• 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

• 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.
  • 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 "hexlobular" 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.
    • Allen head or Hexagonal (hex) socket head has a hexagonal hole and is driven by a Allen 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.
    • 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 are for instance 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 1940's and 50's, 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.

    Tamper resistant screws

    ---

    Many screw drives, including Phillips, TORX, and Hexagonal, are also manufactured in tamper-resistant form. These typically have a pin protruding in the center 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.

    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.

    • Fast and Secure: How much proof is tamperproof? Mechanical Engineering Magazine, June 2005. Publication of the American Society of Mechanical Engineers. Article on tamper-resistant fasteners.

    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).

    Screw measurements

    ---

    ''See also: Screw thread

    There are many systems for specifying the dimensions of screws, but in much of the world the ISO preferred series metric 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 engineering companies 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. This system had two thread sizes: coarse (BSW) and fine (BSF). The thread angle was 55°. 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) gradually converted to ISO preferred series fasteners from the 1970s onward, however most automobiles rely heavily on the use on the inch-based UN or SAE standards.

    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. Typically each increase in each size number will double the double the tensile strength of the fastener, and is based on the fact that size #5 is 3/16" (.187") in diameter. For larger inch-unit size sizes 1/4"-20, 1/4"-28, etc. the first number giving shaft diameter (numeric or inches) and the second number being threads per inch. Most thread sizes are available in UNC or UC (Unified Course 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 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 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 are much interested. However, in the United States they attract different import duties from each other. 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.

    See also

    ---

    • Screw (simple machine)
    • Archimedes' screw
    • Thread pitch gauge
    • Tap and die
    • Screw thread
    • Self-tapping screw
    • Screw propulsion

    References

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    Henry H. Ryffel (ed.), Machinery's Handbook 23rd Edition, Industrial Press Inc. New York, 1988 ISBN 083111200X

    External links

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    • Lara Tools screw types - Specialty tool dealer documents more than two dozen different screw heads, along with their drivers.
    • Nut and Bolt information charts - Bolt grade markings and strength chart, metric and imperial from Bolt Depot.
    • American Fastener - A series of technical documents from an industrial fastener supplier
    • Efunda and Metrication.com provide tables for metric screw dimensions. Dimensions of the pitch versions are also available on the websites.

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