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The idea of the miter lock was to be able to create long rectangular tubes of wood that would have strong, perfect joints. The challenge of using miter lock bits was significant though. Adjustment of the miterlock has to be perfect and the wood must be very consistent in thickness, etc. Also, miter locks have a significant surface area, so a lot of wood gets removed when routing out a miter lock joint with all the consequent high forces and pressures.

I am wondering if all of this is now irrelevant, now that modern glues are "stronger than the wood itself"? Is there any advantage to use this complex joint over just gluing up a normal 45-degree mitre joint?

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    Nit: a properly made glue joint is "stronger than the surrounding wood", not the glue itself. I know it's probably what you meant, but for something like this we ought to be as clear as possible.
    – jdv
    May 14 '21 at 20:16
  • Comment because I don't have a real answer, but if the joinery gives the structure more strength along one or more axes then the whole structure, mechanically, will be stronger. That is, if the forces are enough to want to tear the wood adjacent the glue joint, then mechanically strengthening the joint itself so those forces are spread out instead of concentrating on the weakest part might still have value.
    – jdv
    May 14 '21 at 20:19
  • A lock miter is easier to assemble because it fits together naturally. The same can't be said of a miter. Or a butt joint, but that looks different, so probably isn't in the same realm. May 15 '21 at 4:08
  • About the only benefit I would guess at with a locking mitre joint is that you might not get as many sudden failures under heavy loads. A good 45deg mitre joint will fail, either beside or through the joint depending on how well it was glued up. But the failure mode will be sudden and spectacular. A locking mitre will probably behave more like other joints, which will loosen and tear with less of a sudden failure mode because the forces are spread out. Both, when well done, are as strong as the surrounding wood. I'd expect similar values for load deflection and eventual wood fibre failure.
    – jdv
    May 15 '21 at 20:54
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Joinery is not all about the strength of the joint, though it is certainly an aspect. A large part of what makes woodworking satisfying is the beauty of the construction.

Aside from that, the structure of a joint is important to the strength of the joint as well, because it influences how the glue bonds the pieces together, particularly with relation to grain ... end-grain to end-grain joints tend to be weaker, or at least require more glue as much will be pulled into the grain itself. I typically reinforce a simple miter joint with a biscuit, dowel, or even pocket holes, because even with glue, the joint itself is weak. By using a miter lock joint or similar modification to the profile of the joint, you are adding more edge/face grain for the bonding, which tends to be superior. This is ignoring the obvious benefit of the interlocking members providing a better union.

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  • Almost all the locked mitre joints I've seen use it as a way to showcase different colour wood with exposed joinery, or for mechanical strength. This Answer is spot on.
    – jdv
    May 17 '21 at 21:30
  • I waited a couple of days to see, and lo and behold, no accusations about you not knowing what joint this is (or what a mitre is!) were forthcoming, Curious that :-/
    – Graphus
    May 19 '21 at 7:38
  • Is ... is that a common issue?
    – Daniel B.
    May 19 '21 at 14:43
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    @DanielB. WW.SE is famous for two things: answers in comments; new (and a few old-timer) folks never ticking the Best Answer check box; comment arguments about perceived religious aspects of woodworking usually caused by earlier disagreements. Also, we can't count.
    – jdv
    May 20 '21 at 22:11
  • Man. It's been a while since I was here ... back when it was fresh. Sorry to hear it, though I suppose not surprised. I don't get the fanaticism about some subjects ... it's stupid to tell somebody their art is wrong.
    – Daniel B.
    May 20 '21 at 23:41

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