Screw Thread Definitions
Screw Threads
Screw Thread: A ridge of uniform section in the form of a helix.
External Thread: External threads are on the outside of a member. A chamfer on the end of the screw thread makes it easier to engage the nut. An external thread is usually cut using a die or a lathe.
Internal Thread: Internal threads are on the inside of a member. An internal thread is usually cut using a tap.
Definitions
Major DIA (D): The largest diameter (For both internal and external threads).
Minor DIA (d): The smallest diameter.
Depth of thread: (D-d)/2
Pitch DIA (DP): The diameter at which a line cuts the spaces and threads equally.
Crest: The top surface.
Root: The bottom Surface.
Side: The surface between the crest and root.
Pitch (P): The distance from a point on a screw thread to a corresponding point on the next thread (in/Threads).
Angle of Thread (A): The angle between the threads.
Screw Axis: The longitudinal centerline.
Lead: The distance a screw thread advances axially in one turn.
Right Handed Thread: Advances when turned CW. (Threads are assumed RH unless specified otherwise.)
Left Handed Thread: Advances when turned CCW.