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Thread: Question about warping of walnut/wood in general

  1. #1
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    Question about warping of walnut/wood in general

    About a month ago, I made the small box in the pics below out of some scraps or mahogany, walnut and padauk. The box is about 8" x 5 7/8", and the top is 7/16" thick. The top is made of 3 jointed pieces of walnut. About 24 hours after jointing and gluing up the top, which was about 1/2" thick at that point, I planed it to the final 7/16", & while it was still a rectangle, I made a ~1/16" deep dado for the paduak insert in the middle. I then cut it into the final triangle shape, and glued in the paduak. This all happened in about 3 days. So far so good. After finishing with water based poly, the top slowly began to warp at the outer points of the triangle near the "handle", as shown in the final photo. The top sat nice and flat on the box right after finishing. The gap at the outer edge as shown in the pic is about 1/16". The walnut scrap I used had been in my dry shop for about 3 years, though I did have to resaw it to the 1/2" thickness used in the glue up. I have used the same wood for several tables over the last 3 years with no problems so far. What would have caused the top to warp this way, and what might I have done to prevent this? Should I have left the jointed glue up sit for some days prior to planing to the final thickness? Or should I have left the resawed pieces sit even before jointing/gluing up? Should a waiting period always be allowed prior to final jointing/gluing/planing to allow and pieces to "settle"? I'm planning to remove the cleats on the underside of the top, and try jointing it flat, than fitting a smaller triangle to the flattened underside to replace the cleats. I hope all the above is clear! Any suggestions appreciated.

    It may not be clear in the pics, but the top glue up is 3 pieces, 1 about 1 1/16" wide in the center where the dado is, and one on each side about 2 3/8" wide at their widest points.

    Dan

    box all.jpg

  2. #2
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    I've been working walnut lately and generally find it to be wonderful wood to work. However on my last project I was ripping about a four foot 3/4" board and about 12" into the cut it clamped down on my table saw blade so hard that the blade stopped. There were very strong interior tensions in that board which appeared to be very straight grained. Maybe that is what you encountered. Internal tensions released after you resawed down to 1/2"? If that was the case I guess that you might have been able to correct the problem if you had let the wood relax for a while.

  3. #3
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    Any time you open a piece of wood by resawing or changing the thickness significantly by planing or jointing you need to let the wood acclimate before moving on. The wood may have been in your shop for years, but the inside is a month or more behind the outside on adapting to seasonal humidity changes, depending upon the thickness. Then the water based poly added moisture to one surface at a time in several steps. Finally the lid was placed on the box where one side could breathe and the other could not. When using water based finishes it is important to apply equal amounts to both all sides and make sure it is fully cured before final assembly.
    Lee Schierer
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  4. #4
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    I've been reading "The Fine Art of Cabinetmaking" by Krenov lately and he makes the point that any time you resaw a board you should put it aside for a couple of weeks before working it to let it "settle' due to the fact that the moisture content in the middle of the board is almost always more than at the outside edges.

  5. #5
    Quote Originally Posted by Lee Schierer View Post
    Then the water based poly added moisture to one surface at a time in several steps. When using water based finishes it is important to apply equal amounts to both all sides and make sure it is fully cured before final assembly.
    While it is possible that this is due to internal stress. I will bet that Lee hit it on the head with the quoted portion above. I had the same problem recently. The cure (risky) was to set the box in front of my wood stove with the underside of the lid facing the stove. The lid went back to being flat. So far it has stayed that way.

    James

  6. #6
    As much as anything, its the glue. You have a very small amount of wood and a relatively large proportion of glue joint area. Let the next one sit a while before continuing. And you do need to finish both sides equally, whatever finish you use.

  7. #7
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    You have created a cross grain situation with the cross piece at the bottom of the triangle. As the wood top expands/contracts, the cross grain piece will restrict the movement of the inside surface of the top. This will lead to warping.

    Remove the cross piece and make it two pieces not more that 3/4" long. That will leave enough space so that the top of the top and the bottom of the top can move freely.

