A self-tapping screw is a screw that can tap its own hole as it is driven into the material. More narrowly, self-tapping is used only to describe a specific type of thread-cutting screw intended to produce a thread in relatively soft material or sheet materials, excluding wood screws. Other specific types of self-tapping screw include self-drilling screws and thread rolling screws. Mechanism Self-tapping screws have a wide range of tip and thread patterns, and are available with almost any possible screw head design. Common features are the screw thread covering the whole length of the screw from tip to head and a pronounced thread hard enough for the intended substrate, often case-hardened. For hard substrates such as metal or hard plastics, the self-tapping ability is often created by cutting a gap in the continuity of the thread on the screw, generating a flute and cutting edge similar to those on a tap. Thus, whereas a regular machine screw cannot tap its own hole in a metal substrate, a self-tapping one can (within reasonable limits of substrate hardness and depth). For softer substrates such as wood or soft plastics, the self-tapping ability can come simply from a tip that tapers to a gimlet point (in which no flute is needed). Like the tip of a nail or gimlet, such a point forms the hole by displacement of the surrounding material rather than any chip-forming drilling/cutting/evacuating action. Not all self-tapping screws have a sharp tip. The type B tip is blunt and intended for use with a pilot hole, often in sheet materials. The lack of a sharp tip is helpful for packaging and handling and in some applications may be helpful for reducing the clearance necessary on the reverse of a fastened panel or for making more thread available on a given length screw. Thread-forming vs. thread-cutting Self-tapping screws can be divided into two classes; those that displace material (especially plastic and thin metal sheets) without removing it are termed thread-forming self-tapping screws; self-tappers with sharp cutting surfaces that remove the material as they are inserted are termed thread-cutting. Thread-forming screws may have a non-circular plan view, such as the five-fold symmetry of the pentalobular or three-fold symmetry for Taptite screws. Thread-cutting screws may have one or more flutes machined into their threads, giving cutting edges. Self-drilling screws Some self-tapping screws are also self-drilling, which means that, in addition to the