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 lantern pinion is measured at the moment where the point of contact of the wheel tooth and the trundle 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 trundle that falls on its pitch circle (see illustration). 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 is the circle that traces the points where the addendum rises from the dedendum. The pitch circle for the lantern pinion intersects the center point of all the trundles. 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. 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 a “mantra” for our profession! Setting a depthing to its correct pitch point is!
