First of all, it's only been about 10 days since I roughed out the bowl. I
guess I'm rushing things, but I was just trying to thin down the walls more
so the bowl would dry faster. Initially, the walls of the bowl were about 1
1/4" thick.
A few days after I roughed out, I put the bowl back in the chuck and took a
couple of passes off the inside. Yesterday, about 10 days after initial
roughing, I noticed the bowl was drier and decided to rechuck and thin the
walls a bit more. A bowl with walls over an inch thick is going to take a
long time to season. The tenon and bowl may have distorted some, but its
just not dry enough to have distorted a lot. There is no visible distortion
yet.
My lathe spindle is 1 1/4" (Stubby 750). On the advice of a friend of mine,
I also checked to make sure the threaded insert in my Super Nova Chuck had
not loosened. (It hadn't)
Your comments about mechanical advantage make sense, I'm holding this bowl
on a 2" stub tenon and I'm making cut on a 10" diameter part of the bowl.
Therein probably lies the problem. Thanks.
Barry
"Derek Andrews" wrote in message
...
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