BLADE

In watchmaking, a blade refers to a thin metallic element, generally of rectangular cross-section, flat and endowed with a certain elasticity. The term applies first to spring components: the blade making up the Mainspring or a chime spring stores or releases energy through its controlled flexibility; the Hairspring (balance spring) itself is, in principle, nothing more than an elastic blade of small (rectangular) cross-section coiled in a spiral around an axis. In old compensating balances, a bimetallic blade — brass on the outside, steel on the inside — fitted the Felloe (rim base), curving under the effect of temperature to correct rate variations linked to the expansion of the hairspring. By extension, “blade” also refers to the active, sharp part of a Cutting tool: saw blade, piercing saw (jeweler’s saw) blade, or bench knife blade. In all cases, the quality of a blade depends on the choice of Steel and its heat treatment — hardening then tempering — as well as its geometry, which together determine its strength, hardness, and elasticity: a blade that is too hard becomes brittle and breaks under stress, while a blade that is too soft deforms permanently. The trade-off between hardness and flexibility, specific to each application, guides the choice of material and heat treatment parameters.