Sounds like it's the natural result of increased friction and a grain lift
or two. Are you getting chatter marks on the surface?

Couple things I've done in the past, before a steady, is to turn slow and
ride the left hand outside the bowl, or take two or three rounds of duct
tape to the outside to dampen vibration.

I burn wood, myself, and I've been obliged to stoke four times a day with
what our temps have been - negative 20's Celsius. Of course the wind blows,
the lake snows, and I've had to plow pretty much every day for the last week
and a half. My frozen wood supply is under a hard-packed 66 cm of snow,
which is what two meters of light stuff compacts to.

I swear the road commission has a sensor at the end of my drive. Every time
I remove the snow, they come by with the plow and fill in!

"Denis Marier" wrote in message
...
Thanks for the information George
I have used a screw a screwdriver near the bearing cap. The noise is
constant without any change in pitch.

Lately, the heating system in our house is working hard to cope with
extreme
cold weather (-30). The blanks are stored in between the basement rafters.
When turning the blanks I noticed that they are very dry.
I only hear the low pitch noise when I take light cut/shear with the
scrapper.

"Denis Marier" wrote in message
...
Thanks to everyone for their comments.
Maybe the situation would have been better depicted by using the
"Harmonic
vibration " instead of humming noise.
After about one week I have found that all the comments are valid.
The Harmonic vibration is probably produced by the unbalance in rotating
components of my lathe. This may be further compounded by resonance
caused
by shattering of the scrapper applied on the thin wall of the maple
bowl.
I
was told that in some instances the rotating components may be fairly
balanced but the finishing gouge or the scrapper lightly rubbing on a
bowl
could produce resonance. I was told that the same thing takes place
when
rubbing the lip of a glass with your finger. Some time bowl steady are
used.
Now, If only I can find a way to adjust the bearings I may reduce the
Harmonic vibration a little?



"Ken Grunke" wrote in message
...
Denis Marier wrote:
My lathe is a low end Taiwan made with 3/4 HP and 1" 8 tpi.

The other day I was finishing a 9" OD maple bowl with a 3/8 X 1"
scrapper at
about 800-1000 RPM.
It worked well. Until I reached the 9" OD lip where the wall
thickness
was about 3/8 thick. Then what seem to be the Headstock started
to
hum at
low decibels.

I was going to say maybe your headstock bearings are just a tad too
loose. When your tool gets to the outer diameter of a piece, even if
you
are using the same amount of force as you did towards center, that
force
is amplified by leverage and putting more sideways pressure on the
spindle causing the bearings to vibrate.
But then I thought some more, and figured it could be the wood itself
humming. Probably due to the thinness of the wood, it's hardness, and
the broad-edged scraper.
If you heard the humming while turning the outside (before hollowing)
my first theory makes more sense. If you find a way to adust your
bearings, do so very carefully--a bit too tight can overheat and wear
them prematurely.
I could be wrong on both points, it may be just the quality of the
lathe--but with those cheap imports, you can usually tweak them for
better performance if you have some mechanical know-how.


--
Ken Grunke
SW Wisconsin
http://www.token.crwoodturner.com/
Member, Coulee Region Woodturners AAW chapter
http://www.crwoodturner.com/



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