What is the primary function of a bucking bar?

The primary function of a bucking bar is to back up rivets during installation. This tool is essential in metalworking and structural applications, particularly in aircraft manufacturing, where rivets are commonly used to connect different metal parts. When a rivet is being installed, it is typically spread or flattened on one end to create a secure joint. The bucking bar is placed on the opposite side of the rivet to provide resistance against the hammering action that is applied to the rivet's head. This backing helps to ensure that the rivet securely compresses and deforms properly for optimal strength and stability of the connection.

The other options, while related to fasteners in general, do not describe the specific function of a bucking bar. Aligning rivets or holding fasteners in place pertains to different tools and methods used prior to or during the assembly process, while cutting nails involves tools designed specifically for trimming, which is not the purpose of a bucking bar. Thus, recognizing the relevant context of the bucking bar's use in the installation of rivets clarifies why it serves the function of backing them up effectively.