What factor allows for machine countersinking when flush riveting?

The capability for machine countersinking when flush riveting is fundamentally linked to the thickness of the material in relation to the rivet head thickness. When the material thickness exceeds the height of the rivet head, it allows the rivet to be recessed or countersunk into the material during installation. This correspondence is crucial because it ensures that when the rivet is set, it does not protrude above the surface of the material, achieving a smooth, flush finish that is desirable for aerodynamic efficiency and aesthetics.

In instances where the material is not sufficiently thick relative to the rivet head, countersinking may not be feasible. Thus, the relationship between material thickness and rivet head thickness governs not only the mechanical fit but also the functional and visual aspects of the riveted joint.

Other factors, while they may influence the overall process or quality of riveting, do not directly enable the specific action of machine countersinking in the context of flush riveting.