Many years ago I designed a watch movement using Fusion 360 which had my own arbitrary wheel and pinion sizes (I will upload the designs for this in a future post). Since then life got in the way (moved country, had two children as well as two full-time job changes) and so the manufacture of the movement only got to the early stages.
Having re-evaluated my goals, one of which is to become a competent watchmaker, then I realised a pretty big problem with my first design. It will be very hard to test until most of the movement is built, and when it inevitably doesn't initially perform as well as hoped, it will be quite hard to diagnose exactly where in my new creation the various problems lie.
At the time I probably wouldn't have easily parted with the design which took me many months to complete. But now, 7 years later I'm looking at this with a fresh pair of eyes and I've decided on a new approach.
While the movement layout and many other aspects (such as the keyless works) will be entirely my own, I will ensure the wheels/pinions are compatible in size with another reference movement. This way I can make one wheel/pinion at a time and drop those parts into the existing reference movement as well as swap parts out in my watch to root-cause specific issues. By placing a handmade part into an existing reference movement it's individual performance can accurately be characterised. It's also a more motivating way to work as results can be measured sooner.
Choosing The Reference Movement
The Unitas/ETA 6498 was originally designed as a pocket watch movement, but it has since featured in many wristwatches. For this reason it is on the larger end of wristwatch movements which makes it a good candidate for being handcrafted from scratch.
The original goal was to use the Unitas/ETA 6498-2 which runs at 21,600 BPH. I generally prefer watches with a higher frequency of vibrations, not only because of the potential accuracy improvement, but also I think the second hand just looks better at a higher frequency.
However, after reverse engineering the 6498-2 it turned out to have some pretty small wheel teeth to cut. Effectively, the 6498-2 is a 6498-1 layout with tighter wheel-pinion ratios and a balance frequency. As a result a 6498-2 looks like it will pose some manufacturing challenges. In particular the Second Wheel has an outer diameter of 8.4mm with 120 teeth - giving an approximate module of 0.07. PP Thornton only makes wheel cutters down to a module of 0.09 and I don't fancy manufacturing my own cutter, or cutting the wheel, on that scale for a first-watch project.
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| The Unitas 6498-1 movement used to take reference wheel and pinion measurements from |
The Unitas/ETA 6498-1 on the other hand runs at 18,000 BPH, but has gear train which lends itself more readily to hand manufacture. The smallest module size needed is 0.11 (for the escape pinion) which is covered by readily available cutters from PP Thornton. Hence I have decided to design the movement to be compatible with the 6498-1 wheel and pinion sizes. But this does also give me the option if I'm feeling brave in the future to upgrade my movement to 21,600 BPH if I can find a way to cut wheels at a module of 0.07.
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| The 6481-1 Escape Wheel Pinion (Module 0.11) |
Keeping the Design Unique
As will be shown in a future post, this movement will be particularly unique even though it will borrow aspects from the 6498 movement. At the same time, the rapid feedback and accuracy measurements I can attain by basing aspects of the movement on the 6498 will fulfil my goal of becoming a competent watchmaker in a reasonable timeframe.
