A screw thread is the helical ridge produced by forming a continuous helical groove of uniform section on the external or internal surface of a cylinder or a cone. A screw thread formed on a cylinder is known as straight or parallel screw thread, while the one formed on a cone is known as tapered .
- Screw Thread terminology :-
- Pitch: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis.
- Lead:- The distance a screw thread advances in one turn. For a single start threads, lead=pitch, For double start, lead=2xpitch, & so on.
- Thread Form: The cross section of thread cut by a plane containing the axis.
- Major Diameter: This is the diameter of an imaginary cylinder, co- axial with the screw, which just touches the crests of an external thread or roots of an internal threads. It is also called as ‘Nominal diameter’
- Minor diameter: This is the diameter of an imaginary cylinder, co-axial with the screw which just touches the roots of an external thread or the crest of an internal thread. This is also referred to as ‘root’ or ‘core diameter’.
- Effective diameter or Pitch diameter: It is the diameter of an imaginary cylinder coaxial with the axis of the thread and intersects the flanks of the thread such that width of the threads & width of spaces between threads are equal.
- Flank: It is the Thread surface that connects crest with root.
- Depth of thread: It is the distance between crest and root measured perpendicular to axis of screw.
- Angle of thread: Included angle between sides of thread measured in axial plane.
- Helix angle: Angle that the thread makes with plane perpendicular to thread axis.
- Flank angle: It is half the included angle of the thread.
- Addendum: It is the distance between the crest and the pitch line measured perpendicular to axis of the screw.
- Dedendum: It is the distance between the pitch line & the root measured perpendicular to axis of the screw.