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TWW TWW is offline
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Default bulging bowl bottoms

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?
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Default bulging bowl bottoms


"TWW" wrote: (clip) The
center of the tree on the bowl bottom had bulged out enough to make
the bowl wobble. (clip)

^^^^^^^^^^^^^^^^^^^^^^
I believe that is the reason the bottom is usually turned concave. Wood
moves, even after it is seasoned, as temperature and humidity vary. If you
left the bottom thicker, and then resanded every time the bowl started to
rock, you might eventually reach a condition where it changes from concave
to flat instead of flat to convex.


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Default bulging bowl bottoms

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
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Default bulging bowl bottoms

It warped that way because of grain orientation. On bowls turned with
the pith on the top rim of the bowl, and the bottom of the bowl is on
the outside of the tree, the bowl bottom will go concave as it dries.
When the bottom of the bowl is towards the pith, it will go convex (or
bulge). The interesting thing is that if you get it wet again, it will
warp back the way it was before it dried. They will always move some,
because that is the way wood is. It will change due to humidity
variations because it soaks up some moisture. This is pretty much the
way boards dry. Cut a flat board off the outside of the tree, and it
will warp/cup away from the center of the tree.

robo hippy

On Jan 26, 3:32*pm, Fred Holder wrote:
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


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Default bulging bowl bottoms

On Tue, 26 Jan 2010 06:42:52 -0800 (PST), 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?


Sounds like an end grain project, as opposed to cutting a "normal" cross grain
blank?

You're turning green wood here.. the 3 months changed them (in my terms) from
WET wood to Green wood.. Sill going to have movement..
In an cross grain bowl, it will warp "egg shape", like a banana split dish..
The way you turned yours, the warp will mostly be in the bottom..

I usually leave enough wood at the bottom of the bowl so that when I reverse it
to finish the bottom, I can dish out the foot.. Might have helped in your case,
but not sure with out pictures..

Bottom line is: Did you enjoy the experience and want to turn some more? If so,
it's a winner, right?


mac

Please remove splinters before emailing


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Default bulging bowl bottoms

in addition to what everyone else said, I find that oak does this a lot - so
either feature it (and turn thin to avoid cracks) or do as others suggested
and plan for it.



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