The One Ring (Box)

A brief note to readers: 
This is our very first blog post! We hope you enjoy reading about one of our recent projects. 
In an attempt to balance overall readability and the appetites of the more detail hungry readers, this post has a healthy Footnotes section at the end, where we have relegated many narrative tangents and technical asides. Look for the hyperlinked text, which will jump you to the relevant place in the footnotes. Ok let's get to it!

Instigation

Earlier this year we were asked to participate in the engagement proposal of two important people in our lives. We were honored to be a part of that special moment, and happy for the excuse to plan a trip to visit them in their neck of the woods. We were asked by the proposer if we could also craft up a special ring box to be used in the popping of the question. Of course we jumped at that too! 

The only real requirement (beyond keeping the ring safe and secure) was that it not look too obviously like a ring box. It should appear conspicuous in a natural outdoor setting (e.g. the beach) in order to draw the attention of the proposee, but not immediately give away its true purpose.

Given the mechanical nature of the concepts we began discussing, this was quickly identified as primarily a "Dan task," with Mary advising on color schemes and other esthetics.

Timeline

This request came to us about a month ahead of the big day, providing plenty of schedule margin to accommodate all of the ideas we had in mind with absolutely no stress or worry... How could one possibly expect to produce creative work under such conditions?!? I immediately put this task on the back burner to let it stew a bit and give those anxiety-driven juices a chance to simmer. Some vague concepts were pondered in the intervening weeks, but ultimately the true work did not begin until about one week from game time. In fact, it began in the car during the 4 hour drive to start our week long visit. (For those doing the math: yes, I brought the 3d printer along on our trip in order to complete this project 🙃)

Where to Begin?

In starting a new design task, it is often best to begin by considering your constraints. What are the things that your design absolutely must (or must not) do or have? In this case there was only one truly non-negotiable constraint: the ring. As the designer of the ring box, I have no influence over the size and shape of the ring that it will house. So, first and foremost, I must ensure that my design will accommodate the already-purchased ring. Given that fact, I began by designing the features that will interface with the ring itself and building outward from there. Actually, the first true task was to model and print a stand-in for the ring, to allow me to continue the rest of the work without requiring constant access to the original.

I think its worth noting that this part of the work began before I had any solid idea of how the rest of the box would look or function. I know that I need to design a system to interface with the ring. If I don't have that then I don't have a ring box. That is true no matter whatever else happens between here and the finish line. And in fact, the process and result of this initial design task will inevitably influence or inspire aspects of the next. This domino-esque design process is a great way to build and maintain project momentum in those times where you don't have the luxury of waiting around for inspiration to strike. Use your established design constraints as tools here to zero in on some aspect or component with minimal creative wiggle room.

The Design

Ok, enough of the narrative and metaphor. Lets get down to the juicy details...

Ring Post

The design for the ring interface consists of a vertical post that clamps the ring in place while holding it in a position of prominence above the rest of the opened box. The post splits into two pieces, one of which slides vertically to free the ring and allow it to be removed. The use of a spring tab (with corresponding notches) serves to hold the sliding piece in the clamped and open positions. After a couple of iterations to tune the tab & notch system for ideal position holding, we have a neat and functional perch for the star of the show.

The "Lid"

I knew from the start that I wanted this ring box to have some "life." Not simply an interesting looking box with a simple hinge, but a mechanical device with a personality. This was accomplished in the way that the ring is revealed as the lid is opened. The ring post rotates while the lid opens up and falls away to reveal the ring in its full glory as it spins to catch the light at every angle.

In order for the lid to "fall away" it is split into multiple sections, which would each need their own mechanisms. To enhance the overall transformation effect, the "closed" configuration of the lid was given a simple and innocuous form. I always seem to find myself drawn to hexagons, and for this design six mobile lid segments seemed to strike an appealing balance between mechanical complexity and visual simplicity.

So this is where I began the more detailed portion of the design process: a solid hexagonal prism with a rotating lower section (with the integrated ring post), a stationary base in the middle, and an upper portion split it into six pieces.

Petals & Leaves

As it turned out, six identical moving lid segments just didn't feel interesting enough. Plus I was on track to finish the box design with a day or two to spare. So, I decided to design two different flavors of lid section which would differ in both geometry and motion, to be placed in an alternating arrangement around the assembly. These came to be referred to as "petals" and "leaves" as the expanded state of the box took on a flowery appearance. These names in turn further influenced the final shape and movement of the pieces themselves.

