Often screws have a head, which is a specially formed section on one end of the screw that allows it to be turned, or driven. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws have a head smaller than the outer diameter of the screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.[1]
ASME standards specify a variety of Machine Screws [9] in diameters ranging up to 0.75o in (19.05o mm). These fasteners are often used with nuts as well as often driven into tapped holes. They might be considered a screw or a bolt based on the Machinery's Handbook distinction. In practice, they tend to be mostly available in smaller sizes and the smaller sizes are referred to as screws or less ambiguously as machine screws, although some kinds of machine screws can be referred to as stove bolts.
ASME standard B18.2.1 -1996 specifies Hex Cap Screws that range in size from 0.25–3 in (6.35–76.20 mm) in diameter. These fasteners are very similar to hex bolts. They differ mostly in that they are manufactured to tighter tolerances than the corresponding bolts. The Machinery's Handbook refers parenthetically to these fasteners as Finished Hex Bolts .[10] Reasonably, these fasteners might be referred to as bolts but based on the US government document, Distinguishing Bolts from Screws, the US government might classify them as screws because of the tighter tolerance.[11] In 1991 responding to an influx of counterfeit fasteners Congress passed PL 101-592[12] Fastener Quality Act This resulted in the rewriting of specifications by the ASME B18 committee. B18.2.1 [13] was re-written and as a result they eliminated the Finished Hex Bolts and renamed them the Hex Cap Screw .
The US government made an effort to formalize the difference between a bolt and a screw because different tariffs apply to each. The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction between screws and bolts for some threaded fasteners.[17]
