What does sub countersinking involve?

Sub countersinking is a specific machining process that entails dimpling the surface of a material without removing a significant amount of material or compromising the structural integrity of the part being treated. This technique is commonly used in aerospace applications to prepare the surface for fastening methods, particularly rivets.

The creation of a dimpled outer surface is essential in ensuring that fasteners sit flush with the surface, thus reducing aerodynamic drag and improving the overall aesthetics of the assembly. By using this method, it allows for a more secure fit of the rivets and enhances the load distribution across the material.

The other options do not accurately describe the purpose or the outcomes of sub countersinking. For instance, flushing the skin refers to ensuring a seamless surface finish, which is a different process than sub countersinking. Additionally, using rivets exclusively does not define sub countersinking, as it can be used in conjunction with other types of fasteners. Lastly, integrating heat with the countersinking process is not a standard practice since this process typically involves mechanical means rather than thermal treatment.

Overall, the correct answer emphasizes the key characteristic of sub countersinking, which is creating a dimpled outer surface for improved fastener integration.