Which gauge is used after visually inspecting the rivet to check the flushness of the manufactured head?

The Federal gauge is the correct choice because it is specifically designed to measure the flushness of rivet heads against the surface of the material they're installed in. After a visual inspection, which assesses the overall appearance and alignment of the rivet, the Federal gauge provides a precise measurement that determines if the rivet meets the required specifications for flushness. This is critical in aerospace applications, where any protrusions could create aerodynamic drag or interfere with other components.

The spoon gauge, while useful for measuring certain aspects of rivet installation, does not focus on flushness in the same manner as the Federal gauge. The H-gauge is typically employed for thickness measurements rather than checking flushness, and the NoGo gauge is generally used to confirm whether a rivet has been installed correctly but not specifically for assessing flushness. Therefore, the Federal gauge stands out as the appropriate tool for this specific inspection task in aerospace assembly mechanics.