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).

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.

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.

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.

Although there is no reason to consider this definition obsolete, because it is far from clear that "a bolt by definition takes a nut". Using a coach "bolt" as an example (and it has been a 'bolt' for a very long time). It was not originally intended to receive a nut, but did have a shank. Its purpose was not to pass through the entire substrate but only one piece of it, while the threaded portion bit into the other in order to draw, and clamp the materials together. The 'carriage' bolt was derived from this and was employed more to speed up manufacturing than achieve a different function. The carriage bolt passes through both pieces of materials and employs a nut to provide the clamping force. Both are still, however, bolts.

Bolts have been defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as ISO metric screw thread M, MJ, Unified Thread Standard UN, UNR, and UNJ) such that they can accept a non-tapered nut. Screws are then defined as headed, externally threaded fasteners that do not meet the above definition of bolts. These definitions of screw and bolt eliminate the ambiguity of the Machinery's handbook distinction. And it is for that reason, perhaps, that some people favor them. However, they are neither compliant with common usage of the two words nor are they compliant with formal specifications.

A possible distinction is that a screw is designed to cut its own thread; it has no need for access from or exposure to the opposite side of the component being fastened to. This definition of screw is further reinforced by the consideration of the developments of fasteners such as Tek Screws, with either round or hex heads, for roof cladding, self-drilling and self-tapping screws for various metal fastening applications, roof batten screws to reinforce the connection between the roof batten and the rafter, decking screws etc. On the other hand, a bolt is the male part of a fastener system designed to be accepted by a pre-equipped socket (or nut) of exactly the same thread design.

Early wood screws were made by hand, with a series of files, chisels, and other cutting tools, and these can be spotted easily by noting the irregular spacing and shape of the threads, as well as file marks remaining on the head of the screw and in the area between threads. Many of these screws had a blunt end, completely lacking the sharp tapered point on nearly all modern wood screws.[14] Some wood screws were made with cutting dies as early as the late 1700s (possibly even before 1678 when the book content was first published in parts).[15] Eventually, lathes were used to manufacture wood screws, with the earliest patent being recorded in 1760 in England.[14] During the 1850s, swaging tools were developed to provide a more uniform and consistent thread. Screws made with these tools have rounded valleys with sharp and rough threads.[16][17]

Once screw turning machines were in common use, most commercially available wood screws were produced with this method. These cut wood screws are almost invariably tapered, and even when the tapered shank is not obvious, they can be discerned because the threads do not extend past the diameter of the shank. Such screws are best installed after drilling a pilot hole with a tapered drill bit. The majority of modern wood screws, except for those made of brass, are formed on thread rolling machines. These screws have a constant diameter, threads with a larger diameter than the shank, and are stronger because the rolling process does not cut the grain of the metal.

ASME standards specify a variety of machine screws[18] in diameters ranging up to 0.75 in (19.05 mm). 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 screw can be referred to as stove bolts.

ASME standard B18.2.1-1996 specifies hex cap screws whose size range is 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. 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.

These terms refer to fasteners that are designed to be threaded into a tapped hole that is in part of the assembly and so based on the Machinery's Handbook distinction they would be screws. Here common terms are at variance with Machinery's Handbook distinction.[23][24]

Lag screw, also called a lag bolt

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 20 mm or #10 to 1.25 in (4.83 to 31.75 mm), and lengths from 16 to 200 mm or 1⁄4 to 6 in (6.35 to 152.40 mm) or longer, with the coarse threads of a wood-screw or sheet-metal-screw threadform (but larger).

The materials are usually carbon steel substrate with a coating of zinc galvanization (for corrosion resistance). The zinc coating may be bright yellow (electroplated), or dull gray (hot-dip galvanized). Lag screws are used to lag together lumber framing, to lag machinery feet to wood floors, and for other heavy carpentry applications. The attributive modifier lag came from an early principal use of such fasteners: the fastening of lags such as barrel staves and other similar parts.

These fasteners are "screws" according to the Machinery's Handbook criteria, and the obsolescent term "lag bolt" has been replaced by "lag screw" in the Handbook.[25] However, based on tradition many tradesmen continue to refer to them as "bolts", because, like head bolts, they are large, with hex or square heads that require a wrench, socket, or specialized bit to turn.

A fastener with a built in washer is called a SEM or SEMS, short for pre-asSEMbled.[26][27] It can be fitted on either a tapered or non-tapered shank.

A superbolt, or multi-jackbolt tensioner is an alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs. Tension in the bolt is developed by torquing individual jackbolts, which are threaded through the body of the nut and push against a hardened washer. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.

The field of screws and other hardware for internal fixation within the body is huge and diverse. Like prosthetics, it integrates the industrial and medicosurgical fields, causing manufacturing technologies (such as machining, CAD/CAM, and 3D printing) to intersect with the art and science of medicine. Like aerospace and nuclear power, this field involves some of the highest technology for fasteners, as well as some of the highest prices, for the simple reason that performance, longevity, and quality have to be excellent in such applications. Bone screws tend to be made of stainless steel or titanium, and they often have high-end features such as conical threads, multistart threads, cannulation (hollow core), and proprietary screw drive types (some not seen outside of these applications).

