"Barry N. Turner" wrote in message
...
I put my roughed-out box elder bowl back on the lathe yesterday. Its about
10" in diameter by about 6" tall. It had dried some, so I had decided to
thin down the walls to hasten drying by taking a few hollowing cuts off the
inside of the bowl. Although the bowl walls are well over an inch thick (1
1/8"), I get a low frequency vibration when I start the cut near the rim of
the bowl. As I near the bottom, the vibration diminishes and goes away.

The bowl is mounted in my Super Nova chuck with a 2" stub tenon. The
chuck is tightly seated against the spindle....no plastic washer or
anything. The 2" standard jaws are tight on the tenon. The tenon is
intact....not cracked or anything I am no longer using tailstock support
as the bowl is now hollowed. The Crown PM 5/8" Ellsworth grind gouge is
freshly sharpened. Still I get the vibration. Any ideas would be
appreciated. Thanks.



OK, as others have mentioned, you're not supporting the piece in a way
calculated to keep it from swaying. Gripping a tenon that has certainly
distorted from drying, especially without a true reference face at right
angles to the tenon, won't do it. If you buy a set of jaws that chew the
tenon as you tighten you have all the problems above plus artifact from the
original torn/crushed fibers to contend with. Derek has found a way to get
around some problems of drying distortion. He re-trues the tenon and face to
get full support and hold.

I do the same, but with the pin chuck, I just run a 1" Forstner down the
narrowed (cross-grain drying distortion) hole I used when roughing the wet
blank, put on the lathe, and do the outside. Easy, extremely close to the
original turning axis, and when I'm ready to reverse, I have a circular
mortise and a flat bottom inside for my expansion dovetail. BTW, that
safety talk from the manufacturers is probably pretty well padded for legal
purposes, so use the size mortise or tenon that brings success. It may get
smaller as your proficiency improves. You might even do things that look
dumb to prove you can do it at all.
http://photobucket.com/albums/d160/G...able-Again.jpg
Red oak, and you can bet the bottom is heavy to hold it from tipping.

Last, the bowl has distorted, and until you get it back to circular, a task
which will be difficult if not impossible without proper chucking, you'll be
shaving the side grain and missing the end, which makes the balance worse,
of course, as it shifts the center of mass outward. Your tool hits twice
each revolution, but the effect diminishes toward the bottom of the bowl,
because shrink is in proportion. You are also putting much less strain on
the piece, as the force varies with the square of velocity, and your force
is therefore much less the closer to center you go. Which is why, of course
you're rotating as close to slow as your patience can support, else you
exacerbate all the out-of-balance problems.

Speculation of the walls being too thin is just that. They're not going to
move much until they get below 1/2" green or 3/8" dry and have some
reasonable force placed on them by spinning too fast or "riding the bevel"
too hard. . When you get close to that, it's time to resharpen or rehone,
your preference, and peel easy to desired thickness.

Oh yes, boxelder has approximately the same drying characteristics as soft
maple, so you can get to 20% MC from green in 160 above-freezing days
according to FPL experimental data on _planks_ 1" thick. Means one tenth
of that time through the end grain. Under winter rules, if you've got that
blank open to the warm air, you're probably talking 4-6 weeks to ambient 6%
, tops.