On Jan 26, 6:42 am, TWW wrote:
Since this group has gotten quiet again here is an obscure topic
someone may know about. I cut off some pieces of oak last fall when
turning 8' logs into firewood. I created rough bowl blanks and dried
them for three months at least and then turned some final bowls.
Nothing out of the ordinary there except for two bowl blanks that had
the center of the tree in the center of the bowl bottom (same as
spindle turning but I am not sure if that is the correct term when you
do a bowl). I did not feel like making some sort of jamb chuck so I
used a belt sander to do the bottom of one. My wife loved the bowl
but about two weeks later she told me the bowl was wobbling. The
center of the tree on the bowl bottom had bulged out enough to make
the bowl wobble. I used the belt sander again to smooth off the
bottom but a smooth bottom resulted in a small hole in the center of
the bowl bottom. Does anyone have much experience with 'spindle-
turned' bowls who know any tricks to prevent the bulging?

When you turn an endgrain bowl, the diameter of the bowl is fairly
stable, but greatest warpage is in the direction of top to bottom. In
your case, the bottom of the bowl warped outward giving a bulge on the
bottom. Regardless of the grain orientation, it is always desireable
to recess the bottom of the bowl so that it will set only on the
outside edge of the bottom. This helps reduce the possibility of the
problem that you had and generally makes a more stable bowl. Some
woods warp more than others. Had it been something like black locust,
there is a chance that it would not have become bulged on the bottom.

Fred Holder
http://www.morewoodturning.net