In 2020, we described Grand Seiko's prototype, the "T0 Constant-Force Tourbillon," as the pinnacle of Japanese watchmaking. A year and a half later, Grand Seiko's development team has redesigned most of the movement and added a new exterior, releasing the finished product: the "Kodo Constant-Force Tourbillon." The combination of Grand Seiko's unique practicality and a movement that stimulates the eyes and ears creates an unprecedented sense of unity that will astound anyone who sees it.
Interview and text by Masayuki Hirota (Chronos-Japan)
[Article published in the July 2022 issue of Kronos Japan]
The secret behind the Cal. 9ST1, which stimulates both sight and sound
Grand Seiko's "T0 Constant-Force Tourbillon" was unveiled as a concept model in 2020, garnering attention from watch industry insiders around the world. Its aesthetic appeal is not limited to its visual appeal; the constant-force movement perfectly synchronizes with the regular ticking of the high-precision tourbillon, stimulating the auditory senses as well. The latest complex movement, Cal. 9ST1, elevates the T0 to an even higher dimension. We will explore and explain its secrets, focusing on its key components.
Is it possible to achieve the highest possible precision in a mechanical watch? Grand Seiko's answer to this question was the concept model "T0 Constant-Force Tourbillon" (hereinafter referred to as T0), announced in 2020. By placing a constant-force mechanism that supplies a constant amount of power coaxially with the tourbillon, this movement boasted extremely superior precision both in theory and in practice. In addition, this movement incorporated an unprecedented attempt in the watch industry: synchronizing the constant-force mechanism's operating sound with the ticking of the watch.
Although the T0 was merely a prototype, it was enough to captivate watch enthusiasts. Grand Seiko began a commercialization project for the T0 in September 2020. Just a year and a half later, they completed the caliber 9ST1, which could be considered the final version of the T0.
The structure is exactly the same as the T0, with a hollowed-out movement, twin barrels with strong torque, and a constant-force mechanism coaxially positioned with the tourbillon. However, Takuma Kawauchiya, who completed the 9ST1 following the T0, says, "In the end, we changed more than 90% of the parts." There were three main aims: miniaturization, further refinement of the finish, and improvement of the constant-force mechanism.
Kawauchiya explains, "This was Grand Seiko's first time creating a movement with a visible mechanism. That's why we needed a designer to help with the design." The designer chosen for the role was designer Yu Ishihara. "When we were revising the design for commercialization, Ishihara brought a 3D model. The movement's diameter was 36.3mm. After careful consideration, we concluded that we needed to reduce the movement size." Through collaboration with Ishihara, the 9ST1 was miniaturized, marking the first step toward commercialization. One of the results is the power reserve indicator at 8 o'clock on the dial. By moving the power reserve hand, which faced the outer periphery on the T0, toward the inside, we succeeded in reducing the movement's diameter. Through repeated revisions to achieve a comfortable fit and legibility as a wristwatch, the 9ST1 was able to achieve a small diameter of 35mm, a small size for an ultra-complex movement.
The finishing has also been further refined. The T0 components were finished by Kawauchiya, who is also a skilled craftsman. However, the newly added Seiko Watch engravers have further refined the finish. "This time, I taught my techniques to two engravers and had them work on the 9ST1. They refined their methods and surpassed my level in just two months," says Kawauchiya. What's interesting is that the 9ST1 incorporates new techniques in addition to classic finishing techniques. One example is the matte finish applied to the sides of the plate and bridges. While the creases are usually created with a file, the matte finish on the 9ST1 is unique and originates from metal engraving. The creases are applied little by little by hand using a micro-grinder, creating subtle variations and directional changes in the appearance. Additionally, the chamfered edges of the components have been slightly increased in width, further enhancing the contrast of the skeletonized movement.
The seconds index surrounding the outside of the tourbillon also incorporates a new finish. If you look at the comb teeth lined up inside the seconds index, you can see that the edges shine with a mirror finish. The width of the mirrored surface is just 0.4 mm. To emphasize the difference from the edges, the conical slope has been hand-grained. While it is not difficult to do this on a flat part, it is extremely difficult to apply a grained finish to a part with a concave interior. However, in order to create a movement that appeals to the eye, the development team took on this difficult challenge.
Kawauchiya says, "The 9ST1 is not a movement that boasts of its handiwork," but the finish applied to the entire movement is comparable to that of luxury watches from long-established manufacturers.
The development team also tweaked the unique ticking of the constant-force mechanism. The constant-force mechanism, which supplies stable torque to the balance, is designed to operate once per second. In theory, the interval between ticks is always precise, but Kawauchiya explains that in practice, there were occasions when the ticking intervals were slightly off. The reason for this was a slight eccentricity of the components. He addressed this by improving the stop wheel, the heart of the constant-force mechanism, and fitting a brass bushing to the axis to reduce the eccentricity. He also doubled the machining precision of the tourbillon axis, narrowing the clearance with the jewel, reducing the eccentricity between the tourbillon and constant-force mechanism to just a few microns. As a result, the constant-force mechanism's operating sound is now perfectly synchronized.
The 9ST1 is not only compact and usable, but also achieves T0's aim of appealing to the visual and auditory senses at an even higher level. This is evidenced by the individual components lined up above.
Cal.9ST1 Key Parts Analysis
Grand Seiko's development team sought to further refine the T0, which boasted an outstanding level of perfection. How could they create a movement that would appeal more visually and aurally? The answer was to further refine each and every component. Over 9% of the components were changed. By revising almost everything, the Cal. 9ST1 reached unprecedented heights for a Japanese-made movement.
