Phillips, Flat Undercut, Thread Cutting Screws, Type 25 Thread. 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.