Timepiece assembly wherein a bottom plate is centred and secured relative to a middle part

A timepiece assembly including at least one bottom plate, a middle part arranged to cooperate in abutment with said plate, and a means of securing said at least one bottom plate to said middle part, said securing means including at least one screw, arranged to cooperate in the screwed-in position with at least one axis housing comprised in said bottom plate, to indirectly immobilise said middle part relative to said bottom plate via a flange, each said flange being arranged to transmit the tightening force of said screw by abutting both on a first, conical or spherical shoulder comprised in said bottom plate in proximity to each said housing, and on a second, conical or spherical shoulder comprised in said middle part in proximity to each said housing, said assembly being wherein, in an assembled position wherein said bottom plate and said middle part are assembled to each other and the position wherein said screw is completely screwed into said housing, said first shoulder and/or said second shoulder cooperates with said flange on at most two surfaces or points.