One of the most important skills in horology is correcting/resetting the engagement of wheels with their respective pinions. It is one of the most critical issues necessary to insure the smooth running of any train of gears.
The depthing of a wheel and pinion is measured at the moment where the point of contact of the wheel tooth and the pinion leaf crosses the line of centers (the moment of deepest engagement). The point on the tooth that falls on the pitch circle should be touching the point on the leaf that falls on its pitch circle (see illustration below right). Another way of saying this is to express it in geometric terms: the distance between the centers of the wheel and pinion should equal the sum of the radii of the two pitch circles. The pitch circle for a wheel or pinion is the circle concentric to its axis that traces the points where the addendum rises from the dedendum. If the engagement is too shallow, these points will never touch. If the engagement is too deep,these points will align before the tooth crosses the center line and again afterwards. Both faults will have a negative impact on the running of the clock.
A worn hole in a plate or a worn bushing can change the depth of engagement of the wheel and its pinion. Bushing is not the “mantra” for our profession! Setting a depthing to its correct pitch point is!