Dodecagonal Interlocking Flat-pack Stool

Since learning how to size and cut notches to interlock pieces of plywood (as described in this Instructable), I've wanted to try using that method with different numbers of intersecting pieces or to make a larger object. I had seen simple flat-pack stool designs before, but thought it would be fun to try to make a more complex design.

The result is this stool, made with 6 pieces of 1/2" plywood for the base and a dodecagonal (12-sided) top. It can be disassembled to pack flat or be carried around using the seat as a handle. If you're interested in the template files, those can be found in step 11. I ended up doing all of the cutting manually using a circular saw and jigsaw, but a CNC router likely would have made it much simpler to build.

1. 1/2" birch plywood - the stool is designed to use one half of a full (4'x8') sheet of plywood. I used a sheet that was ripped along the long edge (2'x8') to fit in my car better, but a sheet cut in half the other direction should work fine as well.
2. 1/4" wood dowels - I didn't have a large enough piece of plywood remaining to make the seat of the stool out of a single piece, so I used dowels to fasten multiple pieces together. The dowels are also used to align and hold the seat onto the base of the stool.
3. Glue for the dowels.
4. String for flat-pack carrying (optional).
5. CNC or circular saw and jigsaw. The files included in Step 11 would likely need to be modified for CNC use to account for the width of the bit being used.

Intersecting 6 pieces of plywood to create a 12-pointed shape can be done in a couple of different ways:

2 groups of 3:

One option is to make two groups of material that interlock three pieces together to form a hexagonal shape then slot those together, cutting an additional wide notch where the two groups slide past each other. I made a prototype of this version as shown in the first three images.

3 groups of 2:

If you look down on the 12-pointed shape, you'll notice that each piece intersects with another piece at a 90 degree angle. So it's possible to slot together three sets of two pieces that are at right angles to each other, then stack the three sets at 30 degree angles to each other. I made a prototype of this version that is shown in images 4-6.

Both versions end up with the same overall layout and angles, but it's neat to see the difference in how the pieces meet up with each other in the middle. This is clearly shown from the bottom view of each prototype shown in the 7th image.

If this all sounds a little confusing, please take a look at my other Instructable that I referenced in the Introduction step and it should help clarify the process a little bit.

I ended up choosing the option of making three groups of two pieces that are slotted together at a 90 degree angle. I thought these groups of two would end up making the whole structure a bit stiffer - not sure if that's the case, but I went with it!

Calculating the slot size for the right angle intersections is straightforward. The notch simply needs to be the same as the thickness of the plywood - in my case, that's 1/2"

For the intersections between the groups, we can do the math to determine how wide the slot needs to be based on the 30 degree angle of intersection. This calculation is shown in the image and the result is 1.866". Since I ended up cutting everything by hand, I ended up rounding this to 1-7/8" (1.875"), which worked out fine.

The "tab" that connects the two halves of each base piece is 2-7/8" tall - this is based on the height of the stool base (17-1/4") divided by 6.

Because I was measuring and cutting by hand, I ended up rounding all measurements to the nearest 1/16th of an inch. As mentioned in the Supplies step, the piece of wood that I used was 2'x8' (24"x96").

The height of the stool base is 17-1/4", so I first drew a line lengthwise down the board and cut it at this distance.

The width of the bottom of the base (without the rounded corners) is 16-3/8" and the width of the top of the base is 13-1/2", so I sketched out trapezoids in alternating directions along the plywood as shown in the first image. This image also shows a template that I laser cut for sketching fillets for the inside and outside cuts.

There is a 4" wide "post" in the center of each base piece that the notches are cut out of, and the border thickness of each base piece is 1-1/2".

The bottom corners of each base piece are cut with a radius of 2", and the inside corners all have a radius of 1/2".

The second image shows a sketch of the overall design (without notches) of each base piece.

I used a circular saw to cut out each base piece trapezoid (using a level as a saw guide as shown in the first image), then used a jigsaw to cut the cutouts on each side, the rounded corners, and the notches. I drilled holes using a 3/8" bit in the corners of the notches to make it easier to fit the jigsaw in and make the cuts.

The second image shows one of the base pieces with notches cut out and side holes marked, and the third image shows a completed base piece.

