Explanation: The fastenings have screw threads, which are made by cutting a continuous helical groove on a cylindrical surface. If the threads are cut on the outer surface of a solid rod, these are known as external threads. But if the threads are cut on the internal surface of a hollow rod, these are known as internal threads.

Explanation: The fastenings have screw threads, which are made by cutting a continuous helical groove on a cylindrical surface. If the threads are cut on the outer surface of a solid rod, these are known as external threads. But if the threads are cut on the internal surface of a hollow rod, these are known as internal threads.

Explanation: Helix is the curve traced by a particle, while describing a circular path at a uniform speed and advancing in the axial direction at a uniform rate. In other words, it is the curve traced by a particle while moving along a screw thread.

Lead is the distance, a screw thread advances axially in one turn.

Explanation: Lead is the distance, a screw thread advances axially in one turn.

Explanation: Depth of thread is the distance between the top and bottom surfaces of a thread (also known as crest and root of a thread).

Explanation: If the lead of a screw is equal to its pitch, it is known as single threaded screw.

Explanation: If more than one thread is cut in one lead distance of a screw, it is known as multi-threaded screw e.g. in a double threaded screw, two threads are cut in one lead length. In such cases, all the threads run independently along the length of the rod.

The nominal diameter of a screw thread is also known as outside diameter or major diameter.

The nominal diameter of a screw thread is also known as outside diameter or major diameter.