"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.