It appears simple enough—a metal box without ends—but making a piece of square duct out of a flat piece of sheet metal takes a little practice to master, according to Christian Smith, an HVAC contractor. Making duct, or duct fabrication, is a skill unto itself. In fact, larger HVAC companies often have a full-time employee dedicated to the task, and the first thing a fabricator learns to make, according to Smith, is a piece of square duct.
Place a 4 foot by 8 foot piece of sheet metal on the work table. The width of the sheet, 4 feet, will be the length of the duct when you finish.
Lay the steel ruler on the metal parallel to the edge of the long side. Position it about 10 inches to 12 inches from from the long edge.
Align the end of the ruler with the edge of the metal. Mark the metal with the scribe at 1 inch. Do this in two places. Make sure the marks are at least 2 feet apart.
Slide the ruler away from the end of the metal until the end of the ruler lines up with one of the 1 inch marks. Find the measurement on the ruler that corresponds to the width of your duct. Mark the metal at this point and repeat for the other 1 inch mark.
Slide the ruler until the end lines up with one of the marks you just placed. Find the measurement on the ruler that corresponds to the height of your duct. Mark the metal at this point and repeat for the other row of marks.
Slide the ruler until the end lines up with one of the marks you just placed. Find the measurement on the ruler that corresponds to the width of your duct. Mark the metal at this point and repeat for the other row of marks. Repeat this step using the height measurement and then again using the width.
Slide the ruler until the end lines up with the last mark. Place a mark 1/4 inch from this mark. Repeat for the other line of measurements. This is the pattern you should have now: 1 inch, width of duct, height of duct, width of duct, height of duct, and 1/4 inch.
Use the last marks to draw a straight line across the width of the sheet metal. Cut the sheet metal along this line. Insert the end of the cut piece of metal marked at 1 inch into the lock-former machine. Turn the machine on.
Open the clamp mechanism of your sheet-metal break and insert the end of the metal with the lock-form. Push the metal through until the 1/4-inch marks are lined up with the clamp edge, and clamp the edge down on the metal.
Bend the metal at 90 degrees. Starting with the next mark, repeat the process of bending until all marked sections have been bent to 90 degrees, then remove the piece from the break.
Insert the 1/4-inch lip into the opening of the lock-form. You may have to use the sheet-metal hammer to tap it in. After the lip is inserted, there will be a metal ridge protruding above the 1/4-inch corner bend. Hammer this lip over and down onto the 1/4-inch corner bend. This locks the corner of the duct together.