Two-stage Cryptex: Fabrication and build

As it turns out, it's a lot harder to fabricate things when you aren't a student that has a 24 hour machine shop at your disposal (or a free machine shop anywhere, period). Bemoaning my separation from the CSAIL shop/stockroom and the ever-helpful staff of the Pappalardo machine shop, I began my lengthy search for outside resources. 

First up were the two endcaps, which absolutely needed some portion, if not all, of them to be CNC machined. I shopped around at a number of fabrication services (as an aside, why does every place require you to submit a model and then wait around for a quote? It's incredibly awkward to ask for a quote from someone for a tiny one-off personal project, knowing their answer will be way outside your budget and use case). Disclaimer: I am not affiliated with any of the companies I'm going to mention, I'm just a broke recent-grad who designed a thing way beyond her means to produce at a reasonable price.

Finally, I decided to go with Plethora, largely because they offer a $250 discount on your first order, and because their prices reduce significantly if you aren't in a rush to get your part made. Plethora is one of the only small-batch machining companies to offer a timeline beyond a few days (with price reduced accordingly), and while this makes sense for rapid prototyping reasons, there are few options for people like me that have a relatively simple one-off part but no machine shop to use, but also aren't in a rush to get it produced in 3 days for $600. I also appreciated the real-time cost and machinability estimates available through the SolidWorks plugin, which also saves me the awkward interaction of having to ask for a quote and then finding a nice way to balk at the price.

The endcaps turned out beautifully, and I'm a little sad that the spiral toolpath that the endmill left on the inside is covered by the chamber end on one of the caps and barely visible at the bottom of the cryptex on the other. They came on time and the entire ordering process was very straightforward.

CNC milled endcaps from Plethora. So shiny!

The middle casing turned out to be the hardest thing to explain to online fabricators. My plan was to mill the slots down the length, since the material thickness and the width of cut would likely not result in the shape of the tube opening up. But every supplier I talked to was unconvinced about the structural properties of aluminum and insisted that the part be machined from a solid cylinder of aluminum, resulting in a total machining cost of approximately one arm and two legs. This was particularly frustrating because I purchased the tube stock myself, knowing that if I just got access to a Bridgeport, I was fully capable of machining the part myself, without the failure modes that everyone kept insisting would happen.

In the end, one of my coworkers in the machine shop saved my butt, responding to my request for a machining service by graciously offering to machine the part himself, given that it was simple and not particularly in a rush to be fabricated. The one modification I made, because our fancy circular mill was booked for the next few weeks, was to make the lock slots for the retaining ring planar cuts instead of being concentric around the cylinder. I used the projection of the outline on the outer diameter as the planar cut outline, to prevent the slot being too short for the lock tab.

The middle casing did not in fact open up during machining, and turned out just fine (thanks Sean!). When I tried to piece everything together, however, I realized I'd made a horrible mistake. When I got the tube stock from McMaster, I neglected to ask for it to be turned down to the proper OD; as a result, I had to use the grinding wheel on a Dremel and sand down a significant amount of diameter around the base of the middle casing in order for it to fit in the endcap. I avoided remachining anything on the endcaps in an effort to preserve the nice finishes. After much elbow grease and some hammering (and a little help from an arbor press), the middle casing was pressed into the endcap and it was never coming back out ever again.

Middle casing pressed into the bottom endcap

For the spacer rings, I wanted to have a contrasting color from the 3D printed letter dials, and continuing the theme of aluminum, I designed the spacers to be waterjet. I sent them to Big Blue Saw, who I'd used before for waterjetting a huge 90 tooth sprocket in the go-kart course I took in college, having heard about the company from Charles Guan of Battlebots and silly go kart fame. It was cheaper to get five pieces than the four that I needed, and the online tool gave me an instant quote for my uploaded part, no human interaction or wait time necessary. My first shipment got lost in UPS land, but one email to the Big Blue Saw customer service resulted in a quick resolution, and they were incredibly responsive and helpful, shipping out a replacement order so that I'd receive it before I left the country for travels and walking me through how to proceed with a claim.

I had never considered the word "tolerances" until this project. The large OD of the middle casing continued giving me issues, and I had to take a file to the inside of each of the waterjet spacers in order to take off material until they fit over the tube. A low-tech way to figure out what to file down (no pictures of this part, unfortunately) is to use sharpie (or any non-water based marker) and color around the entire inside border, and then try to slip it onto the mating part. Once you take the part back off since it doesn't fit, the mating part will have scraped off the sharpie at the points of interference, indicating where there are regions that need to be filed down. Re-sharpie and repeat until the part fits! This is basically a budget version of using engineer's blue. (Additionally, felt tip marker can also be used for the other thing often called "engineer's blue", which is marking blue, where you cover a surface in a layer of prussian blue mixed with a solvent and then use something sharp to make high contrast marks for communicating machining rough cuts.)

