In May 2022, someone posted to Hacker News Bartosz Ciechanowski's blog post explaining how mechanical watch movements work. Since then, his blog has been my absolute favorite corner of the Internet. His posts are not just well written and easy to follow, the accompanying interactive illustrations are magnificent.

The first illustration in his blog post about mechanical watch movements allows you to "explode" a ticking mechanical movement and rotate it to inspect its every component from any angle. I owe my foray into the hobby of watchmaking to Bartosz, but that's not what this blog post is about. Instead, I want to scratch an itch I've had for years now: How cool would it be to hold an exploded view of a real mechanical watch in your hand?

Discussed on Hacker News.

Apparently, nobody builds such models

I figured that surely, someone has had this idea and built it before. On eBay you'll find cubes of resin embedding various random components from mechanical watches, but they are typically sold as "steampunk art" and bear little resemblance to the proper assembly of a mechanical watch movement. Sometimes, you'll find resin castings showing every component of a movement spread out in a plane like a buffet---very cool, but not what I'm looking for. Despite my best efforts, I haven't found anyone who makes what I'm after, and I have a sneaking suspicion as to why that is.

Every component making up the movement of a beautiful 1960's Longines Flagship wristwatch I worked on recently, laid out "buffet style".

Building an exploded view of a mechanical watch movement is undoubtedly very fiddly work and requires working knowledge about how a mechanical watch is assembled. People with that skillset are called watchmakers. Maker, not "destroyer for the sake of art". I guess it falls to me, then, to give this project an honest shot.

...how do you build such a model?

Here comes my favorite part: jumping head-first into a project requiring a set of skills I don't even know exist, let alone possess, following a process that develops as the project evolves. So how would one go about building a real-life exploded view of a mechanical watch movement?

The first thing to consider is what type of mechanical watch movement to "explode". Although he doesn't explicitly say, I think Bartosz Ciechanowski's blog post is based on the ETA caliber 2824-2 mechanical watch movement (or one of its many clones, such as the Chinese PT5000). This is a very robust and popular automatic (self-winding) mechanical wristwatch movement found in many watches. It's considered one of the all-time "workhorse" movements of the industry. I highly encourage the curious reader to check out this YouTube video of a very skilled and equally witty watchmaker servicing a Hamilton watch featuring this exact movement. The video will also reveal how many absolutely miniscule components make up the ETA-2824, making it less than ideal for my initial prototyping. A larger and simpler movement would be much better.

Luckily, in the late 1800's to early 1900's, many people carried a pocketwatch about their person. However, as smaller wristwatches gained popularity among men, beginning around the first world war with trench watches, pocket watches lost their value as timepieces. Many were melted down to reclaim the gold used to case the movement. Today, you can find gorgeous, hand-engraved movements from the turn of the 20th century on eBay for next to nothing.

Pocketwatches are the simplest mechanical watch movements out there: they typically don't have any date complications, aren't self-winding, and usually don't even have a central seconds hand---that's a relatively modern feature. There's a reason budding watchmakers start out practicing on pocketwatch movements.

Now for the hard part: How do you suspend 50-100 tiny components to form an exploded view of the assembly?

A beautiful early 1900's pocketwatch movement from eBay. $20 for a piece of horological art.

Layered resin casting

The model I'm planning to build must stand up to being handled, which means it must be solid. In turn, that probably means that I need to cast my components in clear epoxy resin. My first idea was to build the exploded model up, layer by layer, letting the resin cure between each layer. To save time, I experimented with clear resin that cures when exposed to UV light. There are several reasons why this did not work:

1. The resin I purchased ended up with a heavy yellow tint after curing.

2. My puny UV flashlight took forever to cure a pour even 1 mm deep.

3. The seams between layers are very visible.

The first two problems can probably be solved by throwing money at the problem, but the last issue seems to be a problem with varying indices of refraction throughout my casting. The resin art community is well-aware of this problem, and the suggested solution is to pour the next layer of resin before the previous layer has fully cured. This supposedly helps the two layers fuse, and should make the transition between layers much less pronounced. Unfortunately, UV resin seems to cure from the outside in, meaning I needed to use "regular" two-part epoxy instead.

To achieve the effect I want, I need around 20 layers for a regular pocket watch movement. If I were to cast each layer in a transparent container, adding components and epoxy as the previous layer was half-cured, I would be doing nothing else for a solid week. Instead, I figured my best bet was to cast all 20-or-so layers at once, and stack them together once the epoxy was semi-cured. So, my next project was to order some casting silicone and cast myself a mold suited to casting thin disks of clear epoxy. A baking tray studded with poker chips formed the ideal mold for casting the silicone.

