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Thread: Wood Movement

  1. #1
    Join Date
    Jun 2010
    Location
    Vancouver Island, Courtenay/Comox Valley, British Columbia
    Posts
    3,220

    Wood Movement

    Tonight's question is how much can a hardwood book shelf be expected to expand or contract with the seasons?

    I understand that a) it depends on the wood and b) it depends on the orientation that it's cut but I just want to get an idea. If a shelf is 30" X 11" how much might it change across the grain? 1/4"? 1/2"? 1/8"? It's pretty damp here all year round.

    thanks,
    AKA Young Grasshopper Woodworker
    AKA The Rookie

  2. #2
    Quote Originally Posted by Cynthia White View Post
    Tonight's question is how much can a hardwood book shelf be expected to expand or contract with the seasons?

    I understand that a) it depends on the wood and b) it depends on the orientation that it's cut but I just want to get an idea. If a shelf is 30" X 11" how much might it change across the grain? 1/4"? 1/2"? 1/8"? It's pretty damp here all year round.

    thanks,
    Here's a calculator that will tell you exactly what you want.

    http://www.woodbin.com/calcs/shrinkulator.htm

  3. #3
    Join Date
    Dec 2006
    Location
    Bellingham
    Posts
    2,449
    The calculator is great, but it is only half the story. You need to understand the current humidity of the wood when you build the furniture, the humidity of your shop where you will be building it and the humidity swings this furniture will experience in its finally resting place (inside your house).

    If your wood was green, it would under go some significant changes until it became relatively stable with your shop environment. Most likely it is not green, but you always should allow it to become conditioned to your shop's environment. If you do this, then you really only need to understand the difference between your current shop conditions and the conditions of where this furniture will reside. Part of that understanding is knowing that wood humidity changes with the seasons. See the graphs of Seattle and New York below (surprising, isn't it). Another wild-card is that inside conditions are also affected by the mechanical heating and cooling systems which may reverse what is occurring outside.

    So how to make sense of all this? In the long term, keep sample blocks of a couple common wood species in your shop with known dimensions (dated) and occasionally (seasonally) measure, mark and date on the board the dimensions. Do the same for some blocks you keep in your house. It then becomes a no-brainier and you have information that is tailored to your situation. Just one thing, make sure you really mark them well so they don't inadvertently get used or thrown out in a fit of cleaning up the "mess".

    Click image for larger version. 

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