Dumpling Joint aka 3 way Mitre

Motivation

This is a page detailing the construction of a dumpling joint aka 3-way mitre. This is a joint used in traditional Chinese woodworking, dating back 500 yrs to the Ming dynasty. It's unique in that needs no glue and is very strong. As well, it hides all the complexity inside, leaving a clean and simple joint on the outside.

I was inspired in this particular case by an article in Fine Woodworking, which used a different technique to make the same joint. He used a router table. Although I have a router table, I don't have the type that easily cranks up and changes the height. Plus, I hate using the router.

I realized that since I owned a mortiser, I could improve on the method. All I'd really have to do is make sure that the gaps between the tenons matched my mortise bit size - 1/4 in this case. Additionally, I realized that with the use of a jig for my tablesaw, I'd be able to quickly generate the required 45 degree angle cuts.

Naming Things

The dimensions of this article are calculated for a 2" square stock, with the gaps being based on multiples of 1/4" spacing. You could adjust for larger sizes, you'll just want to keep in mind that the gaps you make should be a multiple of your available mortising bits.

For the purposes of this article, I'll name the parts above as following:

1. male rail - the part on top with the tenon
2. female rail - the part on top with the mortice.
3. stile - the part that comes in from underneath and locks the rails in place - the post.

Woodworking for Dummies

Next, a few incredibly important things. Trust me, don't skim this section.

It should be obvious, but the stock needs to be square. Exactly square, to within a tolerance of more than 1/64". Otherwise, you'll get to the last step, and that error of 1/64" is doubled to 2/64", and you have a ridge and a gap. Don't ask me how I know this. I recommend you set the fence, make a test cut, adjust the fence, try again. Rinse lather repeat until you can cut 2" square exactly, then rip your stock.

Make a test piece

Take some wood you don't care about, make a test joint. This is a difficult joint to understand, but once making the test piece lets you make sure the jig is working, the spacers are correct, etc. Plus, you get an extra test joint that you can keep in the living and show all your friends. Here's mine:

I made a few mistakes in it, marked "1" and "2" in the picture. Notably, the style has two tenons, one short, one long. Yah, guess what - I cut the wrong one, then I had to cut the other one. Oops. I also cut one the wrong width when I put the wrong spacer in. But I learned from that which one is the correct one to cut for my real joints. And then I had a spacer incorrect on my first cut. Oops again, I just turned the piece around and used the other end.

Those kinds of things are what you want to work out before you start working on the pieces you care about. Now, on to making the jigs.

Tablesaw Jig

You'll use this jig again and again as you make the pieces, along with a set of spacers. It's a simple jig for cutting the 45 degree angles from end on.

The construction was simple - I cut a 4x4 piece of wood on a 45 degree angle, and glued the result to a sideboard. For the sliding back support, I cut another 45 degree angle in a plank, flipped it, cut again. That made a slot with 45 degree angles and a runner that fit it. Then I glued a plank to the runner to make the back support. One important detail - when you choose a place to put that nut in - make it high enough that the tablesaw blade doesn't touch it. Again, don't ask how I know, but I can say that it takes about 2 hours to sharpen a pair of dado blades by hand after they've encounter a bolt. Just saying...

Here are import details.

1. The sliding back support was very important to preventing the wood from shifting during the cut.
2. It was very important to set the fence once so that the blade always cut the same part of the jig. Then I found that I could use the cut in the jig as a zero tolerance mark for correctly cutting the parts.
3. My jig had a slight tendency to cut too deep (very slight) on the last cut. Modifying the construction to prevent that would be something I'd consider next time. Perhaps wider or taller sliding back support?
4. Critical to make the spacers exactly accurate. I cut, measured, and cut again, throwing them out if they were wrong.
5. The blade in the tablesaw was 2 dado blades, each 1/8, so that the cut was exactly 1/4 wide
6. Important to make several test cuts to get the height exactly correct - it needs to cut at exactly 45 degrees across the end of the parts, not higher or lower.

Spacers

You'll need these spacers, exactly why becomes apparent in the Stile section below.

