The index core is a relatively flat truncated cone made of steel. Its role is to secure the index to the balance bridge and allow it to pivot around its axis with greasy friction. The index core, directly screwed or pressed onto the upper face of the cock, holds the crown in a sandwich. The conical shape of the index’s perforation and that of the circumference of the index core allow for the adjustment of the ideal rotational friction of the index.

The invention of the index core is intrinsically linked to that of the index. However, no specific date or inventor is strictly known to this day. Nevertheless, the first indexes aimed at statically correcting the active length of the hairspring were discovered between the late 17th century and the early 18th century.

Due to its simple shape, the index core is an easy component to produce, regardless of the chosen method. A lathe and a drill are sufficient for artisanal production. The central hole (clearing access to the anti-shock of the upper pivot of the balance staff) and the screw holes are centered and drilled. Turning operations involve thicknessing the core and bringing its circumference to the correct diameter, respecting its conicity. All that remains is to give the core the required finish and carry out any necessary heat treatments.

Due to the simple circular shape of the index core, machining it using an automatic lathe capable of performing all machining operations in a single cycle is by far the fastest, most qualitative, and cost-effective method, regardless of the required quality and production volume. Finishing and decoration are done manually or industrially depending on the desired quality.