Oval heads are countersunk (cone shaped under the head) with a rounded top. Square drive, also referred to as a Robertson drive, are self-centering, reduce cam out, stop a power tool when set, and can be removed if painted over or old and rusty. In industry, they speed up production and reduce product damage. Sheet metal screws have sharp threads that cut into a material such as sheet metal, plastic or wood. They also make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood.