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An Impossible Japanese Joint: Description and Build Notes

Earlier this year, I saw a video on Youtube (https://www.youtube.com/watch?v=GtdQoT7saz0), and I thought that I would try it for myself. This is an example of an “impossible joint”, where it appears that the joint cannot be assembled.

Impossible Joint

I changed the scale, which might have been a mistake. In the video, I would guess that the post is 6 or 8 inches square. I decided to make mine 2 inches square. This has the advantage that there is less wood that needs to be removed, but due to the smaller scale, any errors appear that much larger. I’m tempted to say that doing a clean 2x2 version might be harder than doing a clean 6x6 one.

I decided to make the angle on the sides 30°, which meant that the height of the features would be 0.577 inches. The top of the dovetail is 1 inch wide, and the bottom is 1/2 inch.

This proved to be an exercise in precision and in hand-tool work. All of the cuts are at funny angles, so it would be very difficult to make them with power tools. This ended up being one of the few projects that I’ve made where I carefully laid out lines, and then cut to the lines.

The secret to the joint is that the dovetail doesn’t go straight through the block. Essentially, the dovetails go up at a 30° angle to match the slope of the side. This means that they end up looking funny on the inside.

Design of Joint

They end up being easy to lay out. It ends up that the endpoints of each cut at either 1/4, 1/2, or 3/4 inch from the sides. On the left block (which I’ll call the “male” block), that makes the two top sets of reference lines parallel to each other, with a rise over run of 1/4 inch over 1 inch. On the right block (which I’ll call the “female” block), the reference lines are on the angled face. In one case, the reference lines are parallel, and in the other case they have a rise over run of 1/2 inch over a bit more than an inch.

For my first attempt, I essentially did what was in the video (except for using my table saw to make the V on the end of one block. Then I make all of the other cuts by hand with a small hand-saw.

Alas, my hand-saw skills are not that great. That attempt did not assemble at all, and it looked pretty bad.

For my second attempt, I tried a different approach, which worked much better. This was the approach I took for the second, third, and fourth versions. The second one assembled, but it was slightly loose. The third and fourth ones were acceptable, although not nearly as neat as I would have hoped.

I remembered that I had an old manual miter-saw that I had used 25 years ago, before I had a table saw. It hasn’t seen any use for a long, long time. However, it ended up being ideal for this project.

My plan was to make a jig that would hold the piece of wood, so that one edge of the dovetail was vertical. Then I could rotate the saw the appropriate amount around a vertical axis, and then the saw blade would be in the proper orientation to cut the sides of the dovetails.

To rotate the edges of the dovetails to make them vertical, I made a simple jig to twist the block by 23.4°.

Jig to Tip Post

Then with lots of math that I won’t bother entering here, I concluded that I would then need to rotate the miter saw by 12.9° to actually make the cuts.

On my first attempt, I just knifed the lines, but I found them hard to see. On my second attempt, I knifed the lines, and then colored them in with pencil. This made a broader line, but I found it hard to figure out exactly where on the pencil line to cut. For the third and fourth versions, I tried the following. I put blue tape over the wood, knifed in the lines, and then removed the tape on the half that was to be removed. My thinking was that there would be a crisp zero-thickness edge, and that the cuts should then go up to but not intrude into the tape.

After spending a lot of time laying out lines and adjusting the tape, the blocks looked like this.

Blocks With Layout

The first cuts were the easiest, because they did not have compound angles. For these, I could lay the block flat and just turn the miter saw to 30°. The first pair of cuts were easy, because the bottom of the cut was level.

First "Easy" Cuts

The second pair of cuts were a bit more difficult, because the bottoms of the cuts were at rather an angle. I found that there was a limit in the miter saw as to how much I could angle the blade. Because of this, I could only make these cuts partway through, and then I had to later finish them with my hand saw (using the partial cuts as a guide).

After this, it was a matter of putting the block into the jig, aligning the blade against the edge of the tape, and then making the cut. Sort of.

Making Cuts with Miter Saw

The cuts on the male block were the easiest. One side could be cut all the way through, although it came very close to exceeding the capabilities of the saw. The other side could be cut most of the way through, but not quite all the way. After this, the male block looked like this:

Male Block After Miter Saw Cuts

The cuts on the female block were more problematic. The bottoms of the cuts wanted to be level, but because of the other side getting in the way, the cuts could only be made about halfway through.

Problematic Cuts

Before I started the final hand work, I used the table saw to hog out as much of the dovetail slots as I could.

Block After Hogging Out with the Table Saw

After this, I had to switch to a tiny hand saw to finish the cuts. The cuts on the male block were easy to finish, but the cuts on the female block were again a hassle. The cut needed to extend to the middle of the block, but the saw blade could not extend further than the middle without hitting the other side. This resulted in lots of really short strokes with the saw.

Finishing the Cuts with a Hand Saw

After this, the female block looked like this:

Female Block After Finishing Saw Cuts

The final steps were the chopping out of the remainder of the dovetails with chisels, and then an endless series of fitting the blocks together, seeing where there was interference, trimming one of the blocks, and then repeating.

I found the most difficult part is getting it to close all of the way. It seemed that in all of my attempts, after a bunch of work, I could get it to close to just short of all the way. At that point, it wasn’t exactly clear what was keeping it from closing fully. I had to trim here and trim there, hoping that I was removing the wood that would allow it to fully close, and not removing too much wood that didn’t need to be removed.

The final result looked like:

Final Result (Unassembled)

And you can see how they are assembled as:

Partially Assembled

The final result is:

Final Result After Assembly

Overall, it was a rather humbling project. My hand-tool skills are really not that great. It seemed really easy on the Youtube video, but I found it far from easy. The main saving grace is that if you don’t plan to open the joint, you can hide a lot of ugliness inside, as long as the outside edges are clean.

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