Rough turning salad bowl question

[OneStaple]

Hey all,

I'm on the verge of green turning one of the largest bowls that I've ever attempted, aiming for a diameter of 18"-20". I just added risers to my lathe to accomplish this. The wood is maple and it will be a salad bowl for my wife.

Since it'll be a salad bowl, I'd like the finished bowl to be circular, thus meaning a rough turning, drying stage, and a finish turning. All the wisdom that I've seen is to rough turn the thickness to 10% of the diameter, meaning I'd end up with 2" thick walls after the rough turn. Do I really want them that thick? Wouldn't it be more prone to cracking with such thick walls?

After rough turning, I plan on putting it in a few paper bags for a few months until it's dry enough to finish turn. I'm in humid Virginia, so it may take a while, although we're approaching the dryer winter months, so perhaps not hidiously long.

Thanks,
Tyler

[MichaelMouse]

Turners repeat a lot of silly silly things they've heard as if they were true. Leaving the piece an inch thick should be adequate. Inch and a quarter at the heaviest. If you've got the standard heart up capacity-cut bowl, expect to lose around 8% of your dimension to distortion. that's 4% per side, if the piece is relatively straight grained. Translates to a bit less than a half inch either side and the same half inch off the end grain. Will leave you with a wall thickness of half an inch, which should be adequate.

Make the turning a continuous, or nearly continuous curve in and out. If you go flat/broad on the bottom, you'll probably crack. Put it, naked, in your unheated garage for a month or two, then bring into the house for final equalization. No bags, no boxes, no waxes required. Measure the mortise (or tenon) to determine loss of dimension. When the cross-grain is about 1/16 to 1/8 per inch of long grain, weigh the piece. Week later, weigh again. Same, go final, not, repeat next week.

Here's a 14+ Cherry cut to ~1"for example.

http://i35.photobucket.com/albums/d160/G...age-14.jpg

Nice pattern for a large capacity salad bowl here, with a no slip grip for lifting it on the outside. Keep them wide, not deep, so it's easier to toss edge to center.

http://i35.photobucket.com/albums/d160/G...ok-One.jpg

This has great capacity, but the pattern would make for a difficult toss. Potato chip suitable, however.

http://i35.photobucket.com/albums/d160/G...irch-1.jpg

[Steve Friedman]

MichaelMouse said:

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Make the turning a continuous, or nearly continuous curve in and out. If you go flat/broad on the bottom, you'll probably crack.

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Sorry for the semi-hijack, but I have a related question about this. I'm not a turner, but make hand hewn bowls that are more oval than roundand have flat bottoms. The percentage of bowls that end up cracking is huge. Mostly repairable, but still annoying. This comment had me wondering whether the flat bottoms are causing the bowls to crack so often.

Not sure there's much I can do about it, but what is it about a flat bottom that makes a bowl crack?

Thanks for the help,

Steve

[MichaelMouse]

You're working them green? The wood tries to shrink across the grain - tangentially - more than twice as much as radially while it dries. From the Fiber Saturation Point to say 10% Moisture is ~8%, so the broad bottom is trying to contract as soon as you're done. Broader the bottom, greater the stress on those rounded ends, which, because they're end grain, dry maybe ten times faster than the tangential, and 12 times faster than quarter grain. The first check relieves a bit of stress, and makes a weak point for the mechanical stress of drying to exploit.

When you do a bowl on the lathe, there are normally few points where there's more than three-four inches of continuous wood. Makes the farthest point from open air no more than two inches through the faster drying end grain. With little to no wet, expanded center and dry, contracting ends, you will lose less.

Try a cheat, and don't make a flat bottom, but rather round it so it rocks until you get some drying. You can make a flat spot after it's dry. This will also give you a bit of the benefit of shoulder droop, which tends to close end checks.

