Pitch diameter
Pitch diameter turns up in 2 fields: fasteners, gears. Here is what it means to the people in each one.
Pitch diameter in fasteners
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
The diameter of an imaginary cylinder that cuts the thread where the width of the thread and the width of the groove are equal. It is the key dimension in thread class tolerances, since mating flanks bear on each other around it. It is checked with thread micrometers, thread plug and ring gauges, or the three-wire method, where wires of a best size sit in the grooves and the measurement over them is converted by formula.
"OD's fine, but pitch diameter over the wires is a thou under the 2A minimum, so the whole lot goes on hold."
Pitch diameter in gears
The diameter of the pitch circle, the imaginary rolling circle on which tooth size and gear ratio are based.
The diameter of the reference circle on which tooth spacing is defined, equal to tooth count times module, or tooth count divided by diametral pitch. Two mating gears behave like two cylinders of these diameters rolling without slip, and at standard center distance the center distance equals half the sum of the two pitch diameters. The operating pitch diameter shifts if the gears run at a different center distance.
"Pitch diameter's 4.000 on the 40-tooth, so center distance with the 20-tooth pinion should be three inches flat."