A friction stir welding (FSW) apparatus has an FSW tool adapted for joining work pieces supportable against a backing anvil. The FSW apparatus comprises a pair of clamping rollers maintained in substantially perpendicular relationship to the backing anvil as the FSW tool moves along the tool path. A roller pivot joint of the FSW apparatus maintains a cylindrical roller outer surface of each clamping roller in substantially parallel relationship to the top sides of the work pieces. The roller pivot joint may include a gimballing roller bearing to which the clamping roller is mounted. The roller bearing is universally movable in a variety of directions to maintain the perpendicular relationship of the clamping rollers relative to the backing anvil.