Introduction: Katasage Ari (half Dovetailed Joint)

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This joint is mainly used to connect corner posts to tie beams in classical Japanese architecture

Here I just make one for demonstration. Hope you enjoy it.

As usual, there is a video to accompany the instructable.

Step 1: Marking Out the Mortise

Select a chisel fractionally narrower than the 'tie beam', and set a mortise gauge to the chisel's width

Gauge in the sides of the mortise, and then add a line that represents the base of the 'tie beam', and another line about ½" above the 'tie beam' (this marks the position of the top of the wedge)

Step 2: Chopping the Mortise

Chop the mortise, as usual initially, to a depth of about half the post thickness, leaving the top/wedge end about a ¼" shy of the end

Once that's done, you need to slope both ends of the mortise as follows:

The top/wedge end is sloped from the layout line to the base of the mortise, so that it flares by the ¼" we left initially

The bottom end is sloped with an undercut of about ¾" to the base of the mortise

(These dimensions are suitable for the roughly 2½" square post, and 2½" beam, shown in the photos, and should scaled up or down as appropriate)

Step 3: Beam End

Use a bevel gauge, or similar, to transfer the undercut slope angle to the end of the beam. It should extend to the depth of the mortise minus about 3/8", to allow for the full beam to be supported within the post. This creates the dovetail as shown.

Saw, split, and pare the dovetail, as shown

Step 4: Fit and Notch

Because the chisel was selected slightly narrower than the beam, the fit should be very tight (possibly a no go). (This was deliberate, to allow some tolerance in chopping the mortise)

Ensure the sides of the mortise are clean, flat, and parallel, before adjusting the thickness of the beam for a snug sliding fit using a plane

Now chop out the small notch at the bottom end of the mortise, so that the beam seats well in place. The bottom of the beam should close-up to the bottom of the notch, as the tail slides to the base of the slope in the mortise

Step 5: Wedge

With the beam in position, measure for the locking wedge:

Take the angle for the wedge from the slope at the top of the mortise

Saw a wedge with a point to begin with. Insert it and gauge how much further it needs to go to snug up, then remove that much plus another ¼", to allow for tightening over time

Bevel the striking end of the wedge to avoid spelching, and then drive it home

Step 6: Finished

The joint requires no glue, but the wedge may need to be tapped in a little more if shrinkage loosens things

Don't forget, the dimensions given here are suitable for the roughly 2½" square post, and 2½"
beam, shown in the photos, and should scaled up or down as appropriate.

For posts where more than one beam connect at the same height, be sure to pick post and beam sizes that leave sufficient wood, in the post, around the joint to maintain it's integrity.

Thanks for looking at my instructable.

Cheers, Mitch