I am brand new to bowl turning but I do have 2 credentials for you to
evaluate at your peril:
- Sawed down, cut up and split hundreds of cords of wood over the last
35 years.
-Watching the spring-pole lathe woodturner opposite me in the MN state
fair grounds blacksmith shop as he turned ale bowls. He removed the
part from the lathe every time he took a break and stuck it into a tub
of water to make sure that no differential shrinkage took place.


I don't think the pith is the problem. It seems to me that the bigger
the hole you drill when you remove the pith, the less splitting problems
you will have. It's just basic leverage. The thicker the wall of your
turning, the more leverage there is between inside and outside of the
wall. If the fulcrum of the "lever arm" is to the inside and the
cohesion of the fibers on the outside is exceeded, then the wood cracks
on the outside first.

Pete Stanaitis
---------------

charlieb wrote:

Unlike some here who have BIG lathes, and probably the chainsaw and
bandsaw to go with them, I turn on a JET mini/midi - 10" nominal swing
and maybe 13" between centers. Though I have a chainsaw and a bandsaw
with a 12" throat capacity, I often find myself turning branches that
are readily available from neighbors and friends and require no cranking
up the chainsaw and no bandsawing out a bowl blank.

Problem is The Dreaded Pith. Pith is the pits!? Unlike folks who
chainsaw a blank from a log, being careful to avoid The Dreaded Pith, I
often turn "pith in" branches, usually "green" fresh cut stuff. Lately
the wood has been plum. playing with pinch pot hollow forms, through a
half inch or 5/8" hole. Because of the pinch neck and small opening,
it's hard for me to judge wall thickness, especially the bottom
thickness. Having blown through the bottom and on a few occassions, the
side wall - I err on the side of caution and quit while I'm ahead.

THAT may be part of my problem. I turn a nice form in some nice wood
that happens to still have the pith in it. I drill out the remaining
pith in the bottom of the piece, turn a plug for it out of dry maple or
mahogany, epoxy it in then apply a carnuba finish to the outside using
an unstitched wheel on the buffer, then pour some oil inside, slosh it
around, maybe put t back on the lathe and spin it in hopes of forcing
some of the oil into the wood then pour any excess. Sometimes I give
the bottom a coat of two of thin CA glue. Then it's stuff some paper
towels inside and try
1. paper bagging
2. paper bagging under a pile of semi-damp shavings

Sometimes all that works - and sometimes it'd don't.

So what is it about the pith that seems to make it propogate cracks,
even after it's been drilled out and the space plugged?

Pith is the softest wood - spongee and porous - so it dries quicker than
the heart or sapwood. But even when drilled out and plugged, cracks
seem to propagate from the pith, or where the pith was even after it's
been replaced with a denser plug.

Normally the direction of a crack, outside in or inside out will
indicate a drying rate problem. If the crack starts on the outside and
goes inward then the outside dried too fast. So if the crack starts at
the pith and goes outward then the inside dried too fast.

But some wood I've turned with the "pith in" and left it in haven't
cracked after being inside the house for over a year. Pieces turned
from "pith in" English Walnut with no dark heartwood have held up well.
And the same goes for Magnolia and Black Walnut. Fruitwoods - cherry,
plum and apple all seem proned to pith propogated cracking.

So now I'm thinking it may be the concentricity of the grain that's the
culprit, or at least an active member in The Dreaded Pith Plot to crack
my "pith in" green turnings. Could it be that the smaller diameter
innner growth rings are drying faster than the larger diameter outer
rings?

Now if you've looked at a wood shrinkage table, you'll not that the
Tangential Shrinkage % is ALWYAS greater than the Radial Shrinkage %,
sometimes twice or more greater. Maybe the RATIO of tangential
shrinkage to radial shrinkage is one of the culprits in the "pith in"
cracking problem. If the combined shrinkage mwere closer to being equal
then the original form might be maintained, just made smaller
proportionately in all dimensions,

Here are some Tangential/Radial Ratios for various woods, examples
grouped low to high. Note that the two fruitwoods are close to 2.0
while walnut and magnolia are closer to 1.0. Could this be the
indicator of which woods are proned to "pith in"cracking and which are
not?

Birch, Yellow 1.3
Black Walnut 1.4
Yew Pacific 1.4
Basswood 1.4
Birch,Paper 1.4
Bubinga 1.4
Mahogany 1.4
Magnolia 1.5


Apple 1.8
Cherry, Black 1.9


Holly, American 2.1
Madrone, Pacific 2.2
Maple, Soft 2.2
Teak 2.3
Willow, Black 2.6
Lacewood, Australian 2.9
Pine, Eastern White 2.9

Anyone have a theory about why "pith in" turnings are more proned to
cracking than "pith out" turnings?

charlie b