Inertia setting or poising of a timepiece sprung balance assembly

A method for producing and for setting inertia of and/or for poising a sprung balance assembly mounted in a timepiece oscillator mechanism which includes, mounted on a plate carrying a balance spring stud via a balance cock, the sprung balance assembly including a balance pivoting about a pivot axis and a balance spring whose inner coil is fixed to the balance or to a collet mounted integrally with the balance and whose outer coil is fixed to the stud to be held in the plate or in the balance cock, the method comprising:

• selecting a re-machining technology for setting the inertia of and/or for poising the balance by removal of material, from among tool or abrasive machining or laser beam or plasma projection or liquid jet projection, the re-machining technology being implemented by a re-machining means including a means of ablation and/or of charging and/or of deformation and/or of heat treatment;
• defining, according to the selection of the re-machining technology, a value of a first distance, forming a minimum distance between any point of the balance spring and a re-machining area for setting the inertia of and/or for poising the balance;
• determining a peripheral surface for setting the inertia of and/or for poising the balance, at a periphery of the balance, as a re-machining area, any point of the surface being separated from the balance spring by at least the value of the first predefined distance, to prevent any modification of features of the balance spring during irreversible re-machining of the balance on the peripheral area for setting the inertia of and/or for poising the balance on the assembled sprung-balance assembly;
• making the balance spring, prior to attachment to the balance, with the outer coil thereof bent and/or twisted on the attachment thereof to the balance spring stud to move an active end part of the outer coil away from the peripheral surface;
• manufacturing the balance and the balance spring, and irreversibly assembling to each other by fixing the inner coil to the balance or to the collet mounted integrally with the balance;
• fixing the outer coil in position to the balance spring stud which is immobilized by irreversible attachment with respect to the balance cock;
• defining, according to the selected re-machining technology, a value of a second distance, greater than or equal to the first distance and forming the minimum distance between the balance spring stud and the peripheral surface separated from the balance spring stud by a value greater than a second predefined distance, to prevent any modification of features of the attachment of the balance spring to the balance spring stud during re-machining of the balance on the peripheral area for setting the inertia of and/or for poising the balance on the assembled sprung-balance assembly;
• determining a value and location of any necessary re-machining operations for setting the inertia and/or for poising;
• determining a limit of a trajectory of the re-machining means, the limit being a volume external to a first surface of revolution about the pivot axis and resting on a minimum radius of the peripheral surface and at a third distance from any active point of the balance spring, the determining including comparing the third distance to the first distance to ensure that the third distance is equal to or greater than the first distance, and the first surface is a cone with a first apex centered on the pivot axis and whose first apex angle is lower than a first predetermined value;
• and
• performing the inertia setting and/or poising re-machining operations only on the peripheral surface according to the limit of the trajectory of the re-machining means.