Phillips, Flat, 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 head screws are used when protrusion of the fastener above the mating surface is unacceptable. Typically 100 degree flat or for fastening softer materials, 82 degrees flat.