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Re: [MedievalSawdust] Seasonal expansion - dowels vs biscuits

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  • Rebekah d'Avignon
    It has to do with the way the grain runs.... Wood will absorb moisture from the atmosphere (humidity), when it does - it expands (sometimes ever so slightly).
    Message 1 of 3 , Jun 1, 2007
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      It has to do with the way the grain runs.... Wood will absorb moisture from the atmosphere (humidity), when it does - it expands (sometimes ever so slightly). This is why older doors and windows are loose in the winter (low humidity) and tight or hard to open and close in summer (high humidity) or when it rains. When you join two boards together, the grain can run parallel or perpendicular. Glue, like screws, lock the board and try to prevent it from expanding causing stress and, possibly, warpage or cupping. The pressure has to go somewhere. With biscuits, the grain runs at a slight diagonal allowing for *some* movement both ways. With dowels (and those new tenon-type biscuits that require a special cutter) the grain runs perpendicular to that in the boards and the glue locks them. Any expansion of the boards toward each other causes pressure on the glue holding them....something has to give. This can cause the joint to fail after repeated expansion. Many antique (and modern) chairs have dowels holding the seat frame to the legs. When these joints fail or the old dowels break there is a process for removing the old dowels from the legs - running the drill bit in reverse.

      Su Ralston <suralston@...> wrote:
      ???
      I'm not much of a wood worker, but I have a bit of an idea what a biscuited
      joint looks like (my hubbins uses them) and what a doweled joint looks
      like...

      Why would one of these allow expansion and the other not?

      Su of the Silver Horn, not just a textile person, Caid
      Su Ralston, Fullerton, CA
      (signatures really do simplify too much!)
      .




      When you get down to it about all a man has to call his own is his values. When you sell those out, you don't have anything left.
       
      Col Sherman Potter
      4077 MASH


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    • leaking pen
      that is, of course, if you GLUE your dowels. ... -- That which yields isn t always weak.
      Message 2 of 3 , Jun 1, 2007
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        that is, of course, if you GLUE your dowels.

        On 6/1/07, Rebekah d'Avignon <rebekahdavignon@...> wrote:

        It has to do with the way the grain runs.... Wood will absorb moisture from the atmosphere (humidity), when it does - it expands (sometimes ever so slightly). This is why older doors and windows are loose in the winter (low humidity) and tight or hard to open and close in summer (high humidity) or when it rains. When you join two boards together, the grain can run parallel or perpendicular. Glue, like screws, lock the board and try to prevent it from expanding causing stress and, possibly, warpage or cupping. The pressure has to go somewhere. With biscuits, the grain runs at a slight diagonal allowing for *some* movement both ways. With dowels (and those new tenon-type biscuits that require a special cutter) the grain runs perpendicular to that in the boards and the glue locks them. Any expansion of the boards toward each other causes pressure on the glue holding them....something has to give. This can cause the joint to fail after repeated expansion. Many antique (and modern) chairs have dowels holding the seat frame to the legs. When these joints fail or the old dowels break there is a process for removing the old dowels from the legs - running the drill bit in reverse.

        Su Ralston <suralston@...> wrote:

        ???
        I'm not much of a wood worker, but I have a bit of an idea what a biscuited
        joint looks like (my hubbins uses them) and what a doweled joint looks
        like...

        Why would one of these allow expansion and the other not?

        Su of the Silver Horn, not just a textile person, Caid
        Su Ralston, Fullerton, CA
        (signatures really do simplify too much!)
         
        .




        When you get down to it about all a man has to call his own is his values. When you sell those out, you don't have anything left.
         
        Col Sherman Potter
        4077 MASH


        Get the Yahoo! toolbar and be alerted to new email wherever you're surfing.




        --
        That which yields isn't always weak.
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