Sheet metal screws have sharp threads that cut into materials such as: Plastic, Plexi Glass, Wood, and sheet metal. Sheet metal screws are also the go to fasteners for use with plastic wall anchors in drywall. Tapping screws are considered externally threaded fasteners that "tap" their own mating threads in metallic and non-metallic (such as plastic) materials. Tapping screws, especially Type A, are referred to as "sheet metal screws" because one of their first applications was in ventilation ducts made of sheet metal. They also make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood. Pan heads are slightly rounded with short vertical sides. A Pan head provides a low large diameter head, but with characteristically high outer edge along the outer periphery of the head where driving action is most effective for high tightening torques. Phillips drive is x-shaped for installation with a phillip screw driver. The design is often criticized for its tendency to cam out at lower torque levels than other "cross head" designs. There has long been a popular belief that this was actually a deliberate feature of the design, for the purpose of assembling aluminum aircraft without over tightening the fasteners.