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green thin walled bowl?

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I have a natural edge maple bowl I am beginning to turn. It's green and prone to crack (sitting in a plastic bag with shavings right this minute). If I wanted to turn it down thin enough (1/16"?) to let it finish drying (or maybe help it with a microwave) without fear of cracking, how thin should it be?

What about the base? Can it be thicker than the walls?

Any tips on the procedure here would be much appreciated...


DW
In the High Desert of Central Oregon
 

Steve Worcester

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More important than wall thickness, is wall thickness consistency. (Although a 1/8-116" is great). If it gets thicker towards the bottom, as it drys it will contort and warp and crack easier.

I would also put it in a plain brown grocery bag with the top closed and leave it for a week or so and check progress. If you are in a high humidity area, change the bag every day or so.
 
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There is a fantastic book out there by a guy named something Odonnel (I forgot) called "Turning Green Wood" and he shows several projects turned to 1/16th. He uses a lamp held very near the bowl to shine light through it as a thickness indicator. I've done this and it works well, but I'd use one of those screw-base compact flourescent lights as they generate far less heat, something you don't want near a wet turning (cracking while on the lathe).

I turned a small coffe-cup sized cup out of magnolia recently as a test and it's about 1/16th thick, maybe a tad heavier, and the bottom was a bit heavy of 1/8. I part dried it in the microwave in 1.5 minute bursts, allowing it to air cool for 10 mins or so. No problems. Then again this may just be magnolia's property, as some woods check worse than others. I can't find any info on magnolia as to whether it's "checky" or not.
 
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Thanks for the replies.

Steve, your advice to put the bowl in a paper bag is a good one. I'll try that...next time.

I have read Turning Green Wood but I don't own a copy. I should have taken a copy out of the library but as I mentioned I was in a hurry to salvage this wood.

I got the bowl/vase done...walls about 3/32-1/8" thick--you can see light through them. But I guess I went about it a bit backwards.

Started out between centers. Since the wood almost seemed to be tearing itself apart, and since I had seen this technique illustrated, I thought that perhaps a preliminary rough hollowing while between centers would take off some of the pressure.

But then with the center stub left inside the bowl I had no way to mount it on my screw chuck to turn the outside clean and thoughtfully.

Anyway it degenerated from there...finally got a tenon turned and mounted it. Proceded to hollowing and when I was about half way to the bottom I noticed that the base...just up from the chuck, was cracking. So, after rough hollowing to the desired depth, I tried to turn those cracks off. But by this time the walls of my bowl were thin enough that I couldn't get a really, classically pleasing form without cutting through the side. So the whole thing is a bit "clunky" looking.

I was a good exercise...although I had enough problems with ongoing warping (even as I was turning) that I doubt I'll revisit green wood bowls anytime soon.

Thanks again for the advice...


DW
In the High Desert of Central Oregon
 
M

mkart

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Don't stay away from green wood!

:eek: Don't walk away from turning green wood thats where you learn how to turn better. I don't like the look of warped bowls so I only finish turn natural edge bowls to finish from green wood. Every thing else I rough turn green, dry, then finish turn. Natural edge bowls often look oval to begin with so when the warp a little no one really notices. Another thing is start and finish a green turned bowl in one session. When turning thin it is going to warp rather quick so don't stop til your done. Another thing I do when turning the inside is stop and sand as I go. I'll turn to a depth of 1.5" at 1/16" stop and sand. Then turn another 1" or so and do the same. You don't want to go back out towards the rim when your that thin.

Good luck and keep turning green. :)
 
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Definitely don't give up on green wood, I'll take it over kiln-dried anyday. I'm in a really odd situation in that I have tons of kiln dried available either free or cheap in the cabinet shop I work in, and I still go to great pains to find green wood. I'm a mere rookie but I know a good thing when I see it!
 
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You're right...I turn most of my bowls green--to 10% of diameter wall thickness and then put them up to dry.

But this thin walled stuff is much more tricky than it looks.

For instance, because my natural edge was uneven--as you might expect being the outside circumferance of the log--I had a hard time getting the edges even. Where the edge was high it was thick, where it was low it was very thin. I think...not sure...I solved that problem but it was by accident only---I didn't really know what I was doing.

