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Thread: Questions regarding Warping, Twisting, Cupping etc.

  1. #1
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    Questions regarding Warping, Twisting, Cupping etc.

    We have all been there. We sticker and air dry our rough cut or buy it kiln dried. We use moisture meters to ensure the wood is ready to be used. We mill it and build it and still we have problems sometimes. I made a keepsake Toy Chest for my nephew last spring out of Cherry that had been air dried for more than ten years and had a moisture content of 8% when I started the project. (8% is the best I can get in my shop here in Western NY). It has been in my Nephew's home, out of direct sunlight, away from any heat source and no spillage since he took possession of it. 11 month's later, the lid twisted and cracked. Obviously there was an issue with moisture content likely caused by humidity change but I'm at a loss why it happened after 11 month's. Up until then nothing was wrong with the lid. It was in perfect condition. So why did this happen and why can you buy a store bought piece of furniture that is made in a completely different climate under different humidity levels, made in a different part of the country, sold in a store that also has a different humidity level than your home and when you put the store bought piece in your home, nothing happens to it. We all use the explanation that wood will do what it wants to do but this is baffling to me. Please help me to understand this.

  2. #2
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    What design was the lid, frame and panel or a solid panel? If solid, one board or glued up?

  3. #3
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    Was it flat sawn or quarter sawn how thick? finished on both sides?Or maybe some knots?

  4. #4
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    Wood can be a you-know-what (rhymes with witch); you think you're doing everything right and still it does something you thought you'd guarded against.

    Could have been reaction wood; the lid might have had more finish on the outside than on the inside (or vice versa); any number of things could have caused the lid to move on you. The good part is that a lid is easier to replace than the whole chest.

  5. #5
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    And where did it happen and when? Cold climate, middle of winter, cold moisture-wrung-out air then heated to produce a massively low relative humidity (RH is all that matters really)? Was the chest in an airconditioned house in summer or fall, producing high RH values? If so, then despite your initial shop MC of 8%, the RH the thing was subjected to between an east coast summer (say in NY) vs. winter with maybe summer windows open and winter heat, could be 90%RH dropping to 5%.

    Bruce Hoadley has a great section on this in his book "Understanding Wood", where he gives this specific example of someone mistakenly believing that the shop MC (relatively stable) will hold sway inside the house, while in fact the effect of RH changes between seasons when airco and heat are applied inside a house can be extreme... all the while your garage staying pretty stable.

    Well worth the $$ to buy the book on Amazon, about $23, just for this chapter alone for future design considerations.
    Thread on "How do I pickup/move XXX Saw?" http://www.sawmillcreek.org/showthread.php?p=597898

    Compilation of "Which Band Saw to buy?" threads http://www.sawmillcreek.org/showthre...028#post692028

  6. #6
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    Wood is hygroscopic which means it will absorb or desorb moisture depending on the RH of it's environment. I think that either your moisture meter is wrong or, more likely, the lid absorbed moisture over the high-humidity summer months and then lost the moisture during the winter, resulting in the movement you saw.

    It's not unusual for a modern home to see RH's of <20% during the winter without humidifying the inside air. That equates to a 4.5% MC which is a pretty big change from even the 8% you measured, much less something higher.

    Also, the 10 years of air drying does not get the wood any dryer than it was the first year. Once it reaches EMC, which 4/4 wood will do in about 60 - 80 days of good drying weather, it only reacts to relative humidity changes.
    Cody


    Logmaster LM-1 sawmill, 30 hp Kioti tractor w/ FEL, Stihl 290 chainsaw, 300 bf cap. Solar Kiln

  7. #7
    Quote Originally Posted by Frank Drew View Post
    Wood can be a you-know-what (rhymes with witch); you think you're doing everything right and still it does something you thought you'd guarded against.

    Could have been reaction wood; the lid might have had more finish on the outside than on the inside (or vice versa); any number of things could have caused the lid to move on you. The good part is that a lid is easier to replace than the whole chest.
    I'm not sure about that after reading the link below.
    Flexner explains it.
    http://www.woodshopnews.com/columns-...arped-thinking
    Last edited by Brian Penning; 04-21-2011 at 8:51 AM.

  8. #8
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    I am not so sure I agree with Flexners theory here. In slow humidity changes yes, and I will buy the tabletop explaination, but at the extremes where wood can see big changes you will see more movement on one side tan the other until it is equalized. I have played with this in my shop and just do not agree that in all situations this is correct.

    I did experiments with poplar panels. I put them under my tube heater, unfinished, finished one side, and finished both. All cupped, but the unfinished and finished both sides cupped less in the same amount of time. Not empirical evidence, but enough that I decided to finish both sides of all panels. For example for economic reasons many people leave their air off until they can't stand it. So the house gets up to 70% humidity, then in a matter of an hour is brought down 20-30 points. These are the kinds of changes that I worry about. One long cold snap up here with forced air can plunge humidity down to very low numbers. Our area sees humidity ranges inside from the twentys to 100% as the seasons change. I hate callbacks, so I am going to stick with what I know works.

  9. #9
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    Lid design could be a factor but so could changes in humidity. I normally try to put equal amounts of finish inside and outside on furniture projects and boxes in particular. A closed box represents the worst case situation as one side of the lid is open to the surrounding air and the inside is in a closed almost constant environment. If the lid is framed, you need to let the panel portion float or most likely the lid will warp or crack.

