8×1/2 phillips flat undercut thread cutting screw type 25 fully threaded zinc. Thread-cutting type-25 screws provide excellent chip clearance. Steel: used in molded or through holes in plastics and other soft materials. Stainless: 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.