Pith - The Pits!?

SNIP

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.

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.

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 summerwinter growth, compressiontension 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

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.

re: 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.

Re: 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

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

+||||+ +||||++||||++||||++||||++|| --->| d |

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

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charlieb wrote:

charlieb skreiv:

[snip]

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

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.

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

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.

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.

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

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.

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