Intermeshing pitch circle - Gear Nomenclature
The pitch circle of each gear when a pair of gears are in mesh is called the intermeshing pitch circle (alternately known as the operating pitch circle or the working pitch circle).
As the meshing of gears becomes shallower (or deeper), the intermeshing pitch circle also changes.
d1, d2 : Reference diameter
dw1, dw2 : Intermeshing pitch circle diameter