Messages Archive

Subject:
doomed to failure *PIC*

Bill Tindall, E.Tn.
Why:

the tangential shrinkage of oak is 8% and the radial 4% (approx. to make the calculation easy to follow).

what do these values mean? They mean the circumference of the disk will shrink twice as much as the diameter as the wood dries. The circumference can't shrink twice as much so it will open cracks to be able to shrink this amount as illustrated in the picture of a pine disk.

As stated this disk of wood had a diameter of 5', and hence a circumference of 15' (C=piD). When dry the diameter will shrink 0.04 times 5 or 0.2 feet. To match this shrinkage the circumference would need to shrink 0.04 times 15 feet equal 0.6 feet. If the diameter and circumference both shrank 4% the disk would not split. But, the circumference in fact shrinks twice as much. When the diameter shrinks 4% the circumference will have shrunk not 0.6, feet but 1.2 feet. A crack, or cracks, of 0.6' will open up around the circumference to relieve the stress.

I have no idea what makes the white but it likely has something to do with water making bubbles or crazes in the epoxy.

PS that pine disk was cut in 6 seconds with a cross cut saw in a competition.

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