What is the primary function of a reamer in machining?

The primary function of a reamer in machining is to enlarge or smooth previously drilled holes. Reamers are specially designed tools used after drilling to improve the accuracy, tolerance, and finish of a hole. When a hole is drilled, it may not be perfectly to size or may have rough edges, which can affect the performance and fit of components.

By using a reamer, the operator can achieve a more precise inner diameter and a smoother surface finish. The cutting edges of a reamer are arranged in such a way that they gradually remove small amounts of material, refining the hole to the desired specifications without significantly enlarging it. This capability makes reamers essential for achieving tighter tolerances and better surface qualities than drilling can typically provide.

In contrast, the other options describe different machining tasks that are not the purpose of a reamer. For example, drilling new holes is a function of a drill bit, cutting external threads relates to tap tools, and shaping flat surfaces typically involves milling or grinding operations. Each of these tools has a distinct purpose in machining processes, emphasizing the unique role of reamers in enhancing already existing holes.