buck
To press a heavy, shaped bucking bar against the bucktail of a rivet, while the opposite end (the rivet factory head) is hammered by a rivet gun, to upset the bucktail into an appropriate shape, most commonly a pancake-shape.
buck: support a rivet while the other end gets hammered
Bucking is the act of holding a shaped steel bar, called a bucking bar, firmly against the shank side of a rivet while a pneumatic or manual rivet gun strikes the factory head on the opposite side. The bucking bar absorbs the hammer blow and prevents the rivet from moving backward, allowing the impact energy to deform the protruding tail into a secure, load-bearing head. Without bucking, the rivet would simply be driven through the joint without forming a proper clinch.
The bucking bar itself is typically a steel block weighing 2 to 8 pounds, with one face shaped to match the desired final rivet head form, usually a flush or domed profile. The bar must be held perpendicular to the rivet axis and kept in firm contact throughout the strike sequence. Different bar shapes are needed for different rivet types and applications, particularly the distinction between flush rivets used in aircraft skin and protruding rivets used in structural assembly.
Common problems and technique
Improper bucking creates weak, asymmetrical rivet heads that fail under shear or tensile load. If the bucking bar bounces away or is held at an angle, the deformation becomes uneven and the rivet does not fully fill the hole. Timing matters: the bucking bar must make contact just as the gun fires, and the bar must not be withdrawn too quickly or the rivet will cool and harden before the tail is fully upset. Vibration and recoil can also cause the bucking bar to slip, especially in tight or blind riveting locations where access is constrained.
The term derives from the action of a horse bucking, the vigorous resistance or reaction. In riveting, the bar metaphorically bucks back against the hammer blow. The operator holding the bucking bar is called the bucking team member or bucking helper, and the role demands precision, hand strength, and timing. In modern aerospace and structural fabrication, bucking remains essential for riveted joints, particularly where blind riveting (using a mandrel pull) is not acceptable and where maximum strength and reliability are required.