Isuka Tsugi

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A little warm up joinery? Looks simple enough. This is a simple form of isuka tsugi, or halved rabbeted oblique scarf splice. I’m hosting a Japanese carpentry study group meetup this Saturday to study these joints, so I wanted to get in a bit of practice such that I’ll be useful explaining what I’ve learned.

This joint is mainly used on cosmetic applications, like the interior ceiling rafters on a drop ceiling in traditional Japanese construction. Its also used as a ware-away on the top of batter posts when laying out a foundation, not a friendly surface to sit on and throw off the layout, haha.

I’ve never seen a western carpenter cut spikes on the top of a foundation layout posts for stretching string lines, I wonder why. OSHA may have a fit! When I recently layed out the foundation post holes for the greenhouse I built I actually ran in to the problem of people wanting to sit or lean on my batter posts.  Hell, I wanted to sit on them, or grab the top to stand myself from a seated position. Good idea, and good practice for apprentices new to the saw.

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What you notice right away is that the central rip shares a cut line on the end face. At first I assumed that I’m missing something important in cutting this joint. But consider, if you first cut half way down the end grain face for either side, most of the joint is well defined on its face grain cheek surfaces. I tried my best to ride the edge of the adjacent kerf without slipping into it, but its all but impossible. The saw slips into the kerf and undercuts the cheek surface a bit.

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Surfaces from the saw, marking with sumisashi and ink on 1-1/2″ square stock, using 210mm ryoba saw.

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For such small cross section material it made more sense to use a marking knife and kebiki for the joint layout. I also used my dozuki saw and a piece of kumiko as a cutting guide. I’m slow with my sashigane, I know that from watching better carpenters than I at work via Youtube. It took me longer to lay out the joint than to cut it.

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On to a more complicated variation. Here’s what it looks like when you draw it incorrectly, haha.  While in the midst of this useless layout a small bird flew into my shop and slammed into some window glass trying to chase a fly. The poor little fellow was so dazed I was able to pick him up and find a tree nook for him to recuperate. It wasn’t a great moment, the bird might have had a broken neck. Thankfully a little fresh air and he took back to the skies. And I was able to draw the joint correctly, well sort of.

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I scaled up the lumber a bit to 2″ square. This variation has a cool parallelogram key to lock the joint, sachi-sen. I didn’t get the ratio of length to cross section correct, and I’m actually missing a bunch of cut lines, but I started cutting to find out. Made a bunch of cuts actually, all over the place.

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With not much wood removed as a result.

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Most of the material removal came down to being chopped away with a 1″ bench chisel. This blue stain pine is very soft, the chisel work was fast.

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The mating joint is identical in all respects. I wised up a bit and added more cut lines. I shouldn’t have taken out the waste portion of the ‘v’ in this photo, it removed my cut lines for the central rip before I had a chance to saw to the line! Still not much of a problem though.

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Looks like the subject of an Escher print.

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The assembled joint, planed out and keyed with a tapered oak wedge. The joint showed a tendency for the two halves to assemble with twist if there was any proud surfaces. Getting the joint to come together in plane seems to be the major challenge of doing an acceptable job.

Feeling warmed up? Time for hip roof joinery.

2 thoughts on “Isuka Tsugi”

  1. Im travelling again so a short one.

    I was going to as you to post a drawing of the joint too, when I realized that the whole joint can be drawn from 1/4 of it by reflections along different axes. Very economic joint. Will try next week.

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