What caused this cracking? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 18 Old 04-07-2012, 10:30 PM Thread Starter
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What caused this cracking?

This is the top of the hollow form I turned a few days ago. The picture of looking down on the end grain and the center of the log was right in the middle of the top home. The log was very green when i turned it. The cracking pattern doesn't make sense to me, it doesn't radiate out from the center and doesn't seem to be parallel to the growth rings, but rather curve away from the center. Any analysis from someone who knows better?
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post #2 of 18 Old 04-07-2012, 10:40 PM
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Looks like stress cracks where the stress was at one time, Its happens a lot all depends what part of the tree it came from an how green it was

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post #3 of 18 Old 04-07-2012, 11:25 PM
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Can you wet it and take a picture if the growth rings show up better?. Ideally draw on the grain direction and the growth rings with a dark pencil?
-----
It's almost alway going to related to drying stresses if the wood did not have hidden cracking relating to the fellin of the tree.
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post #4 of 18 Old 04-08-2012, 12:11 AM Thread Starter
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Here are 3 additional pics. The first is the raw log on the lathe so you can see the pith is right about where the tail center is. I believe the end you see ended up being the base. The second pic shows the growth rings the best, it was taken right after the form came off the lathe while the wood was still naturally wet. The grain direction is straight out the top of the piece. It just happens where the fractures formed was facing away from the camera, but the lines were visible back then, but didn't appear to be cracks. The third pic is when I just wetted the top, but the growth rings are hard to see, but they are concentric around the hole in the top.

I'm pretty sure this log was the trunk of the tree, but it was a relatively small tree.
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post #5 of 18 Old 04-08-2012, 04:10 AM
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That's too bad. I agree with the stress crack theory, either from felling or storm damage. Assuming that you made a relatively even wall thickness it's probably nothing you did wrong. I'm glad it didn't give way while on the lathe. Unfortunately I'd put the rest of that log in the firewood stack if you have more.
I wonder if you could rewet the wood to get the cracks to close up then flood them with thin CA. I had a rough out starting to check the other day and I heated it in the microwave then put it in a plastic bag just to even out the moisture. The cracks closed right up. just a thought.
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post #6 of 18 Old 04-08-2012, 10:28 AM
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Next time put the piece in a paper bag and throw some shavings in with it. Close it up and let it dry at a much slower rate and see if it cracks. Another method is to put the whole thing in a pot of water and boil it to relieve the stresses and speed up the drying process afterwards.
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post #7 of 18 Old 04-08-2012, 10:32 AM
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I also believe they are stress cracks. Could have been there when you started turning. Was it close to where the tree was cut down. Sections further up may not have that problem. I tried to turn some bradford pear that had gone down in a storm. I had lots of stress crack problems and just ended up tossing the wood.
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post #8 of 18 Old 04-08-2012, 11:47 AM
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Turning issues aside, that's quite a shop you've got there. Looks cleaner than my house:).
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post #9 of 18 Old 04-08-2012, 08:09 PM
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I would say it looks like stress fracture, I assume this piece of wood is from a tree limb? When the wood no longer had to support the weight of the branch, the same forces pulled one side of the vessel away from the other side causing the cracks you see.

