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The basic principles of the ISO metric screw thread are defined in international standard ISO 68-1 and preferred combinations of diameter and pitch are listed in ISO 261. The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO 262. The most commonly used pitch value for each diameter is the coarse pitch. For some diameters, one or two additional fine pitch variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimetres (e.g. M8). If the thread does not use the normal coarse pitch (e.g. 1.25mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign (e.g. "M8×1" if the screw thread has an outer diameter of 8mm and advances by 1mm per 360° rotation). Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations. [11] [ failed verification] Some of these issues are discussed below: This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw [6] [7] and bolt. [8] [9] [10]
Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature. These abbreviations have jargon currency among fastener specialists (who, working with many screw types all day long, have need to abbreviate repetitive mentions). The smaller basic ones can be built up into the longer ones; for example, knowing that "FH" means "flat head", it may be possible to parse the rest of a longer abbreviation containing "FH".
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A screw will usually have a head on one end that allows it to be turned with a tool. 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. A carriage bolt has a domed head that is not designed to be driven. A set screw may have a head the same size or smaller than the outer diameter of the screws thread; a set screw without a head is sometimes called a grub screw. A J-bolt has a J-shaped head that is sunk into concrete to serve as an anchor bolt. Find sources: "Screw"– news · newspapers · books · scholar · JSTOR ( October 2020) ( Learn how and when to remove this template message)
There is no universally accepted distinction between a screw and a bolt. Part of the confusion over this is likely due to regional or dialectical differences. Machinery's Handbook describes the distinction as follows: Mild steel bolts have property class 4.6, which is 400MPa ultimate strength and 0.6*400=240MPa yield strength. High-strength steel bolts have property class 8.8, which is 800MPa ultimate strength and 0.8*800=640MPa yield strength or above. Screws are often self-threading (also known as self-tapping) where the thread cuts into the material when the screw is turned, creating an internal thread that helps pull fastened materials together and prevents pull-out. There are many screws for a variety of materials; materials commonly fastened by screws include wood, sheet metal, and plastic. The first person to create a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required. Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was introduced in 1908 because the Whitworth thread was too coarse for some applications. The thread angle was 55°, and the depth and pitch varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). Spanners for Whitworth bolts are marked with the size of the bolt, not the distance across the flats of the screw head. The distinctions above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural language use of the words by machinists, auto mechanics and others. These differences reflect linguistic evolution shaped by the changing of technology over centuries. The words bolt and screw have both existed since before today's modern mix of fastener types existed, and the natural usage of those words has evolved retronymously in response to the technological change. (That is, the use of words as names for objects changes as the objects change.) Non-threaded fasteners predominated until the advent of practical, inexpensive screw-cutting in the early 19th century. The basic meaning of the word screw has long involved the idea of a helical screw thread, but the Archimedes screw and the screw gimlet (like a corkscrew) preceded the fastener.
Types of Screw Points or Tips
Screw head shapes [ edit ] (a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head) Combination flanged-hex/Phillips-head screw used in computers Pan head A low disc with a rounded, high outer edge with large surface area. Button or dome head Cylindrical with a rounded top. Round head A dome-shaped head used for decoration. [46] Truss head Lower-profile dome designed to prevent tampering. Flat head Conical, with flat outer face and tapering inner face allowing it to be countersunk into the material. The angle of the screw is measured as the aperture of the cone. Oval or raised head A decorative screw head with a countersunk bottom and rounded top. [46] Also known as "raised countersunk" or "instrument head" in the UK. [ citation needed] Bugle head Similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle. Cheese head Cylindrical. Fillister head Cylindrical, but with a slightly convex top surface. Flanged head A flanged head can be any of the above head styles (except the countersunk styles) with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer. Hex head Hex shaped, similar to the head of a hex bolt. Sometimes flanged. The international standards for metric externally threaded fasteners are ISO 898-1 for property classes produced from carbon steels and ISO 3506-1 for property classes produced from corrosion resistant steels. Metric hexagon bolts, screws and nuts are specified, for example, in International Standards ISO 4014, ISO 4017, and ISO 4032. The following table lists the relationship given in these standards between the thread size and the maximum width across the hexagonal flats (wrench size): Old USS and SAE standards defined cap screws as fasteners with shanks that were threaded to the head and bolts as fasteners with shanks that were partially unthreaded. [13] The relationship of this rule to the idea that a bolt by definition takes a nut is clear (because the unthreaded section of the shank, which is called the grip, was expected to pass through the substrate without threading into it). This is now an obsolete distinction, although large bolts still often have unthreaded sections of shank. This article is about the fastener. For the screw as a mechanism, see Screw (simple machine). For other uses, see Screw (disambiguation). An assortment of screws, and a US quarter for size comparison A screw in macro view A bolt (with a nut) and a screw
