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

"charlieb" wrote in message
...

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.

SNIP

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


Only the ones from the Hoadley book and the FPL site. The shrinkage depends
on the curvature of the annual rings. Faster curvature in areas of tighter
rings.

Since I regularly turned things like Darrell's angel wings with the pith in,
and they survived, I figured there might be another factor at work in
failure when making bowls along the grain.

I looked at some, and discovered that they cracked from the inside out, and
some were even obvious enough to have visibly wet grain where they had been
sitting. So I started elevating them on stickers to provide free
circulation underneath, and the problem seems very much under control. Last
dozen or so pieces I've done that way have done just fine. Makes sense that
an expanded sheltered bottom will stress the rapidly contracting top.

Other thing you can do is use your contour to let the outer portion contract
at least partially into air, rather than wood. Means no flat bottoms. Also
seems to help a lot. You may have to weight the base of a goblet afterward,
but it seems worth it.

"charlieb" wrote in message
...

I'm using the same lathe and have the same problems using green branches. I
don't know the answer to your question but I've recently been turning some
Bradford Pear. I had one piece that split at the pith on both sides of the
bowl (I was turning crosswise). It was a minor crack on each side. It was
rainy so I had left them outside and things were great until we got a drying
wind. That's when the cracks appeared. But the weather turned cold and
they closed up. Warm they opened again and cold they closed again and
pretty much stayed closed. Then I finished with oil and in two days one of
them split seriously. These had fairly thick walls

One thing I am currently trying that has worked so far is I turn to finish
size (much thinner walls) and place the item in a plastic bag and leave like
that for a week although I do check wipe out the water that has seeped out.
That's more a matter of I got to see what's happening rather than thinking I
need to wipe out the water. I then open the bag up so a little air can get
it and spend another week letting it dry that way. So far, two Bradford
Pear bowls with lids have dried without warping or cracking and three oak
(two with lids) dried without issue. The oak is a branch that fell and took
out part of the roof on my shed -- I'm seeing some minor spalting in this
but there is enough water in it that an earlier attempt split from the
moisture in it. They have all been sitting out on the table without issue
for more than a week although that wasn't by plan, just worked out that way.
Generally I've seen cracks long before now so I am hopeful that this is
going to work. the paper bags, damp shavings, etc. have not worked.



--
Posted via a free Usenet account from http://www.teranews.com

Just musing with no scientific reasoning or experimental backup: Perhaps
the pith itself, doesn't play a causal role in the internal stresses of
drying timber, it's just the area where the growth rings begin. Maybe
the stresses are owing to constricted growth rings due to an intact
drying log or limb or distorted growth rings due to summer-winter
growth, compression-tension reactions or cell
alignment-'dis'alignment wind.

Cutting the rings radially to their beginnings at the pith or that 'one
big longitudinal crack' allows for their free movement tangentially that
would be otherwise constricted with resultant severe stresses. Should
higher Tan/Rad ratios relieve stress and result in less cracking?

What am I talking about, Charlie? I dunno except I think that drilling
out the pith doesn't affect the rings (the real culprits) the same as
relieving their tangential stresses by cracking or cutting radially to
the pith. Whether it's removed or not may not matter as much as we all
believe. You did ask for theories so let the laughter begin!


Turn to Safety, Arch
Fortiter


http://community.webtv.net/almcc/MacsMusings

All good information.

Bradford pear is apparently semi-notorious amongst solid
wood furniture makers as proned to splitting, cupping, warping
etc. - and I'm betting it T/R ratio is much closer to 2.0 than
1.0.

As for keeping the green turned piece in a plastic zip-lok type
bag and opening to wipe out condensed water on the inside of
the bag - a week's worth of this probably wouldn't start mold
from forming. Longer periods and warmer temperatures sure
seem to get mold growing in damp wood, especially in a sealed
plastic bag.

Wall thickness seems to be one of the significant factors in
cracking of "pith in" pieces. Makes sense - thinner walls mean
less material in which differential shrinkage strains can build
up. Thin walls are easy to turn - in bowls and cups and other
open forms. Not to easy with "pinched neck" more closed
forms, especially if there's a longer narrow neck. Short of
attaching a laser pointer to the tool there's no way to measure
wall thickness in these types of forms. And it's the bottom
thickness - that still contains the pith - that's the hardest
to measure - while the piece is still in the chuck. But the last
thing you want to do if you're getting walls thinnned down
is to have to rechuck the piece.

growth rings
Frank Klauzs, an "old world" trained furniture maker has a
rule about grain and drawer parts - I.D.I.O.T. - Inside of
Drawer Is Outside of Tree. Boards, if they will cup, will cup
with the concave side being the "outside of the tree" face.
His explanation if that the growth rings are like rubber bands
- the ones on the outside of the tree are stretched a lot
more than the ones towards the inside of the tree. So the
"outside of the tree" growth rings want to get shorter
while the growth rings on the "inside of the tree" aren't
stretched much if at all, so they're not trying to get shorter.

So if the T/R ratio is higher, the rubber bands towards
the outside of the tree, or in this case the branch, will
be stretched tighter than it would if the T/R ratio were
lowers. The tighter they're stretched, the more strain
they put on the wood cloer to the center of the log/
branch - the outside growth rings shrinking to relieve
stretching tension - compressing the grain towards
the inside of the log/branch. Now, because the grain
closer to the center is heartwood, and heartwood is
stronger than sapwood, the heartwood cells should
be able to stand more compression than sapwood
furhter out towards the perimeter of the log/branch.
In that case, I'd expect the cracks to begin at the
outside and travel inwards towards the center.

