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. In some variants the pin is placed slightly off-center, requiring a correspondingly shaped bit. However, the bits for many tamper-resistant screw heads are now readily available from hardware stores, tool suppliers and through the Internet. There are also many commonly used techniques to extract tamper resistant screws without the correct driver — for example, the use of an alternative driver that can achieve enough grip to turn the screw, modifying the head to accept an alternative driver, forming one's own driver by melting an object into the head to mould a driver, or simply turning the screw using a pair of locking pliers. 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[1]. An example of this would be the attachment for the wheels and/or spare tires of some types of car; one of the nuts may require a specialized socket (provided with the car) to prevent theft.
The break away bolt is a high security fastener that is extremely difficult to remove. It consists of a counter-sunk flat head screw, with a thin shaft and hex head protruding from the flat head. The hex head is used to drive the bolt into the countersunk hole, then the wrench or hammer is used to knock the shaft and hex head off of the flat head, leaving only a smooth screw head exposed. Removal is facilitated by drilling a small hole part way into the outer part of the head and using a punch and hammer at a sharp angle in a counter-clockwise direction. This type of screw is used primarily in prison door locks.
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 but 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.
Thread standards
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.
ISO metric screw thread
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 known as 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 millimeters, 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 shaft, and the nut or threaded hole is made by tapping threads in a 5 mm hole.
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 in 1908 because the 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 (i.e., the bigger the bolt, the coarser the thread). The spanner size is determined by the size of the bolt, not the distance between the flats.
The most common use of a Whitworth pitch nowadays is in all (UK) scaffolding where a 7/16" spanner size is required. A 21mm spanner is frequently used, and works for this application. 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.
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, except in equipment made prior to the 1970's 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. 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 of America has its own system, usually called the Unified Thread Standard, which is also extensively used in Canada and in most other countries around the world. At least 85% of the world's fasteners are dimensioned to Unified thread dimensions, and the biggest selection of fastener sizes and materials are found supplied in this standard.[2] 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 for some assemblies from the 1970s onward, because global parts sourcing and product marketing favor international standardization. However, all automobiles sold throughout the world contain both metric (engine assemblies) and Imperial fasteners (for example, lug nuts, oxygen sensors, internal electrical assemblies, body fasteners, lamps, steering, brake and suspension parts).
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 16. The first number can be translated to a diameter using a formula, 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 slightly greater strength is desired.
The numbering system follows a roughly logarithmic series where an increase in each screw number size approximately doubles the tensile strength of the screw and the screw number is found by , where "d" is the nominal diameter. Using this formula a #5 screw has a major diameter of .125" (1/8"), a #10 screw has a diameter of .190" (or 3/16" in practical terms), etc. The formula applies for screw thread numbers #0 and higher, but does NOT apply to smaller Unified miniature screw thread series. Typically screws smaller than size #0 are supplied in the Unified Miniature Series. The formula for number sizes smaller than size #0 is given by , with the zero size being the number of zeros after the first. So a #00 screw is .047" dia, #000 is .034" dia, etc.
The number series of machine screws once 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 less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12.
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 or UF (Unified Fine Thread, example 1/4"-28).
A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watches, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68.
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