What must be aligned with the center of the rivet shank while driving a rivet?

When driving a rivet, it is crucial for the bucking bar to be aligned with the center of the rivet shank. This alignment ensures that the force applied during the riveting process is even and effective, which is essential for forming a proper rivet head and achieving a strong, structurally sound joint.

If the bucking bar is not centered on the shank, it can lead to improper deformation of the rivet, potentially resulting in a weak or incomplete connection. This misalignment can cause issues like rivet snapping, insufficient setting, or distortion of the materials being joined.

While the rivet gun provides the initial force needed to drive the rivet, its alignment is less critical compared to that of the bucking bar, which has the primary responsibility of properly supporting the rivet while it is being set. Other options like the drill bit and measuring gauge are not involved in the direct action of driving the rivet and do not impact the immediate alignment needed during this phase of the riveting process.