Making Koushido 格子戸- Japanese Lattice Doors

This month I’ve been working down in Okayama with Somakosha again, this time making 格子戸 koushi-do, or lattice doors. A friend of mine bought an old kominka and he’s been fixing it up over the past year (I posted a bit about the project in this post from last year.). He is closing in on the final details before moving in, one of which is to deal with the doors. The house is big, and in total I think there are over a 100 plus doors, many of which are old sliding shoji. Some of the old doors can be reused, but others are either damaged or no longer fit the space after making different adjustments to the structure. So I’ve been tasked with making the remaining new doors. In total there are some 37 doors of different varieties. Some are simple frame and panel doors, others are glass doors, and then there are a bunch of 格子戸 koushi-do, lattice doors. The koushi-do are the most involved to make, and the most unique, but I’ll share details from the fabrication of all the doors in this series of posts.

The overall set of steps involved were:

• wood selection
• rough cutting
• milling parts to dimension
• layout
• cutting parts to length and cutting tenon shoulders
• cutting mortises
• cutting tenons
• men-tori / chamfering edges
• hand planing
• assembly

Below is a graphic showing one of the types of koushi-do I was working on. In the image there are four doors, and there are two sets of these, so in total 8 doors. With 16 koushi per door there are a total 128 koushi. With so many pieces to make we decided to keep the joinery fairly simple. The door rails join the stiles with double mortise and tenon joints, and the koushi join the rails with short stub tenons.

Sumi-tuke 墨付け – Layout

After rough cutting and milling all the parts to size, I started to lay out the joinery for the doors. Most of the doors are 30 mm thick, which is a convenient size for laying out an even spacing of 6mm double tenons and mortises. I set one ball pen kebiki to 6mm and another to 12mm. Using those two marking gauges I was able to work off of each side of the workpiece and layout a set of lines spaced 6 mm apart. On the end of the rails pictured below you can see four lines spaced 6mm apart. These will become the double tenons. The other layout marks are for the koushi lattice pieces. I used the same overall process and set of ball pen kebiki to mark out the corresponding mortises as well.

When laying out the mortises for the koushi I clamped the door parts together and marked across all the pieces at once.

This is one of 6 ballpen kebiki that I used for the layout. These tools are fun to make and incredibly handy. It’s nice to have a lot of these available when doing layout so that you can set them to a dimension and leave them at that setting for the duration of the layout. Having to change and readjust them is not only a pain, but also creates more possibility for inconsistency in your layout.

Cutting the shoulders

With the layout complete I moved on to cutting the shoulders of the tenons. This process first involved cutting all the parts with tenons to length and then setting up a stop block for cutting the tenon shoulders. I used a sliding table saw for most of this work.

Below I’m cutting the shoulders for one batch of koushi 格子. In total there are 128 of these pieces. Each stick requires both ends get shoulder cuts on all faces which makes for 128*2*4=1024 cuts total. That’s way too many cuts to do one piece at a time, so I cut a piece of mdf to use as a wide stop that would allow me to gang up a group of koushi and cut the shoulders in batches.

I could have used a wider piece of mdf and tried to do cut even more koushi at the same time, but I was a little bit worried about small imperfections in my setup that could translate to bigger and bigger errors. So I worked in batches of 15 koushi each which was quite manageable and still sped up the process greatly.

After the koushi I also cut the rail pieces of the door frames to length and cut their tenon shoulders as well. I was tasked with getting these doors done within the time that I was here (1 month) so to speed up the process I simplified certain areas. One such area was the tenons on the rails. In the past I have used haunched tenons on doors, but it does add a decent amount of time to the overall process. So this time I skipped the haunch. Instead each rail consists of a double set of wide tenons.

I didn’t batch cut the rails like the koushi pictured above. The quantity was less and I also felt I’d have more accurate results if I cut them one at a time.

Most of the koushi 格子 were just single vertical slats with no cross members of any kind. But one set of doors consists of tightly spaced vertical koushi with three horizontal cross members that join with lap-joints. In the graphic below you can barely make out the three horizontal koushi in each door but they are there. They will add stability and keep the long vertical koushi straight over time.

Here again I had a lot of cuts to make. This batch consisted of 60 vertical koushi with tenons on each end and 3 lap joints cut into the back of each for the horizontal pieces. The table on the sliding table saw was too short for me to cut the shoulders using the same method described above. So I came up with a different method to batch cut these koushi, this time using a groover. The video below shows the overall process.

The video is a bit light on details so I’ll try to describe the process a bit more. You can see the overall setup below. I used a straight edge and piece of mdf to create two reference edges that were 90 degrees to one another. I could then secure all the koushi tight to those edges and batch cut all 15 koushi at once. (there are actually 17 koushi pictured but the 2 outer pieces are more or less reference pieces and also prevent the groover from blowing out the grain.)

For those unfamiliar with a groover, it’s basically a beefy circular saw designed to cut grooves. Here is the groover I used for this project, an old Hitachi model that I picked up at a flea market. In this picture it has an edge guide attached, but i removed that when cutting the koushi tenons and lap-joints. (for any readers who may be trying to search for these tools, in Japanese they are called 溝切カッター, mizo-kiri-katta)

There are a variety of cutters available but one common variety looks like this. I could have used a wider cutter (they go up to 33 mm wide!) but the wider the cutter the more dangerous the cut becomes. For these koushi the half-lap joints were actually 30mm wide, but I chose to use this 12 mm cutter and make the 30 mm lap-joints in three passes. I did this partly because this cutter was one of the only fresh and sharp cutters I had available.

Here you can see a bit more of the setup. After clamping the koushi together and holding them tight to my reference edges, I used a clamp-on edge guide to guide the groover for making the cuts forming the tenons and for the half-lap joints. Here I’ve already cut one side of the tenons, and I’ve made the first cut for one of the lap joints.

Here is the setup after completing the first of the lap joints in this batch of koushi. Overall this setup wasn’t perfect. For the lap joints I had to move the edge guide 3 times, meaning little errors have lots of opportunity to creep in. I did my best to carefully measure and locate the edge guide each time, and in the end I erred on the safe side by making the half-lap joints a bit tight. That allowed me to use a plane to fine-tune and fit the horizontal cross-pieces for gap-free joints.

The tenons were also a challenge since I had to rotate the koushi 4 times to cut the shoulders and tenons all the way around. Again not the best setup for reducing the possibility of errors, but given the timeline and the tools at my disposal this was the best I could come up with. In the end things worked out quite well.

Here’s one end with the tenon shoulders cut on two sides.

For cross grain cuts in a very soft wood, the groover really cut cleanly.

And here are the finished tenons and lap joints for this batch of koushi 格子. I labelled them so that I will be able to assemble them in the same order they were in while batch cutting.

Coming up I’ll cover cutting the mortises and tenons, and then it’s on to hand planing. So stay tuned for that work in the next post. Thanks for following along.