A small opus replete with horological finesse. The impressive details of the TEFNUT calibre at a glance:
The calibre 102.0 movement was newly conceived from the bottom up. With a diameter of only 26 mm and a height just under 4 mm, it is distinctly smaller and flatter than previous Moritz Grossmann movements. Featuring a straight cut of the train bridge and the newly configured going train, the calibre looks very well organised and showcases the deliberately arranged inner workings of the watch. The ARCAP alloy of the wheels contrasts well against the German-silver movement elements. The large mainspring barrel suspended between bear-ings on both sides is an eye-catcher as well.
Despite its compact dimensions, the classic pillar movement with the main plate and the 3/5 plate features all Grossmann hallmarks such as the cantilevered balance cock with the Grossmann micrometer screw, the raised chatons, the separately removable winder module, and the mass-optimised Grossmann balance. Because of the smaller diameter of the balance, the new calibre generation breathes at a frequency of 21,600 semi-oscillations per hour and assures dependable rate stability.
In the 19th century, Moritz Grossmann already realised that small pinions with six leaves produce very unfavourable transmission ratios. To avoid this phenomenon, he used a 16-tooth escape wheel. This concept was adopted for the escapement integrated in the calibre 102.0 movement. Due to the somewhat higher frequency of the smaller balance, it was fitted with an escape wheel that has 18 very slender, classically shaped teeth.
THE INDEX ADJUSTER
The Grossmann balance has four interchangeable inertia screws that can be used for a first-pass balance-spring matching process. Two precision poising screws make it possible
to fine-tune the frequency. The balance-wheel rim features 22 equidistant bores. For material removal purposes, they can be countersunk or broached to accurately true the balance without damaging the finished rim surface.
The index pointer enables the daily rate to be corrected without jeopard-ising the integrity of the balance. It is easily accessible once the caseback has been removed.
THE WHEEL TRAIN
Departing from the mainspring barrel, where according to Glashütte regleur Alfred Helwig the adjustment of a movement must begin, the wheel train of the calibre 102.0 reflects a new concept. The barrel is bilaterally supported in jewel bearings, which reduces friction, optimises the energy balance, and improves the rate accuracy of the watch.
Normally, the barrel engages with the wheel train directly at the centre wheel, located at a height that lies between the barrel and the ratchet wheel. To achieve a thinner silhouette and make the best possible use of the space in the round movement available to the wheel train, the centre wheel was shifted to a different position in the TEFNUT calibre. The power produced by the mainspring barrel is now transmitted to an intermediate wheel by a single pinion and from there passed on to the repositioned centre wheel.
A copper-nickel-zinc alloy called ARCAP assures the long-term brilliance of the wheels.
THE GOLDEN CHATONS
The white sapphire bearing jewels are set in prominent gold chatons. Together with their brown-violet annealed pan-head screws, they stand out over the datum level of the plate. This raised arrangement, inspired by historic Grossmann pocket watches, makes it possible to remove and clean the bearing jewels individually without risking damage to the plate when resetting them.
THE ENGRAVINGS BY HAND
All engravings and inscriptions on the 3/5 plate, the fourth-wheel cock, and the balance cock are always engraved by hand.