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Thread: Half-lap joints

  1. #1

    Half-lap joints

    My project has quite a few half-lap joints. I'm thinking of reinforcing them with dowel pins to increase the pull-apart strength. On the other hand I once saw an article that showed where dowel pins actually weaken a mortise and tenon joint (the mortise part broke due to the pins). Rogowski in his book "Joinery" makes no mention of the pins. Any opinions pro or con?

    Thanks, DKT

  2. #2
    Anything you can do to help out a crosslap is an advantage, I fail to understand why a pegged Mortise and tendon would be weakened. It was common practice back when the adhesives were not too reliable or not available.

    Crosslap relys on the adhesive to secure two face planes together, not much else holds the two pieces together (especially on end lap joints) Wood fibers in a cross direction are hard to secure by adhesive alone because of the changing ambiant moisture content and the two directions of expanding and contraction. Any mechanical fastener be it a screw, nail, or peg will help hold the pieces in place during trying times.

  3. #3
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    Like Bill, I was surprised that joint tests under controlled conditions proved some assumed joints to not be what I expected. It only seems logical that dovetails would be stronger or pegged joints would be stronger. It just didn't prove out in the lab. The pegged M&T were only a little bit weaker than the non-pegged in FWW's test and Wood mags test also showed them to be very close.

    To quote FWW magazines article:

    "So what does all of this mean? I think some woodworkers may need to rethink commonly held beliefs regarding mortise-and tenon and dovetail joinery—both in terms of strength, as well as in how the joints fail."

    I use bridle joints now and again for door frame and panel construction. I also use them on web frames for drawers. I have had no problems with them but I've only been doing this for a few years so none of my pieces has 'stood the test of time'. If you can sub bridles for the laps, I would. If you decide to go pegs but don't want them to show, sink them from the back (or the unobserved) side without going all the way through the front face (?). Just an idea.
    Last edited by glenn bradley; 03-20-2008 at 04:01 AM.
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  4. #4
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    Quote Originally Posted by glenn bradley View Post
    Like Bill, I was surprised that joint tests under controlled conditions proved some assumed joints to not be what I expected. It only seems logical that dovetails would be stronger or pegged joints would be stronger. It just didn't prove out in the lab. The pegged M&T were only a little bit weaker than the non-pegged in FWW's test and Wood mags test also showed them to be very close.
    ...
    If I recall, either that article or a similar one half-laps coming out ahead of the other joints they tested due to the increased glue surface area. They didn't test bridle joints but I would think that would have been even stronger.
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  5. #5
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    In the days of old when knights were bold uh er, where was I? Most pinned M&Ts of those days of yore were actually drawbored with the pins (and possibly other parts depending on the work being done) being of green wood. Some appear to have been glueless as well as they make very strong mechanical joints. I've used drawbore M&Ts in situations where clamping was difficult, i.e. curved table aprons into legs. The drawbore pulls the joint tight and no clamps are needed.

    Depending on the wood being used and the width of the pieces, the half lap is really quite a strong joint. Even though the grain of the pieces run perpendicular to each other, they are still long grain to long grain. I don't know that adding dowels would detract from the strength of the joint, but I also don't know that there would be any advantage added. But then, I'm not much on dowels anyway.
    Jerry

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