       

Each petal and leaf moves in a complex path of combined translation and rotation with the help of a very handy and ubiquitous mechanism known as a 4-bar linkage. By varying the relative lengths of these four "bars" or "links" (the orange lines in the image below) we can tune the motion to suit our purposes. The most critical aspects of that motion are the start and end positions, to achieve the collapsed hexagon and expanded flower arrangements. Additionally, the pieces need to move in a way that does not cause them to collide with either each other or the stationary base. It was this second aspect that required the most tweaking of both the motion path (via link lengths) and the geometry of the petals and leaves themselves.

Actuation

The final critical step in this design is to incorporate the system that will drive the motion of the entire device. We'd ideally like to actuate all six of our 4-bar linkages with a single input motion, which can be achieved with some vertical sliding action. We can transform the manual input rotation into linear motion easily with some screw threads. So we add some external threads to the bottom of the ring post, and a new component with corresponding internal threads. As long as we prevent it from rotating along with the ring post, this new piece will travel up and down the post as the manual rotation is applied. The last step here is to connect this new vertical-moving piece to our linkages and transfer its motion to them. That connection comes in the form of.... another link!

And with that, we have our mechanisms completed. The GIF below demonstrates the full system described above, and shows the difference between the petal (right) and leaf (left) versions.

Bait Hatch

The ring box is essentially complete at this point, but there was one additional feature added to accommodate an extra piece of the proposal plan. The proposer wanted this box to be discovered by the couple's adorable pup. For this purpose, an additional component was added to the design which would serve as a place to stash some bait to attract the interest of the (usually) food-obsessed pooch. This piece took the form of a cap to be installed on the top of the closed box, with a small space to hold some kibble and a vent hole to release the enticing aroma. This cap also had the secondary benefit of keeping the box securely closed when not in use, as the linkages were a little loose in the closed configuration.

       

Assembly

The last step is to put it all together! 

      

The Final Result!

And here it is in all its glory! I am quite pleased with the result, especially for what ended up being a total 5-day project. It is certainly not perfect, but I love the way it transforms from its unassuming collapsed form, blossoming mechanically to present the ring at its center. The Leaf and Petal motions provided by the linkage mechanisms are quite satisfying, and the difference in the paths of two variants makes for an interesting visual effect.

Did it Work?!?!

So, how did it fair in its singular purpose? Pretty good! The goal of misdirection and surprise were certainly achieved when the time came (despite a misfire in the kibble bait plan). The device kept the ring safe prior to the big moment, it did not break or jam during actuation, and the ring was easily removed when it was time to try it on. And most importantly, our two special "customers" are now engaged!

Product Worthy?

So is this something that might end up for sale in our store? Not in its current state, but it has some potential. With some effort devoted improvements, its conceivable that this box (or one like it) could be available for for purchase one day. If you'd like to be able to buy a ring box like this in the future, let us know with an email or a comment below! It may motivate us to make it happen sooner!

Let's walk through a few of the characteristics of this device that I feel would need to be improved:

Robustification

There are a couple places where the device is susceptible to mechanical failure. In particular, the linkage joints can be somewhat delicate, especially the ones with non-ideal print orientation. Failures can occur during assembly/disassembly, or if manual rotation continues beyond the limits of operation (including if mechanisms get jammed mid-operation). Improving the robustness of these features would go a long way toward product-worthiness. I'd also want to add some "hardstop" features to prevent over-rotation in general.

General Utility

This box was designed to only ever hold one very specific ring. This meant I could measure that ring and design the interface to hold it perfectly. However, in order to be a useful product for more general use, it would need to be able to accept a much wider range of ring sizes and styles.

Smooth Operation

The overall operation could be made a bit smoother. For one thing, because of the surface quality on some features, the rotation action is a little "crunchy", and there are places where the linkages can get hung up on features of the other components. Steps could certainly be taken here to make the operation a bit more reliable in this sense.

Mechanical Stability

As noted before, the linkages are a bit loose, which is particularly noticeable in the collapsed configuration. With more time to devote to design and prototyping, I would have added some form of pre-loading (most likely one or multiple elastic bands) to hold the mechanisms in their collapsed and expanded states.

Manufacturability

Improvements to the printability would help as well. Some of the pieces could be made more efficient, in terms of print time and material consumption. And surface quality could be improved in a few places in the name of both functionality and esthetics. It may even be worth incorporating non-printed parts as well (certainly so if it ever came to be produced at any decently large volume).

Epilogue

This project was a lot of fun, and a great opportunity to put some of the creative and technical skills to work for a meaningful purpose. I also enjoyed the process of putting together this blog post about it! This was our first, but there will definitely be more. There are a host of past and in-progress projects that are longing to be memorialized, and there is so much more I could write about many of the sub-topics in the footnotes below and others. 

