What measured part of a thread is indicated by the pitch diameter?

The pitch diameter is an essential concept in threading that represents a diameter where the width of the thread is equal to the width of the spaces between the threads. It is considered the effective diameter for the purposes of measuring and maintaining thread accuracy. In a screw thread, the pitch diameter provides a critical parameter for ensuring proper fit and function between mating components.

When a threaded part is measured at its pitch diameter, it indicates where the thread geometry effectively interacts with the surfaces of the mating threaded part. This is important not only for the strength of the joint but also for minimizing issues such as cross-threading or excessive wear over time. Knowing the pitch diameter helps in calculations for thread tolerances, assembly, and performance of fasteners.

The other diameters mentioned—major, minor, and root—serve different purposes in the thread design and cutting process but do not define the specific point of equal engagement between the crests and roots of the thread profile as the pitch diameter does. Thus, the pitch diameter is crucial for understanding and working with threaded fasteners effectively.