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    1. 12-24-2008, 10:35 PM12-24-2008, 10:35 PM 12-24-2008, 10:35 PM12-24-2008, 10:35 PM #1
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      long term integrity of different glue joints

      After reading the FWW article on joint strength, and the thread that followed on this forum, I am curious....

      It makes sense the lap joints, M&T would win the test, they have the most surface to surface area, as well as the the obvious structural joining at critical stress areas.

      However, since wood expands on its width, and glue is unforgiving... over time, why would the glue on these joints, specially the half lap not give to expansion / contraction. You have two woods joined at 90 degrees, one will expand on the X axis, the other on the Y axis. What prevents the glue line from weakening by natural expansion / contraction?

      Also, this makes a good argument for loose tenon joinery with compressed woods, or woods that don''t tell us what really matters. Furniture built with any of these methods will be plenty strong immediately after completion.

      What most people who use mortise and tenon, or any people who care about joint strength really want to know, is which joint will stay strong over time. Will my chair still be strong in 10 years, 20 years, 100 years?

    2. 12-25-2008, 6:26 AM #3
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      Question about Mortice & Tendon

      I''ve seen pictures of tendons with draw pins.

      Would using both make for the strongest joint of all?

      Thanks - Eric

    3. Woodworking Planshow to Woodworking Plans for 12-25-2008, 7:07 AM #4
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      joint longevity

      The longevity of the joint is determined by many more factors as well.

      In my short-lived experience:

      Dowel joints subjected to racking give way the soonest.

      draw-bored and through-keyed mortise and tenon joints with no glue have been know to last hundreds of years even in bad weather and total neglect.

      I''m taking my best guess at these measurements, don''t move in a direction opposed to the wood it is glued to. Dowels and Dominoes meet much of these objectives. Make sense?

      Will

      Nothing will stop the glue from weakening. All glued joints will eventually fail. It may take years, decades, or centuries to happen, but eventually the glue will break down. Furniture makers centuries ago learned this, and designed their joints to maintain mechanical integrity between the two pieces of wood.

      A glued M&T joint is not as exposed to ambient environmental conditions as the rest of the piece that is exposed. Wood absorbs and releases moisture through the end grain, for the most part.. Having the end grain protected in an enclosed space, the mortise, and filled with glue, the rate of expansion/contraction will be diminished at the joint.

      M&T joints designed for longevity will always utilize some form of a mechanical device in the joint. A tusk tenon, a wedge, a haunch, or locked shoulder, or pin to drawbore, etc. I''s place and usage. Used in the wrong application, the joint will fail early.

      The "traditional" M&T joint is a labor intensive joint, even with a machine it still takes time. Individuals and furniture makers have tried for decades to find a joint that speeds up production and that is more forgiving than cutting M&T''t happen. The joint needs to be designed for the application. They all have their respective place.
    Last edited by Dennis Peacock; 12-26-2008 at 8:35 AM.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)
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  • 12-25-2008, 8:09 the 1 last update 2020/08/06 AM8:09 AM #6
    for 1 last update 2020/08/06 Member Member
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    Originally Posted by Eric Brown
    I''ve seen pictures of tendons with draw pins.

    Would using both make for the strongest joint of all?

    Thanks - Eric
    Eric

    Strongest can have multiple meanings. It can be related to compressive and tensile strength. It can be applied to quantify shear load, or related to an individual materials overall modulus. It really kind of depends. Strength is a relative term.

    The joint that you referred to above is used in a lot of the Mission Style, Arts and Crafts Style, and Art Nouveau Style furniture from the late 1800''s

    In the Stickley and Morris furniture the through tenon was a massive joint that got it''s me, and the way I was taught. Not that I don''t do it. All the half laps I''re doing the area calculation, you only get half the surface area of the dowel as face grain - the other half is facing end grain in the hole. So, a 3/8" dowel 4" long (2" in each piece of wood), only has about 0.59 sq inches of face grain surface area in each piece of wood. (the formula for the surface area of the sides of a cylinder is 2*PI*r*h - remember the radius is 3/16")

    Compare to a tenon about 2" by 2", which has 8 sq inches of face grain to face grain contact. You''s also a consideration of repair. which is why you see those lap joints on older windows and short mortises on muntins in divided light doors and such. should a piece be broken it would be preferable to be able to reproduce and replace the broken piece without trashing the rest. so while those lap joints and 1/8 or 1/4 deep muntin mortises might not be initially stronger than a hidden mortise or deeper mortise in the case of the muntins, they are serviceable.
    Last edited by Neal Clayton; 12-25-2008 at 7:21 PM.
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  • 12-25-2008, 8:58 for 1 last update 2020/08/06 PM12-25-2008, 8:58 PM #14
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    Mike, fully agreed on the dowel contacting "some" end grain.... but the dowel is compressed so tight, the overall strength at the dowel to end grain will probably be at least 50% as strong. Remember, the dowel itself is not end grain, so its not end grain to end grain. But good point....


    >
    Woodworking Planshow to Woodworking Plans for You''t get much mention, of course mainly when the joint is not visible. The popular one is "figure 8''s a good chance that the things we build will be around for many lifetimes and even go up in value. Not many people get to say that about the work they do.
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