Barry N. Turner wrote:
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.

Barry

PS Is it time for me to replace my Super Nova with a new Stronghold
chuck? Do I just need to put this blank on the rack, wait a few weeks and
let it finish drying before I come back to it?


There could be any number of things going on here.

First, the wood is now drier and therefore harder than before. The
cutting action is therefore going to place more force on the bowl and
hence the tendency to create vibration. Sharp tools and light cuts are
in order.

The tenon, and the shoulder that the chuck jaws mate against, will have
distorted as the bowl dries. The flat surface is probably no longer
flat, and the tenon is no longer round. This may be enough to allow the
bowl to move a little in the chuck.

The bowl will have distorted, so as it rotates the depth of cut is going
to vary. This might cause vibration.

I use a SuperNova and don't think I would have a problem. What size
lathe spindle do you have? When I moved up from a Delta (1 inch) to the
Nova (5/4 inch), I noticed a lot of vibration problems disappeared and
my turning improved considerably.


FWIW, when I skim roughed out bowls, I usually do the outside first.
Holding the inside of the rim of the bowl in cole jaws allows me to work
on both the walls and the recess (in your case the tenon). Gripping the
rim gives a much bigger mechanical advantage compared to the base.


--
Derek Andrews, woodturner

http://www.seafoamwoodturning.com
http://chipshop.blogspot.com - a blog for my customers
http://www.seafoamwoodturning.com/TheToolrest/ - a blog for woodturners