Use a thread cutting screw or a thread forming screw to create a tight fit in heavy gauge sheet metal, aluminum, cast iron and other hard materials. Both are a type of self tapping screw. Thread cutting screws have tapered entering threads and multiple cutting edges to dig into the boss (tapped hole). Thread forming screws are generally used for plastics. Grainger has thread cutting, thread forming and other self-tapping screws for secure fastening.