A duct making apparatus bends a metal web to form a duct blank having a lock tab at a forward end and having a Z-fold at a rearward end. Prior to shearing off the duct blank from the metal web, the apparatus bends the duct blank to insert the lock tab into the Z-fold, and further bends the Z-fold to form a clinch tab holding the lock tab into the Z-fold. After shearing off the duct blank from the metal web, the apparatus clinches the clinch tab over the lock tab to form a completed duct blank.