Phillips, Oval 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. Oval undercut head screws are preferred over a flat head when a more decorative look is desired or when fastening onto a conical surface and the screw is short.