Phillips, Flat, Thread Cutting Screws, Type 23 Thread. Thread-Cutting Type-23 screws provide excellent chip clearance with minimum tightening torques. Steel: Used in cast iron and zinc, aluminum die castings, and plastics. Stainless: Material into which the screw is to cut threads should have a lower 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.