Method and production support tooling for measuring the torque of a timepiece balance spring

A method for measuring a torque of a balance spring for a timepiece oscillator, wound around a balance spring axis and comprising at its outer end an outer coil, and at its inner end a collet with an aperture whose inner surface comprises centering surfaces, which are arranged to cooperate with an arbor of revolution about said balance spring axis and to center said balance spring on such an arbor, and which, in a free state of said balance spring, define a passage diameter, wherein:

• dimensions of said inner surface or of said passage diameter are determined;
• as a function of said dimensions or of said passage diameter, a main tool is chosen having a mock arbor of continuously increasing cross-section, on a rectilinear tool axis, from an apex in an upper part where an envelope diameter of said mock arbor is smaller than said passage diameter, to its base in a lower part where the envelope diameter of said mock arbor is greater than said passage diameter, said mock arbor comprising in the upper part an insertion guide, every section of which, in projection onto a plane perpendicular to said tool axis, is inscribed in said aperture of said collet, and wherein the envelope diameter of said mock arbor is always smaller than said passage diameter, said insertion guide being arranged for a first centring of said collet of said balance spring placed on said upper part and allowed to slide freely under its own weight along said insertion guide, and said insertion guide surmounting at least a lower shank of substantially frustoconical or obelisk shape and which is arranged to complete the self-centring of said collet on said tool axis, and to hold said balance spring without stress on said lower shank, and said lower shank including drive means which are arranged to cooperate with said inner surface of said collet to drive in rotation said mock arbor, without slipping, via said collet or vice versa;
• said tool axis is aligned on a vertical of the place in the field of gravity;
• a balance spring to be measured is gripped with a gripper, with which said balance spring to be measured is placed simply by gravity on said insertion guide and allowed to drop down onto said lower shank until stopped simply by the effect of its weight, in a self-centring axial position referred to as a measuring position;
• a coaxiality of said balance spring axis of said balance spring with said tool axis is then checked in said measuring position;
• when said balance spring axis of said balance spring is aligned with said tool axis in said measuring position, a holding tool, which is arranged to grip the outer coil of said balance spring, is placed level with the measuring position in a direction of said tool axis, and said outer coil is fixed to said holding tool;
• an adjustment is made to a vertical position of said holding tool in the vertical direction of the place in the field of gravity, and, using measuring means, a measurement is made of the torque of said balance spring by rotating, about said tool axis, said main tool and/or said holding tool, without stressing said balance spring.