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Thread: Three Piece Slab Top - cupped

  1. #16
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    The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

    I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).

  2. #17
    Is it possible it's in the joint itself? Best to joint face in, face out, face in,rather than depending on fence being 90 degrees. But it could be something else.

  3. #18
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    It was flat a few months ago.

  4. #19
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    Quote Originally Posted by Joe Jensen View Post
    If he ripped and flipped ever other one over it would be stable. This is because the warp would alternate for each board.

    https://www.woodworkingnetwork.com/w...269418501.html
    Logic suggests that it would work that way, but I've read many studies that say it just doesn't make much of a real world difference. My first priority is arranging the grain in the most pleasing manner. If I can get the grain alternating without sacrificing the look, I'll do that.

  5. #20
    Quote Originally Posted by andrew whicker View Post
    The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

    I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).
    Increasing humidity will cause wood to cup one direction. Decreasing it will cause it to cup the other direction. This is caused by the curve in the growth rings. Depending on what the moisture content of the wood was when it was planed, a change in humidity can either cup the board, return the board to flatness, or cup the board the opposite direction.

    The wood is going to move with changes in humidity no matter what you do. The design of the table will need to restrain the cupping. Messing around with blankets or wet towels or anything like that will only help with getting it flat enough to mount the top to the framing. They will not cause the wood to become permanently flat, nothing will.

  6. #21
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    Quote Originally Posted by andrew whicker View Post
    The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

    I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).
    I have experienced this and it was dramatic. I wondered if it was more than just airflow (heat on one side that was covered, especially if that faced the sun, changed relative humidity on that surface, or even condensation type effect since you mentioned it was out overnight and temperatures cycled).

  7. #22
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    It is simple, really. You put, what is the equivalent of a big sponge- the blanket, on top of the table, and set it outside. During the night, the condensation and frost saturated and soaked through the blanket into the other big sponge below it- the wood top.
    You now are seeing the extreme effects of moisture differential on a wide top. It will probably take many days, to a couple weeks for it to equalize and come back hopefully to flat or very close.

  8. #23
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    Quote Originally Posted by Andrew Seemann View Post
    Increasing humidity will cause wood to cup one direction. Decreasing it will cause it to cup the other direction. This is caused by the curve in the growth rings. Depending on what the moisture content of the wood was when it was planed, a change in humidity can either cup the board, return the board to flatness, or cup the board the opposite direction.

    The wood is going to move with changes in humidity no matter what you do. The design of the table will need to restrain the cupping. Messing around with blankets or wet towels or anything like that will only help with getting it flat enough to mount the top to the framing. They will not cause the wood to become permanently flat, nothing will.
    Exactly correct. Everyone should read the book "Understanding Wood". After you understand that finish will not prevent changes in humidity from changing the shape of wood.

  9. #24
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    Quote Originally Posted by Carl Beckett View Post
    I have experienced this and it was dramatic. I wondered if it was more than just airflow (heat on one side that was covered, especially if that faced the sun, changed relative humidity on that surface, or even condensation type effect since you mentioned it was out overnight and temperatures cycled).
    It is due to changes in the moisture content in the wood. If you put a piece in the sun it dries one side faster than the other and it warps. If you put a piece on a damp basement floor it will absorb some moisture and warp. This warpage is not a mystery. The book "Understanding Wood" has extensive tables of any wood you are likely to encounter with data on how much the wood will move in each direction with changes in humidity.

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