- 1/4"
- 1/2" (2 of these, I used 1/2" plywood I had lying around)
- 1 1/4" (building this from 1/2 + 1/2 + 1/4 each time, the spacers tend to be unreliable and shift when you cut with them)

Each spacer needs to be about 4" long and slight under 2" wide (otherwise gets in the way of the stock)

Order of Cuts

It's important to label all of your parts carefully, so that you know where to make a female rail and male rail. Otherwise, it's all too easy to get one of the pieces in the wrong orientation and cut wrong. That said, I cut the female rails first. This is because once we get to the cutting the male rails and stiles, we'll be setting the tablesaw fence very carefully and we won't want to move it again until we're finished with those cuts.

Female Rail

No pictures here, since it's just a 45" cut on the tablesaw. However, it is VERY important to make the mortise before cutting the 45 degree angle, otherwise the mortiser has no support when making the mortise, and a simple mortising job becomes basically impossible.

Stile

Here I'll take you through making this, step by step. It helps immensely if you keep the sample pieces around, otherwise it's very easy to get confused and cut wrong. Also, once you get one piece in place, you'll be making the same cut for all the other stiles in the piece - it's an assembly line.

First, here is the layout of the tenons in the stile. It's completely symmetrical, and notice that the tenons are 1/2" square, the gaps between them are 1/4". I've shown you the end of one of the pieces, with the test piece beside it for comparison. I've number the cuts on the paper - cuts 1&2 will use the same setup, as will cuts 3&4. Note that the long tenon here will go into the male rail, which means the mortice for the male rail needs to be 1/2" from the top edge.

Order of Spacers

The tablesaw jig is carefully setup to exactly cut 2" in, so that if you insert one of the part without a spacer and push it past the blade, nothing happens. Here, we need the first cut to be 1/4" in, so we put that 1/2" spacer in for accuracy. All of our cuts will work this way, and the beauty is that you can look at the sawblade coming through the throat, compare it to the markers on your part, and be confident of success.

When you've done cuts 1-4, you should have something that looks like this:

Cuts 5&6 are the outer edges, and are done last, since after cutting those the part becomes unstable in the jig. As well, we'll use the mortiser to chop out the corners now, since before we make cuts 5&6, the edges serve as handy guides.

Remove Waste

Here we're using the slots we cut to easily guide the mortiser to the right spot. I've also clamped a stop-block to the guide, making this quick and fast.

After the waste is removed, it looks like this.

And now, back to the tablesaw to finish the outside edges

Cuts 5&6

Here is the tablesaw jib setup for cutting Cut 6. Really all the cuts look like this, with different spacers. Note that I've carefully tightened the nut on the back support to prevent our part from shifting.

That'll give you something like this, with 4 tenons. Note that the tenon in the front corner isn't needed, and neither is the one in the back, we'll remove those in the next steps.

Remove the front corner tenon.

I used the tablesaw and fence to carefully part of the way through, and then a dozuki to completely remove front corner, and cleaned up with a chisel.

Carefully cutting ...

Giving this:

which I finished this way.

Remove the back corner tenon.

I tilted the piece at 45 degrees, and then used the 1/4" bit on the mortiser, cleaned up again with the chisel. I did it all freehand, it was ridiculously easy.

Cut the edges (Cuts 5&6)

Order of operations is important here - if you remove these edges too early, you'll lack support for some of the earlier steps. As well, I'd consider cutting these by hand and a bit high, so that you'd clean up the last bit with a chisel during final assembly. The Fine Woodworking article did it that way, and it might be necessary to get a perfect fit. I didn't have a perfect fit, but then again I failed to make the parts exactly 2" square.

Trim the right tenon short

Seriously, I got this wrong even though I was looking at the test piece. Look 3 times, then cut the tenon short that will be blocked by the male rail. Nothing hard about trimming the tenon - I carefully used the table saw to start the cut, and then the dozuki to finish it.

Male Rail

Again, set up the tablesaw jig. It's easy to set the fence correctly again, since the slot the blade cut in the jib is the reference point.

Make the Mortise

Nothing fancy here except for measuring - use the mortiser and a 1/2" bit to make this mortise. I set the mortiser fence flat against the piece, and then used one my handy spacers to offset it by 1/2"

Make the Tenon

On the tablesaw jig, cut away the left side

Then cut the right side. That leaves you with this, which sends any self respecting woodworker to the bandsaw.

Bandsaw

First cut out the tenons, then 45 degrees and freehand it. Feel the force, you can do it. Clean up with a chisel.

Cut 1:

Cut 2:

Assembly

Things should just fit. That's why we mucked about with the test piece, right?

First, assemble the top frame:

Then insert the stiles, giving us this result

Yay!

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