Machine Drawings, Continued
Terminology used to describe screw threads is a worldwide industry
HELIX: The curve formed on any cylinder by a straight line in a plane
wrapped around the cylinder with constant forward progression.
EXTERNAL THREAD: A thread on the outside.
INTERNAL THREAD: A thread on the inside.
MAJOR DIAMETER: The largest diameter of an external or internal thread.
AXIS: The centerline running lengthwise through a screw.
CREST: The surface of a thread corresponding to the major diameter of an
external thread and the minor diameter of an internal thread.
ROOT: The surface of a thread corresponding to the minor diameter of an
external thread and the major diameter of an internal thread.
DEPTH: The distance from the root of a thread to the crest measured
perpendicularly to the axis.
PITCH: The distance from a point on a screw thread to a corresponding
point on the next thread as measured parallel to the axis.
LEAD: The distance a screw thread advances on one turn as measured
parallel to the axis. On a single-thread screw, the lead and the pitch are
identical. On a double-thread screw, the lead is twice the pitch; on a triple-
thread screw, the lead is three times the pitch.
Continued on next page