The most common kind of screw has threads covering all of the bottom part of the screw. These screws are designed to drive directly into the material being fastened or through a pre-drilled hole in the material. The self-tapping screw drills its own hole, and you use it when the material isn't suitable for a regular screw or the location doesn't allow for pre-drilled holes. The front part of the self-tapping screw resembles a drill bit. It literally creates its own guidehole.
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Examine the thickness of the material. You can't use self-tapping screws if the material is thicker than the drill portion of the screw. Self-tapping screws aren't advisable in situations where the screw won't go all the way through before the first thread touches the material.
Examine the location for proximity to electrical connections. Self-drilling screws break off bits of material that will fall on the other side. If the material is metal, this could cause small pieces of a conducting medium to fall into an electrical connector.
Position the screw perpendicular to the material.
Stabilize the screw with your fingers or some other material so that the screw doesn't tilt during driving. You may have to wrap your fingers around the shaft of the screwdriver as well as the screw. Self-tapping screws don't have a guide hole to keep them in line.
Put pressure on the screwdriver and turn clockwise. Continue twisting the screwdriver until the screw is completely into the material. Watch carefully and stop twisting when the head of the screw touches the material. Forcing the screw to turn will strip the threads.