A method for the fine adjustment of the rate of a mechanical oscillator with an oscillating inertial mass, equipped, in a first step, with an actuator made of material suitable for irreversible local micro-expansion under the action of laser fires, to impart to an inertia-block a radial travel during suitable laser fires on a writing zone of the actuator, in a second step, the initial rate of the oscillator is set and measured, in a third step, the direction and the value of the deviation required to achieve a predetermined rate range, and of the travel to be imparted to inertia-blocks are calculated, in a fourth step, a writing zone is subjected to femtosecond laser fires to create expansion lines by local molecular expansion to radially deform the actuator, in a fifth step the rate is measured and the third step and fourth step are repeated if required.

A balance spring (1) of a sprung balance assembly of a mechanical horological movement, formed by a succession of coils (S1, . . . , Sn) which extend between a first free end, referred to as inner first coil (2), and a second free end, referred to as outer last coil (6). The are arranged off-centre when the balance spring is in the free state, the outer last coil (6) terminating in a stop means for the attachment thereof to a stud (14). The balance spring (1) is centred and its spires concentric when the spring (1) is in the mounted state inside the sprung balance assembly, the coils being rearranged concentrically when the spring is in the mounted state. The attachment of the spring to the stud inducing, in the coils, a resilient stress as a result whereof the stop is attached in a captive manner to the stud (14).

A method for the fine adjustment of the rate of a mechanical oscillator with an oscillating inertial mass, equipped, in a first step, with an actuator made of material suitable for irreversible local micro-expansion under the action of laser fires, to impart to an inertia-block a radial travel during suitable laser fires on a writing zone of the actuator, in a second step, the initial rate of the oscillator is set and measured, in a third step, the direction and the value of the deviation required to achieve a predetermined rate range, and of the travel to be imparted to inertia-blocks are calculated, in a fourth step, a writing zone is subjected to femtosecond laser fires to create expansion lines by local molecular expansion to radially deform the actuator, in a fifth step the rate is measured and the third step and fourth step are repeated if required.

An assembly (1) for attaching a free end (8) of an outer last coil (10) of a balance spring (12) for a horological movement, this attachment assembly (1) including a stud (2) and a blocking element (14), the stud (2) being provided with a groove (6) in which the free end (8) of the outer last coil (10) of the balance spring (12) is engaged, the blocking element (14) also being engaged in the groove (6), in contact with the outer last coil (10) of the balance spring (12), the attachment assembly (1) further including a clamping member (16) which presses the blocking element (14) against the free end (8) of the outer last coil (10) of the balance spring (12), such that the free end (8) of the outer last coil (10) of the balance spring (12) is immobilised in the groove (6) of the stud (2).