My resin disks solved problems 1 and 2 above, but problem 3 is still unsolved. In addition, this was a very messy and challenging way to cast resin: the half-cured disks are floppy, resin runs everywhere and bubbles get stuck between the layers. I bought a small vacuum chamber to combat the bubble issue, but escaping gas weaked havoc on the disks of resin. I learned enough about resin castings to finally realize that layered casting is not the way to go.

1st attempt: embedding some nails in layers of UV resin.

2nd attempt: Embedding more nails in disks of clear resin in a silicone mold...

yeah, no.

Suspending each component with fishing line

It took a while to accept that the only hope I had was to cast the entire model at once. This is difficult because I need to somehow suspend each tiny component in a way that is robust enough for me to pour resin all over it and then pull out any air bubbles.

Scarred and burnt by resin's index of refraction, I went looking for very thin rods of plastic or acrylic that I could cut to length and glue my components to. Fishing line fits the bill. Specifically, monofilament nylon leader used in fly fishing. It comes in many thicknesses, has an index of refraction very similar to epoxy resin, and is cheap. The only major challenge with using fishing line is that it "remembers" its spooled shape, so every segment I cut off has a slight bend to it. I was able to remedy this somewhat by stringing it repeatedly across the grill pan in my oven and baking it at 150°C (300°F) for an hour or so. When done, the middle segments are reasonably straight and significantly stiffer than what I started with.

The assembly process actually resembles "proper" watchmaking a lot. Of course, tweezer control and steady hands are important. But applying tiny drops of CA glue to components with a pinhead is a lot like applying oil to bearing surfaces and jewels. I just find it amusing to do it with glue instead of oil---CA glue being the complete opposite of a lubricant.

Armed with a pair of helping hands and a set of self-closing tweezers, I found the process of building up the exploded model of a scrap watch movement to be very satisfying.

0.7 mm monofilament fishing line attached to bridge screws.

Monofilament fishing line maintains the distance betwen components. A jig holds everything still while the CA glue hardens.

Resin casting at home

At this point I've tried a number of different epoxy resins claiming to cure crystal clear. As far as I can tell, they all turn out appreciably clear. Their main differences are viscosity, cure time and how much air they trap when mixing. Some resins claim to expel bubbles when curing. They manage this to some extent, but to get completely clear castings you either need a vacuum chamber to pull out all the air from your cast, or a pressure chamber to completely squash any pockets of air. I went for a vacuum pump because it's a fun thing to have around (boiling water at room temperature doesn't get old). Another benefit of the vacuum pump is that you don't need to leave your cast in the chamber for the entire cure time. My castings have a lot of voids that love to trap air, and the vacuum chamber does a good job of pulling out the trapped air.

My casting method goes like this: Mix enough of part A and B of your resin to fill the mold plus 10–15 %. Mix thoroughly for 3 minutes. Pour the resin into another mixing container to ensure that no unmixed resin is stuck to the walls of your container. Use a fresh stirring stick, and mix for another 3 minutes. Pull the whole container under vacuum (I manage around -0.96 bar) and leave it there for 30 minutes. Depending on your resin, the froth may very well overflow your mixing container: cycle between pulling a vacuum and letting air back in a couple times to pop most of the bubbles.

Now pour the resin into the mold and go through the vacuum process again. This time, most of the bubbles you're pulling out come from around the embedment, not from air trapped in the resin, which helps keep the frothing down.

Some resins froth up to several times the liquid volume under vacuum! Make sure there's enough room to accomodate this.

Prototype #1: lessons learned

My first somewhat promising cast was in a borosilicate class cylinder. I won't dwell on the looks of this, because I mostly wanted to experiment and answer a few burning questions with this prototype:

Q: How visible is the fishing line in the final result?

A: Not very! It's visible in the right light, but I don't think I'm able to do any better.

Q: Does CA glue interfere with the resin in any way?

A: Most resin datasheets say that CA glue can interfere with the resin's curing process, but I can't tell from this test.

Q: Does casting in a cylinder make sense?

A: Definitely not. Light refracts on the curved cylinder surface making it difficult to understand the geometry inside, defeating the purpose of the model. Cast in a cube.

A week or so later, the cylinder shattered. Resin shrinks as it cures. On to the next prototype!

First prototype cast. Fishing line all but disappears in the resin.

Prototype #2: promising results

At this point I have settled on a somewhat structured process.

