime in.

There is no doubt in my mind that "runout" with respect to the bottom and top surface, is THE most important consideration with respect to grain orientation and brace strength, and that IS supported by loads of data. Known formally as "cross grain" or "slope of grain", wood scientists distinguish between "radial slope of grain" (easily assessed as deviation of the lines of latewood from the surface) and "tangential slope of grain" (MUCH more difficult to see). Only the former is considered in visual grading rules for commercial lumber. If we use a vertical grain orientation for the beam (brace), then tangential slope of grain becomes the single most important material characteristic affecting bending strength. A builder or tonewood supplier would normally assess tangential slope of grain by splitting the billet/piece at right angles to the growth rings.

The other thing I would evaluate in brace wood is straightness of the lines of latewood. Pronounced deviations or wobbles from straight indicate the cambium laying down xylem around a branch or occlusion. These areas are much more likely to contain "reaction wood" in response to local compression stresses. Reaction wood is MUCH weaker than "normal" xylem.

With regards to species, my experience is limited to Sitka, Lutz and Norway spruce. I think my comments on grain orientation would still apply, but I would expect dimensions yielding WRC braces of strength equivalent to spruce would be much different. Personally, if I am to gain experience with another material, it will more likely be with carbon-fibre composites.

The integrated effect of all these characteristics on brace strength can be assessed with a simple bending test as deflection under load, the same principle as used for Machine Stress Grading (MSG) is some lumber markets. If your brace stock is prepared to a uniform dimension, you can obtain highly relevant data with simple measurements and no math at all.

I'll also take this opportunity to acknowledge a PM received from another member who questioned my previous statement with respect to the term "runout" that "the only references I've seen in woodworking have related to music instrument making". He pointed out HIS familiarity with the term used in boat building and longbow construction. Good thing I qualified the statement as my own experience, as I really wasn't familiar with the term until my entry into lutherie. I've been unable to find the usage in the scientific literature, and when I searched with Google, I had to run through several pages to find a small number of references to steam bending, boat building and aircraft construction. So, it's there, but I would still urge caution in assuming that all woodworkers will be familiar with or have the same understanding of the term.

(Apologies for typos -- the forum editor has been giving me grief. Of course I accept responsibility for any factual errors, but please be gentle!)

Another very thoughtful reply Tim. Thank you again.

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Joe Beaver
Maker of Sawdust

Tim thanks for the info. Could not get a better explanation than that.

On the other side of it, some of the terms I had to look up. That stupidness feeling went up 10% and I just felt like I went to microbiology class. Lol.

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久保
美智え