How to Cut Sheet Metal for a Duct

Using the correct tools to cut sheet metal ductwork decreases your project time.

Sheet metal ducts for routing air conditioning and heating to your home are sturdy and retain their shape. Choosing the correct tools for the type of cuts that must be made enables you to cut through the sturdy metal as if it is paper. Compound tin snips, or aviation tin snips, have an inner spring to apply more pressure with each cut and work with thicker materials with less hand fatigue. Straight-cut and offset tin snips also are useful depending on which kinds of cuts are needed..

Things You'll Need

• Leather work gloves
• Measuring tape
• Permanent marker
• Straight-cut tin snips
• Straight-cut compound tin snips
• Air-conditioning collar
• Hammer
• Flat screwdriver
• Offset compound tin snips

Instructions

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

1. Put on leather work gloves.

2. 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.

3. 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.

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

Tips & Warnings

• Offset tin snips have horizontal handles paired with vertical jaws so you can see where you are cutting, and keep your hand above the sharp metal edges of cut sheet metal.

• Compound offset tin snips with green handles cut clockwise curves, and the red-handled version cuts counterclockwise curves.

References

• The Family Handyman: How to Use Tin Snips to Cut Sheet Metal