I previously posted this on Reddit, since it reaches more people there (and I didn't want to post everywhere at once, as it makes it harder to keep up with the comments). Sorry about that.
This is a tool for measuring the radius of a circle or fillet from the outside; it uses a moire pattern of slots and lines to enable a direct reading of the values from a vernier scale.
A video of a broken-open version makes it a little easier to see how the moire and vernier features operate: https://i.imgur.com/Ku2nBkq.mp4
More photos of a slightly earlier version are here, including the tool being used for actual readings: https://imgur.com/gallery/moire-vernier-radius-gauge-design-3d-printing-ajy0GBg
I was inspired by this post: https://makerworld.com/en/models/1505553-adjustable-chamfer-gauge#profileId-1575605
which is a gauge which measures chamfers using a sliding probe. The same user had also posted a radius gauge, which worked similarly, but it was much larger, using gears and two racks in it to amplify the motion, which I didn't initially understand. I asked about it, and he pointed out that, because of the geometry of the probing, the slider only moves a small proportion of the length of the actual radius being measured--about (sqrt(2)-1), or 0.414mm per mm of radius. Since we're drawing the marks with a 0.4mm nozzle, it's not really possible to make marks that close together and still have them readable.
So I thought, I bet you could fix that with a vernier scale. And then I had several thoughts all at once--that a lot of people are kind of scared off by vernier scales, and also that I bet you could fix that with 3d printing using the relationship between moire patterns and vernier scales. I don't think I've seen this done before, but it probably wasn't really practical before 3d printing. Arguably it's not entirely practical now, as the deep slots and parallax effects can make it a little hard to actually see the markings. But it was a fun experiment, and I think the result is eye-catching enough that it's probably got some educational value in getting people to actually think about how it is that vernier scales work. (It might even have educational value for things like number theory...e.g., it's important that the vernier factor involve relatively prime numbers, in this case 9 and 10. Can you see why?)
Anyway, hope folks here find it interesting too.
As a math nerd, this bothers me way more than it should. The reason we say "hundred" when we read a base-ten number that ends with two zeros is because that is the place value of the final non-zero digit--it is literally one hundred times the number you've already read aloud. But in the military time version, a) the hours are not hundreds of minutes, they're groups of sixty minutes, and b) it's groups of minutes, not hours, so the units also get messed up. If someone tells you it's currently 0 hours and you should meet again at 800 hours, logic would suggest they're asking you to go away for more than a month, but in fact they're saying 8 hours, despite the difference being apparently 800 hours.
I'm aware how pedantic this is, and I'm perfectly capable of understanding what they mean because I've heard it so often in movies and whatnot. But I swear these stupid games with units contribute to keeping us dumb.