Minor diameter

Illustrated explanation

Diameter, profile angle and taper describe different geometry

1Minor diameter
Diameter, profile angle and taper describe different geometryCallout 1 shows the external minor diameter between opposite thread-root envelopes. Callout 2 separates the included profile angle from one flank angle measured to the perpendicular. Callout 3 compares diameters at two axial planes to describe taper. Current term: Minor diameter, callout 1. 1 External minor diameter Roots define the external minor envelope 2 Included and flank angles Included One flank Dashed line: perpendicular to the axis 3 Diameter taper d₁d₂ Axial length L Taper = |d₂ − d₁| / L Diameter change, not radius change Profile angle is a separate feature
  1. Minor diameterThis term

    The shown external minor diameter spans the root envelope. For an internal thread, the minor diameter is at the crests; it is not the cable passage size.

  2. Thread angle / flank angle

    The included angle spans both flanks. One flank angle is measured from the perpendicular to the axis; on a symmetric profile, the two flank angles sum to the included angle.

  3. Thread taper

    Taper compares the difference in diameters at two planes with the axial distance between them. It is independent of the profile angle and does not identify the thread family on its own.

An external thread section locates its minor diameter at the roots. A separate profile distinguishes the included angle from one flank angle, while the tapered envelope compares diameter change with axial length.

Conceptual external-thread geometry with exaggerated dimensions. The profile sketch illustrates a symmetric form; actual root shape, tolerances, gauge planes and thread standard determine the manufactured interface.

ReferencesISO 5408:2009 — Screw threads — VocabularyISO 965-1:2026 — General-purpose metric screw-thread tolerancesASME B1.20.1 — Pipe Threads, General Purpose, Inch