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THE FIGURE-OF-EIGHT CALLIPER

The figure-of-eight calliper is a testing tool used in horology to check the geometry of an arbor or a balance, and more particularly the out-of-flat (hairspring defect) and the out-of-round (hairspring defect) of its pivots or its rim. It consists of two spindles or points whose spacing is adjustable, between which the part to be tested is mounted between centers so that it can rotate freely without excessive play. The horologist (watchmaker) imparts a slow rotation to the arbor, generally using a blower (small bellows) to avoid any contact with the fingers, and observes whether its profile deviates from the reference plane or describes a perfect circle relative to a tab or a fixed index placed in the immediate vicinity of the rotating part. An out-of-round or an out-of-flat, even a minor one, unbalances the balance and can cause parasitic friction against the regulator assembly or the bridges, altering the amplitude and the regularity of the running in the various positions. Some figure-of-eight callipers feature an adjustable tab acting as an index, making it easier to visually assess the deviation. This testing generally takes place after a truing operation on the lathe and before thepoising of the balance, of which it constitutes an indispensable preliminary step.