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".

These abbreviations are not universally standardized across corporations; each corporation can coin their own. The more obscure ones may not be listed here.

The extra spacing between linked terms below helps the reader to see the correct parsing at a glance.

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.

Head markings and properties for metric hex-head cap screws[32]
Head marking	Grade, material and condition	Nominal size range (mm)	Proof strength		Yield strength, min.		Tensile strength, min.		Core hardness (Rockwell)
			MPa	ksi	MPa	ksi	MPa	ksi
	Class 3.6[33]	1.6–36	180	26	190	28	330	48	B52–95
	Class 4.6 Low or medium carbon steel	5–100	225	32.6	240	35	400	58	B67–95
	Class 4.8 Low or medium carbon steel; fully or partially annealed	1.6–16	310	45	340	49	420	61	B71–95
	Class 5.8 Low or medium carbon steel; cold worked	5–24	380	55	420	61	520	75	B82–95
	Class 8.8[34] Medium carbon steel; quench and tempered	Under 16 (inc.)	580	84	640	93	800	120
		17–72	600	87	660	96	830	120	C23–34
	Class 8.8 low carbon Low carbon boron steel; quench and tempered
	Class 8.8.3[35] Atmospheric corrosion resistant steel; quench and tempered
	ASTM A325M - Type 1[36][37] Medium carbon steel; quench and tempered	12–36
	ASTM A325M - Type 3[36][37] Atmospheric corrosion resistant steel; quench and tempered
	Class 9.8 Medium carbon steel; quench and tempered	1.6–16	650	94	720	104	900	130	C27–36
	Class 9.8 low carbon Low carbon boron steel; quench and tempered
	Class 10.9 Alloy steel; quench and tempered	5–100	830	120	940	136	1,040	151	C33–39
	Class 10.9 low carbon Low carbon boron steel; quench and tempered
	Class 10.9.3[35] Atmospheric corrosion resistant steel; quench and tempered
	ASTM A490M - Type 1[36][38] Alloy steel; quench and tempered	12–36
	ASTM A490M - Type 3[36][38] Atmospheric corrosion resistant steel; quench and tempered
	Class 12.9 Alloy steel; quench and tempered	1.6–100	970	141	1,100	160	1,220	177	C38–44
	A2[34] Stainless steel with 17–19% chromium and 8–13% nickel	up to 20			210 minimum 450 typical	30 minimum 65 typical	500 minimum 700 typical	73 minimum 100 typical
	ISO 3506-1 A2-50 304 stainless steel-class 50 (annealed)				210	30	500	73
	ISO 3506-1 A2-70 304 stainless steel-class 70 (cold worked)				450	65	700	100
	ISO 3506-1 A2-80 304 stainless steel-class 80				600	87	800	120

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.

Head markings and properties for inch-system hex-head cap screws[34]
Head marking	Grade, material and condition	Nominal size range (in)	Proof strength		Yield strength, min.		Tensile strength, min.		Core hardness (Rockwell)
			ksi	MPa	ksi	MPa	ksi	MPa
	SAE Grade 0[39]	Strength and hardness is not specified
	SAE grade 1 ASTM A307[40] Low carbon steel	1⁄4–1+1⁄2	33	230			60	410	B70–100
	ASTM A307 - Grade B[40] Low or medium carbon steel	1⁄4–4					60 minimum 100 maximum	410 minimum 690 maximum	B69–95
	SAE grade 2 Low or medium carbon steel	1⁄4–3⁄4	55	380	57	390	74	510	B80–100[41]
		Greater than 3⁄4	33	230	36	250	60	410	B70–100[41]
	SAE grade 4[42] Medium carbon steel; cold worked	1⁄4–1+1⁄2			100	690	115	790
	SAE grade 3[40] Medium carbon steel; cold worked	1⁄4–1	85	590			100	690	B70–100
	SAE grade 5 Medium carbon steel; quench and tempered	1⁄4–1 (inc.)	85	590	92	630	120	830	C25–34[41]
		1–1+1⁄2	74	510	81	560	105	720	C19–30[41]
	ASTM A449 - Type 1[40] Medium carbon steel; quench and tempered	1–1+1⁄2 (inc.)	74	510			105	720	C19–30
		1+1⁄2–3	55	380			90	620	Brinell 183–235
	SAE grade 5.1[43] Low or medium carbon steel; quench and tempered	No. 6–1⁄2	85	590			120	830	C25–40
	SAE grade 5.2[43] Low carbon martensitic steel; quench and tempered	1⁄4–1	85	590			120	830	C26–36
	ASTM A449 - Type 2[43] Low carbon martensitic steel; quench and tempered								C25–34
or	ASTM A325 - Type 1[40] Medium carbon steel; quench and tempered	1⁄2–1 (inc.)	85	590	92	630[42]	120	830	C24–35
		1–1+1⁄2	74	510	82	570[42]	105	720	C19–31
[44]	ASTM A325 - Type 3[40] Atmospheric corrosion resistant steel; quench and tempered	1⁄2–1	85	590	92	630[42]	120	830	C24–35
		1–1+1⁄2	74	510	82	570[42]	105	720	C19–31
	ASTM A354 - Grade BC[40] Medium carbon alloy steel; quench and tempered	1⁄4–2+1⁄2 (inc.)	105	720	109	750[42]	125	860	C26–36
		2+1⁄2–4	95	660	99	680[42]	115	790	C22–33
	SAE grade 7 Medium carbon alloy steel; quench and tempered	1⁄4–1+1⁄2	105	720	115	790	133	920
	SAE grade 8 Medium carbon alloy steel; quench and tempered	1⁄4–1+1⁄2	120	830	130	900	150	1,000	C32–38[41]
	ASTM A354 - Grade BD[45]	1⁄4–2+1⁄2 (inc.)	120	830	130	900[45]	150	1,000	C33–39
		2+1⁄2–4	105	720	115	790[45]	140	970	C31–39
	SAE grade 8.2[41] Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered	1⁄4–1	120	830			150	1,000	C33–39
	ASTM A490 - Type 1[40] Medium carbon alloy steel; quench and tempered	1⁄2–1+1⁄2	120	830	130[42]	900	150 minimum 170 maximum	1,000 minimum 1,200 maximum	C33–38
[44]	ASTM A490 - Type 3[40] Atmospheric corrosion resistant steel; quench and tempered
	18/8 Stainless Stainless steel with 17–19% chromium and 8–13% nickel	1⁄4–5⁄8 (inc.)			40 minimum 80–90 typical	280 minimum 550–620 typical	100–125 typical	690–860 typical
		5⁄8–1 (inc.)			40 minimum 45–70 typical	280 minimum 310–480 typical	100 typical	690 typical
		over 1					80–90 typical	550–620 typical

