Recreating the clever 3-way joint (Kawai Tsugite)
A number of people have sent me a link to this video of a clever joint on imgur.com. This joint is also sometimes called a "Kawai Tsugite".
I thought it was novel, but not useful for woodworking. But eventually I figured, if there is so much interest in this type of joint, I might as well play around with it.
The imgur video didn't show the name "Kawai Tsugite", so I initially wasn't able to find out anything else about the joint, so I worked from just the video.
The three-way aspect of this joint is based on the rotational symmetry of a cube about an axis going through opposite corners. If I spin this cube between my fingers by 120 degrees (1/3 of a turn), it will occupy exactly the same space as before.
Now, if I cut the cube in half, through a plane perpendicular to that axis, I can rotate one side of the cube by 120 degrees and the two halves will still form a cube. This is the basis of this joint.
Now imagine each half of the cube not being just part of a cube, but the end of a longer square piece of wood. I can now put this together forming a straight joint or a right angle joint in two ways (with the piece at left fixed, I can make the piece at right face towards the camera or away from me).
But the cube sliced through in a plane doesn't interlock so it doesn't make for much of a joint. Ideally, we'd join the cube halves with some joint geometry that can be assembled at three angles. The simplest would be something like the joint at left. So let's make the cube halves join like this.
They can be slid together in three different orientations.
So I have the geometry recreated, but crudely with extra cuts and hot glue!
I started by slicing a triangular corner off a piece of square stock, much like I sliced the triangular pyramid corners off my cube.
I needed a precise template of the joint. I started by cutting six exactly equilateral triangles. This was tricky, and I didn't get it accurate enough until the third try (note the two hexagons with a gap on the right). I glued three triangles on the template while using the other three to make sure the spacing was correct.
I then drilled a small hole in the middle of the template so I could stick my follower pin in there for alignment purposes. I used my metal pantorouter for this. The easier vertical adjustment ability came in handy.
Depth of cut is set to the rear line that I drew on the stock.
It needed a bit of cleaning up. Fortunately, much of that could be done just with a hand saw. Where the corners meet, I could just run a hand saw along the plane that was cut. This cuts a tiny bit off the converging edges, but I'd do much more damage if I tried to clean it up with a chisel.
But this joint is really not a useful woodworking joint. Wood is strongest along the grain, and this joint doesn't take advantage of that. I stressed an earlier prototype and it broke very easily. The part at the very end of the joint breaks off first and once that part is broken, the rest of the joint doesn't hold together very well.
So the whole thing was more scratching an itch and an exercise in geometry. I certainly won't be using this joint in any practical project. It's way too difficult to get it right and it's not a strong joint.
It was only after building the joint that I searched for more stuff
and realized people call it a "Kawai Tsugite". Once I knew that, I was
able to find more about it online. For example:
Wooden chain carved using only power tools
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