Over the past few days I’ve started work on the “heavy duty” shelves that I discussed last week. I started with the posts, milling them to size, laying out the mortises for the kata-sage-ari joints, and from there started cutting out the joinery. Here’s a look at the layout of the joinery on the side of one post.
On the left side is the layout for the wedge, and on the right the layout for the half-dovetail which includes a recessed haunch as well. In the previous blog post about the design of this shelf I shared a cutaway view of the kata-sage-ari joint (below left). Rather than having the small shoulder of the dovetail butt directly into the face of the post I thought it would be better to cut a haunch and inset the shoulder into the post (below right). Recessing that point of contact should make the finished look of these joints a bit cleaner, since the horizontal stretchers will now be fully housed without any exposed shoulder.
There are 8 posts and three levels of shelves, so that makes a total of 24 mortises to cut. That’s a fair amount of mortises and I only had a few days available to make this shelf. So in order to move things along relatively quickly I used a variation of a method that I learned when doing carpentry work with Somakosha.
I started by rough cutting the mortises with a mortiser. The finished width of the mortises is 15mm, and I used a 12mm mortising bit for roughing out. With a good mortiser and a sharp mortising chisel in the machine, you could potentially go straight to the line and skip a lot of what I do here. But since these mortises are going to be visible and exposed on both sides, and since the mortiser that I’m currently using has a fair amount of sloppiness in the cut, I decided to use a method that ensures a really clean finish around the perimeter of the mortise.
Below you can see one mortise after rough cutting with the mortiser. This view is of the outside face of the joint.
And here is the inside face of the same mortise.
This is were I made my first mistake with these joints. I should have cut the through portion of the mortise, followed by the sloped cutouts for the wedge and dovetail, and then cut the hauches. Instead as you can see above, after roughing out the through portion of the joint, I reset the depth stop on the mortiser and proceeded to rough out the haunches as well. This wasn’t a project ending sort of mistake, but it did make final cutout of the dovetail part of each joint a little more troublesome to cut.
The kata-sage-ari joint is a good example of joinery that really benefits from thinking about the order of operations when doing cut out. The three images below show the path that makes the most sense to me. Start by cutting the through portion of the mortise (red), followed by cutting the sloped portions for the wedge and dovetail (green), and finally cutting the haunched part of the joint last (blue).
What I ended up doing was something closer to this:
It wasn’t the most efficient way to cut out the joints. That’s what happens when you start getting lost in a funny podcast while making spur of the moment decisions cutting joinery… I’m not a good multi-tasker.
Back to mortising, after roughing things out, I used a chisel to clean each mortise to the line. These cuts can be done really quick with a chisel placed right on the line and a couple of light taps with a hammer. The point is to cleanly cut to the line in order to establish a refence surface for a router and bearing bit. In other words, there’s no need to spend time trimming with the chisel all the way through the mortise, you can focus just on the top edge. The image below shows the inside face of one mortise after a bit of trimming with a chisel.
From there I use a trimmer router and bearing bit riding on the very upper edge of the mortise to clean up the interior.
Here’s the mortise after routing. The bit doesn’t go all the way through the joint (and if you ask me for best results it shouldn’t). So after cutting one side, I flipped the piece over and trimmed from the opposite face as well.
Here’s another mortise routed from the inside face of the joint. You can see a big step down in the mortise showing the material that I still need to cut from the opposite side of the joint. Here you can also see why waiting to cut the haunches would have been a better choice, since I lost a reference edge for guiding the bearing bit, and needed to be careful to stop cutting as I approached the haunch.
After routing out the joint, I started rough cutting the sloped cut for the wedge. I did this process by hand with a chisel and hammer, aiming the angle of cut by eye. At this stage the sloped cuts for the wedge and dovetail are still rough. I came back later, to do the final trimming on the sloped parts.
Here is the same joint after roughly chiseling out the slope for the dovetail part of the joint.
Next I cut the final depth of the haunches.
This is where I had to do a bit more marking and cutting on the interior face of the haunch to establish the correct final dimension for the dovetail part of the joint. (Had I not pre-cut the haunches this wouldn’t be an issue.) After calculating the right dimension, I milled up a little reference stick and inserted it in the joint to use as a reference for marking the final cut on the face of each haunch.
