   #copyright

Screw

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Screws come in a variety of shapes and sizes for different purposes.
   U.S. Quarter coin (diameter 24mm) shown for scale.
   Screws come in a variety of shapes and sizes for different purposes.
   U.S. Quarter coin (diameter 24mm) shown for scale.

   A screw is a shaft with a helical groove or thread formed on its
   surface. Its main uses are as a threaded fastener used to hold objects
   together, and as a simple machine used to translate torque into linear
   force. It can also be defined as an inclined plane wrapped around a
   shaft.

Threaded fastener

   A screw used as a threaded fastener consists of a shaft, which may be
   cylindrical or conical, and a head. The shaft has a helical ridge or
   thread formed on it. The thread is essentially an inclined plane
   wrapped around a shaft. The thread mates with a complementary helix in
   the material. The material may be manufactured with the mating helix (
   taps and dies), or the screw may create it when first driven in (a
   self-tapping screw). The head is specially shaped to allow a
   screwdriver or wrench (UK: spanner) to grip the screw when driving it
   in. It also stops the screw from passing right through the material
   being fastened and provides compression.

   Screws can normally be removed and reinserted without reducing their
   effectiveness. They have greater holding power than nails and permit
   disassembly and reuse.

   A screw that is tightened by turning it clockwise is said to have a
   right-hand thread. Screws with left-hand threads are used in
   exceptional cases, when the screw is subject to anticlockwise forces
   that might undo a right-hand thread. Examples include rotating items
   such as the left hand grinding wheel on a bench grinder or the left
   hand pedal on a bicycle (both looking towards the equipment).

   Threaded fasteners are traditionally made by a cutting action such as
   taps and dies provide, however recent advances in tooling allows them
   to be made by rolling the blank (a section of rod) between two
   specially machined dies. The thread form and shape of the fastener are
   squeezed onto the blank. This method work hardens the threads and saves
   material. A rolled thread is obvious after manufacture because the
   outside diameter of the thread is greater than the diameter of the
   blank material. Bicycle spokes, which are just very long thin bolts,
   always use rolled threads for strength.

Bolt

   Bolt with nut
   Enlarge
   Bolt with nut
   An 87.5 kg (192.9 lb) nut and bolt, one of sixteen used to join
   sections of the generator shaft of a 75000 kW generator at Grand Coulee
   Dam in 1942
   Enlarge
   An 87.5 kg (192.9 lb) nut and bolt, one of sixteen used to join
   sections of the generator shaft of a 75000 kW generator at Grand Coulee
   Dam in 1942

   A bolt is a cylindrical (as opposed to conical) threaded fastener that
   passes through the work piece and is held in place by a nut or a
   threaded hole on the other side. This is a very common way of holding
   together temporary and permanent constructions. An unthreaded hole is
   known as a clear hole. The thread on a bolt sometimes occupies only
   part of the shaft, the remainder of the shaft being clear; if the
   thread continues up to the head it is known as a 'set'.

   See also the article on the bolt manufacturing process.

Other fastening methods

   When screws and bolts cannot be used, nailing, riveting, roll pins,
   pinned shafts, welding, soldering, brazing, gluing, and duct tape
   (taping) are some alternatives.

Materials and strength

   Screws and bolts are made in a wide range of materials, with steel
   being perhaps the most common, in many varieties. Where great
   resistance to weather or corrosion is required, stainless steel,
   titanium, brass or bronze may be used, or a coating such as brass, zinc
   or chromium applied. Electrolytic action from dissimilar metals can be
   prevented with aluminium screws for double-glazing tracks, for example.
   Some types of plastic, such as nylon or Teflon, can be threaded and
   used for fastening requiring moderate strength and great resistance to
   corrosion or for the purpose of electrical insulation. Even porcelain
   and glass can have molded screw threads that are used successfully in
   applications such as electrical line insulators and canning jars.

   The same type of screw or bolt can be made in many different grades of
   material. For critical high-tensile-strength applications, low-grade
   bolts may fail, resulting in damage or injury. On SAE-standard bolts, a
   distinctive pattern of marking is impressed on the heads to allow
   inspection and validation of the strength of the bolt. However,
   low-cost counterfeit fasteners may be found with actual strength far
   less than indicated by the markings. Such inferior fasteners are a
   danger to life and property when used in aircraft, automobiles, heavy
   trucks, and similar critical applications.

