BALANCE WHEEL POISING TOOL
What is a balance wheel poising tool used for?
The balance poising tool is a precision instrument designed to check and correct the static balance of the balance wheel of a mechanical watch. It is used to verify that the mass of the balance wheel is evenly distributed around its axis of rotation, an essential condition for the regulating organ to maintain a stable rate in all positions of the watch.
In its classic configuration, the tool consists of a massive base — generally made of cast iron or steel — onto which two perfectly parallel knife-edges (jaws) are set, most often made of ruby or polished hardened steel. The knife-edges form two sharp ridges on which the pivots of the balance wheel rest. A system of micrometric screws positioned beneath each of the bench’s three feet enables the entire assembly to be perfectly levelled. A further micrometric screw allows the spacing between the knife-edges to be adjusted to the length of the staff being examined.
To carry out the check, the watchmaker places the balance wheel between the two knife-edges and then imparts a slight rotation to it. If the balance wheel is not in poise, it will systematically come to rest in the same position, with its heaviest portion (commonly called the heavy spot or imbalance) settling directly beneath the axis of rotation. Correction is carried out by removing material at the identified point, generally by milling the underside of the rim with a dedicated cutter. The operation is repeated until the balance wheel remains stationary in all successive stopping positions.
Poising is performed prior to the pairing with the hairspring and the running-in of the movement. When carefully carried out, it determines the isochronism and the final precision obtained during timing/regulation. While industrial workshops today resort to static balancing machines capable of automatically detecting and correcting defects at high speed, the traditional poising tool remains a reference instrument for artisanal high-end watchmaking and for restoration work.