The federal government of the United States made an effort to formalize the difference between a bolt and a screw, because different tariffs apply to each. [12] 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. The document also reflects (although it probably did not originate) significant confusion of terminology usage that differs between the legal/statutory/regulatory community and the fastener industry. The legal/statutory/regulatory wording uses the terms "coarse" and "fine" to refer to the tightness of the tolerance range, referring basically to "high-quality" or "low-quality", but this is a poor choice of terms, because those terms in the fastener industry have a different meaning (referring to the steepness of the helix's lead). Often a surface coating is used to protect the fastener from corrosion (e.g. bright zinc plating for steel screws), to impart a decorative finish (e.g. japanning) or otherwise alter the surface properties of the base material. The nominal diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1mm, is made by threading a 6mm shank, and the nut or threaded hole is made by tapping threads into a hole of 5mm diameter (6mm – 1mm).The word bolt is also a very old word, and it was used for centuries to refer to metal rods that passed through the substrate to be fastened on the other side, often via nonthreaded means (clinching, forge welding, pinning, wedging, etc.). The connection of this sense to the sense of a door bolt or the crossbow bolt is apparent. In the 19th century, bolts fastened via screw threads were often called screw bolts in contradistinction to clench bolts. A screw and a bolt (see Differentiation between bolt and screw below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a male thread (external thread). This section possibly contains original research. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed. ( June 2022) ( Learn how and when to remove this template message)
Lag screws (US) or coach screws (UK, Australia, and New Zealand) (also referred to as lag bolts or coach bolts, although this is a misnomer) or French wood screw (Scandinavia) are large wood screws. The head is typically an external hex. Metric hex-headed lag screws are covered by DIN 571. Inch square-headed and hex-headed lag screws are covered by ASME B18.2.1. A typical lag screw can range in diameter from 4 to 20mm or #10 to 1.25 in (4.83 to 31.75mm), and lengths from 16 to 200mm or 1⁄ 4 to 6in (6.35 to 152.40mm) or longer, with the coarse threads of a wood-screw or sheet-metal-screw threadform (but larger). Bear in mind that these are just examples and the width across flats is different for structural bolts, flanged bolts, and also varies by standards organization. There are many standards governing the material and mechanical properties of imperial sized externally threaded fasteners. Some of the most common consensus standards for grades produced from carbon steels are ASTM A193, ASTM A307, ASTM A354, ASTM F3125, and SAE J429. Some of the most common consensus standards for grades produced from corrosion resistant steels are ASTM F593 & ASTM A193. The most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, the standard photographic tripod thread, which for small cameras is 1/4" Whitworth (20 tpi) and for medium/large format cameras is 3/8" Whitworth (16 tpi). It is also used for microphone stands and their appropriate clips, again in both sizes, along with "thread adapters" to allow the smaller size to attach to items requiring the larger thread. Note that while 1/4" UNC bolts fit 1/4" BSW camera tripod bushes, yield strength is reduced by the different thread angles of 60° and 55° respectively. Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate, and most are classed as screws. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shank are generally designed to mate with a nut or to be driven into a tapped hole, and most would be classed as bolts, although some are thread-forming (eg. taptite) and some authorities would treat some as screws when they are used with a female threaded fastener other than a nut.
ASME standard B18.2.1-1996 specifies hex cap screws whose size range is 0.25–3in (6.35–76.20mm) 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. Machinery's Handbook refers parenthetically to these fasteners as finished hex bolts. [19] 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. [20] In 1991, responding to an influx of counterfeit fasteners, Congress passed PL 101-592, [21] the "Fastener Quality Act". As a result, the ASME B18 committee re-wrote B18.2.1, [22] renaming finished hex bolts to hex cap screw– a term that had existed in common usage long before, but was now also being codified as an official name for the ASME B18 standard.
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