To those who've made it this far, we hope you enjoyed this peek into the life of one of our recent endeavors. If you'd like to be notified when new posts are published, consider subscribing (see the bottom of this page)!

Footnotes

1. ⇧

   Welcome to the Footnotes section! This is a footnote. When you're ready, click the arrow above to jump back to where you left off in the main post.

2. ⇧

   One of the best ways to "unserious" a primarily functional object is to incorporate some needlessly complicated mechanisms!

3. ⇧

   To be fair, I had originally planned on doing the design and printing at home that week, prior to the visit. But an unfortunate plumbing issue had motivated us to start our trip early, while we let the fans and dehumidifiers do their work without us there. So we made up an excuse for why we were bringing a 3D printer (which basically boiled down to "Dan's a nerd"), and we hit the road.

4. ⇧

   This is a useful lesson I've learned in the course of my career: You don't necessarily have to see the finish line in order to start the race. Sometimes you just need to know that its its there and trust that you'll find it. Until then, turn your focus to the first steps that are right in front of you.

5. ⇧

   I almost just wrote an entire 2nd post in this foot note about spring tabs, beam bending analysis, and compliant mechanisms... This topic may get its own blog post at some point (will link it here if so), but for now I will simply note that these are very useful design elements that I use a lot as assembly features for 3D printed designs (as well as injection molded, machined, sheet metal designs, and more).

   In this design, there is a single tab that holds the ring post halves together as well as three that keep the stationary base component secured to the ring post axially, while still allowing rotation (thanks to a circular groove in the ring post shaft).

      
6. ⇧

   4-Bar Linkages: Another topic that is ripe for its own dedicated blog post (stay tuned)! For now, I'll simply stress just how useful and deceptively simple this family of mechanism is, and offer a quick description and example. A 4-bar linkage consist of four "bars" or "links", connected by four joints. Linkage joints can be revolute (rotating) or prismatic (sliding). The GIF below shows a stripped down representation of the 4-bar linkage found in each Petal mechanism, which uses four revolute joints.
   

          

   One of the four links is known as the "fixed" or "ground" link (the vertical black link in our example above). This link remains stationary, while the others move relative to it. In our ring box design, the non-rotating "base" component serves as the ground link for all six linkages on the assembly.

   The ground link shares its joints with two other links: the "input" link (red) and "output" or "follower" link (blue). As the names imply, the input link is the one that actuates the rest of the mechanism, and the output/follower moves as a result of the input's rotation. In some cases, the motion of this blue link is the purpose of the entire linkage (hence "output"), but sometimes its just along for the ride ("follower"). Both of these links can only ever pivot about their fixed joint (the one connecting it to the ground link).

   The Petal itself represents what's called the "coupler" or "floating" link (green), which connects ("couples") the input and output. This link ends up with the most interesting and complex motion, which is why its used for the Petals & Leaves in this design.

   Our Petal example is of a sub-type commonly called a "Double Rocker", a name which refers to the fact that neither the input nor output links are capable of completing a full rotation. They can only kind of "rock" back and forth.

7. ⇧

   Some of the compromises here can be seen in the final geometry, and while some of it actually lends well to the desired misdirection and ambiguity of the collapsed form (e.g. the complex seam lines and the blunted corners), certain features are clearly undesirable. For example, the rectangular holes in the top face of the box. These are necessary in the current design in order to allow each of the Petals to expand fully without colliding with one of its own links.

       
8. ⇧

   These linkages are "planar" mechanisms, which means among other things that the input motion used to drive them must move within the same plane as the rest of the linkage. Our manual input rotation is not in the same plane of any of our linkages, so it needs to be converted to a different form. 

   A key thing to consider here as well is that our six linkages are oriented on three distinct planes (each plane includes an opposing petal and leaf pair), which means that if we want to use a single motion to drive all six, that motion must be in a direction that is a common to all three planes.

   As it happens, our planes intersect such that they create a single common line, which also happens to be the axis of rotation for our manual input. Luckily there is a pretty easy way to transform rotation about an axis into linear translation along that same axis: That's what screws do!
   

9. ⇧

   The addition of this new link actually gives us a 6-bar linkage with six revolute joints and one prismatic (or sliding) joint. If you count the physical links you might only find 5, but the sliding path is actually the sixth. You can think of it like a different type of link that changes its length instead of its angle. 
   

10. ⇧

    Even after a couple practice runs to introduce the pup to the device and demonstrate to her that it contained a tasty snack, the excitement of the actual event proved too distracting, and necessitated an improvised plan B.

11. ⇧

    A single kibble was chosen at random and measured in order to precisely size the bait hole.

    

12. ⇧

    It took me longer to write this blog post than it did to design and build the ring box! But I am an engineer after all, not a writer.