Bowl with heart centered, drooping almost 1/4" in 5.

http://i35.photobucket.com/albums/d160/G...-Droop.jpg


Here's what to expect with different orientation of the annual rings.

http://i35.photobucket.com/albums/d160/G...LogEnd.jpg

[NCPaladin]

I agree with Michael but I would anchorseal the end grain.
I made a couple of bowls near 16" when I started and they were the last. Just too darn big.
I checked and the largest serving bowl my wife has is about 11.5", she does have some mixing bowls in the pantry that are larger.

If you can picture it, diameter wise, a 30 gallon drum is 18-1/4". She may want one that large.

[jerickson]

I agree with MM that 2" is overkill. However, keep in mind that, if you go down to 1", you'll have to be very accurate when you remount it. In other words, you'll have less wiggle room. So you'll want to pay attention to how much figure is in the wood, whether there are knots, etc., because those factors can affect how much and what type of warping you'll get.

I don't know what your experience level is, so sorry if you already know all this. Early on I had some rough turned bowls that were tricky to finish turn because I wasn't very accurate at reading the wood or remounting them.

If you haven't done many big bowls, maybe spit the difference and go with 1.5"? I agree that 2" will make cracks more likely. Personally, I have done 16" bowls at 1" thick without problems.

[MichaelMouse]

Note the joy of the pin chuck (or pin jaw) mount. A one inch bore yields a 1" by ~15/16 hole to run that 1" bit down the dead center of the cured bowl blank. If you are careful, even a woodworm screw will do the same. I used to mark center in the mortise (tenon) as well to have a crosscheck, but with the pin chuck, it never goes off, so I no longer bother. If you do the dangerous, in my opinion, method of using the fanged drive center and a tail center to start, leave a pillar and the center reference at both ends to mount the cured bowl.

When I got the 3000, I HAD to do a 16" piece at once. I learned that a 400mm lathe is really 15 3/4, and a bowl cut that large was, as SWMBO informed me, as big as a chamber pot. I still do some, but they are not in great demand.

Some were even, as you can see by her face, used to relieve the bowels.

[robo hippy]

How much it will move as it dries depends on the particular wood, and also, how wet it is. If you get early spring wood, it will have a lot higher water content, so will move more than fall/winter wood. Some species warp insanely, like our Pacific Madrone. I had one come off the lathe at 22 inch diameter, and finished moving at 25 by 17 inches. So, what kind of wood is it? Do make sure to round over your rims. You can boil or steam woods to take just about all of the drying stress and shrinking out of them. I have also heard of drying them in a form, where you put a collar on the outside. The compression will hold it in place, then when you return it, most of your second turning will be leveling off the top. 2 inches would be a bit thick for drying without cracking, 1 inch would most likely not leave you with enough wood to return it. Most will anchor seal the pieces, or at least the end grain. You may get a lot of experienced help from your nearest club if you are involved with it. Another method fro drying is to put it in a large paper bag, and put that into a large plastic bag. Change out the paper every day. Lots of variables. Personally, I love the warped bowls.

robo hippy

[MichaelMouse]

robo hippy said:

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How much it will move as it dries depends on the particular wood, and also, how wet it is. If you get early spring wood, it will have a lot higher water content, so will move more than fall/winter wood. Some species warp insanely, like our Pacific Madrone. I had one come off the lathe at 22 inch diameter, and finished moving at 25 by 17 inches.

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PLEASE, don't mislead the new folks. Wood begins to shrink once it reaches and goes below the fiber saturation point, no matter where it started. Simple wood science. Doesn't matter when it was cut. Madrone, from my experience, is the screwiest wood out there, and does not belong in any discussion of the 99.9% of woods.

Compressing is silly. The wood will shrink anyway, leaving the ring tight only across the end grain. Had some Kiwi turner maybe 20 years back who put boards across on the inside, hoping to control that tangential movement. Big fad, like alcohol "drying", for a while, then people found out, as I did, that it was one way to virtually guarantee a split. Faded to a bad memory.

[Arlin Eastman]

Tyler

It looks like the guys have helped you. It is nice to see you posting here and hope you do not make yourself a stranger.

Arlin