Several common methods such as rivets, screws, and spot welding are used to fasten materials to ductwork. Three types of screws are most often used. The first is self-tapping only and requires a hole to be drilled before the screw can be utilized. The others are both self-tapping but also create their own holes. Self-piercing screws have very sharp points which act much like a scratch awl in making a small opening in the steel for the threads to engage and work the sheet metal into a coned threaded hole. These work well for light gage steel. Self-drilling screws have an integral drill point which creates a large hole for tapping. This allows the screws to be used on heavy gages of steel. The maximum total thickness of steel that the self-drilling screw can be used with is determined by the drill point.