Mechanical analysis

   Rotating screw and fixed trough
   Rotating screw and fixed trough

   A screw is a specialized application of the wedge or inclined plane. It
   contains a wedge, wound around a cylinder or shaft, that either fits
   into a corresponding inclined plane in a nut, or forms a corresponding
   inclined plane in the wood or metal as it is inserted. The technical
   analysis (see also statics, dynamics) to determine the pitch, thread
   shape or cross section, coefficient of friction (static and dynamic),
   and holding power of the screw is very similar to that performed to
   predict wedge behaviour. Wedges are discussed in the article on simple
   machines.

   Critical applications of screws and bolts will specify a torque that
   must be applied when tightening. The main concept is to stretch the
   bolt, and compress the parts being held together, creating a
   spring-like assembly. The stretch introduced to the bolt is called a
   preload. When external forces try to separate the parts, the bolt sees
   no strain unless the preload force is exceeded.

   As long as the preload is never exceeded, the bolt or nut will never
   come loose (assuming the full strength of the bolt is used). If the
   full strength of the bolt is not used (e.g., a steel bolt threaded into
   aluminium threads), then a thread-locking adhesive may be used.

   If the preload is exceeded during normal use, the joint will eventually
   fail. The preload is calculated as a percentage of the bolt's yield
   tensile strength, or the strength of the threads it goes into, or the
   compressive strength of the clamped layers (plates, washers, gaskets),
   whichever is least.

Tensile strength

   Screws and bolts are usually in tension when properly fitted. In most
   applications they are not designed to bear large shear forces. For
   example, when two overlapping metal bars joined by a bolt are likely to
   be pulled apart longitudinally, the bolt must be tight enough so that
   the friction between the two bars can overcome the longitudinal force.
   If the bars slip, then the bolt may be sheared in half, or friction
   between the bolt and slipping bars may erode and weaken the bolt
   (called fretting). For this type of application, high-strength steel
   bolts are used and these should be tightened with a torque wrench.
   Rusty hexagonal bolt heads
   Enlarge
   Rusty hexagonal bolt heads

   High-strength bolts usually have a hexagonal head with an ISO strength
   rating (called property class) stamped on the head. The property
   classes most often used are 8.8 and 10.9. The number before the point
   is the tensile ultimate strength in MPa divided by 100. The number
   after the point is 10 times the ratio of tensile yield strength to
   tensile ultimate strength. For example, a property class 5.8 bolt has a
   nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile
   yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400
   MPa.

   Tensile yield strength is M10, property class 8.8 bolt can very safely
   hold a static tensile load of about 15 kN.

Types of screws and bolts

   Typical Phillips head screw used in computers
   Enlarge
   Typical Phillips head screw used in computers
     * A hex cap screw has a protruding hexagonal head, designed to be
       driven by a spanner or wrench.
     * A socket cap screw has a hexagonal recessed drive, usually with a
       cylindrical head, but can also be found with a rounded button head
       or a countersunk flat head. Socket cap screws can be torqued more
       tightly than other drives without stripping, and they are usually
       made from a high strength steel alloy.
     * A Wood screw has a tapered shaft, allowing it to penetrate
       undrilled wood, or a wallplug in brickwork etc.
     * Lag screw, lag bolt or coach screw refers to a large wood screw
       with a hexagonal head, driven by a wrench rather than a
       screwdriver.
     * A Machine screw has a cylindrical shaft, threaded its entire
       length, and fits into a nut or a tapped hole; a small bolt.
     * Self-tapping screws or thread cutting screws have sharp threads
       that cut into a material such as sheet metal or plastic. They are
       sometimes notched at the tip to aid in chip removal during thread
       cutting.
     * A Self-drilling screw is similar to a self-tapping screw, but has a
       drill-shaped point to cut through the material without prior
       drilling.
     * Thread rolling screws have a lobed (usually triangular) cross
       section. They form threads by pushing outward during installation.
       They may have tapping threads or machine threads.
     * A Drywall screw is a specialized self-tapping screw designed to
       bind drywall to wood or metal studs, but it has proven to be a
       versatile construction fastener.
     * A Set screw, used to prevent loosening due to vibration, is
       available with thumb screw, square head, Hex head (inset socket)
       and, most commonly, headless (a grub screw in UK parlance, designed
       to be inserted flush with or below the surface of the work piece).
       Alternatively defined to be a screw whose thread reaches the head
       (if any), as opposed to a bolt.
     * Dowel screw is a wood screw with two pointed ends and no head, used
       for making hidden joints between two pieces of wood.
     * A stud is similar to a bolt but without the head. Studs are
       threaded on both ends. In some cases the entire length of the stud
       is threaded, while in other cases there will be an unthreaded
       section in the middle. It may be anchored in concrete, for example,
       with only the threads on one end exposed. (See also: screw anchor,
       wedge anchor.)
     * A carriage bolt or coach bolt has a domed or countersunk head, and
       the shaft is topped by a short square section under the head. It is
       used in wood where the square section is pulled into the wood as
       the nut is tightened, preventing the bolt from turning. The rib
       neck carriage bolt has several longitudinal ribs instead of the
       square section, to grip into a metal part being fixed.
     * A stove bolt is similar to a carriage bolt, but usually used in
       metal. It requires a square hole in the metal being bolted to
       prevent the bolt from turning.
     * The Superbolt is a form of very large fastener where tension in the
       bolt is developed by a special nut containing individual
       jackscrews. This is applied in large structural joints where hand
       tools or portable tools are insufficient to develop the required
       tension of a simple bolt and nut.
     * Acme screw form has threads that are stronger and broader than
       standard V-profile threads, making them much better for load
       carrying, linear actuating (also see ball screws), and quick
       threading. It is often used for vise screws.

