Mechanical Iris Engagement Ring Box (February 2017)

When I was machining my wife's engagement ring from a bar of titanium, I knew a store bought ring box just wouldn't do. I wanted to make something really intricate to test my machining skills, so I settled on a mechanical iris. Similar to the engagement ring, I filmed the whole process and put together a video.



My first thought was to make a normal mechanical iris where rotating an outer ring pulls the petals open via little arms. I spent quite a while modeling this in Rhino before finally accepting it would just be too big. 6" diameter in this case!

Mechanical Iris Engagement Ring Box


Eventually it came to me that if the petals were mounted to the rotating ring, I could have some little pins mounted in the stationary base protrude up through slots in the moveable ring and move the petals via a slot on the underside. If the pins had a larger band on them, they could also act as "nails" to hold down the rotating ring. It turned out to be a really compact design once I figured it out and I'm quite proud of it.

Mechanical Iris Engagement Ring Box


And here's a model showing how it opens and closes. The rotating ring has a little protrusion on it to help rotate the ring and open the petals. The wedding and engagement ring fit into little slots in the walnut base. The rotating ring and petals will be made of bronze.

Mechanical Iris Engagement Ring Box


I started with a piece of 3" X 1/4" 954 aluminum bronze.

Mechanical Iris Engagement Ring Box


This material is extruded at the factory, so I surfaced both sides with a fly cutter on my Powermatic Millrite 7x27.

Mechanical Iris Engagement Ring Box


Next I mounted my rotary table and drilled 5 holes around the circumference of the rotating ring. I then tapped these #5-40 for the screws that hold on the petals.

Mechanical Iris Engagement Ring Box


Then I came in with a 3/8" end mill and cut out the inside of the ring.

Mechanical Iris Engagement Ring Box


Next up were the slots for the pins. The first pass was with a 3/16" end mill halfway through the material thickness. Then another pass with a 1/8" end mill all the way through. The larger diameter slot will allow a larger diameter on the pin to retain the rotating ring. Each of the five slots are 40.5º equally spaced around the center point. I wanted to use the rotary table to cut the outside of the ring, but I couldn't clamp the workpiece very well and it shifted on me. After that, I cut it out with a bandsaw and sanded it to shape.

Mechanical Iris Engagement Ring Box


Next up were the petals. These were probably the hardest to do with manual machines. I'm sure with enough setup time I could use a boring head to get the proper radii, but I thought it would be easiest to just use a taped on template and cut them out by eye.

Mechanical Iris Engagement Ring Box


I shifted the vice around until a pointer in the chuck showed the slots parallel with the X axis. I did use coordinates for the holes and slots, so all I need to do is cut the petals out and sand to the line. The slot is 1/8" to accept the other end of the pins that hold the rotating ring onto the base.

Mechanical Iris Engagement Ring Box


After milling the features I cut the petals out on a bandsaw and sanded them to shape.

Mechanical Iris Engagement Ring Box


I made the pins that hold the rotating ring down and operate the petals out of brass. They have a larger diameter that fits into the 3/16" slot in the rotating ring. I'm parting this one off and it will be flipped around and taken down to 1/8" on the very end.

Mechanical Iris Engagement Ring Box


Next I made the base out of a piece of walnut. It has 5 holes to accept the pins and two slots for the wedding and engagement rings.

Mechanical Iris Engagement Ring Box


I pinned on the rotating ring with some drill bits and sanded the walnut base to fit. This insured I had a perfect fit.

Mechanical Iris Engagement Ring Box


Next up was channeling my inner Clickspring and polishing the bronze parts.

Mechanical Iris Engagement Ring Box


It was a lot of work, but definitely worth it. Here you can see the slot on the underside of the petal a little better.

Mechanical Iris Engagement Ring Box


Next I started assembling the mechanical iris. The rotating ring is placed onto the walnut base and a pin inserted.

Mechanical Iris Engagement Ring Box


The holes are slightly smaller than the pin, so the pins are driven down with a hammer until they're just below flush with the rotating ring. But not so tight that the ring is nailed down making it hard to turn.

Mechanical Iris Engagement Ring Box


All five pins installed and the rotating ring is held captive to the walnut base, but it is free to turn through 40.5º.

Mechanical Iris Engagement Ring Box


After making sure the rotating ring was working properly, I started installing the petals. Due to the hand sanding they aren't all identical, so I stamped them 1-5 and put them on in order. I had to do some fitting on each petal to make sure it didn't bind with the previous one and I was careful to make sure they close very tightly. These are screwed down with some #5-40 x 3/8" stainless screws (McMaster #91792A126). I put some Loctite on the threads and backed them off 1/8 of a turn so they are snug, but can easily pivot.

Mechanical Iris Engagement Ring Box


And here's the finished product. It was an incredibly satisfying feeling when the petals opened just like I thought they should when I designed it. This was a really involved project, but so worth it in the end.

Mechanical Iris Engagement Ring Box


With the rings.

Mechanical Iris Engagement Ring Box