Disassemble and clean

Start with a clean, disassembled movement. For instructions on how to properly disassemble and clean a watch movement, I highly recomment Mark Lovick's Watch Repair Course. Assemble the train of wheels with their bridges and seize them by depositing small amounts of CA glue on the pinions. Just like with the watchmaker's oil, a sewing needle with a flat spot is perfect for picking up a small droplet of CA glue and depositing it where it's needed. Capillary action wicks it between the components. Less is more when it comes to CA glue: the bond between surfaces is stronger when no excess is used, and the cure time is around 30 seconds even without the use of an activator.

I made a small jig for cutting pieces of fishing line accurately and squarely out of a janky hand press tool.

Assemble trainwheel side

Work starts on the train wheel side of the movement (often called the watchmaker's side) because that's where most of the complexity lies. While working here, the movement can rest in a movement holder. I attach long strands of fishing line to the end of each of the bridge screws. Self-closing tweezers hold the bridges in place above where they seat on the mainplate, and the screw/fishing line combo is threaded through the screw hole in the bridge into the corresponding holes in the mainplate. More glue binds the bridges to the fishing line.

Subassemblies such as of the keyless works, motion works, balance assembly, etc. can be constructed separately and fastened as a unit. Small, flat components such as cover plates, intermediate wheels and cap jewels each get their own short studs of fishing line. I dip the studs in CA glue and place it near the center of mass of the component I'm working on: the fishing line's flat ends allows it to stand upright.

Flip and assemble the dial side

Before starting work on the dial side of the movement, it must be flipped over. I transfer the assembly into the jaws of some tweezers and apply closing force with a rubber band. The tweezers are clamped by "helping hands" glued to a piece of cardboard which allows me to spin my work around. Work continues similarly on the dial side. Finally, some thin transparent nylon sewing thread attaches the mainplate to a pegwood stick that allows me to suspend the whole assembly over a mold, ready for casting resin.

Make the mold, prepare and pour resin

Speaking of the mold: I bought some 2 mm thick 20x30 cm acrylic sheets. Foamcore or wood could work as well, but I don't want the vacuum pump to pull air from the mold into the casting. I cut them into 7x10 cm rectangles and lined them with "epoxy mold tape" (fancy packing tape that epoxy resin does not bond to). Then I used some Tec7 construction adhesive to form a cube.

Prototype #2 shows a lot of promise! It's far from perfect, though. Here are some of the things I'll improve for prototype #3 in no particular order:

• I struggled with attaching subassemblies to the mainplate because I need to precisely control the distance between the subassembly and the target surface: too snug and the assembly ends up crooked, too far away and the glue doesn't adhere properly. A proper lab jack (tiny scissor lift) might solve this, so one is on the way.

• The resin shrunk a lot while curing, most likely because it overheated. I'll focus on proper airflow for my next cast.

• People noted that it's difficult to see between the components, so I'll "explode" the next model even more.

• The hands are set to an invalid time. 10:10 is the way to go.

Protoype #3: nailing the technique

The third iteration incorporated the scissor lift lab jack into the process which, together with helping hands, allowed for much greater precision in bringing components together true and square. I'm also committing another watchmaking sin by placing a small magnet in the jaws of my self-closing tweezers to gently hold tiny screws. This allows me to lower a screw into its target drop of CA glue and simply lift the tweezers once the glue has set. This is a much more reliable method than trying to hold onto a sub-millimeter screw with self-closing tweezers whose gripping force I have little control over. Magnets are sinful in this case because magnetism can cause all sorts of timing issues in mechanical watch movements, but that's obviously not an issue in my case.

I also started using a CA accelerator to speed up the assembly, because waiting for glue to cure is painful. Spraying the accelerator onto my work would create a huge mess, so instead I spray some into a lidded container and use tweezers to pick up droplets of the stuff to deposit accurately onto the glue joints. This stuff is too volatile and runny to be picked up by a needle. For me, the most effective use of the CA accelerant is to first place a drop of CA glue on one of the two mating surfaces, then dip the other surface in accelerant and quickly bringing them into contact. I believe this works well because the joint cures along the interface instead of curing from the outside-in as is the case when spraying accelerant after the joint has glue in it.

You may see that the balance wheel is hanging from the hairspring in this casting. The balance wheel marks the end of the train of wheels and releases tiny amounts of energy from the mainspring 18,000 times per hour. The balance assembly is the heart of the movement and is also the most delicate component, and I want to highlight that by stretching the hairspring to show its form. These components are not glued in place, and the balance wheel hangs freely from its spring, meaning I have to cast the whole assembly upside-down to achieve this effect.