    Wood preparation is also critical. If you started with rough lumber and milled it yourself, you need to remove equal amounts of material from each side to maintain balance in the piece. Removing all the stock from one side can create uneven moisture within the piece of wood which will cause the wood to move. Remember that it takes about a year per inch for the moisture level to change in most unfinished wood. The 11 months was probably just the wood adjusting to the final location moisture content. Most houses this time of the year have been heated for 4 months or more and the air and contents are very dry. Wood right now in my shop in NW PA is at 7% because it shares the house heating system.
    Lee Schierer
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  10. #10
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    Brian

    I've never seen a panel that was unevenly finished swell on the more finished side (that is, create a hollow on the less finished or unfinished side). As an extreme example, apply plastic laminate to one side of an unsecured panel and watch what happens.
    Last edited by Frank Drew; 04-21-2011 at 10:01 AM.

  11. #11
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    I think it sort of, kind of, maybe ought to start with the conditions -- if you know them:

    The conditions where the piece will live.
    The conditions where the piece will be made.

    Quarter-sawn moves less than flat-sawn.
    Some species move less -- radially and tangentially -- than other species.

    Alternating grain pattern (rip wide boards into thinner boards, and do glue-ups, alternating grain pattern) minimizes overall movement.

    Joinery techniques -- which grain pattern is joined to which grain pattern (long grain, face grain, end grain), etc. -- will play a big role in the strength of the joint -- particularly as concerns its seasonal movement.

    To me, this subject is like dust collection: you can learn a little about it, and do a fair job -- relying a lot on luck, or ....

    You can learn a lot about it, and do an incrementally better job of dealing with it.

    As the guy that just found an autographed Michael Jordan picture in HIS copy of Hoadley's book, I truly recommend that book

    But .... I think beginning with the end in mind is critical. If your piece isn't going to see wide swings in humidity, then ... go nuts.

    If, on the other hand, it is likely TO see wide swings/seasonal variation, then -- much like dust collection -- I think it's smart to take the time to learn about wood movement from any of the wonderful texts, out there.

    Me ? I'm spending a fair bit of time trying to understand that, now, myself.

    LIKE dust collection, you'll have a very distinct weak link if -- say -- you focus, primarily, on "finishing both sides," to the exclusion of all the other elements.

    There ARE a lot of elements. The furniture that lasts for generations, and doesn't seem to care about seasonal variation .... probably does at least a fair job on all these elements.

    Woodworking Darwinism

    In retrospect, this should have been one of the FIRST things I learned, when I started down this woodworking path, but -- silly me -- I wanted to make stuff, not read stuff

  12. #12
    Quote Originally Posted by Dave MacArthur View Post
    And where did it happen and when? Cold climate, middle of winter, cold moisture-wrung-out air then heated to produce a massively low relative humidity (RH is all that matters really)? Was the chest in an airconditioned house in summer or fall, producing high RH values? If so, then despite your initial shop MC of 8%, the RH the thing was subjected to between an east coast summer (say in NY) vs. winter with maybe summer windows open and winter heat, could be 90%RH dropping to 5%.

    Bruce Hoadley has a great section on this in his book "Understanding Wood", where he gives this specific example of someone mistakenly believing that the shop MC (relatively stable) will hold sway inside the house, while in fact the effect of RH changes between seasons when airco and heat are applied inside a house can be extreme... all the while your garage staying pretty stable.

    Well worth the $$ to buy the book on Amazon, about $23, just for this chapter alone for future design considerations.
    Everything you say, esp. about Hoadley's book. It's amazing to see 80%RH winter air turn into 5% in a heated house, yes. This is exactly why I keep my shop at about 35% year round. I'm not building for the desert. I'm building for New England winters!

  13. #13
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    Thanks for all of the input. Since so many of you asked questions or had ideas what could have went wrong, I'll try to explain here. I've also included a picture of the chest that shows the lid to help illustrate.

    John TenEyck

    It's 4 pieces glued up with three cross braces underneath

    Andrew Hughes

    It was flat sawn and was 5/4 before I started and the finish thickness of the lid is 3/4

    Frank Drew

    Very true. The finish is equal on all parts of the lid, as well as on the chest so that wasn't the problem.

    Dave MacArthur

    I don't know how the house was venitlated/heated/cooled. I would assume windows were open in the summer and forced air heating in the winter but I can't swear to this. I don't believe the house has air conditioning.

    Coby Colston

    My Moisture meter could be off but it certainly wasn't wet wood. I mentioned that the wood air dried for ten years only to say that it wasn't green. I didn't air dry it that long. The person I got the wood from had it stickered that long and just never got around to using it.

    Lee Scheirer

    There were equal amounts taken from each side of the boards when I milled it. That is something I'm pretty insistant about.

    Here is the chest
    chest.jpg

  14. #14
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    an old time woodworker told me the most stable wood is that in which the sap has been replaced with ordinary water and then dried to 6 or 8%. But it is important to drive out all of the sap completely for the best stability. At least so he told me.

  15. #15
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    How were the cross braces attached? If you screwed them to the panel at both ends, w/o elongating the holes in the battens for wood movement, that could explain why it warped as the humidity changed.

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