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Thread: Maybe I Don't Understand

  1. #1
    Join Date
    Feb 2003
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    Oxford, NC
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    384

    Maybe I Don't Understand

    I always hear, and I think I understand, about the negative aspects of gluing end-grain, or cross-grain, to long grain. Wood movement, stress and possible cracking/splitting etc.

    So, mortise/tenon (including bridle joints), half-lap joints, and any rail and stile joints (cope and stick or otherwise) all violate this adage. What keeps the stresses from breaking the joint when you glue up any of these, not allowing for wood movement?

    George

  2. #2
    Join Date
    Mar 2003
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    SE PA - Central Bucks County
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    65,694
    Size of the jointery makes the difference, George. Movement across small distances is proportional to the size of the piece. A narrow tenon, for example, while it will move, will do so minimally. But this is also why you never do a very wide tenon...multiple tenons are prefered over one wide one to avoid expansion/contraction issues. An extreme example of that is a breadboard end on a table top. When you think about it, it's merely a very wide mortise and tenon joint that we let a good portion of it "float" to avoid movement issues. In the case of my cantilevered miter station, the two end supports have the tenon divided into two narrower protrusions. One wide one could have caused splitting from expansion/contraction in the joint.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
    Join Date
    Apr 2005
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    There are hundreds of variations on the mortise and tenon joint. No one machine either portable or stationary is a cat's meow here and in the end analysis, hand tools become the weapons of choice for the odd variations which are actually useful for the purposes for which they were designed. I think some of the most beautiful of these are from the japanese timber framers.

    Having said this, there are a couple of things to note which apply to the orig. post question. First of all, there are no end grain glue joints. Often a standard mortise and tenon joint will leave a space between the bottom of the mortise and the actual tenon. This space is used to take up some of the hydraulic pressure that can be generated on glue up. Because end grain forms such a horrid glue joint, there is virtually no loss in strength by doing this.

    The second thing to consider is the space on the top of the tenon and the bottom of the tenon. Here, it does not hurt to have a slightly loose fit because the face of the tenon will expand and contract 90 degrees to the gain run. These are the minor sides of the mortise.

    The major sides of the mortise is where the main surface area that forms the glue joint is located. Here, you want a piston fit. If you have done it right, you should be able to yank the tenon out and hear it "pop". It is also nice to be just tight enough so that you force the joint with your own strength. Some say you should use a mallet. To make life easy this is true esp. when the glue goes in the joint. But remember this, a mallet can generate enough hydraulic pressure to pop an opening on the outer side of your mortise and leak glue. And if your to tight, you can strave the joint.

    Now, you want a nice little exterior surface area forming the exterior joint of the mortise and tenon. The cheeks. These can hide any imperfections but they also serve a loftier goal. Any force applied to the joint with one exception will create an assortment of compressive loads on these cheeks. The exception is a linear load trying to pull the joint apart. All others will have to deal with a portion of the cheek surface area digging into the outer edge of the mortise. In english, this means that most moment generating forces will tend to compliment the strength of the joint and not work against it. So by keeping the major faces as tight as possible with the major sides of the mortise and by keeping the cheeks as tight as possible, this joint is a killer joint and will take some serious effort to cause it to fail. Even if you have an aft hydraulic pocket and a bit of expansion room on the top of the minor faces.

    And of course, if you draw bore the joint at the time of glue up, then the joint becomes both naturally tight at the cheeks and LOCKING. So now, my draw bore pin will work against that one last force when you try to pull the joint apart. It also works on keeping the cheeks tight as I dont want any movement at the cheek interface lines.

    So how does goober stuff (i.e. glue) improve this joint? Well, glue does fill in some of the imperfections left over at the time of fitting. It also glues the joint together and offers some pretty good strength. But the majority of the strength comes of the joint itself. So if done right, a glue line failure may go unnoticed for 50 or 70 years! The glue does restrict any loose movement in the joint and this helps to improve strength. Not by glue film strength but by forcing the joint elements to compress the glue that is in the way of a loose fit.

    The major glue of choice by the early masters was hide glue. Mixed up by adding water and glue flakes, pellets, bars, etc. and bringing it up to temperature in a double boiler. I love this glue and I use it in my joints for furniture type work. For kitchen work, I often use resourcinol. Its a different application. But hide glue can be reversed and the joint tinkered with later on.

    There are dozens of antique furniture pieces in which the joints have had glue failure years ago and the joint continues to hold up today. Hot and cold, dry and wet atmospheres, etc. take their toll on the glue. But the woodwork of the joint was done so well that even with a bit of residual (i.e. film has failed) glue in the air gaps, the joints are still sound today.

    So hopefully, this gives you a better idea of what really holds a good mortise and tenon joint tegether. And if you do the joints tight enough and you do the draw bore precedure correctly, you can join cabinet doors with no glue at all!
    Had the dog not stopped to go to the bathroom, he would have caught the rabbit.

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