When threading a pipe, cutting oil is applied to:

When threading a pipe, cutting oil is primarily applied to reduce friction and heat during the threading process. The act of cutting threads into a pipe can generate significant friction, which can lead to increased temperatures. This elevated heat not only can damage the pipe but can also affect the quality of the threads being cut, leading to issues with joint integrity and performance.

By applying cutting oil, the lubricant reduces the resistance encountered during the threading operation, allowing for smoother movement of the cutting tool across the pipe's surface. This results in cleaner, more precise threads and prolongs the life of both the tool and the workpiece, ensuring a better fit when joining pipes.

The other options may have some relevance to other contexts in plumbing or pipe work, but they do not directly pertain to the primary purpose of cutting oil in the threading process. For instance, while cutting oil could potentially improve the appearance of the threads by providing a smoother finish, this is a secondary benefit rather than the main purpose. The notion of enhancing flow is not related to the threading process, as it concerns the internal movement of fluids rather than the preparation of pipe threads. Preventing rust is also not a direct function of cutting oil; while some cutting oils may have rust-inhibiting