Introduction: How to Cut Thin Sheet Metal and Keep It Flat
If you use metal shears to cut thin sheet metal such as duct metal, it curls and can be difficult to flaten properly. These photo-steps show that it can be done using a utility knife. The illustrations are for a small bracket, but I have cut 3 foot long pieces using this method. You just need to be careful and take your time making several scoring lines and not pressing too hard. Snap off a piece of blade as necessary and don't use the thinest blades or the extra heavy duty types.