Uillinn: a Pair of Stools

Uillinn was created for a school project in which I was tasked with producing a stool that fit into a 20"x20"x20" box, supports up to 200 lbs, and weighs no more than 20 lbs. I created this pair of stools with my parents in mind- they often host lots of friends and need to pull out extra chairs, which can be cumbersome if they're stacked in a closet. I wanted to design a storage solution that would allow them to leave out all the seats they needed without looking cluttered. That way, when they have company they just have to unstack a couple pairs of stools, and their seating will double!

Uillinn (pronounced ill-in) is the irish word for elbow. The form I was originally inspired by was elbow pasta, so I named it after that!

1. 4x8'x3/4" ash plywood
2. domino hole cutter
3. 6x40 floating tenons
4. wood glue
5. Six10 thickened epoxy adhesive
6. caulk gun
7. table saw (that you can adjust the height of the saw on)
8. stain
9. brush for stain
10. various grits of sandpaper
11. CNC machine, 1/4" bit
12. orbital sander
13. belt sander
14. band saw
15. as many clamps as possible (22+)

In the above sketches and models, I began to consider forms I might be interested in building, and realized I wanted to make a nesting pair of stools (rather than stacking, nesting is more about the interaction between the objects, and how the parts exist on their own but still make a whole).

I developed my design further in CAD, which helped me check how heavy it would be and get it under my target weight. It also helped me determine how much material I would need.

I then practiced kerf cutting, a method to curve plywood: by making many cuts along a board almost up to its top layer, you can gently bend the piece and then glue it in place. Above are my practice pieces- and me practicing! I also made a 3D print, which is just a quarter scale of my CAD model I showed in the previous step. With that I tested the positions I wanted to place my floating tenons, and made sure everything lined up.

Next, I made a full scale model out of cardboard and chipboard to make sure the two stools fit together and were a good height for me to sit on. Satisfied with it, I made a quarter scale wood model to practice gluing the kerf cuts- which turned out to be challenging! Once I cut the kerfs, it was easy to get the center piece into the right shape, but difficult to hold it there while the wood glue set, so I gathered a lot of clamps for the final model.

Now you are ready to begin!

First, with the files below, use a CNC machine to cut the pieces you will need (all the pieces for two stools are in these files, no need to duplicate them). When cutting the middle sections, be aware of which way the grain is going- you want it to be going the long way along the middle, where the cuts will be perpendicular to it. This is crucial for the stability of the stools- if you cut with the grain, it is much more likely to break.

Sand all the pieces so they are smooth to the touch to avoid splinters. Start with a belt sander for the the edges, then go from a lower to higher grit with the orbital sander on all the sides of each piece, including the grooves. Set aside one of the U shapes and the thicker U shape.

Use the 6 mm bit for the domino and set it to go 12 mm deep. Make sure the smoother, more consistent grain is on top while you are doing this- it is unlikely you will get the cuts exactly in the middle, and this will help us make sure all the parts line up correctly. Line up the domino to hit in the center of the edge of your middle piece as best you can, and make holes at 2.5" and 6" from each edge on each side, and one in the center of each side. Repeat for the other middle piece.

Make sure to keep the domino at the same height so these cuts line up with the middle! Make cuts at 2" and 5.5" on each side and one in the middle, coming at it from the outside of the curve with the tool. Pay attention to which side you are working on- the wood grain will be slightly different on each side, so make sure you like the side you are not making cuts into.

Working on the table saw, adjust the height so that the saw will just barely penetrate the veneer of the top of the plywood (it's a good idea to practice this on a spare piece of wood). Starting 7.5" from a short edge, make 24 cuts every 1/2", then after those make 4 cuts every 3/8", then 2 cuts every 1/4". This allows us to get more of a curve close to where the tenon will go- we don't want to slice into the domino cut because then it will not add as much stability at the top of the stool. Repeat this starting on the other end of the same middle piece, then repeat the whole process on the other middle piece.

Helpful tips:

1. Take your time on this step- one wrong cut will break the whole piece. It will feel like this is cutting too much off, but it's not- you need to cut up to the veneer or else it will not curve.
2. Make sure to hold down the wood tight to the table as you cut it so it doesn't lift.
3. As tempting as it is, do not try and curve the piece until you are done cutting all of it- it is difficult to get it to lay flat again once it has been bent, and never bend it backwards.

This step is really up to you. I stained the middle of one of mine and the sides of the other, leaving the rest its natural color which I think created an interesting pair and added to their relationship. Just make sure if you want to use a stain, you apply it before gluing, and be careful with the glue because removing spills will damage the stain later!

I made a silly mistake in my process that must be remedied in this step. Get rid of that thick U shape- it was supposed to be a jig you could clamp the middle piece to while its glue dried, but I made it too small. Instead, take your extra U shape and cut off 3/4" from the entire perimeter. This is easy to mark with a compass, because you can set it to that length and trace the shape, then take it to a band saw and belt sander.

Now that that's sorted, trim your floating tenons to be 24 mm long. Fill each of the holes on one side of the middle and one U with wood glue, and put glue along the edge of the middle piece. Put the floating tenons in the U holes and gently bend the middle piece so the tenons fit into its holes. Clean up excess glue, and then place the jig in the middle of the U (far enough from the top that glue doesn't get on it) and use every clamp you can to hold it in place. Also be sure to clamp the U shape to the middle piece. Once dry, repeat on the other side, and then repeat on the other stool.

NOTE: The middle section will be longer than the U pieces- this is intentional! Don't panic!

Once everything is glued, fill the kerfs on the inside with epoxy using a caulk gun. I did 3 passes to fill each cut, and scraped over the edges with an old gift card to remove excess glue. Once dry, use nail polish remover and a soft cloth to remove any residue.

Above are some opportunities for improvement on the steps I took, and some options for staining the two stools. Sand off excess wood glue and any imperfections. Finally, use the table saw to cut off about 1/2" on each leg of each stool so they will sit evenly- sand the bottom until it doesn't rock on the floor.

Take some nice photos of your models, and maybe build some more so you have plenty of extra seating!

I brought the stools home, and my parents loved them! They are comfortable to sit on whether they are stacked or not. I had a great time with this project, and I hope others can too!