Derek Andrews sagely responded:


1- the bowl has been remounted. Even with 'perfect' technique there is
still likely to be some lack of concentricity, but there is plenty of
scope for poor technique and some woods lend themselves to better
accuracy than others.

2 - the wood is green and warping as it dries out.

3 - the turning preocess is releasing stresses in the wood causing it
to
distort.

--
Derek Andrews, woodturner


I am with him. Check for runout as indicated by others above. But for
me personally, I am thinking along the lines of Mr. Andrews. When
remounting a piece, it is ALWAYS out of balance. How can you remount
in exactly the right place with exactly the right pressure on your
chuck to give you tolerances that are within a few thousandths?

When giving my off center turning demo, I show the class how little it
takes to get a kiln (OK microwave) dried piece off center. I take a
cylinder that is about 3" across and 6 inches long. I turn it true
between centers, and turn a 2" tenon on it to mount in the chuck. I
mount it in the chuck with the shoulders of the blank firmly against
the chuck, then turn on the lathe. It is now no longer balanced or
true. But then to drive in the point of alignment, I loosen the scroll
chuck a bit, and bring the tailstock up to the piece and put the point
in its old hole. Add tailstock pressure, tighten scroll chuck, and
remove tailstock and turn on the lathe. It is still not perfect, but
it is 100% better.

When you turn a bowl, you have no such realignment tool. Mount the
blank, turn away, and then you go from there. I have also found that
turning woods with large knots, or other features such as holes,
occlusions, or punky areas are particularly bad about being out of
round in just a few hours. Then if I fill a large void with some epoxy
mixed with who knows what, watch out.

When I started turning, I found this out these things well as the fact
I needed to put an index mark on the piece and the chuck so I would
remount as closely as possible.
Still in frustration, I started doing things like leaving the piece on
the jaws while out in the shop. Yup, after a day or two it was out of
balance. It had begun to dry. So I tried bringing the piece in the
house, still mounted on the jaws; same results, not as dramatic.

With really green wood, I have turned it out for a while, gone to
lunch, and some back and it is out of balance!

Now I just accept the fact that they will be out of round and a couple
of passes with the correct tool will get them back on track and ready
to finish up.

Robert