The seat is 13-1/2" from flat to flat, so it can be made from gluing together pieces from the 6-3/4" strip (24" wide original strip minus 17-1/4" stool height) remaining after ripping the plywood down in Step 3. The seat is made of two layers of plywood.

Each layer is made from two strips doweled together, then the layers are stacked and glued at a 90 degree angle to each other. The bottom layer has holes in it that the dowels from the base line up with, as described in Step 7.

The overall process of creating the seat is as follows:

1. Cut plywood strips roughly to length (at least 13-1/2").
2. Dowel and glue two sets of two strips together.
3. Sketch out dodecagon and dowel locations on one layer.

A dodecagon has 12 sides, so points are separated by 30 degrees. I used the connecting line of the two strips of wood as my 0 degree reference mark and sketched out 30 degree increments on what will become the bottom layer. I then measured 7" out from the center (flat-to-flat distance is 13-1/2", so the point-to-point distance is roughly 14", so centerpoint to point distance is 7"), made a mark at each point, and connected the points to form a dodecagon.

I also marked out points on these lines 3-3/8" from the center for dowel holes to be drilled. I unintentionally marked and drilled holes along the wrong lines, so you'll notice in 5th image that I have twice as many holes as I should!

Clamp the two layers of the seat together and cut out the dodecagon shape. You may need to shift clamps around a few times to be able to cut all the way around! Do not glue the layers of the seat together yet, since the holes in the bottom layer will be used as a guide for drilling holes in the base for dowels.

As shown in the prototype image in Step 1, each piece of the base has a pair that slots into it at a 90 degree angle. Slot the first pair together, then line the next pair up at a 30 degree angle to the first. The third pair then slots into the remaining gap. The image shows the completed base with no dowel holes in it yet.

Once the base is assembled, line up the bottom layer of the seat with the base. Each hole in the seat should line up with the top of one of the pieces of the base. Drill down through the seat into each base piece, ensuring that your drill stops far enough down that the dowel will stick up just under 1/2" above the base. It's kind of hard to see, but the second image shows the dowels sitting just shy of the surface of the bottom layer of the seat.

I started by drilling two holes and placing the dowels in them right away. Doing this guaranteed that the seat wouldn't shift around as I drilled the rest of the holes.

It is definitely possible to drill the holes in the base by just measuring and drilling, but I wanted to ensure that the holes from the seat lined up perfectly with holes in the base.

I made sure to use four clamps to glue the seat layers together. The doweled joints may flex a bit, so this helps ensure that they stay flat while the glue dries.

This step isn't totally necessary. I thought carrying the stool around using the seat as a handle would be a neat feature, but it didn't end up working out as well as I had hoped. It was challenging to get the pieces all lined up on the string, feed the string through the holes in the seat, etc.

If you do want to add a handle, just make sure to cut it between the pieces of the base, otherwise you'll see the base through the top of the seat. For the string holes, I drilled a hole and cut a slot so a knotted string can pass through the hole, but the knot gets held by the slot.

Pop the seat onto the base. This can be done by wiggling the base pieces around until the dowel/hole pairs line up properly, but sometimes it's challenging to do it this way.

The easiest way to assemble it is to set the seat down with the holes facing up, then add the base pieces one at a time. This way, you only have to get a couple holes lined up at a time instead of all at once.

I haven't tested the weight limit of the stool, but it easily holds my toddler :) It feels sturdy when I (~200lbs) sit on it as well!

Overall, I'm pretty happy with how it turned out! I had never used "regular" birch plywood before, and it is quite different than the Baltic birch that I'm used to. This birch has a very unforgiving thin veneer on either side (while the veneers on Baltic birch are all the same thickness), so any chipouts in the wood were much more obvious than they are with Baltic birch. Better planning would likely minimize this issue, so I'll have to keep that in mind for the next one I build!

The image attached here shows the dimensions that I used for sketching out my design by hand. The svg file includes the precise measurements - the page size of the svg file is set to 24"x96". Note that each base piece in the svg includes its full border, so if you're cutting on a CNC, you'll have to delete lines as needed to avoid going over the same line multiple times.

If you make one, please let me know how it goes! I'm especially curious to see how it would go using a CNC. My thought was that with a CNC, you could just use tabs instead of dowels to hold the seat onto the base. You could also only use a single seat layer by cutting a blind mortise into the seat.

Please don't hesitate to reach out if you have any comments or questions!