    Finally, leave the top off the box for a couple of days whenever you move its location. This way air can freely get to both surfaces and they will expand/contract equally.
    Howie.........

  8. #8
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    Quote Originally Posted by Dan Barber1 View Post
    I've been reading "The Fine Art of Cabinetmaking" by Krenov lately and he makes the point that any time you resaw a board you should put it aside for a couple of weeks before working it to let it "settle' due to the fact that the moisture content in the middle of the board is almost always more than at the outside edges.
    *or less* than the outside edges - Krevnov used air dried - for kiln dried wood, the inside is likely to be drier than the outside. You can tell by the way the wood cups after resawing. The face with more moisture will become convex.
    JR

  9. #9
    I think the biggest contributing factor is the cut of the piece of wood chosen for the top. It was cursed from the beginning because it's flatsawn. Instead of using a flatsawn scrap, the lid would have been better picked from a quartersawn piece. Given the thin top and the design that tapers to a point, the more perfectly vertical the end grain, the better, from a stability point-of-view.

  10. #10
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    There is a fantastic book titled "Understanding Wood" that will help you understand wood movement. All wood moves with changes in humidity. Quarter sawn wood (where the grain is perpendicular to the face) moves the least, and boards where the grain runs edge to edge moves the most. Unfortunately most boards have grain running somewhere between, and this is where you get warping or cupping. I have a 6/4 oak kitchen table top that is 36" wide. I live in Arizona where the humidity varies from mild to very dry. My oak top changes width by 3/8" from the dry season to the humid season. If it were quarter sawn it would move much less. The reason we are told to alternate the growth rings on a glue up is so that warping on one board will offset the warping on the other boards. Anyway, the book is fantastic and it also has tables for each species so you can see exactly how much the wood will move over a given humidity range. Walnut isn't terrible, pretty average. By looking at the growth rings in your picture, the wood moved exactly like it is expected too. If you had flipped one of the boards in the glue up so the other face was up, the warp would have offset and you would not notice it. Also, finishes won't keep wood from moving with humidity, they are not hermetic...joe
    Last edited by Joe Jensen; 01-18-2012 at 3:08 PM.

  11. #11
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    Dan - When I first started taking woodworking classes there was a sign in front of the room that said:



    WOOD MOVES


    It took me about four classes before I really believed it, but after 30 of working wood, I know it to be an undeniable fact.
    When you resaw , unfortunately you learn that a week or two for the wood to stabilize is about right. After that, movement will be less, but it doesn't stop. The best thing to do is to plan for and allow room for expansion and contraction.

    Have fun with your project.
    Doug



  12. #12
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    Quote Originally Posted by Joe Jensen View Post
    Also, finishes won't keep wood from moving with humidity, they are not hermetic...joe
    They don't stop it, but equal coats of finish on all sides will definitely slow down the changes due to humidity. With the advent of water based finishes, they tend to have the same effect as wiping down one side of a board with a soaking wet rag.
    Lee Schierer
    USNA '71
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    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  13. #13
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    Dan, Are the cross-braces under the lid jjust nailed in place or are they glued also? By the way, that is a nice looking box.

  14. #14
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    Thanks for the great replies. I finally got a chance to rework the top, as in the pics, below. I removed the cleats (which were glued & nailed), and jointed the top flat, then fitted a triangular piece of walnut to replace the cleats. I think that should work much better. The triangle was more difficult to size as exactly as I want, but the end result fits the box quite well with the triangle, rather than the cleats, and looks much more "professional", too, I think.

    I'm pretty good at cutting/joining/fitting wood, but I do need to think more about "movement". I also need to get it through my head that a piece of wood (and the final product) is not like, say, a cylinder head; it doesn't need to have a tolerance measured in a couple thousands of an inch.

    Dan

    box2.jpg

  15. #15
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    Quote Originally Posted by Pat Barry View Post
    Dan, Are the cross-braces under the lid jjust nailed in place or are they glued also? By the way, that is a nice looking box.
    Pat - They were both Glued and nailed, and thanks for the compliment!

    Dan

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