The pitch diameter (also referred to as the effective diameter) is the diameter of an imaginary cylinder, which intersects it so that the distance between two intersection points at opposite flanks of a thread flanks equals half the nominal pitch of the thread. (This definition is valid only for n parallel threads)
The allowance between the pitch diameters of the female and male parts of a threading assembly determines whether they can be successfully mated together.
The nominal value of the pitch diameter is a function of the pitch and the thread angle. For example, in a 60° thread (ISO metric and Unified Inch standards), it equals to:
D Pitch=D Nom − 38×3×P
For example, for a UNC 1/4-20 screw, the nominal pitch diameter is:
0.25−0.6495∗0.05=0.2175”(5.525mm)
Determining the required pitch diameter in each case is complicated and depends on the threading standard and the selected class.
You can find it out by referring to the thread standards or with our Threading Calculator.