Screw thread is a continuous helical groove of specified cross-section produced on the external or internal surface. A screw thread formed on a cylinder is known as straight or parallel screw thread, while screw thread formed on a cone or frustum is known as the tapered screw thread.
Axis of a thread: This is an imaginary line running longitudinally through the center of the screw.
Crest: Crest of the thread is the top most surface joining the two sides.
Root: Root of the thread is the bottom of the groove between the two flanks.
Flank: Flank of thread are straight edges which connect the crest with the root of the thread.
Pitch: Pitch of a thread is the distance measured parallel to the axis from a point on a thread to the corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis.
Depth of thread: Depth of a thread is the distance between the crest and root of the thread.
Major diameter: It is an imaginary largest diameter of the thread which would touch the crests of an internal or external thread.
Minor diameter: It is an imaginary smallest diameter of the thread which would touch the roots of an external thread.
Pitch diameter: It is a theoretical diameter between the major and minor diameter of screw threads.
Lead angle: On the straight thread, It is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis. Lead angle is measured in an axial plane.
Flank angle: Flank angle is the angle made by the flank of a thread with the perpendicular to the axis of a thread.
Included angle: Included angle is the angle between the flanks or slope of the thread measured in an axial plane.
The lead: It is the distance the nut moves parallel to the screw axis when the nut is given one turn. For a single thread as shown in the figure above, the lead is the same as the pitch of the screw thread.