In thread geometry, which measurement is used to describe the outer boundary of the external thread?

In thread geometry, which measurement is used to describe the outer boundary of the external thread?

• A. Major diameter
• B. Minor diameter
• C. Pitch diameter
• D. Root diameter

Answer: (A)

The outer boundary of an external thread is described by the major diameter. This is the distance across the thread crests—the largest diameter of the thread—and it defines the nominal size of the external thread. The other diameters describe other features: minor diameter spans the thread roots and is the smallest diameter, pitch diameter is the effective diameter where the thread thickness and space are equal, and root diameter refers to the bottom of the thread groove. Since we’re concerned with the outer envelope of the thread, the major diameter is the correct measurement.

The outer boundary of an external thread is described by the major diameter. This is the distance across the thread crests—the largest diameter of the thread—and it defines the nominal size of the external thread. The other diameters describe other features: minor diameter spans the thread roots and is the smallest diameter, pitch diameter is the effective diameter where the thread thickness and space are equal, and root diameter refers to the bottom of the thread groove. Since we’re concerned with the outer envelope of the thread, the major diameter is the correct measurement.