What is the purpose of using a bucking bar during riveting?

The purpose of using a bucking bar during riveting is primarily to shape the bucktail of a solid shank rivet. When two pieces of material are being joined together using a rivet, the rivet is inserted into pre-drilled holes and then deformed at its tail end to create a strong bond between the materials. The bucking bar is held on the opposite side of the rivet from the hammer or rivet gun being used to set the rivet. This provides a solid backing against which the rivet can be deformed, ensuring that the bucktail spreads out evenly and creates a secure fastener.

This shaping process is essential for ensuring the integrity and strength of the joint, as it affects how well the rivet will hold the materials together under various loads and stresses. The other choices pertain to different operations that are not related to the primary function of a bucking bar, thus reinforcing that the main role of the bucking bar is specifically to shape the rivet's bucktail effectively.