What is one key factor that permits machine countersinking when flush riveting?

Machine countersinking is a critical process in flush riveting, and one key factor that allows this technique to work effectively is that the material thickness should be greater than the rivet head thickness. When the material is thicker than the rivet head, it ensures that enough material is available for creating the countersink recess that accommodates the rivet head, allowing it to sit flush with the surface. This flush configuration is essential for aerodynamic efficiency and aesthetic considerations on aircraft surfaces.

If the material were thinner than the rivet head, it would not provide sufficient support to accommodate the countersink, risking suboptimal installation and potentially compromising the structural integrity of the assembly. Conversely, if the material thickness were equal to the rivet head thickness, it might also limit the countersinking capability, preventing the rivet from achieving the desired flush profile. Thus, thicker materials are required to ensure effective countersinking and proper rivet integration into the structure.