Phillips, Flat Undercut, 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 undercut heads are used when a flat head is desired on shorter length screws.