Shapes of screw head

   Image:Screw head types.png
   (a) pan, (b) button, (c) round, (d) truss, (e) flat, (f) oval
     * pan head: a low disc with chamfered outer edge.
     * button or dome head: cylindrical with a rounded top.
     * round head: dome-shaped, commonly used for machine screws.
     * truss head: lower-profile dome designed to prevent tampering.
     * flat head or countersunk: conical, with flat outer face and
       tapering inner face allowing it to sink into the material.
     * oval or raised head: countersunk with a rounded top.
     * bugle head: similar to countersunk, but there is a smooth
       progression from the shaft to the angle of the head, similar to the
       bell of a bugle.
     * cheese head: disc with cylindrical outer edge, height approximately
       half the head diameter.
     * fillister head: cylindrical, but with a slightly convex top
       surface.
     * socket head: cylindrical, relatively high, with different types of
       sockets (hex, square, torx, etc.).
     * mirror screw head: countersunk head with a tapped hole to receive a
       separate screw-in chrome-plated cover, used for attaching mirrors.
     * headless (set or grub screw): has either a socket or slot in one
       end for driving.

   Some varieties of screw are manufactured with a break-away head, which
   snaps off when adequate torque is applied. This prevents tampering and
   disassembly and also provides an easily-inspectable joint to guarantee
   proper assembly.

Types of screw drive

   Pozidriv compared with other screws: (a) Slotted, (b) Phillips,
   (c) Pozidriv, (d) Torx, (e)  Hex, (f) Robertson, (g) Tri-Wing,
   (h) Torq-Set, (i) Spanner
   Enlarge
   Pozidriv compared with other screws:
   (a)  Slotted, (b)  Phillips, (c)  Pozidriv, (d)  Torx, (e)  Hex, (f)
   Robertson, (g)  Tri-Wing, (h)  Torq-Set, (i) Spanner

   Modern screws employ a wide variety of drive designs, each requiring a
   different kind of tool to drive in or extract them. The most common
   screw drives are the slotted and Phillips; hex, Robertson, and TORX are
   also common in some applications. Some types of drive are intended for
   automatic assembly in mass-production of such items as automobiles.
   More exotic screw drive types may be used in situations where tampering
   is undesirable, such as in electronic appliances that should not be
   serviced by the home repair person.
     * Slot head has a single slot, and is driven by a flat-bladed
       screwdriver. The slotted screw is common in woodworking
       applications, but is not often seen in applications where a power
       driver would be used, due to the tendency of a power driver to slip
       out of the head and potentially damage the surrounding material.

