on Screw Threads
The pitch diameter (often called
the effective diameter) of a parallel thread is the diameter
of the imaginary co-axial cylinder which intersects the surface
of the thread in such a manner that the intercept on a generator
of the cylinder, between the points where it meets the opposite
flanks of a thread groove, is equal to half the nominal pitch
of the thread.
The major diameter of a thread is
the diameter of the imaginary co-axial cylinder that just
touches the crest of an external thread or the root of an
The minor diameter is the diameter
of an imaginary cylinder that just touches the roots of an
external thread and (or) the crests of an internal thread.
The crest of a thread is the prominent
part of a thread, whether internal or external.
The root is the bottom of the groove
between the two flanking surfaces of the thread whether internal
The flanks of a thread are the straight
sides that connect the crest and the root.
The angle of a thread is the angle
between the flanks, measured in an axial plane section.
The pitch of a thread is the distance,
measured parallel to its axis, between corresponding points
on adjacent surfaces, in the same axial plane.