On "Mechanical Watch"

I'm not even entirely sure where the idea to write about mechanical watches came from. I think part of it was inspired by drawing the watch in the GPS article, and part came from seeing ads for watchmaking sets.

I myself don't even own any traditional watches, so the topic was very new to me. Thankfully, it was easy to understand the gist of how mechanical watches work just from watching some YouTube videos and I decided to give the topic a go.

The watch movement I present in the article is ETA 2824-2 which seems to be a reasonably popular Swiss movement used in many watches. I'm happy with that choice as it contains some features like the date ring and automatic winding that don't exist in simpler devices, but it also wasn't too packed to make the mechanism overbearing.

When it came to recreating the parts in CAD, I primarily relied on basic drawings found in the movement's technical specification. However, I also bought a replica of this movement on eBay – that turned out to be invaluable. By measuring with calipers I was able to model everything in real units, I could look at sections that weren't visible in the PDF drawings, and having the physical object itself also helped me understand how some of the components work.

Unfortunately, in the process of disassembling the movement, the click spring jumped out and vanished, never to be found again. I ordered another copy of the movement as a functioning backup. This actually ended up being a blessing in disguise since I could dedicate the first one to a more invasive analysis. I had to irreversibly tear apart some of the components to see what's inside and understand how they work, but I could keep the second movement working to test its features. I don't think I'm actually planning to buy a proper mechanical watch, but that second movement I have will serve as a neat reminder of the work I did.

Watchmaking absolutely requires use of magnification – the parts are really tiny. Here's a picture of the pallet fork resting next to a US penny which has a diameter of 0.75 in or around 19 mm:

In a few cases, even using a 7x magnifying glass wasn't enough. I had to resort to taking a picture of the part through that magnifying glass, and then look at the photograph itself, e.g. to count the number of teeth in some gear. I've certainly gained a lot of appreciation for the steady hands and patience that people working on watches must have.

The CAD modeling process took a lot of time – I ended up with almost 80 individual parts. Some complex components, like the mainplate, took longer than others, but even some simple ones, like the stop lever, required a few iterations to make them fit into the final assembly.

Similarly to the article on Internal Combustion Engine, I ended up making models in Shapr3D on my iPad. I still like the app, but I feel that it's quite easy to get disorganized in it. The final design ended up being a total mess, which is perhaps more of a testament to my CAD management abilities:

I originally planned to include a watch case as well, but they're surprisingly much larger than the movement itself, so to make it fit I would have to make all the other parts smaller and harder to see. The idea of having to work on another full featured CAD part definitely played a part in the decision to stick to just the movement.

Modeling the parts is one thing, but making them realistically interact with each other on the website is another. For example, I spent a big chunk of time making sure that the teeth of the escape wheel rub against the jewels in the pallet fork without any gaps, or that the date corrector actually slides up and down in its groove and only changes the date when turned one way. These little details sell the simulation, but they were taxing to make.

For the model rendering I reused a bunch of tech from the Internal Combustion Engine article. I originally wanted to do something more ambitious with more realistic renderings of metal surfaces, but I think I'm happy with the current clean design. The colors work reasonably well together, although not all pairs of hues combine nicely, so it took some iterations to end up with the current state.

One of the hardest parts of writing the article was, as it's often the case, coming up with a convincing progression of topics. It was a real struggle to decide how to introduce the concept of time tracking in the first place. I originally wanted to start with a pendulum clock to present the escapement and the notion that some force has to act on the shafts for a clock to run.

I ultimately ended up abandoning that idea since I convinced myself that I can explain these concepts using the watch itself, but even then I had a hard time deciding if I should start with the balance as the source of actual time tracking and then show how it's used to unlock the escapement and how that escape is powered, or start from the other end by showing how things are powered first and then show how to make them track time. As you may have seen, I opted for the latter approach. I'm reasonably happy with that path, but I'm not 100% sure that I've cracked this explanation.