Thread Glossary

Angle of thread - the angle between the flanks.

External Thread - A thread formed on the external surface of a cylinder (e.g. a screw).

Flank - The flanks of a thread are the straight sides that connect the crest and the root. 

Major diameter - The diameter of an imaginary cylinder that just touches the crests of an external thread or roots of an internal thread.

Minor diameter - The diameter of an imaginary cylinder that just touches the roots of an external thread or crests of an internal thread.

Pitch - the nominal distance between two adjacent thread roots or crests; or between the centers of other repetitive shapes.

Right hand thread - a screw thread that is tightened by rotating clockwise when viewed from the head. Most screws are right handed.

Rolled thread - A thread formed by plastic deformation of a rod instead of cutting. Most fasteners are formed this way.

Root diameter - Same as Minor Diameter.

Shoulder screw - A screw with an unthreaded portion used for more precise and secure locating or as a bearing surface such as in a pulley.

UNC/UNRC - Unified National Coarse is a thread form with a 60 degree flank angle, rounded roots and flat crests. The unified thread is based on inch sizes and was first standardised in 1948. It is the most commonly used thread system used in the majority of screws, bolts, and nuts.

UNF/UNRF - Unified National Fine is a thread form using a finer pitch than UNC. It is used when a higher tensile strength is needed than the coarse thread series; when smaller length of engagement is needed; when a thinner wall is available; and when instruments and parts require fine adjustments.

UNEF/UNREF - Unified National Extra-Fine is a thread form using a finer pitch than UNF. It is used for the same reasons that UNF is used over UNC but for even more stringent requirements.