Grain orientation question

[Harvey Schneider]

I have always oriented the grain on my bowl blanks so that the center of the tree is nearer the mouth of the bowl. I feel that this gives a more interesting appearance to the finished bowl. I remember reading somewhere that green wood will move less if the orientation is with the center of the tree nearer to the foot of the bowl. Is this true? What does everyone else do?

[Kim Ford]

Harvey;

Great Question .. . . . . . . . I will enjoy hearing the input from the "creek". As for me:

I turn bowls both ways. It just depends upon the graining in the piece and what works. I kinda stick to the adage that the wood itself determines not only which way you orient the bowl but also the shape. Not always true, but allowing the grain and the color to be accented can produce some stunning results.

Shrinkage: The primary shrinkage from wood is in direct proportion to the length of the grain (or growth ring from a cross section) in the piece. So when you orient the wood with the mouth or rim of the bowl at the center of the tree, the length of the grain at the outer edge of the bowl perpendicular to the heart is the longest and hence it shrinks the most. Think of the growth rings as rubber bands that contract as the piece dries. The longer the rubber band you have the more it will contract or shrink. That is why when you look at the dryed blank, if it was wet wood it will appear as if the center or heart has jumped up but what has happened is the rest of the rim has contracted down.

When you orient the base of the bowl at the center of the tree the growth ring lengths are shorter because the shape of the bowl interupts their length and the movement in the piece will generally be less. It's hard to describe, but is really simple when you think about it.

[Rasmus Petersen]

im not quite shure what you meen. a quick pic, or draw would be helpfull.

[Frank Drew]

Harvey,

Since you generally get the largest bowl blank by having the bowl mouth at what was the center of the log, as you describe, that's been my preference; I never thought about the shrinkage differences, though. I do believe that you'll get a different grain pattern in the bottom of the bowl depending on the original orientation with respect to the center of the log, either concentric circles or "figure eights" (for want of a better term).

[Steve Trauthwein]

Good description Kim. Another way to visualize this is to picture the rim of the bowl as a circle that intersects with the growth rings at a pependicular angle. If you start this circle at the center of the tree the growth rings are a full cirlce and under more tension, as in the rubber band analogy. Then mentally move the circle to the outside of the tree and the intersected growth rings become shorter and broken, thus presenting less tension.

Regards, Steve

[Kim Ford]

This is a two dimensional drawing that shows the relationship of the grain pattern to the bowl in both applications.

[Frank Kobilsek]

Harvey
When a log has significant contract in color from heart wood to sap wood and the sap wood is very thick, for example apple or locust, I will put the base of the bowl in the center of the tree. That gives a dark center to the bottom of the bowl with a lighter surrounding rim. I think that look is well worth the reduced size of the finished bowl.
Frank

[Reed Gray]

In my turnings, I have noticed little or no difference in how much the bowls will move relative to grain orientation, other than if you turn one end grain, it will distort less than ones turned flat grain. How much they move depends more on tree type, and water content (spring with lots of water, or fall with less water). the direction they move can change some with turning from the inside of the tree to the outside (normal way), or outside in.
robo hippy