Protype #4: ETA-2824

At this point I figured I was ready to tackle the final boss of this project: the ETA 2824 wristwatch movement that we've all seen in Bartosz Ciechanowski's blog post. Well, I'll tackle the Chinese PT5000 clone movement instead, because I can't justify spending €300 on a genuine movement only to ensure it will never run again.

Comparing my PT5000 to Bartosz's render.

This movement arrived from China in good working order---I was actually suprised at its performance out of the box. It ran with good amplitude and little positional variance between horizontal and vertical positions. I became less and less impressed as I disassembled the movement prior to cleaning, though, because it was absolutely drenched in oil. Many of the bridges had sharp burrs that broke off during cleaning. Nothing was broken though. All in all, these movements really need a proper service before putting them to service. That's not an issue here though.

I was worried that the much smaller components would be challenging to work with, but I found the process to be essentially the same as for larger pocket watch movements. My 0.7 mm nylon fishing wire still fits through most of the screw holes and the lab jack makes alignment a breeze.

Some components needed special care, in particular the balance shock springs that protect the delicate pivots of the balance staff from shocks. These aren't normally found on older pocket watch movements which is why a broken balance staff is one of the most common failure modes of pocketwatches. Shock springs are fragile and are, in my opinion, among the most difficult components to handle when servicing a watch. Of course, I want to also explode the balance assembly, so I needed a way to suspend the shock springs above the capstone jewel. By laying the spring down flat on silicone and placing a drop of CA glue on it, the surface tension of the glue fills in the inner disk. Once cured, the spring with its hard and transparent interior can be lifted off the silicone. This is how the luminous material (lume) is applied to the hands of a watch---another watchmaker's trick.

The assembly process was luckily uneventful, and I finished the build off with a black dial and a random set of hands from eBay. Unfortunately, the casting process went completely south. Only after I had created a mold, mixed, degassed, and poured resin, did I realize that my mold was just barely too tall to fit comfortably in the vacuum chamber. With some hasty modifications to the scaffolding that supported the exploded model, I was able to pull a reasonable vacuum on it, but it left the exploded model crooked. To add insult to injury, the resin seems to have dissolved the paint on the date indicator ring, which left milky streaks throughout the casting.

All in all, I spent roughly 18 hours stripping, cleaning and assembling the exploded view of the PT5000. With improved technique I might get this down below 15 hours, but it's very tedious work, and rushing means I'll knock something off and have to redo work. Good to know, because I will be doing this again until I get it right.

The PT5000 disassembles nicely into a steampunk whale.

Installing the barrel bridge assembly.

Assembly complete!

Final casting. The paint on the date ring, dial and hands dissolved. Also, the model is crooked.

End game: the wristwatch

Prototype #4 has a couple flaws which I'd like to fix before calling this project done. The process of assembling the model is more or less nailed down: I just need to find a way to seal any painted surface before casting, as well as try to remember that my vacuum chamber has limited volume. I bought another PT5000, this time in a complete watch with case and a metal band, and started experimenting with sealing the painted surfaces. Here's what didn't work:

• CA glue dissolves paint just as readily as epoxy resin. It might work on simple paint-jobs like the chapter indices and the hands, but the date ring has crisp lines (the numbers) which I need to preserve.
• UV curable CA glue didn't cure on top of the paint. No idea why.
• Same with UV curable epoxy resin.

Clear spray lacquer from the hardware store worked well and didn't dissolve the paint. It did turn yellow as you'll see in the final casting, but I'm okay with that. I also built a better jig to cut the fishing line into equal lengths with square ends. It's a bit janky but worked very well.

There's not much more to say. I'll stop blabbering and make way for an image series.

Janky but effective cutting jig for fishing line.

Painting the dial, hands and calendar ring...

The watchmaker's side of the movement is assembled and fastened to the case.

Our watch is held by a stiff cardboard tube (okay, a toilet roll core) to keep the finished half suspended mid-air while work continues on the dial-side.

Aligning the date ring...

Done! I wish I could keep it in this form, but it's too fragile.

Final result

I don't have the tools or knowledge required to sand this down to a perfect mirror finish, but that's okay. After 2.5 years of noodling around with this, I've achieved what I set out to do, and I'm pretty happy with the result, even if it is difficult to photograph. Figured I'd write this post to get some honest feedback, maybe even some tips, in case I decide to continue working on this... I mean, more interesting wristwatch movements do exist!

Bartosz, if you are reading this, contact me and I'll send you the final casting. This project would never have happened without your blog post.