BUT - because the pith is in the center, and pith
is the weakest of the wood cells - you basically
have a "hole in the middle donut". Now you have
a compressive force being applied and accumulating
from the outside to inside - maxing out at the
last inside rings of the donut. If that compressive
force exceeds the cell walls/lignen strenght, then
they will fail/crack and the cracking will radiate
OUTWARD rather than inward.

With "pith in" cracking, that's exactly what happens,
the crack initiated around the pith and radiating
outward.

So drilling out the pith and plugging it with solid
wood seems to be a way of keeping the cells/lignen
adjacent to where the pith WAS might help.

rate of curvature based on how the distance
between growth rings changes - tighter grain/
tighter radius curve - looser grain/longer radius

This seems intuitively right in a convoluted way
when viewed from a "strain on the wood" perspective.

Look at FIGURES 1 & 2 below (sorry about the ASCII
drawing). Not that both hollowed objects have the
same maxium diameter and the same "walls" and
bottom thickness. But FIGURE 2 has more growth
rings in its bottom than does FIGURE 1.

If the amount of shrinkage is proportional to the
thickness of the wood - ACROSSED THE GRAIN
- then the strain on the bottom of FIGURE 2 will
be considerably greater than for FIGURE 1 -
about four and a half times greater.



FIGURE 1 FIGURE 2

|-------- D ---------| |-------- D ---------|

| | | | | |
| |
\\ // | | | |
| | | | | |
| |
\\ // | | | |
| | | | | |
| |
\\ // | | | |
| |_| | | | |
|
+||||+ | | | | | | | | | | | | | | |
| |

+||||+ +||||++||||++||||++||||++||
---| d |--- |-------- D ---------|

One problem with FIGURE 1 - it's not very stabile.

Barry Irby, over in WoodCentral had a great suggestion
- do a concave "dome" ala the bottom of a champagne
bottle to get the bottom wall thickness thinner, while
maintaining a stabile base

| | | |
| | | |
| | | |
| | +||||+ | |
| | // \\ | |
| | // \\ | |
+|+ +|+

Again, from WoodCentral, John Jordan recomended
R. Bruce Hoadley's book Understanding Wood. I
have that book but skipped over "pith in" stuff
since I was interested in how BOARDS behaved.
not logs and branches. Will go over that information
much more thoroughly looking for why "pith in"
cracks - and what can be done to avoid that type
of cracking. Will report back.

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

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

charlie b
[snip]

If you haven't already you should read this book:

URL:http://www.amazon.com/Understanding-Wood-Craftsmans-Guide-Technology/dp/1561583588/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1204792325&sr= 8-1

"charlieb" wrote in message
...
All good information.

Wall thickness seems to be one of the significant factors in
cracking of "pith in" pieces. Makes sense - thinner walls mean
less material in which differential shrinkage strains can build
up. Thin walls are easy to turn - in bowls and cups and other
open forms. Not to easy with "pinched neck" more closed
forms, especially if there's a longer narrow neck.

You've missed a point of reality here. If the piece grabs air rather than
wood, it has no place to pull against. It can only pull against itself.
All shrinkage is local, just like politics. If the rate is 5%, then it's 5%
of whatever thickness you have, with a more or less symmetrical shrink
toward center.


growth rings
Frank Klauzs, an "old world" trained furniture maker has a
rule about grain and drawer parts - I.D.I.O.T. - Inside of
Drawer Is Outside of Tree. Boards, if they will cup, will cup
with the concave side being the "outside of the tree" face.

Yep, they curve toward the sapwood. See the FLP diagram, or the one in
Hoadly for good visualization.


So if the T/R ratio is higher, the rubber bands towards
the outside of the tree, or in this case the branch, will
be stretched tighter than it would if the T/R ratio were
lowers.

Nope, they will just have more distance before they are intersected by a
ring. 5% of a 4" long is greater than 5% of a 2 inch long span. Game
starts more or less over at the next ring.


With "pith in" cracking, that's exactly what happens,
the crack initiated around the pith and radiating
outward.1

That's because you're drying that area faster than the outside. When the
opposite happens, as on your woodpile, you get radial cracks originating on
the dryer surface and running to center.


So drilling out the pith and plugging it with solid
wood seems to be a way of keeping the cells/lignen
adjacent to where the pith WAS might help.

No. Giving it an open place to contract is one answer, not putting something
strange in there to press against


rate of curvature based on how the distance
between growth rings changes - tighter grain/
tighter radius curve - looser grain/longer radius


Again, from WoodCentral, John Jordan recomended
R. Bruce Hoadley's book Understanding Wood. I
have that book but skipped over "pith in" stuff
since I was interested in how BOARDS behaved.
not logs and branches. Will go over that information
much more thoroughly looking for why "pith in"
cracks - and what can be done to avoid that type
of cracking. Will report back.

Look at Hoadley, or at The Wood Handbook Fig 3-3 at the FPL site for a great
depiction of the direction and dimension of shrinkage for pieces taken from
different places in the log. You will notice that the rings are the key.

Do what Barry said, and what I said in my post above and you'll get better
results. There's a whole genre of similar forms out there, all with the
pith. http://i35.photobucket.com/albums/d1...e/4c0d5a44.jpg
http://i35.photobucket.com/albums/d1...ch-Upright.jpg

Cut thin, balance the rate of drying and you'll get there.