Mechanical Heart Locket
In middle school, my mom introduced me to a movie called The Illusionist which contained sci-fi technologies designed to trick an audience. The movie was great in my opinion, but much more important than the movie, to me, were the seemingly impossible devices that it was centered around. The main character designs a locket within the first few minutes of the movie which can twist about two separate axes seemingly magically in order to open and reveal a photograph.
At the time, I re-watched the scenes containing the locket repeatedly and mentally tested the diagram drawn in the film to decide whether the way they had worked it out was mechanically possible. Sadly, I found that they’d simply used two separate props for each twist of the locket. I concluded that it was impossible to make such a locket in the real world and set out to make my own replica but with a realistic two step opening process as a replacement for their unrealistic process, as I was still interested in the device. I have since produced many versions of the heart locket, each time narrowing down on a more optimal design.
Using a wooden dowel and some nails and glue, I whittled down the necessary sections of the locket with my pocket knife, carved out sections for the axles to ride in and glued the components together. It was the best that I could do at the time but was a far cry from what I’d seen in the movie which was disappointing. I hung up the project to one day re-approach it with a new mind and new manufacturing techniques. Without precision manufacturing I would have had to settle for my unimpressive first prototype of the locket. I realized during my sophomore year in college while looking back at my old prototype that using some of the precision forming techniques that I’d learned about, with some extreme care, one could create a working replica of the locket from the movie.
The locket is shaped like a small tablet normally, a heart in the intermediate state, and a vertically mirrored heart when opened. In order to achieve this, each axle about which the four sections of the locket rotate has to be cleanly sliced through its center to allow the axle to rotate apart but still function as an axle when in the correct configuration. Through precision manufacturing methods such as 3D printing or a combination of a router or mill and a lathe, this is a feasibly manufacturable design.
That year I had just begun to learn how to use Creo Parametric for my 3D modeling needs and decided to model my own version of the locket to become more familiar with the program and eventually conceive a working, testable version of the mechanism. After designing the first iteration, I submitted a 3D print request to the engineering machine shop at the university. Upon receiving my print, I realized how low the resolution of their fused deposition modeling printer was and had to use my fine-metalworking files to grind the surfaces to specification, so the locket would actually go together. After hours of filing, sanding and allowing epoxy to set, the locket worked exactly as intended.
In the future I would like to 3D print many more with my own SLA 3D printer, once I acquire one, which will have far better printing resolution than the university’s Maker Bot. Based on what I’ve seen from currently available SLA printer models, I shouldn’t have to clean any of the surfaces for it to mesh nearly perfectly. I plan to model a different version once I have easy access to a lathe so that I can construct one from wood, steel and bronze (bushings), effectively replicating the actual locket from the movie as I set out to do years ago.
To finish it off, I added magnets to holes that I included in the printed model so the mechanism would snap into its two main configurations. I have since modeled three different iterations, experimenting with different axle versions and positions, section thicknesses, and the addition of two cutouts inside for small photographs. Using those models, I printed around six more prototype mechanisms, only three of which I have spent the time to file and assemble, because I could tell that the others would fail due to mechanical inadequacies in the 3D printing. I have given two lockets away as gifts, lost one outside and never found it, and stopped halfway through filing a third to tolerance because in testing it, it didn’t mesh or hold together well as a result of a modification that I had made to its design.