An ultrasonic welder for splicing a plurality of workpieces has four anvils, each formed with a meeting surface and displaceable to form a workpiece nest, which has a rectangular shape of a predetermined width. The width of the workpiece nest is sufficient to receive only at least one vertical column of workpieces to be welded, so as to produce a vertical splice. The ultrasonic welder further has a controller controlling a starting position of tooling, which forms the predetermined width of the workpiece nest, and displacing at least one of tools, which form side faces of the nest in reciprocal and time controlled manner. The displaceable tool is brought back to its starting position corresponding to the predetermined width of the workpiece nest before a new bundle of workpieces is loaded therein.