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Cracking rough turned items

Discussion in 'Getting Started' started by bsaks, Mar 17, 2010.

  1. bsaks

    bsaks

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    I am new to turning and have been experiment with boxes with some good success. However I have some green wood, plum to be exact, and I have rough turned a few boxes so they would dry quicker. However I am getting a lot of cracks. So fare I have been able to either turn it down so the cracks are gone or used an inlay of turquoise to full them.

    My question is how do I keep it from cracking out during the drying process? and what is the quickest way to get the wood from green to a dry so I can turn them?

    Thanks
    Brian
     
  2. john lucas

    john lucas AAW Forum Expert

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    That doesn't look like a box in the photo at the bottom. You probably need to seal the end grain with either wax or end grain sealer. Wood loses moisture fastest out of the end grain. It is uneven drying of the wood that causes the cracks. By sealing the end grain you even out the drying. For box blanks, unturned I seal the ends and put them on the shelf to dry. It will take a year or more for a 2" piece to dry.
    To speed this up rough out the box including a rough hollowing, tape them back together and seal the ends. This speeds it up to about a month or so. You can microwave it after roughing. I use many short low power bursts until it stops losing weight. Then I let it sit for a day or so to pick up the ambient moisture from the room.
    Fruitwood moves a lot and is more likely to check than other woods. Dry it slower by putting it in a box to restrict the air movement. It doesn't have to be a sealed box I simply fold the lids over. For Bowls from fruitwood I will use a paper sack for the first month or so.
     
  3. Bill Weaver

    Bill Weaver

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    I had same problem with a fruit wood. I was told by local wood turner to stick away in paper bag with the top folded over several times and stapled shut to slow drying process.
     
  4. MichaelMouse

    MichaelMouse

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    I don't tape them back together. Keeps the inside wet while the outside drys. I use witness marks to match tops and bottoms and store them open. One further recommendation is to keep the outside bottom open to the air so it doesn't stay wet sitting on a shelf but contracts at the same pace as other exposed surfaces. Stickers under or on its side will do. If the RH is below about 50 or the wood a known stinker to dry, you might want to give a bit of bag time initially. Otherwise, open to the air seems fine.

    "Fruitwood" is not the most descriptive term, just like a lot of others you might encounter. Cherry grown in the woods behaves better than a lot of other woods. Even the volunteer apple trees along the old road behave pretty well. Fruit trees which are trimmed and forced to bear fruit low and widely develop a lot of problems because they're subject to some strange stresses. There's even a big difference between open-grown and woods-grown trees of the same species which don't bear what we might call fruit. The best figure seems to result from the worst circumstance, so plan your rough thickness accordingly.

    The microwave is fine, but I hedge my bets even a bit beyond John and work the piece inside a plastic bag. The condensation forms inside, the bag is turned inside out, and readied for the next cycle. When only a slight haze results, it's done.
     
  5. Dave Moore

    Dave Moore

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    I don't have a microwave, and most of what I make would be too big to fit in one anyway, so speeding things up that way is not on the menu for me.

    Relative humidity around here is commonly below 15%, sometimes even less, so I have to slow things down as much as possible. Depends on the species. I've never turned plum, but have used a fair amount of "fruitwood"; apple, peach, cherry and recently some apricot. They're not all the same at all but I start by assuming they will all be uncooperative.

    In the summertime I put everything I rough turn down under the house next to the water cistern, where it is more humid and cool. Anything I suspect will be troublesome goes in a paper bag as well. I stand things on edge or use slatted racks so that no part of the piece is sitting flat on a shelf or whatever.
    As I've said before, it's so dry around here that I have to put a plastic bag over work in progress on the lathe if I get interrupted for more than a couple of minutes. In the summertime I work with the door open so I get a nice breeze. Nice for me, not for the work.
    I've not found a way to speed things up; I spend most of my energy finding ways to slow things down.