You can just make out my ink line right against the reference stick.
Here’s how the joint looked after chopping out the extra bit of material.
From that new cut, I could trim the final slope for the dovetail part of the joint.
And that was more or less it for the kata-sage-ari mortises. But there are more mortises of another variety to come. I made a slight change to the design, eliminating the x-brace on the back of the shelf in favor of some horizontal cross bracing, and I’ll share more on those details next week.
See you all again in 2023!
As always, well-written and helpful. I hadn’t seen the router method for the mortises, I’ll have to try that!
Thanks Spencer, it’s a pretty neat trick especially when you’ve got a lot of mortises to deal with. With Somakosha we used the trimmer a lot to clean up joinery, especially when cutting frames. Hope you’re doing well, and have a great new year.
Now you tell me!
I did a bunch of these for my garden shed build (not through mortises but the same idea). The beams were too large to take to my mortiser so I used a router template and routed to the haunch depth. That then drilled waste and chiseled the sides using the routed sides as a guide as for the mortise sides and a paring guide for the slopes. But determining where to clamp the paring guide over the haunch was tricky since now I had to pare across a gap. I got it done but it was fussy.
Sounds like you had a similar experience, but maybe even a bit more challenging! I imagine you probably had quite a few more of these mortises to cut for your shed than I did for this shelf project.
How did the kata-sage-ari joints work out for you? Did they feel pretty solid when you hammered in the wedges?
I was impressed with how solid these joints were after assembly. Despite the fact that the dovetail is one-sided, they really tightened up and secure once I tapped the wedges in.
I have done only test fits so far using a dummy tenon. Those seem really solid but I can’t tell about the stress of what will happen in the completed building. I did 21 of them. One aspect that is easier is that I didn’t have to fuss over a clean fit on a through tenon. I could leave the bottom of the blind tenon rough.
One thing I did not do, since the joinery drawings I was working from weren’t clear, was taper the wedge side of the mortise to match the taper of the half dovetail. I wish I had since I think that would be a much firmer grip than what I have. Now that I have finished I may go back and simply glue in a tapered insert to correct that.
I’ll send you pix separately.
This is the first time I’ve come across your blog. Nice work, nice pictures, nice writing. Keep up the good work.
Much appreciated
Tanks to your blog and this article on kata sage ari i recently get some tips and got courage for building a bunk bed with a variant of this type of joint. It has been an amazing and challenging experience, i love the result.
In that project, for some particular joints with trough tenons (most of them were not), i had a small problem wich i’ll try to explane: in some cases, for design purposes, i wanted the wedge being in an exact place; i tried to guess the dimension of the wedge to locate it exactly, then, after the bed spent some days in the heated room, the wood shrinked and the wedge went further, definitly off place! Preventing this I made some wedges more and (one day) i will replace them with some bigger ones.
The teaching i got from this is that, by designing joints locked with wedges, i must always accomodate for a certain unpredictable position of weges.
If you have any tips for manage this issue i’d be glad to hear about.
I hope i did explane well my toughts and I apologize for my poor english!
Hi Marco, thanks for the comment. Really glad to hear you put this joint to use for your own project!
I completely understand the issue you faced with the wedges getting loose. That’s especially a problem when the wood used for the male part of the joint may not be entirely dry, or at least not as dry and the environment where the finished piece ends up. You tap in the wedge and think it’s nice and tight and then come to find it’s becomes loose as the wood continues to dry and shrink a bit. Another situation can pop up where even if the wood has reached equilibrium moisture content, if there are extreme swings in humidity throughout the year repeated expansion and contraction can cause the wedges to get loose as the male part of the joint and/or wedge become overcompressed during humid part of the year only to shrink during the dry season leaving a loose wedge. I’ve found softwoods to be more problematic in regards to overcompression than hardwoods.
Two approaches I try to take to alleviate loose wedges is first by leaving the wedges long (if possible) and cutting them to length after the wood has fully acclimated to the environment allowing the wedges to be tapped in further as the wood acclimates. When you’re confident the wood has fully acclimated and the wedges seem fully seated you can then trim them to length. Another important factor is to try and select quarter sawn wood grain for the male part of the joint so that wood movement will be minimal and the material should be more stable overall with less movement throughout the seasons.