What tool is commonly used to chamfer holes for fillet relief?

A countersink tool is specifically designed to create a conical hole which allows for a smooth transition from the hole to the surrounding material. This is crucial in aerospace assembly where fillet relief is needed. The countersink provides the necessary angle and shape for fasteners or components that will be flush with the surface, ensuring that the load is distributed properly and minimizing stress concentrations around the fastener. This enhances the structural integrity and aerodynamic performance of the assembly.

Other tools listed, such as drills, coupling tools, or lathe tools, do not fulfill the specific requirement for chamfering holes for fillet relief in the same way. Drills create cylindrical holes and are not designed for the conical cut needed for countersinking. Coupling tools typically relate to joining or connecting components rather than modifying holes. Lathe tools are used for shaping materials but do not address the need to create a chamfered edge around a hole. Therefore, the countersink tool is the most appropriate choice for this task in aerospace applications.