    The single most important thing I've found here is to take some trouble to ensure that the thickness of any rough turned work is consistent; if it's supposed to be an inch or whatever then it needs to be an inch all over, I'm careful to ensure I'm not too thick anywhere, especially around the foot.

    good luck
     
  6. john lucas

    john lucas AAW Forum Expert

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    I agree with Dave on the thickness. I had a conversation with John Jordan years ago when I was trying to stop my bowls from cracking and he told me to leave everything the same thickness, especially the bottom. The shrinkage at the bottom isn't as bad as the shrinkage at the lip of the bowl so you can leave it the same thickness. I was leaving my bottoms thick enough to account for a long tenon because I was using faceplates. Once I quit doing that my problems dropped tremendously.
     
  7. MichaelMouse

    MichaelMouse

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    Please, can any of the supporters of this uniform thickness myth explain why, given that end grain dries 10-15 times faster than face grain, which dries almost twice as fast as quarter, it would make ANY difference in the scheme of things?

    It's how broad the bottom and how steep the sides that count most in a bowl as pictured, followed by the length of continuous wood in a chord through the walls. A bit of time with a ruler and roughs would show how the Wood Handbook fig. 3-3 , has direction and magnitude of shrinkage based on grain orientation down pat.

    My faceplate roughs have bottoms as thick as they are deep, sometimes even more, but it's difficult to remember last time I had one pop.

    For spindle orientation, as is the case with most common boxes, wouldn't the bottom have to be 10 times as thick as the sides to even things out?

    EDIT: Oh yes. The shrink at the bottom of what is normally turned as a bowl is GREATER than almost anyplace else. This because the orientation is normally close to perfectly tangential, where the percentage is twice the radial, and its breadth is normally greater than the length of a chord through the walls, meaning the absolute distance is greater.
     
    Last edited: Mar 17, 2010
  8. Dave Moore

    Dave Moore

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    Heh heh, Micheal.

    No, I cannot explain it, certainly not in terms of chords or tangents or anything else. For all I know your reasoning may be perfectly rational.

    What I do know, and fer darn sure, is that if I leave the bottom of a rough bowl significantly thicker than the walls, or if the walls gradually thicken toward the foot, the damn thing will crack 100 times more often than if I don't.

    At one time I had almost a pickup truck full of proof, but they're long gone in the woodstove at this point.....

    ......unless it was all just a bad dream ........?
     
    Last edited: Mar 18, 2010
  9. bsaks

    bsaks

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    Thanks

    Thanks for all the information. I have tried the microwave thing once and it worked good. Looking back on the ones I have had the most trouble with it is fruit woods. I will have to get some sealer and try that.

    I think one of my biggest problems is that I live in a place with high humidity, in the 90%, and it is also cold this time of year. So I have been rough turning things and bringing them into the house to get them to stabilize to the environment they will be in. However it is really warm in my house, my wife is always cold so the wood pellet stove is doing overtime, and the humidity in the house is around 70%.

    I rough turned one the other day and have it sitting in a paper bag and did put it in the microwave for a few minutes on defrost once I saw it cracking, probably too late with that idea, so we will see how that comes out.

    Also John there now boxes in the bottom picture :)
     
  10. john lucas

    john lucas AAW Forum Expert

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    MM Can't explain it, it just works. I have roughed a lot of bowls and lost a lot over the years. After considerable playing and reading I settled on the overall even thickness and sealing the endgrain portions with anchorseal. Now I have very few failures. I'll stick with what works for me.
    My failures have almost always been at the bottom with a long tenon or on the lip. Making tenon and bottom thinner helped that. When you re-turn a bowl you remove less wood from these areas so you can get away with much thinner wood here.
    The anchorseal on the endgrain both inside and outside the bowl stopped the lips from checking. If I have a knot or other odd inclusion I coat those areas also.
    I've come to the conclusion that each person has to figure out what works in their environment and methods of working. What works for me may not work for someone in Denver, Washington state or Vermont.
     
