Turning Archive 2008

Subject:
Pith - The Pits!?

charlie belden
>(also posted this question to rec.craft.woodtrurning so if you saw it there, skip it here)

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

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