Sprung balance

The sprung balance consists of a flywheel called the balance and a return spring called the balance-spring or hairspring. It provides the regular period needed to subdivide time into seconds, minutes and hours.

The balance used in the NM3 timepiece is highly unusual, inspired by those used in the production of 18th-century marine chronometers. It is a split bimetallic variable-inertia balance of the Guillaume type, also known as an "integral" balance. At the time, this type of balance was the only solution to the isochronism problem caused by temperature variations. Indeed, under the effect of heat, the elasticity modulus of the hairspring becomes more flexible, which slows down the oscillations of the balance wheel; conversely, in cold weather, it becomes rigid, accelerating the movement. The bimetallic balance compensates for these effects thanks to its split arms made of two metals with different thermal expansion coefficients.

The balance of Naissance d'une Montre 3 is made of invar which has a very low coefficient of expansion along with brass featuring a higher expansion coefficient – with these coefficients basically cancelling each other out.

Producing this bimetallic balance alone required a complete re-learning process, since its use had fallen into disuse decades earlier, due to the introduction of modern balances made from alloys enabling the springs to self-compensate. Manuscripts dating from 1940 written by professors and students at the Geneva Watchmaking School provided some clues as to how to achieve this. Reference works such as Watchmaking (1981) by English watchmaker George Daniels (1926-2011), and the Traité d'horlogerie moderne (1887) by Claudius Saunier (1816-1896), were consulted on the subject of the bimetallic balance.

The 18-part balance-spring of Naissance d'une Montre 3 – measuring 10.94 mm in diameter at the balance rim and 12.56 mm at the screw – is the heart of the watch. The quality of its execution is of the utmost importance and takes on a special character because of the meticulous care and sensitivity with which it is produced by the micro-mechanical engineers, before it is assembled and adjusted by the watchmakers. The variable-inertia balance-spring of the NM3 movement incorporates four 18-carat gold adjustment screws, two gilded nickel silver weights adjustable on the cut balance rim and necessary to adjust the thermal compensation, as well as fixed weights ensuring the inertia of the balance.

STAGES

• Production of the bimetallic balance required numerous tests before a perfectly homogeneous invar and brass disc was achieved. Following a complex iterative process punctuated by unsuccessful and unstable tests, the decision was taken to use welding instead of fusion.
• The results of the fusion experiments showed that the materials held up well, but that their random homogeneity did not meet the pre-defined quality requirements.
• Slightly less invasive for the material, the welding process yielded excellent results in terms of both homogeneity and material strength, enabling effective control of the balance-shaping operations.
• Nonetheless, the welding solution chosen requires real control over the parameters of the soldering temperatures, heating time and cooling speed.
• Once the balance has been machined from the bimetallic invar and brass disc by turning and milling, it is decorated, taking care to respect its geometry and balance.
• The blank for the balance-staff is turned on a lathe. It is hardened and tempered before being shaped by turning, polishing and burnished. A small bench lathe and a swivel lathe are used for this purpose. Burnishing the pivots is a manual feat requiring particular dexterity. After burnishing, the diameter of the pivots must measure 0.085 mm to ensure that the oscillations of the balance reach the predefined amplitude.
• The use of a bimetallic balance necessarily entails the use of a traditional balance-spring. The latter is made of carbon steel from 0.06 mm wire that is drawn, rolled, shaped in a dedicated winder and finally hardened and tempered to the colour blue. This hue is not an aesthetic choice, but the result of respecting to the nearest degree the exact temperature required for tempering.The eight balance screws serving to adjust the balance’s moment of inertia and to poise it are machined and threaded on a lathe to 0.35 mm – the smallest possible thread diameter achievable by conventional means.
• Assembling the balance involves fitting the weights and screws. It is then poised, while avoiding any distortion of is shape. The balance-spring is cut at the centre to be pinned up to the collet, after being placed on the balance. The subsequent “counting” operation consists in determining the length of a hairspring coupled to a balance in order to obtain the desired oscillation frequency, i.e. 21,600 vibrations per hour or a frequency of 3 Hz for the NM3 timepiece) with the help of a standard balance. After being cut to the desired length, the balance-spring is pinned up to the stud and its terminal curve formed. The geometry chosen for the curve of the NM3 balance-spring corresponds to the one characterising the Breguet hairspring, featuring a Phillips terminal curve, in which an external curve is coiled back over the balance-spring. Finally, the balance-spring was set in motion and adjusted, including thermal compensation tests.