  11. MichaelMouse

    MichaelMouse

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    Can explain it, and it works if you realize that keeping the bottom narrow, not thin, is the answer. For example, look at the included photos. You will see that the bottom has contracted an eighth of an inch over two inches , for a 6.25% shrink rate. Looks pretty consistent with an average from green to oven of ~7.1%, remembering that these pieces were at EMC with ~5% MC.

    When you look at the larger diameter, you'll find that John's presumption was inaccurate, as the smaller has lost <5%, the larger<3% across the top. Even some of that loss is illusory, becuse the sides have dropped over an eighth of an inch. Now note the thickness of the walls and you'll discover that if they're close to the same, the total dimensional loss will be close. That's the chord. Wood can only contract upon itself, and when it meets air, it's done.

    It's a great thing, science, because its essence is repeatability, not chance.

    Yes they're both "fruitwood," and the bottoms are >1 1/2" thick, exclusive of my remount pillar.
     

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    Last edited: Mar 18, 2010
  12. bsaks

    bsaks

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    Thanks John, I think I am going to try your method to see how it works. I only have 1 more question. When you do the microwave thing do you still use the anchorseal?
     
  13. john lucas

    john lucas AAW Forum Expert

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    MM I don't do a good job of explaining some things. When you leave the bottom thin it moves just like the rest of the bowl. However you have to remove very little wood when truing up this area so you can get by with it being thinner here. Since my bowl damage dropped after doing this I assume that the moisture is leaving the wood more evenly. Since I put feet on my bowls I return this area anyway so movement here isn't critical either.
    I don't use the anchorseal when I know I'm going to microwave. I heat the bowl until it's quite hot but you can still handle it. Then I let it cool completely and do it again. I don't rush this. I usually do this while doing other projects so sometimes it sits for a while between bursts. Everytime I walk by I'll pop it in the oven again. They are usually dry by the end of the day.
     
  14. Dave Moore

    Dave Moore

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    Micheal, I think I understand what you're saying here;
    "You will see that the bottom has contracted an eighth of an inch over two inches , for a 6.25% shrink rate. Looks pretty consistent with an average from green to oven of ~7.1%"
    I would call that a 6.25% shrinkage, rather than a shrink rate, since we're talking distance only, not time.

    Speculation time;
    We're contemplating cracking while drying, so we're taking about the forces exerted when some part of the wood moves more rapidly than another part that it is attached to. The forces are a function of shrinkage (% change in linear measurement with MC) and also rate of shrinkage (% change with time).
    Yes?
    A 2 by 6 will shrink in width by whatever, and 1 by 6 will shrink the same amount, but the 1 by 6 will do it faster given the same drying environment.

    If my bowl cross sections are of substantially uneven thickness it introduces another variable into the mix does it not? In addition to the shrinkages from the different grain orientations, and the MC loss rates attendant to those orientations, I have added another shrinkage rate variable due to thickness. Is this not so?

    Wedges can be pretty tough to dry without checking for this very reason, no?

    I agree science is great. Certainly my empirical results with drying and cracking (or not) are repeatable, I've repeated them more often than I care to.
    My speculative attempt to explain those results here may not hold water for reasons someone may point out, but whatever the explanation, it's not just chance. Things are too empirically repeatable for it to be that........
     
    Last edited: Mar 18, 2010
  15. john lucas

    john lucas AAW Forum Expert

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    Here are my last batches of green bowls. I turn most green bowls start to finish but I do some bowls as twice turned. My bowls vary in shape and size a lot which may be why I had to come up with a formula that works for me.

    They start out on the floor. The batch on the workbench just came off the floor. They will go up on the shelf near the ones in the other photo. The flourescent lights don't add enough heat to cause a problem. They just look closer than they are in this photo.
     

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  16. MichaelMouse

    MichaelMouse

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    IF the material is maintained with sufficient moisture it will be plastic enough long enough to take the stress developed by shrink. Less stress, greater survivability. The surface, if it dries significantly below the fiber saturation point, will produce checks. I'm sure we've all pressed a bit too hard when sanding and realize what a surface check is, and that less the mechanical stress, it will close as the wood rehydrates. Matters not if the moisture comes from within during the drying process or without from a damp rag, if it's not being pulled (inside is a squeeze, and can be disregarded) apart simultaneously, good to go.

