Which tool is used for cutting external threads?

The correct answer is the die. A die is specifically designed for cutting external threads on cylindrical objects such as bolts or rods. When a die is applied, it forms the helical grooves needed for threading. This tool is used in a process called threading, where the die is rotated around the workpiece to create a consistent external thread profile.

In contrast, a lathe is primarily used for shaping and cutting materials, but it is not specifically designed for creating external threads by itself. While it can be used to create threads when combined with the right tooling, it is not the tool most commonly associated with cutting external threads on its own.

A tap, on the other hand, is used for cutting internal threads, making it suitable for creating threaded holes rather than external threads. Lastly, a mill is primarily for machining flat surfaces or complex shapes and does not directly cut threads like a die does. Each of these tools has a distinct function that caters to different aspects of machining, which clarifies why the die is the correct choice for cutting external threads.