On 2/22/2012 2:28 PM, Michael Joel wrote:
Swingman wrote:
You don't want to argue because you were mistaken, charitably, either
in your wording, or your understanding.
And you are very mistaken in your contention that you "doubt anyone
knows for sure" ... a first semester, college level, Botany 101 class
would prove to you that this is well understood, and well documented
with empirical evidence.
For your future benefit, and so this ends on an instructive note, you
will certainly want to explore the definitive work on the matter:
http://www.amazon.com/UNDERSTANDING-.../dp/B000UQH160
Every woodworker can learn something from reading Hoadley, guaranteed.
See - that is why I get pulled back in. Because the logic is so illogical.
If I said I don't believe they really know what is happening - you say
take a "first semester, college level, Botany 101 class
would prove" - The only thing it would prove is they believe it. Very
circular thinking (as the "scientists" would say).
....
Well, that's self-serving to the max. You might as well say that the
physicists on believe Newton's Principles; after all, it's "only" a theory.
I suggest you do read Hoadley (or any of several references from the US
Forest Products Lab, these can be downloaded at no cost); specifically
Chap 4, "Wood and Water".
The short story is---wood shrinks as it dries and it does so
non-uniformly. The tangential/radial shrinkage ratio from green to
oven-dry has been measured for some hundreds of species and averages
about 2 (roughly an average of 8%/4% T/R) but shows a significant
variation between species of from just barely above 1:1 to highs
approaching 3:1. The higher the ratio the more the particular would
will distort as it is dried as the relative shrinkage in the two
directions competes at a different rate.
The difference between tangential and radial shrinkage isn't random nor
magic; it's caused by the anatomical structure, principally the effect
of wood rays whose lengthwise axes are oriented radially outward.
Species w/ more predominant rays are more stable.
Over the range of moisture content shrinkage is roughly proportional to
moisture loss. This doesn't matter too much as raw lumber is dried; the
roughsawn stock is oversize to begin with and if dried uniformly will be
stable after it is milled given a uniform environment. This is why it
is so important to finish both sides of a furniture panel, say--if one
side is finished and the other not, differential moisture absorption is
highly likely to cause movement.
Cupping after the fact is owing to one of two causes--either the piece
wasn't at equilibrium to begin with and dried after milling with the
resultant change in dimension as determined by the species' particular
T/R ratio and the percentage change in moisture.
The second is that the piece has subsequently absorbed moisture and
therefore grown. The relative amount in the direction is also dependent
on T/R and how uniform (or un-uniform) the moisture absorption is.
Cupping in flatsawn boards results in concavity away from the pith, the
result of greater tangential than radial shrinkage. The magnitude is
greater as the location of the board from the original trunk is closer
to the pith on a surface. This face is completely radial while the
opposite is tangential in the portion across from the location of the
pith and will therefore shrink at twice the rate. Woodworkers tend to
say the "rings flatten out" which an easy way to remember the direction
the board will cup but the reason for the cup has virtually nothing to
do w/ the growth rings themselves.
Cup is reversible on swelling which is why the idea of wetting is given
as a cure. Of course, the end then has to be to get the whole board in
equilibrium again at that point which goes back to point a) above--if
the piece was milled in inequilibrium, it's an insoluble problem w/o
mechanical repair.
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