Unfortunately, the problem with waterjetting is that the rough cut surface was visible on the outer diameter of the part, which is the part that would be exposed. I used the sanding band once more to grind away the seam where the jet entered the material to make the cut, and also around the outer perimeter of the band.

Rough as-is waterjet finish on the left, dremel sanded on the right

I used scotch brite metal polishing pad for a final polishing pass because these would be up next to the nice CNC'd endcaps and I wanted them to achieve a relatively close level of shininess.

Scotch-brite polished spacer on left, compared to dremel sanding only

I used the Fortus 360MC Stratasys FDM machine at my workplace to print the initial version of the letter dials, but was fairly dissatisfied with the surface finish and crispness of the letters. In the machine's defense, it had the lowest resolution print heads (0.01in layer size), but it was "free" to use so I figured I'd give it a shot at least. In an effort to remedy the finish, I sanded down the outer layer of the rings with increasingly fine levels of sandpaper (60, 100, 220, 600 grit).

Sanded letter dial from the Fortus on the left, raw print on the right

 Because the sanding left a rough matte surface finish, I used black touch-up lacquer as a filler and finish on the outside of the sanded rings.

Lacquer finished ring on left, versus sanded on right

The retaining rings needed a little bit of sanding and some shaving with an exacto blade to get them to fit, and I colored in the alignment symbols with thematically appropriate silver paint to make them clearer.

Bottom retaining spacer with contrast painted alignment arrow

The chambers needed some smoothing as well to slide past each other because of the low resolution of the surfaces. Additionally, I had to shave away a significant amount of the material along the sides of the keyed ridges. I had left enough width for the letter dials to rotate freely, but I failed to consider that the waterjet spacers would have a very small tolerance for the width of the key ridge. After a lot of shaving and sanding, I painted the inner chambers as well. It would have cost too much to reprint them on a different printer or with updated dimensions, since both pieces needed either a long print time or a large amount of support material.

Sanded inner and outer chambers

The only modification I needed to make for the retaining ring was to carve a little bit away from the J-shaped hooks that were used to lock the ring onto the middle casing slots. The resolution of the 3d print meant that everything was slightly fatter than it should have been, so there wasn't enough clearance both axially due to the flat ring face, and in the width of the piece that was meant to slot into the milled casing.

Finished retaining ring

Even after expending my store of elbow grease, the letter dials still didn't turn as easily as I would have been satisfied with, the height of the rings was too large given the tolerance on the thickness of the waterjet spacers, and the surface finish wasn't great either. So I uploaded my parts (after increasing the inner diameter slightly) to Shapeways and got the rings printed from strong and flexible black plastic, since the parts were small and relatively cheap. After one mixup where they sent me a duplicate of one of the rings (customer service set things straight by sending me the right ring eventually), and after a long delay, I received my new rings in the mail. Shapeways seems to often be behind schedule even if their material schedule didn't say so; it took my parts 11 days to ship after ordering, despite estimated time of 5 days, and with the printing mixup adding another week to ship after they ok'd the printing of the missing ring, total time before I received all my parts was about 3 weeks, so I definitely don't recommend their services unless you're ok with a nebulous timeline that's unlikely to be on schedule (or if you're willing to pay 1.5x the price for rush production).

That being said, the resolution on the shapeways rings was much better, with sharper letters and no further polishing or sanding needed to fit. After a tiny bit of rework to deburr the lip from SLS printing, I fed the rings onto the casing and assembled the cryptex.

Assembled cryptex with shapeways SLS rings on top, compared to the sanded and lacquered FDM rings loose on bottom

I was planning at first to attach the outer chamber top to the lid endcap using some sort of fasteners, but then I decided drilling holes and adding fasteners would look messy and be kind of unnecessary. Instead, I just used some superglue to attach it, since it wouldn't be seeing a huge amount of force (as long as my raccoons didn't try to open the cryptex too hard with the wrong answers).

Top of outer chamber glued into the lid endcap

Here are some shots of the final product and a video of the cryptex being solved. I left a USB stick with puzzles in the space that existed between the inner and outer chambers when the cryptex was closed, so that the puzzles for solving the inner chamber would only be available once the outer chamber was opened. The present that was locked in the inner chamber will remain a secret; rest assured that it was a small but useful little trinket.

Final cryptex, with the first stage solved

Cryptex with both stages solved and removed

Inside the cryptex; note the aligned slots on the left side that allowed the inner chamber to slide out