Which method of countersinking is less commonly used?

Dimpling, also known as press countersinking, is less commonly used in aircraft metal structural repair compared to machine or drill countersinking. This method involves creating a shallow indentation in the material that allows for the head of a rivet to sit flush with the surface. While it has its applications, especially for specific aircraft designs, it is generally not the preferred method when compared to machine countersinking, which provides a more uniform and precise hole for rivet installation. Machine countersinking involves using a drill or a milling machine to create a specific profile in the material that accommodates the rivet, resulting in a more controlled and consistent outcome.

In contrast, riveting is a fastening technique rather than a countersinking method, which means it doesn't fit into the categories of countersinking as presented in the options. Hence, the focus is clearly on the methods of creating countersunk rivet heads, where press countersinking is less prevalent in practice. This distinction underscores why the choice of press countersinking is considered less common in the context of aircraft repair procedures.