Self Tapping Screws are designed to advance when turned, creating their own threads in the material. They are often used in sheet metal and plastic.
Steels are designated by a 4 number SAE Steel grade. The first two digits indicate the primary materials used to form the steel. The last 2 digits identify the percentage of carbon for the alloy (in hundredths). The steel used in these fasteners has a tensile strength ranging from 100,000 to 150,000 psi (pounds per square inch). Zinc Plating helps to prevent corrosion.
Flat head fasteners are designed to fit flush to the surface when used with countersunk holes. Length is measured from the top of the head. The Phillips drive style was originally designed so that the driver would slip out under extreme torque, preventing over-tightening and damage to the fastener or the material.
