Making Koushi-do 格子戸 – Part 2

In the previous post after completing the joinery layout, I proceeded with cutting the koushi and door rails to length. I also cut the shoulders of the tenons in preparation for cutting the tenons themselves. Finally I used a groover to cut a special set of 60 koushi that were too long to batch cut on the sliding table saw.

So in this post the main tasks at hard are cutting mortises and tenons. Before getting into the details I thought I’d share another video. The tenon saw that I use for cutting the tenons is a particularly unique tool, and I think it’s helpful to see it in action to fully understand how it works.

https://www.youtube.com/watch?v=9IanDYSD4k8&ab_channel=BigSandWoodworking

Mortising

Now on to cutting mortises. I like to cut the mortises before the tenons so that I can adjust the fit of the tenons to the mortises. It’s much harder to do the opposite, particularly when using mortising chisels that have a set size.

So on to the hollow chisel mortiser. For those interested in Japanese tools, the Japanese term for a hollow chisel mortiser is kaku-nomi 角ノミ, which literally means square chisel. I don’t have much experience with hollow chisel mortisers in the states aside from cheap benchtop models which leave a lot to be desired. It seems like the only decent hollow chisel mortisers available nowadays in the USA are either old machines or imports from Europe. This Japanese kaku-nomi, is fantastic, and makes processing mortises a breeze. The table is adjustable front to back, up and down, and has a really convenient lever lock for clamping the workpiece. The giant wheel controls left to right movements making it easy to quickly move the table/workpiece and cut wide mortises as I’m doing here. My only complaint about these machines is they are short. I’m 5’10” (~178cm) and I have to hunch over a bit while using it. Taller folks would likely have a sore back after a long day of using this machine.

The majority of the double mortise and tenon joints were 6 mm thick. So I started with a 6 mm hollow chisel and processed all of those mortises first. Then I switched over to a 9mm bit for cutting the koushi mortises. There were also a few larger doors that I made from thicker 36 mm stock and those had 9mm thick double tenons/mortises.

Below you can see a set of door stiles with double 6mm wide mortises cut. In the video when cutting these mortises you can see me sliding the mortise chisel back and forth in the mortise. This is another technique that I learned here in Japan, and applies particularly to narrow deep mortises. The purpose of doing this is to clean up the bottom of the mortise and to clear out the chips. After hollowing out the mortise a lot of little chips still tend to be lodged inside, and they are particularly hard to get out on narrow 6mm mortises. So by carefully sliding the chisel left and right you can use the hollow chisel and bit to draw out the chips. At the same time the bottom of the mortise will become much more flat and clean as the chisel will scrape away the remaining material. The technique works well, but you need to be sensitive to any resistance when using narrow chisels. If you push them too much they can snap or bend.

Cutting tenons on the Shoukouban – 昇降盤

Now on to a really unique machine. This is one that I have not seen outside of Japan. It’s called a shoukouban 昇降盤, which more or less means ascending/descending table (saw). Unlike most table saws where the blade and arbor are raised and lowered relative to the table, on a shoukoubann the table is raised and lowered. Meaning the table moves and the blade and arbor are fixed. That aspect of these machines is kind of awkward, and I can’t say I’m a huge fan. But what is really special about these saws is the small extension table on the right side which is paired with an externally mounted blade. This auxiliary saw is primarily used for cutting tenons and it’s fantastic. It’s incredibly fast, accurate and easy to adjust for different size tenons.

Here is the business end of the tenon cutting saw. It’s basically a small table with adjustable fence. The table itself is adjustable up and down and front to back. There is also a really simple L-shaped stop that you can see sticking out from the table, close to the blade. This stop is also adjustable and is used to set the depth of cut.

Here are the adjustments for the table. It’s adjustable for tilt, up and down and front to back. It’s a really convenient and adaptable saw.

Now on to the actual tenon work. For double tenons there are a total of four cuts to make. The first cut I made established the inside face of the double tenons.

Then I cut the outer face. These first two cuts ultimately determine the tenon thickness. I try to make the tenon thickness an easy slip fit. Meaning not too tight, but not loose either.

The next cut I made was the first edge. This cut was more or less 15mm back from the outside edge of the rails.

For the last cut, I reset the fence and cut 6 mm back from the inside edge of the rails. It’s a bit hard to see but the tenons are fully cut at this point. One detail to note about the tenon saw is that it undercuts the end grain portion of the tenon shoulder. Depending on who you ask, some people believe undercutting is a bad practice, while others do it all the time. I think it’s pretty helpful, and can really ensure a clean fit upon assembly. Usually you have to do undercutting by hand with a chisel which can be kind of tedious. Having a saw that cuts tenons this easily while also producing a slight undercut is really handy.

Compression Fit Tenons

While I cut the tenon thickness to be an easy fit, I cut the tenon width to be oversize. The result is that the edge of the tenon will compress into the end grain of the mortise when assembling making for a really snug and solid connection. This is a technique that I first learned here in Japan. The first time I saw this was when my friend and boss of Somakosha was making a set of chairs in oak using double tenons. I watched as he cut the joints and the tenons were 2mm oversize from the mortises. I thought he was crazy. Especially with a hardwood like oak I never imagined the joints would go together. But I watched as he assembled the chairs with clamps and all the joints closed up nice and tight. Amazing!

Basically the method ensures a really strong mechanical grab between the tenon and mortise. The key point is to cut the tenon width oversize, but never the thickness. If you cut the tenon thickness too fat it’s quite likely that you will split the stiles when assembling. It’s also important to leave a bit of length beyond the end of the mortises when assembling, so that the pressure from the oversize tenons doesn’t break out the end grain of the stiles. For the doors I left all the stiles uncut and long until after assembly.

When I cut the tenons to size for the doors I was aiming to make the tenons 2mm wider than the mortises. Somehow some of the tenons ended up being even wider than that. Given the number of parts I had to cut I’m guessing I just made a mistake and miscalculated on a few of the tenons. In any case some of the tenons were closer to 4mm oversize. Nevertheless we forged ahead and assembled them, and the joints went together with no problem. It’s pretty impressive how much wood can compress, and sugi in particular is a very soft compressible wood.

Koushi-do Tenons

I used the shoukouban tenon saw to cut the koushi-do tenons as well. The main difference with the koushi-do tenons is that I made them more or less the exact size as the mortises. It’s one thing to fit one or two oversize mortise and tenon joints but when you have to fit 16 koushi during glue-up there is a lot of compounding resistance.

Mentori 面取り/ Chamfering

One step that I didn’t photograph was mentori/chamfering. For these doors all of the koushi and all of the rails got a 3mm chamfer. I planned to use a mentori-ganna to plane the chamfered edge, but with so many pieces thats a lot of handplaning to take a 3mm chamfer. So I used a chamfer bit and router table to rough cut the chamfer on all the necessary pieces.

With chamfering done, the main machine work was complete. Next up handplaning! I’ll have more on that in the next post. So stay tuned and thanks for reading.