Phillips, Truss, Thread Cutting Screws. Thread-Cutting Type-F threaded screws are used in heavy gauge sheet metal, aluminum, zinc and lead die castings, cast iron, brass and plastic. In stainless steel, material into which the screw is to cut threads should have a lower hardness by 10-20 Rockwell hardness. Truss head screws are weaker than pan or round heads but preferred in applications where minimal clearance exists above the head. Modified truss heads offer low clearance and an extra large bearing surface. Commonly used to attach metal lathe to metal studs.