What is a primary characteristic of an open countersink tool?

An open countersink tool is primarily designed to create a conical hole that allows the head of a screw or bolt to sit flush with or below the surface of the material being worked on. This function is essential for ensuring a smooth finish and preventing any protrusions that could interfere with the assembly, operation, or aerodynamics of aerospace components. Edge breaking refers to the process of smoothing out sharp corners or edges of materials, which is effectively what the countersink tool accomplishes when it creates a beveled edge around a hole.

The other options describe tools or processes that do not align with the primary function of an open countersink. Drilling is done with specific drill bits designed for creating holes, measuring involves tools such as calipers or micrometers, and polishing is typically carried out with abrasives or buffing tools to enhance surface finish rather than shaping holes. Therefore, the characteristic of being used for edge breaking accurately identifies the purpose of an open countersink tool.