I bought a clock for a quarter and 3D printed new hands for it
I bought this wall clock at a garage sale on Saturday for 25 cents. The aluminum clock hands were bent and damaged beyond repair, but I figured I could 3D print some replacements.
The clock was a single piece of molded black plastic. It was dusty but was otherwise in great shape. I decided to remove the battery powered quartz movement so I could wash the body in the sink. To do that I had to remove the hands from the movement. There was a little pin, which looked like a thumbtack, at the end of the shaft. I had to get it off to remove the hands. I inspected the pin with a magnifying glass (this awesome illuminated one that costs $1.85 including shipping). I thought I saw threads on the shaft. I tried to unscrew it but the shaft just rotated. So, foolishly, I removed the back cover on the quartz movement in order to hold the shaft with pliers, and a bunch of the gears fell out. At the same time I also learned that it didn't help to lock down the shaft to unscrew the pin. It wouldn't unscrew. But I stopped worrying about getting the pin off for the time being. I worked in getting the gears back in place. I thought I succeeded, but when I closed the cover, the movement didn't work any longer. Only some of the gears were rotating. After a while I discovered the problem -- one of the gears was missing. I found it on the floor (it was the only clear gear -- the others were white) and put it where it belonged. This time when I closed the cover, I was happy to see all the gears turning.
Next, I did what I should have done at the outset, learn from YouTube. There are many videos about quartz movement clock repair. I learned that you can simply pull the pin out with a pair of pliers. I was able to pull it out with my fingers. Oh well, at least I got a tour of the inner workings of a quartz clock movement. It's amazing how simple they are. And inexpensive! You can buy them on eBay for $1:
Once the pin was off, it was easy to get the hands off and remove the single nut keeping the movement secured to the clock body. I cleaned the body with soap and water and spray rinsed it.
Finally, I got to work on making new hands. I used this $13 pair of digital calipers to get the diameter of the holes in the existing hands. I measured the length and width of the hands. With these measurements, I went to Tinkercad and modeled a new set of hands. It took a few iterations to get everything right - I had to make the hole diameter a bit bigger, and I made the hands less floppy by adding a reinforcement beam along the length. I also decided to use red plastic inside of the metallic silver I started with:
If, for some reason, you want the 3D model, here you go.
The end result:
Later that evening, I became curious about quartz movements. Here's a good article that presents a high level overview. Basically, when a quartz crystal is electrically stimulated, it oscillates 32,768 times a second. And when it oscillates, it produced electrical pulses at the same frequency. A small circuit with a microchip divides the frequency to produce a 1 Hz signal, which drives a stepper motor. The gearing provides the rotation of the minute and hour hands. Quartz movements are very accurate - to within 5 seconds a month!