   Phillips vs. Frearson
   Enlarge
   Phillips vs. Frearson
   BNAE driver bit
   Enlarge
   BNAE driver bit
     * Cross-head, cross-point, or cruciform has a "+"-shaped slot and is
       driven by a cross-head screwdriver, designed originally for use
       with mechanical screwing machines. There are four types:
          + The Phillips screw drive has slightly rounded corners in the
            tool recess, and was designed so the driver will slip out, or
            cam out, under high torque to prevent over-tightening. The
            Phillips Screw Company was founded in Oregon in 1933 by Henry
            F. Phillips, who bought the design from J. P. Thompson.
            Phillips was unable to manufacture the design, so he passed
            the patent to the American Screw Company, who was the first to
            manufacture it.
          + A Reed & Prince or Frearson screw drive is similar to a
            Phillips but has a more pointed 75° V shape. It is found
            mainly in marine hardware. The tool recess is a perfect cross,
            unlike the Phillips head, which is designed to cam out. It was
            developed by the Reed & Prince Manufacturing Company of
            Massachusetts.
          + A JIS ( Japanese Industrial Standard) head, commonly found in
            Japanese equipment, looks like a Phillips screw, but is
            designed not to cam out and will, therefore, be damaged by a
            Phillips screwdriver if it is too tight. The standard number
            is JIS B 1012:1985
          + French Recess, also called BNAE NFL22-070 for Bureau de
            Normalisation de l'Aéronautique et de l'Espace, a French
            standards organization.
     * Pozidriv is patented, similar to cross-head but designed not to
       slip, or cam out. It has four additional points of contact, and
       does not have the rounded corners that the Phillips screw drive
       has. Phillips screwdrivers will usually work in Pozidriv screws,
       but Pozidriv screwdrivers are likely to slip or tear out the screw
       head when used in Phillips screws. Pozidriv was jointly patented by
       the Phillips Screw Company and American Screw Company.
     * TORX is a star-shaped "hexalobular" drive with six rounded points.
       It was designed to permit increased torque transfer from the driver
       to the bit compared to other drive systems. TORX is very popular in
       the automotive and electronics industries due to resistance to cam
       out and extended bit life, as well as reduced operator fatigue by
       minimizing the need to bear down on the drive tool to prevent cam
       out. TORX screws were found in early Apple Macintosh computers, to
       discourage home repairs. TORX PLUS is an improved version of TORX
       which extends tool life even further and permits greater torque
       transfer compared to TORX. A tamper-resistant TORX head has a small
       pin inside the recess. The tamper-resistant TORX is also made in a
       5 lobed variant. These TORX configurations are commonly used in
       correctional facilities, public facilities and government schools.

   Hex socket screws
   Enlarge
   Hex socket screws
     * Hexagonal (hex) socket head has a hexagonal hole and is driven by a
       Hex Wrench, sometimes called an Allen key or Hex key, or by a power
       tool with a hexagonal bit. Tamper-resistant versions with a pin in
       the recess are available. Hex sockets are increasingly used for
       modern bicycle parts because Hex Wrenches are very light and easily
       carried tools.
     * Robertson head, invented in 1908 by P.L. Robertson, has a square
       hole and is driven by a special power-tool bit or screwdriver. The
       screw is designed to maximize torque transferred from the driver,
       and will not slip, or cam out. It is possible to hold a Robertson
       screw on a driver bit horizontally or even pendant, due to a slight
       wedge fit. Commonly found in Canada in carpentry and woodworking
       applications and in Canadian-manufactured electrical wiring items
       such as receptacles and switch boxes.
     * Square-drive head is an American clone of the Robertson that has a
       square hole without taper. Due to the lack of taper, the hole must
       be oversize relative to the screwdriver, and is much more likely to
       strip than the Robertson.
     * Tri-Wing head has a triangular slotted configuration. They were
       used by Nintendo on its Game Boys to discourage home repair.
     * Torq-Set or offset cruciform is an uncommon screw drive that may be
       confused with Phillips; however, the four legs of the contact area
       are offset in this drive type.
     * Spanner drive uses two round holes opposite each other, and is
       designed to prevent tampering. Commonly seen in elevators in the
       United States.
     * Clutch Type A or standard clutch head resembles a bow tie. These
       were common in GM automobiles of the 1940s and '50s, particularly
       for body panels.
     * Clutch Type G head resembles a butterfly. This type of screw head
       is commonly used in the manufacture of mobile homes and
       recreational vehicles.

Combination drives

   Some screws have heads designed to accommodate more than one kind of
   driver. The most common of these is a combination of a slotted and
   Phillips head. Because of its prevalence, there are now drivers made
   specifically for this kind of screw head. Other combinations are a
   Phillips and Robertson, a Robertson and a slotted, and a triple-drive
   screw which can take a slotted, Phillips or a Robertson. Combination
   head screws are becoming more and more popular.

