Witnessing shrinkage...

[mrcaptainbob]

Watched the sawyer as he was cutting through a 20" diameter ash log. He stopped the saw at the midpoint which was about eight feet. That plank was a bit over a thumb nail thickness shorter. It was twice that by the end, just PRIOR to the blade exiting. The far end of the plank was still attached to the log. Yep, it shrunk. His kid flips it to saw another side and it happened again. Each plank, as sawn, would shrink about two or so nail thicknesses. So, he gets down to where the center growth is on the bottom. The last cut will be to halve that. Now this is the REALLY strange part. It was the bottom piece, the one with the center growth in it that shrunk! Not at ALL what I would have expected. So...as was suggested in another post, it's because that part of the tree nearer the bark is under tension during growth. So how does a tree grow? Certainly, a clothesline hook at the seven foot mark does not travel outward during growth, nor does it get to be higher than that seven foot mark. The tree does grow 'up' and, of coarse does add width by growing new bark each year. But how DOES a tree grow then? Does it linearly grow slower the further from the center? Would that be the cause of tension nearer the edge? Give me your opinions...

[Tom]

Have you ever seen a candle made by dipping a string in wax?  The wax covers the entire surface, and on a tree, there is a growing bud at the top to insure height.  It's a series of cones upon cones.

Have you considered the possibility that the it's not the board that shrank but the cant below that lengthened?  After all, you are taking weight off of a compressed structure. According to your theory anyway. 

[HOOF-ER]

As Tom says. A tree grows in 2 areas one is the tip of the branch which elongates the stem. The second is the cambium under the bark which adds to the diameter of the trunk or branch. the rest of the "non growing wood" stays where it is for the rest of the trees life.

[Magicman]

That plank was a bit over a thumb nail thickness shorter. It was twice that by the end, just PRIOR to the blade exiting. The far end of the plank was still attached to the log. Yep, it shrunk.

If the board that was being sawn from the cant was still attached on the far end, then you have created a triangle.  Albeit, a tall thin one.  Separated at the "bottom" by the thickness of the kerf.

If this board is then compared with the cant end, it will appear shorter.  Geometry.

[Slingshot]

And then, also, to take a measurement that precise, the cant to start with would have to be totally, 100 per-cent
squared top to bottom. If on the slightest angle, a board taken off would not compare with the cant because of the thickness of the saw kerf. (just my thought on the subject)



______________________________
Charles

[mrcaptainbob]

The geometry of it was the first thing I thought of, too. But that can't be what's happening here. The tangent distance is just to great for the length. A move that great for a kerf that thick would need almost a 45* angle. The angle of the board drop is considerably less than 1*. Supposing the kerf is an 1/8" and the board cut at eight feet, the tangent is .00130208 =<0*5'. This is hardly enough of a drop to display that much cant reveal. Further, while the the saw advances through the cut, the distance of reveal increases. In fact, it should decrease if the angle theory were true. So that, at the very end of the cut, when the blade exits the kerf, the angle would return to zero and the board and cant would again be the same length. This is not what appeared to happen. The reveal remained, though the board did not move. A really interesting (to me) phenomena. I will check the next time there to verify a totally sawn board that it is in fact touching at one end and short at the other while laying parallel to the cant. Has any other sawyer also found this?

[LeeB]

Never looked for it. Wood moves. It ain't steel, which also moves although not as much.

[sigidi]

Never looked at it while swinging, but maybe that's why customers keep ringing asking for their timber to be longer ;) ;D

Will try and get a look at it next time I'm doing something with a good length - don't think anything would show up in less than 10'?

[mrcaptainbob]

I appreciate the corraboration, sigidi. I do believe it would show in 10', as it appears on these logs at the halfway point of a 16'.