How to Cut Sheet Metal for a Duct

Straight Cuts

• 1

  Put on leather work gloves.

• 2

  Measure the length of metal duct that you need for one joint. Mark the measurement with a permanent marker on one side of the ductwork. Turn the duct over, and mark each of the remaining three sides on square ductwork or around the ductwork for round ducts.

• 3

  Lay the duct on a flat surface. Hold your nonprimary hand on the duct, and pick up a pair of straight-cut tin snips in your primary hand.

• 4

  Pierce the bottom jaw of the tin snips into the sheet metal on the permanent marker line on one side of the thicker seam. Squeeze the two handles of the tin snips together to cut a small line. Continue cutting along the line by opening the jaws fully and making cuts the length of the jaws until you reach the thick seam.

• 5

  Turn the duct over when you get to a corner, and cut each side along the line, turning the duct as needed.

• 6

  Insert a pair of straight-cut compound tin snips over the duct seam. Open the jaws fully, and squeeze the handles to cut through the seam.

Round Cuts

• 7

  Put on leather work gloves.

• 8

  Place the sheet metal duct on a flat surface. Place a collar for your air conditioning or heating plenum on top of the duct. Trace around the outside of the collar with a permanent marker.

• 9

  Remove the collar. Place the tip of a flat screwdriver inside the circle of the permanent marker about 1 inch from the exterior. Strike the screwdriver with a hammer to punch a hole in the sheet metal. Move the screwdriver down into the slot, and tilt it at a 45-degree angle to the sheet metal. Continue striking the screwdriver, and moving it down to form a 2- to 3-inch slot.

• 10

  Insert a pair of compound offset tin snips in the slot. Cut over to the permanent marker line, and cut the circle out.