The inner bearing surface of the watch jewel is one of the most 'perfect' surfaces on Earth! Made of synthetic ruby, jewels are laser-drilled, then polished with tungsten wire and diamond paste. Both the inner and outer diameter are perfect as well. This one is R=0.500 mm and the inner radius is just 50 microns. However, the modern jewels are still the pale shade of jewels used in the finest 1800's marine chronometers which were made of real diamonds, hand-drilled and hand-polished - and still perfectly functional today!
Last week we started machining our first watch main plate. The challenge: to figure out both jewel hole sizes and the distances/angles between the jewels in the main train. These are watchmaking machining fundamentals - and yet another of the many best kept secrets in the watchmaking industry. While you can tell your mill where to create a hole, and while you know what the outside diameter of the jewel is, finding the 'perfect' friction fit between ruby and brass is something you can only figure out after a number of attempts. No machine or software can tell you how 'just right' feels like; it is the years behind the watchmaker's bench which ultimately determines the outcome.
The bottom line is this: If your shaft is the size of a hair, how much off centre can you afford to be?
After last week's exercise, Josh started making a bold prediction how soon we could potentially have our in-house movement. I am still very cautious and conservative, and reluctant to be drawn into speculation, but there isn't the slightest doubt in my mind that one day we will have a true made in Australia calibre. Exciting days ahead!