Tamper-resistant screws

   Tamper-resistant TORX driver
   Enlarge
   Tamper-resistant TORX driver

   Many screw drives, including Phillips, TORX, and Hexagonal, are also
   manufactured in tamper-resistant form. These typically have a pin
   protruding in the centre of the screw head, necessitating a special
   tool for extraction. However, the bits for many tamper-resistant screw
   heads are now readily available from hardware stores, tool suppliers
   and through the Internet. What is more, there are many commonly used
   techniques to extract tamper resistant screws without the correct
   driver — for example, the use of an alternative driver than can gain
   enough purchase to turn the screw, modifying the head to accept an
   alternative driver or forming ones own driver by melting an object into
   the head to mould a driver. Thus, these special screws offer only
   modest security.
   One-way slotted screw
   Enlarge
   One-way slotted screw

   The slotted screw drive also comes in a tamper-resistant one-way design
   with sloped edges; the screw can be driven in, but the bit slips out in
   the reverse direction.

   There are specialty fastener companies that make unusual, proprietary
   head designs, featuring matching drivers available only from them, and
   only supplied to registered owners.

Tools used

   The hand tool used to drive in most screws is called a screwdriver. A
   power tool that does the same job is a power screwdriver; power drills
   may also be used with screw-driving attachments. Where the holding
   power of the screwed joint is critical, torque-measuring and
   torque-limiting screwdrivers are used to ensure sufficient and not
   excessive force is developed by the screw. The hand tool for driving
   cap screws and other types is called a spanner (UK usage) or wrench (US
   usage).

Mechanics of use

   When driving in a screw, especially when the screw has been removed and
   is being placed again, the threads can become misaligned and damage, or
   strip, the threading of the hole. To avoid this, slight pressure is
   applied and the screw is driven in reverse, until the leading edges of
   the helices pass each other, at which point a slight click will be felt
   (and sometimes heard.) When this happens, the screw will often assume a
   more aligned position with respect to the hole.

   Immediately after the 'click', the screw may be driven in without
   damage to the threading. This technique is useful for re-seating screws
   in wood and plastic, and for assuring the proper fit when screwing down
   plates and covers where alignment is difficult.

Screw measurements

   There are many systems for specifying the dimensions of screws, but in
   much of the world the ISO metric screw thread preferred series has
   displaced the many older systems. Other relatively common systems
   include the British Standard Whitworth, BA system (British
   Association), and the SAE Unified Thread Standard.

Metric screws

   Metric screws are specified by the ISO 261 and ISO 262 standards. The
   diameter of an ISO preferred series screw is specified in millimetres
   (mm) prefixed by the capital letter M, as in "M6" for a 6 mm diameter
   screw.

   The pitch of metric threads varies according to the diameter, but not
   absolutely regularly. Some examples: a M3 thread has a 0.5 mm pitch,
   M4: 0.7 mm, M6: 1 mm, M10-12: 1.5 mm, M14-16: 2 mm, M18-22: 2.5 mm. As
   the size increases, it is more likely that alternative pitches are in
   use; M12 is available in at least four different pitches, whereas M5 is
   only available in two. If a pitch is quoted (e.g. "M12 x 1.75") it
   usually is because it is one of the non-standard ones.

   The 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 shaft, and the nut or threaded hole is made by tapping threads in a
   5 mm hole.

   Metric screw threads are also available in "fine pitch" versions,
   sometimes several pitches for one diameter (example: M18/fine in 1, 1.5
   and 2 mm pitches). The fine thread series is deprecated and not
   recommended for use in new designs. The fine metric threads were once
   found in equipment made in the Far East, but that has changed with the
   standardisation of the ISO preferred thread series.

Non-metric screws

   Before the metric system was common, many countries and areas of
   engineering specialisation had their own standard screw sizes.

Whitworth

   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 the in 1908 as Whitworth thread was a bit
   coarse for some applications. The thread angle was 55°and a depth and
   pitch of thread that varied with the diameter of the thread (ie: the
   bigger the bolt, the coarser the thread). The spanner size is
   determined by the size of the bolt not distance between the flats. The
   most common use of a Whitworth pitch nowadays is the standard
   photographic tripod thread, which for small cameras is 1/4" Whitworth
   (20tpi) and for medium/large format cameras is 3/8" Whitworth (16tpi).

