A Pittsburgh lock consists of three bends that form an extended "S" shape which creates an airtight seal after completion. In sheet metal shops, a rollforming machine is used to fold the Pittsburgh seam on one end of a piece of ductwork, for example; this increases productivity while keeping each seam consistently formed on multiple pieces of duct. Manually forming a Pittsburgh seam will result in a joint with the same strength as a rollformed seam, and it does not require any measuring before forming.
Hand-forming a Pittsburgh Seam
Open the jaws of a sheet metal hand brake by pushing the top handle away from the brake's jaws.
Slide a piece of sheet metal into the opening in the brake jaws. Push the sheet metal into the brake until the edge of the metal sits even with the front edge of the lower brake die. Close the brake to hold the metal in position. Lift the lower die handle until the metal has a 90-degree bend. Open the sheet metal brake.
Pull the bent sheet metal out from the open brake jaws. Flip the metal over so that the bent flange faces the ground. Push the flange tight to the bottom brake die. Close the brake.
Lift the lower die handle until the metal sits tight to the angled top brake blade. Lower the bending handle. Open the brake. Pull the metal forward 1/2 inch to release it from the top die. Push the bend under the top die. Pull the top handle closed to squeeze the bend tight and complete the female side of the Pittsburgh seam. Release the brake and remove the sheet metal from the brake.
Insert another piece of sheet metal into the sheet metal brake. Align one edge of the sheet metal with the line located between the two sections of the lower bending die. Close the brake. Lift the handle until the metal has a 1/4-inch high bend sitting at a 90-degree angle to the lower bending die. Open the brake to remove the sheet metal containing the male side of the Pittsburgh seam.
Connecting a Pittsburgh Seam
Set the 1/4-inch side of the Pittsburgh seam with the flange located at the top on the Pittsburgh's female edge.
Push the 1/4-inch flange into the female side of the Pittsburgh seam. Set the large, flat side of a rivet set on the bend of the short flange. Hit the smaller end of the set with a hammer to force the flange into the female socket of the Pittsburgh seam.
Slide the rivet set along the seam while hitting it constantly with the hammer. Bend the flange of the female Pittsburgh over every 12 inches to keep the connection tight.
Bend the entire length of the female Pittsburgh seam over the tight connection with the hammer to complete the connection.