One of the oldest clock designs on thingiverse is this model by user TheGoofy. It has many makes and several remixes. I made this once before several years ago, and never got it to run reliably. At the time, I had a printer which was much less accurate and well-tuned than my current one. TheGoofy notes in his description that it was designed for older, more inaccurate printers. I think that I couldn't make it work as a combination of what my printer was able to produce and my ability to figure out what was going wrong and fix it.
I recently had another go, with some success. Here are some pictures and video:
The video was taken while I was still tuning it, and it's quite obvious that the beat and timing are off. Here is a later version that is a bit better:
It worked pretty much straight off with only a few changes. Some of the parts came out undersized. In most cases this does not matter, but in the pentagonal connector between the escapement spring and the balance wheel, it is important to get a good fit. The original spring is 1mm thick, and I found that the coils flopped around too much. I found a remix with a 1.3mm spring. The end of the spring has a triangular piece which fits into the frame, and this was also far too loose. At first I held it in place with some tape, and then adjusted the model to make a tighter fit. One other adaptation I needed was to drill out the holes for the arbors in all the gears and moving parts, as they were too tight. My preferred way of doing this is with a drill bit in a pin vise. It allows you to go slowly and carefully control the drill so that you don't end up skewing the hole.
I used the v1 ratchet and no planetary drum. This gives the shortest running time in the sense that the weight falls a greater distance for a given run time. As I was regarding this a more of a clock demonstrator rather than something I intend to use as a real timepiece, I didn't worry about this too much. I like the idea of the servo driven version, and may try it out later. I found that the clock ran quite reliably with 600g of weight. I think 500g is also OK, but less than that wasn't enough. Note that TheGoofy recommends 1.2kg, and that may be needed for a different ratchet/drum combination: for a higher gear ratio you need more weight, and also get greater run time. With the version I used the weight dropped about 5 cm in 10 minutes (measured very approximately).
One issue I had is that the clock would sometimes stop dead. It took only a slight touch on the balance wheel to get it going again. After a while I realized that this was because I had some screws for setting the time on the balance wheel, and they protruded just enough to occasionally catch on one of the gears. Countersinking the holes on the balance wheel so I could screw them in a tiny amount further was all that was needed to fix this.
I also tried a variant version of the anchor in response to getting an occasional stall. I'm fairly sure that something about this throws the beat out (that is, the ticks and tocks are uneven), and I switched back to the original one.
There are two features of this clock which make it different from the Peterson and A26 clocks. It has a seconds hand, with a little extra mechanical complexity as a result. More importantly, the timing element is a spring/balance wheel combination, with an anchor between this and the escapement. I think this is called a Swiss or lever escapement. It's a much more compact arrangement than using a pendulum. In a 3D printed version, it's less practical as the spring will wear out over time. It's also harder to tune the period. I haven't found any very good guide on this, so here is my understanding of the physics and some observations about the practical reality.
Some noodling about balance wheels
In theory, the balance wheel acts as a harmonic oscillator. Wikipedia gives an expression for the period. The important factors are:
- it is inversely proportional to the square root of the spring stiffness. So a thicker spring makes the period shorter, resulting in less time between ticks. It speeds up the clock, making it run faster.
- it is proportional to the square root of the moment of inertia of the balance wheel. If you imagine the balance wheel as being made up of lots of tiny masses, the moment of inertia is then the sum of each mass times the square of its distance from the axis (mr^2). So a heavier balance wheel or moving some of the mass outwards makes the period longer and the clock runs slower.
- gravity is acting on the screws attached to the balance wheel. The direction of the gravitational force relative to the balance wheel changes as the balance wheel moves. So the resulting moment on the balance wheel is also different. This implies you get different effects depending on which of the balance screws you use.
- when the nub on the spring hits the anchor, it loses some energy, and it then gains some energy back as the trailing edge of the anchor hits it. It also interrupts the smooth motion. I've no idea how this would affect the period, if at all.
- screws in A, B and C: 92 seconds per revolution of the seconds hand (and somewhat uneven).
- screws in B and C: 77s.
- screws in C only: 71s.
- screws in D only: 56s. Note that this case has the same theoretical moment of inertia as the previous one, and so supports some of my speculation above.
- no screws at all: 75s. On a second run I got 68s.
- D: 56s.
- C: 61s.
- stiffer spring means slower.
- more weights means slower.
- position of the weights matters.