British Association screw threads (BA)

   A later standard established in the United Kingdom was the BA system,
   named after the British Association for Advancement of Science. Screws
   were described as "2BA", "4BA" etc., the odd numbers being rarely used.
   While not related to ISO metric screws, the sizes were actually defined
   in metric terms, a 0BA thread having a 1 mm pitch. These are still the
   most common threads in some niche applications. Certain types of fine
   machinery, such as moving-coil meters, tend to have BA threads wherever
   they are manufactured.

Unified Thread Standard

   The United States has its own system, usually called the Unified Thread
   Standard, which is also extensively used in Canada and in many other
   countries around the world. A version of this standard, called SAE for
   the Society of Automotive Engineers, was used in the American
   automobile industry. The SAE is still associated with inch-based
   fasteners by the public, even though the U.S. auto industry (and other
   heavy industries relying on SAE) have gradually converted to ISO
   preferred series fasteners from the 1970s onward, because global parts
   sourcing and product marketing favour international standardization.
   However, automobiles sold in the U.S. still contain some inch-based
   fasteners (for example, lug nuts).

   Machine screws are described as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32,
   8-32, 10-32, 10-24, etc. up to size 12. The first number can be
   translated to a diameter, the second is the number of threads per inch.
   There is a coarse thread and a fine thread for each size, the fine
   thread being preferred in thin materials or when its slightly greater
   strength is desired.

   The machine screw number can be converted to its equivalent inch
   diameter using the formula:

   D = ((N × 0.013) + 0.060)), where N is the machine screw number.

   For example, a #10 screw has a diameter of (10 × 0.013) + 0.060, or
   0.190".

   Formerly, the number series of machine screws included odd numbers (7,
   9, etc.) and extended up to #16 or more. Standardization efforts in the
   late 19th and the early part of the 20th century reduced the range of
   sizes considerably. Now, it is rare to see machine screws larger than
   #10 or odd number sizes other than #1 and #3, even though #5 and #12
   screws are still available.

   Sizes 1/4" diameter and larger are designated as 1/4"-20, 1/4"-28, etc.
   the first number giving the diameter in inches and the second number
   being threads per inch. Most thread sizes are available in UNC or UC
   (Unified Coarse Thread, example 1/4"-20) or UNF (example 1/4"-28 UNF or
   UF).

Other Standards

   Other thread systems include Acme thread form, BSP ( British standard
   pipe thread which exits in a taper and non taper variant; used for
   other purposes as well) and BSC (British Standard Cycle) a 26tpi thread
   form, CEI (Cycle Engineers Institute, used on bicycles in Britain and
   possibly elsewhere), British Standard Brass a fixed pitch 26tpi thread,
   NPT ( National pipe thread) and NPTF (pipe threads), and PG (German:
   "Panzer-Gewinde"), used in thin plate metal, such as for switches and
   nipples in electrical equipment housings.

History

   In antiquity, the Greek mathematician Archytas of Tarentum ( 428 – 350
   BC) was credited with the invention of the screw. By the 1st century
   BC, wooden screws were commonly used throughout the Mediterranean world
   in devices such as oil and wine presses. Metal screws used as fasteners
   did not appear in Europe until the 1400s.

   The metal screw did not become a common woodworking fastener until
   machine tools for mass producing it were developed at the end of the
   18th century. The British engineer Henry Maudslay patented a
   screw-cutting lathe in 1797; a similar device was patented by David
   Wilkinson in the United States the next year.

   Standardization of screw thread forms accelerated during WWII so that
   interchangeable parts could be produced by any of the Allied countries.

   Though not screws, perhaps also see cotter or pin bolts, and "clinch
   bolts" (now called rivets), used in ship building prior to the mid 19th
   century.

Legal issues

   The difference between a screw and a bolt may not seem something in
   which governments would be much interested. However, in the United
   States the two have different import duties. The difference between
   them is therefore of keen interest to importers and customs
   authorities.

   This was the subject of a court case Rocknel Fastener, inc v. United
   States: 34 page PDF. The position is outlined in a current US
   government document Distinguishing Bolts From Screws: 21 page PDF.

   Retrieved from " http://en.wikipedia.org/wiki/Screw"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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