What is the conical surface at the starting end of a thread called?

The conical surface at the starting end of a thread is called a chamfer. A chamfer is a beveled edge that is created to facilitate assembly or to provide an aesthetic finish. In threading, chamfers are commonly used to help guide the start of the screw into a nut or a hole, reducing the risk of cross-threading and improving the ease of engagement.

The other terms mentioned have specific meanings that do not apply in this context. For instance, the pitch cone refers to the theoretical cone formed by the pitch of the thread, which helps in understanding the geometry of threads but is not the surface at the start of the thread. The crest is the highest point on the thread profile, while the flute typically refers to the grooves in a cutting tool design, which are unrelated to the threading process itself. Thus, the appropriate term for the conical surface at the starting end of a thread is indeed chamfer.