    Point of science is to take those measures which promote successful drying. Difficulty is that we run up against folklore or statements endowed with the full aura of an "expert" quotation which are ill-considered or erroneous. The 10% rule is one piece of drying folklore, the even thickness rule is yet another. Investigation and observation, coupled with a bit of scholarly data researched from those folks who do it 24/7/365 gives us a chance to do better, but we have to be able to recognize what we've done when we evaluate. Otherwise we'll end up turning a bit thinner than the 10% rule, perform some magic incantation or accidentally drop the wood in ethanol and trumpet a new drying process when all along it's been known that thinner wood with the same grain presentation dries faster than thick, (not quite the inverse square rule, but close) and that wood contracts on itself, it can't hook on air. Thinner walls will dry faster and distort less overall as long as we minimize the contiguous wood by tapering to the bottom.

    "Did he say thinner bottoms were better?" No, the bottom is the bottom and when the rise reaches perhaps 25% of the run, it's no longer the bottom, but the side. If you take the 1/8 in 2" and make it into a robust tenon of 4", it's going to shrink a full quarter plus, because you'll shoulder, of course. That can be a lot of stress exerted at 90 degrees, less at 75, and even less at 60. So leaving enough extra thickness in the bottom so you can make the tenon longer and broader can hurt you. But it's the breadth, not the thickness that does it.

    I use a mortised dovetail hold which allows a deeper interior on the same thickness blank or a fair inside curve that doesn't match the outside. It's a good way to go, allows me to taper in to avoid splits and I have to enlarge it back to optimum hold size after drying, rather than make it larger and then turn down after. It's been good to me.
     
  17. Dave Moore

    Dave Moore

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    MM
    As a practical matter I think we are in agreement.
    The "hurt" can be mitigated by avoiding the extra thickness, that's the empirical point I was trying to make.
    Choosing a form which is less "broad" at the base helps too, I agree, by "minimizing the contiguous wood", though for me there are often times when I might not wish to do that.

    More than one way to skin a cat, etc etc.
    Onward.
     
  18. john lucas

    john lucas AAW Forum Expert

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    Well I don't think the 10% rule is a myth. It's not about drying doing this part. It's about unequal shrinkage of the bowl. The 10% gives you enough wood to be able to re-turn without losing part of the bowl. :)
     
  19. MichaelMouse

    MichaelMouse

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    The myth is the uniform thickness business. You always have a choice to make when you rough between speeding up drying by turning thinner or having a lot of restyling room. Folks who don't turn a lot tend to be more interested in the former, while those who turn a lot favor the latter. Toss them in the corner and re-turn them "someday" not next Sunday.

    .
     
  20. Vaughn

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    Thus spake the "expert". Not saying you're wrong, but you've apparently missed the fact that methods other than your own do indeed work.

    I'll stick with my incantations, chicken bones, and tea leaves, thankyouverymuch. :D
     
  21. hockenbery

    hockenbery AAW Advisor Staff Member

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    Working with green wood I have success following a

    1. Use wood that isn't cracked already fresh cut.

    2. make flowing curves

    3. turn an even wall thickness

    4. control the drying

    all woods will shrink and warp as they dry. The wood has to move, pieces with even walls and curves can move together. Pieces with straight walls and a flat bottom restrict movement. Thick and thin walls the thin parts dry faster an want to move the thick parts haven't dried and restrict movement.
    if the wood cannot move cracks are likely.

    smaller pieces are easier to dry than larger ones. I turn 3" thick balls in demos from green wood. I toss them in a corner. They rarely crack.

    for rough turned bowls The 10% wall thickness rule works for me. I generally turn the bottom a little thinner to account for the chuck tenon. I also use a 2.5 inch diameter tenon so I can turn a 2" diameter tenon in the dry wood.

    happy turning,
    Al
     

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