I found the following on another luthier forum. I found it pretty interesting. Not sure of reposting rules, but if this violates anything let me know or just remove it.
OK, here we go again. This may get a bit long and complicated. Hope everybody can follow. Here are some things to consider, both when buying backs and sides and lumber for resawing yourself.
First some definitions. In the lumber trade, boards with the growth rings at an angle of 90 to 45 degrees from the face are generally called quartersawn (Q/S). Those with an angle of 0 to 45 degrees to the face are generally called flatsawn (F/S). Sometimes the breakdown is 90 to 60, 60 to 30, and 30 to 0 degrees, with those in the 60 to 30 degree category being called rift or bastard sawn. For simplicity, we will use the first definition.
There are many reasons for using either Q/S or F/S wood. An obvious factor is appearance. Q/S wood generally has a straight stripe pattern, often with ray figure in some woods (sycamore, lacewood, oak, and "silk" in spruce and cedar). Some curl or "fiddleback" figures show more prominently in Q/S wood. F/S wood, on the other hand, can have a wide variety of figure, often quite striking. Some grain patterns such as "birdseye" and "quilt" only show well on F/S faces.
However, in lutherie, the primary consideration is stability, ie. dimensional changes related to changing moisture contents (in soundboards, strength and stiffness are a consideration, also). Wood shrinks and swells less in the radial (Q/S) direction than in the tangential (F/S) direction. Dimensional stability is of primary importance in the thin wood of guitar backs. Excessive shrinkage can easily cause cracks, and swelling can sometimes cause other problems.
Wood movement occurs within the range of moisture content from 0 % (oven dry) to approximately 28 % (the fiber saturation point) Moisture contents above the fiber saturation point do not cause any increase in dimension. The moisture content of wood varies with the relative humidity of the air. For example at a relative humidity of 25 %, wood will equalibrate to around 5 % moisture content. At a relative humidity of 75 % wood will equalibrate to a moisture content of about 14 %.
There are vast differences in dimensional changes between species. Following are the % shrinkage of several species. I have grouped these first by shrinkage radially, and second by shrinkage tangentially.
Radial shrinkage:
2 % - Kelobra, Monkeypod
3 % - Bolivian Rosewood, Central American Laurel, African Mahogany, Ovangkol, Purpleheart, Wenge, Cocobolo Rosewood, East Indian Rosewood, Espave, Brazilian Rosewood, Nigerian Satinwood, Padauk, Catalpa, Granodillo, Honduran Mahogany, Myrtle, Redheart
4 % - Balsamo, Black Limba, Bloodwood, Bocote, Chechen, Spanish Cedar, Pau Rosa, Bubinga, Goncalo Alves, Machiche, Red Maple, Black Cherry
5 % - Beli, Sugar Maple, Yellow Poplar
6 % - Black Walnut
7 % - Basswood, Zebrawood
Tangential shrinkage:
3 % - Monkeypod
4 % - Granodillo, Honduran Mahogany
5 % - Cocobolo Rosewood, Espave, Brazilian Rosewood, Nigerian Satinwood, Padauk, Catalpa, Kelobra
6 % - Balsamo, Black limba, Bloodwood, Chechen, Spanish Cedar, Pau Rosa, Bolivian Rosewood, Central American Laurel, African Mahogany, Ovangkol, Purpleheart, Wenge, East Indian Rosewood,
7 % - Bocote, Black Cherry
8 % - Bubinga, Goncalo Alves, Machiche, Red Maple, Redheart, Black Walnut, Yellow Poplar
9 % - Myrtle, Beli, Basswood
10 % - Sugar Maple
12 % - Zebrawood
OK, what does all of this mean on a typical guitar back. First let's assume that we can generally control the environment of a finished guitar to a range of 25 to 75 % relative humidity (since there is a lag in the wood, short fluctuations above or below this of say less than 12 to 24 hours can be ignored). This relates to a wood moisture range of between 5 % and 14 % (9 % change). The actual dimensional change in wood for that moisture range is given below in 0.0000" per inch of width.
% shrinkage/dimensional change per inch of width
2 % / 0.0065"
3 % / 0.0096"
4 % / 0.0129"
5 % / 0.0161"
6 % / 0.0193"
7 % / 0.0225"
8 % / 0.0257"
9 % / 0.0289"
10 % / 0.0321"
11 % / 0.0354"
12 % / 0.3857"
A few things are apparent from all of this. First, if the stability of Q/S Sugar Maple is adequate, then any perfectly F/S wood from those with a shrinkage of less than 5% should be equally as stable. And since perfectly F/S wood is very rare (since growth rings curve), partially to mostly Q/S wood of many species should be as stable as Q/S Sugar Maple. For example, the shrinkage in boards with growth rings as in examples A, B, and C in the sketch below can be readily approximated by apportioning the amount of radial grain [R] and tangential grain [T] to get an average or the total.
Attachment:
woodcs.jpg
In summary, quartersawn wood is great to find and wonderful to work with. But flatsawn wood in some species and partially quartersawn wood in many species can be just as dimensionally stable. There are obviously many additional considerations, but I hope this offers a bit of insight into this confusing area. Further information on this subject can be found in Chapter 6 of the book Understanding Wood by R. Bruce Hoadley.
Login
Register
FAQ
![[headinwall]](./images/smilies/headbangwalluf8.gif "Mad")
