1/4-20×3/4 phillips flat undercut thread cutting screw type 1 fully threaded zinc. Thread-cutting type-1 threaded screws are used in steel sheets, sructural shapes, special alloy steels, cast iron, brass or plastics. Stainless steel material into which the screw is to cut threads should have a lower hardness by 10-20 rockwell hardness. Flat undercut heads are used when a flat head is desired on shorter length screws.