I want to die quietly in my sleep like my grandfather, not screaming in terror like the passengers of his car.
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post #10 of 18 Old 04-08-2012, 09:03 PM
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Thanks for the new pictures.That is drying cracks.
. If you want to turn a green end grain log, you have to turn it very thin like 1/8 inch.
Any wood that includes the pith and centre of the log has large changes and stress from the uneven drying. The end grain section that makes the top of your vessel loses water very quickly relating to the sides of the vessel. The stresses are very great leading to "checks ".If you turn it thin, it will distort instead of cracking.
Also if a few times if you notice early cracks, you can keep them from propagating by a few drops of CA glue.
All the measures to make the drying slow help also after you have finished.
Melvyn Firmager did a lot of work teaching the process. Essentially he turned to very thin walls.
You also try to get the whole thing turned turning from the top down to a thin wall. Once you leave a section you can not go back as it will already have started to deform. Plastic bags and freezing can sometimes buy time for a meal or maybe overnight. Once you start , you want to get the sections turned thin.
Another approach is to not include the centre of the log. Turn a vessel from a third to 1/4 of the log and stay a few inches from the centre of the "blank".
The more uniform the gran structure of the log the better as the distortion will be symetrical.
Let me know if this is still a bit unclear. It's hard to describe in a few words.
Bob
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post #11 of 18 Old 04-08-2012, 09:21 PM
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The other drying advice applies mostly to side grain green turned bowls. Drying them in paper bags, using endseal or pentacryl and doing the first drying in a more humid environment like shelves in a garage or shed for the first few months helps avoid losses to drying cracks. When the bowl or vessel is down to 12 % or so you bring it into the house to finish te drying. A moisture meter is ideal but you can get by with the sequential weighting. Weight carefully on a weekly basis and move to the next stage when the wet is stable.
Ther are good books and articles on green bowl turning.
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post #12 of 18 Old 04-10-2012, 10:10 PM Thread Starter
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Thanks Bonanza, I'm pretty happy with my workshop/man cave. I just put updated pictures of my shop here: http://www.woodworkingtalk.com/membe...s/my-workshop/
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post #13 of 18 Old 04-11-2012, 10:35 AM
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That's an impressive shop. Hoe do you get time for projects. Reminds me of Andy Rooney's comment when asked what he builds. He repelied my shop.
You should be able to acquire a moisture meter. An invaluable tools that is rarely talked about.
Mike Mahoney also has a good DVD on hollow forms in green end grain timber. Thin!
The direction that the cracks propagated is interesting. They are usually radially but can be anywhere if there is an inherent weakness at a growth ring or adage in the log.
Anyway the vessel need to be able to deform as it dries or it will split.
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post #14 of 18 Old 04-11-2012, 11:04 AM Thread Starter
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Thanks Midlandbob. I enjoy spending time there so sometimes making a tool holder is just something I can do quickly as an excuse to be hanging out down there. Once I was practicing dovetail joints a lot, I'd start with a good sized board, cut dovetails, then cut off the ends and do it again. Turns out those cutoff angles come in handy as tool holders and as stops for jigs. I'm gradually moving from open shelves to closed cabinets (just finished building those wall cabinets) so dust doesn't collect on what I'm storing. I don't run the overhead air cleaner as much as I probably should because I like to have the TV on while I'm down there. Someday I'll probably end up with a Festool dust collector to replace the Craftsman Shop Vac because it'll be much quieter to run. My wife is obsessive about keeping the upstairs clean so justifying fancy cleaning equipment is relatively easy in my household.
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post #15 of 18 Old 04-11-2012, 09:34 PM
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Yep, those are stress cracks and I come up on those from time to time. You can always fill the cracks with inlace chrystals as long as the bowl is dry. Also, GREAT looking shop.
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post #16 of 18 Old 04-11-2012, 10:20 PM Thread Starter
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So it seems like the opinions are divided between stress cracks and drying cracks. I'd much rather believe stress cracks because that would imply that I did things right otherwise and the thickness I turned the form to was OK, but with the split opinion I don't know what to think.
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post #17 of 18 Old 04-12-2012, 12:20 PM
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Both answers have the same solution. When wood dries it changes dimension. If it is locked in a form that does not allow deformation, something has to give and cracks/splits will happen. Where they happen depends on the weakest spot. Radial checking or other splits. If the wood was damaged it will split on the fault line. If the centre of a log is involved, the radial slits are likely.
Hollow forms including end grain turning need to be turned thin enough to allow the wood to deform. If you leave it thick it resists deformation and must check.
If doubt remains, get one of the DVDs or attend a seminar where closed turning of end grain. Is the object.
That is nice wood for end grain turning because of the dramatic colour in the maple heart. Try again. The shop shows attention to detail.
Good luck, bob
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post #18 of 18 Old 04-12-2012, 05:49 PM
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I just noticed something else. If this is the same vessel in your "first HF", the shape is probably a factor. The hard chine near the top of the vessel would restrict the deformation of drying. The chine makes a boat stronger from deforming but the urn shape would be better if the transition was more smooth. The circle at the chine would hold the shape out solidly so the only way it could move was the way it did opening up the stress relieving cracks.
I hope this helps. A rounded shape of uniform thin wall will work.
The maple is good. I hate oak and it's hard on tool edges. Cherry is good and turns like butter.
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