From DT Online
Features and Uses
A Grooved Seam Joint is one of the most widely used methods for joining light and medium-gauge sheet metal. Choose a Grooving Iron slightly wider than the joint seam. The interlocking seam is supported on a Bench Block, Blacksmith's Anvil or Mandrel Stake. The Grooving Iron is placed over it and struck sharply with a hammer to lock the seam.