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. 1.25 mm 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 8 mm and advances by 1 mm per 360° rotation).

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 1 mm, is made by threading a 6 mm shank, and the nut or threaded hole is made by tapping threads into a hole of 5 mm diameter (6 mm – 1 mm).

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):

In addition, the following non-preferred intermediate sizes are specified:

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.

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 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.

British Association (BA) screw threads, named after the British Association for Advancement of Science, were devised in 1884 and standardised in 1903. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft. BA threads are specified by British Standard BS 93:1951 "Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A."

While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factors 0.9 and 1.2. For example, a 4BA thread has pitch ${\displaystyle \scriptstyle p=0.9^{4}}$ mm (0.65 mm) and diameter ${\displaystyle \scriptstyle 6p^{1.2}}$ mm (3.62 mm). Although 0BA has the same diameter and pitch as ISO M6, the threads have different forms and are not compatible.

BA threads are still common in some niche applications. Certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured. BA sizes were also used extensively in aircraft, especially those manufactured in the United Kingdom. BA sizing is still used in railway signalling, mainly for the termination of electrical equipment and cabling.

BA threads are extensively used in Model Engineering where the smaller hex head sizes make scale fastenings easier to represent. As a result, many UK Model Engineering suppliers still carry stocks of BA fasteners up to typically 8BA and 10BA. 5BA is also commonly used as it can be threaded onto 1/8 rod.[47]

The Unified Thread Standard (UTS) is most commonly used in the United States, but is also extensively used in Canada and occasionally in other countries. The size of a UTS screw is described using the following format: X-Y, where X is the nominal size (the hole or slot size in standard manufacturing practice through which the shank of the screw can easily be pushed) and Y is the threads per inch (TPI). For sizes 1⁄4 inch and larger the size is given as a fraction; for sizes less than this an integer is used, ranging from 0 to 16. The integer sizes can be converted to the actual diameter by using the formula 0.060 + (0.013 × number). For example, a #4 screw is 0.060 + (0.013 × 4) = 0.060 + 0.052 = 0.112 inches in diameter. There are also screw sizes smaller than "0" (zero or ought). The sizes are 00, 000, 0000 which are usually referred to as two ought, three ought, and four ought. Most eyeglasses have the bows screwed to the frame with 00-72 (pronounced double ought – seventy two) size screws. To calculate the major diameter of "ought" size screws count the number of 0's and multiply this number by 0.013 and subtract from 0.060. For example, the major diameter of a 000-72 screw thread is .060 – (3 x .013) = 0.060 − 0.039 = .021 inches. For most size screws there are multiple TPI available, with the most common being designated a Unified Coarse Thread (UNC or UN) and Unified Fine Thread (UNF or UF). Note: In countries other than the United States and Canada, the ISO Metric Screw Thread System is primarily used today. Unlike most other countries the United States and Canada still use the Unified (Inch) Thread System. However, both are moving over to the ISO Metric System. It is estimated that approximately 60% of screw threads in use in the United States are still inch based.[48]

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