Answer:

The largest diameter of an external or internal screw thread is known as the major diameter. It is denoted by the symbol "D" and represents the distance between the crests of the thread. The major diameter is one of the key dimensions that determine the size and fit of a screw thread.

Key points:
- Major diameter: The largest diameter of an external or internal screw thread.
- Denoted by symbol "D".
- Represents the distance between the crests of the thread.

Explanation:
To understand the concept of major diameter, let's first discuss the basic components of a screw thread. A screw thread consists of two main parts: the male thread (external) and the female thread (internal). The major diameter is relevant for both types of threads.

- External thread: In an external thread, the major diameter refers to the largest diameter of the screw. It is measured across the crests of the thread. The major diameter determines the size of the screw and is important for matching it with the correct nut or threaded hole.

- Internal thread: In an internal thread, the major diameter refers to the largest diameter of the threaded hole. It is measured across the crests of the thread. The major diameter determines the size of the threaded hole and is important for matching it with the correct screw or bolt.

The major diameter is crucial for ensuring a proper fit between the screw and the nut or threaded hole. It is also used in various calculations and specifications related to screw threads, such as determining the pitch diameter, thread depth, and thread engagement.

In summary, the major diameter is the largest diameter of an external or internal screw thread. It plays a significant role in determining the size and fit of a screw thread, ensuring compatibility between the screw and the nut or threaded hole.