Also many "ridges" that I couldn't seem to get rid of (until I started sanding) within the bowl. I could feel them and then I'd mark them with a heavy lead pencil and, holding tghe pencil steady, turn on the lathe for a second to extend the mark all around. Seemed like when I'd try to shave it away I'd never get much joy--the bowl was, by that time, already warped.


Well...it's all part of the game I guess. I did get a bowl...it has thin walls and I didn't blow it out. Tung oiled it last night and this morning I can't even see the cracks. So...I'm still ahead of the game, I suppose.


DW
In the High Desert of Central Oregon
 

hockenbery

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DW

Keep turning the green wood.

Humidity is the woodturners freind and I'm supposing that High dessert means low humidity.

So you may have to take some extra care to keep your blanks from cracking before you start turning and to prevent cracking during the turning process. See what other turners in your area are doing. Think about the following

1. Be sure your wood has no cracks before you start. A log section will begin to crack as soon as it is cut. Try putting your bowl blanks in heavy plasitc contractor bags as soon as you chain saw them. This will help prevent cracking. As the bowl dries pre-existing cracks will open up.

2. Turn even walls like others have mentioned. Also use flowing curves in your design. The wood in work with sharp corners and flat walls can't move easily as it dries and stress will cause cracks.

3. Keep the piece wet while turning. Use a pump sray bottle to keep the surface wet with water. If you let the the thin rim dry before you have finished the base it will be likely to crack.

4. Turn the piece in one session.

5. Slow the drying of the finished piece. put it in paper bags or a cardboard box. Keep it away from moving air.

Keep at it! A bowl with a nice curves and even 1/2" or less walls shouldn't crack.

I see a lot of beginning turners have trouble with cracking because their walls aren't as even as they should be, their shapes are too angular to allow the wood to move, they work slowly so the wood dries out on the lathe, they don't always have fresh wood so it has cracks they can't see when they start. The good news is that the more you turn the better you get.

Finally Cracks aren't all that bad in some work. Highlight them and make them a feature.

- Al
 
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hockenbery said:
DW

Keep turning the green wood.

Keep at it! A bowl with a nice curves and even 1/2" or less walls shouldn't crack.

- Al

That's it. Under 1/2" shouldn't give you cracking problems unless they were preexisting, you cut a shape which favors cracking, used a wood predisposed, or you just plain loused up in the drying.

Thickness of the bottom is much less important than its shape. I have a lot of bowls that wouldn't stand on their own if the bottom wasn't an inch thick, because they taper to such a small bottom. Fortunately it's radial shrinkage which predominates with heart up, which normally means larger diameter, and that's usually half or less the tangential, which affects broad, bark up bottoms. Thin wood will tolerate stress much better than thicker, but thicker doesn't necessarily mean wall thickness. It means continuous wood in the direction of shrinkage. Thus a set of thinner, more vertical walls is much more likely to split than a thicker sweeping set of walls, because there is a greater distance of continuous wood to pull against, just as the broader bottom will take almost the full measure of normal shrinkage. Nature helps us again, though, because the bottom of a bark-up bowl is predominantly quartered figure, which moves less.

You want to believe your fingers or calipers over your eyes on wall thickness. Since the angle is changing, there's a naturally broader look as you begin to transition to the freshly cut portions of the bottom which can fool you into cutting too thin there. Sometimes you cut so thin you can't recover on the wings. Especially when the wood's already got problems like spalting.

Spin all the unbound moisture you can out of the piece prior to putting it into the paper bag or newsprint wrap. Even at that, you'll want to open it next day to make sure you're not growing mildew. I don't bag, but I'm not in the desert.

I find sanding a problem. The moist wood clogs up paper pretty rapidly, and wet wood doesn't show areas where the bevel compressed the surface, so I generally run a single 120 grit pass and sand after drying. Sanding with fine grits is a double pain, because I have to keep cleaning them, and the chances of making surface checks increases. So I sand after the wood is dry, where I can use a light touch more easily.
 
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