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Aself-tapping screw is ascrew that cantap its own hole as it is driven into the material. More narrowly, self-tapping is used only to describe a specific type ofthread-cutting screw intended to produce a thread in relatively soft material or sheet materials, excludingwood screws. Other specific types of self-tapping screw includeself-drilling screws andthread rolling screws.
Self-tapping screws have a wide range of tip and thread patterns, and are available with almost any possiblescrew 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 intendedsubstrate, oftencase-hardened.[1]
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 aflute and cutting edge similar to those on atap. Thus, whereas a regularmachine screw cannot tap its own hole in a metal substrate, a self-tapping one can (within reasonable limits of substratehardness 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 anail orgimlet, such a point forms the hole by displacement of the surrounding material rather than anychip-forming drilling/cutting/evacuating action.
Not all self-tapping screws have a sharp tip. Thetype B tip is blunt and intended for use with apilot 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.[2]
Self-tapping screws can be divided into two classes:[3] those thatdisplace material (especially plastic and thin metal sheets) without removing it, known as "thread-forming" self-tapping screws, and self-tappers with sharp cutting surfaces thatremove the material as they are inserted, termed "thread-cutting" self-tapping screws.
Thread-forming screws may have a non-circular plan view, such as the five-fold symmetry of thepentalobular or the three-fold symmetry of Taptite screws, while thread-cutting screws may have one or more flutes machined into their threads, yielding cutting edges.[citation needed]
Some self-tapping screws such as the Tek screw brand, are also self-drilling, which means that in addition to the thread-forming section there is also a fluted tip much like the tip of acenter drill. These screws combine hole drilling,threading and fastener installation into one driving motion (instead of separate drilling, tapping, and installing motions); they are thus very efficient in a variety of hard-substrate applications, fromassembly lines toroofing.[4][5] Some types incorporate a sealing washer, for fastening roofing sheets to purlins.[6]
Sheet metal screws (sometimes called "sheet-metal self-tappers",P–K orPK screws from the brand nameParker Kalon, the company which pioneered[7] the manufacture of, but did not invent, these screws) are a type of screw which can form a thread in thin sheet metal.Pan-head self-tapping screws are common in metal cases for electrical equipment,[8] while flatter-headed truss or flat countersunk headed self-tapping screws are found in aviation applications.[9]
Winged self-drilling have thin wings wider than the thread formed just behind the self drilling head. These cut a clearance hole in soft materials (such as wood or plastic), but are destroyed by more robust materials (such as metal). Thus, to clamp some material to metal, the clearance drilling, tap drilling, thread tapping, and fixing itself can happen in a single operation from one side, with the materials in their final position.[10]
Wood screws are technically self-tapping, but are not referred to as such.
Self-tapping screws are used in a variety of applications ranging fromDIY carpentry tosurgery.
Dental implants andorthopedic bone screws are both examples of self-tapping screws used in surgery. Different thread profiles are used for either densercortical bone or the spongycancellous bone.[11]
