Every threaded fastener needs a way of turning it. It may have a head with a shape that a driver can engage, as a wrench fits a hex-head bolt or a nut, or it may have a shaped hole into which a driver can be inserted (fastener engineers call the hole the “recess”).
Using a slot in a screw’s head to turn it is an old idea: drawings from the 16th century show screws with slotted heads. The advantages of the slotted head are that:
Otherwise the slotted head is the worst screw drive system, and is generally obsolescent, largely because it is utterly unsuited to automated driving. Some of its deficiencies:
To add to the shortcomings of the slotted head, screwdrivers for slotted screws are usually described by the length of the shaft and the width of the tip; the crucial measurement, the tip's thickness, is not given. Any given tip width is sold in a range of thicknesses; the longer shafts usually have the thicker blades. If a driver with the right tip thickness is not available, the tip of a driver can be ground to match the screw's slot width, which is not possible with more complicated recesses.
A table showing tip widths and slot widths for American wood screws is here.
A tamper-proof slotted head design is available. It is used in low-tech areas where vandalism and theft are feared, such as window fixtures and public toilet stalls. Opposite quarters of the head are cut away so that a flat blade driver rotating counterclockwise has nothing to push against. Special bits are sold that are capable of removing these screws.
Watches sometimes contain small screws whose heads have three parallel slots. The center slot is for driving the screw; the slots to the side are shallower and are a sign that the screw has a left-hand thread.
John Frearson, an engineer in Birmingham, England, invented a cross drive screw head, being granted a U.S. patent in 1873. His patent describes the major advantages:
It is well known to persons who use screws that if the nicks are narrow and shallow it is difficult to drive the screw without the screw driver slipping out of the nicks, and if the nicks are wide and deep to afford a good gripe, the head of the screw is weakened, and the screw-driver is liable to slip out sidewise and deface the finished surface of the work, and if the screw-driver is the same width as or wider than the head of the screw, the countersink work is liable to be defaced, and the angles of the screw-driver are often broken.
In all cross drive systems the driver will self-align with the fastener. Both the driver and fastener recess are tapered. Camout is possible and can ream the recess and destroy the bit.
The licensor is the American Screw Co. The Phillips system was invented for use in assembling aluminum aircraft, with the object of preventing assemblers from tightening screws so tightly that the aluminum threads strip. The driver will cam out before that happens. The driver has a 123° point with a blunt tip, tapered wings.
|Fits these machine
& tapping screws
|#0||#0, #1||#0 and #1|
|#1||#2 – #4||#2, #3, #4|
|#2||#5 – #9,
|#5 – #10|
#11 – #16
|#12, ¼ inch,
5⁄16 inch if roundhead
plus 5⁄16 inch flathead
|#5||—||5⁄8 inch, 3⁄4 inch|
Consumers are likely to think that any screw head with a cross drive recess is a Phillips, which can lead to problems.
A cross drive system referred to in ANSI standards as type II recess.
In the United States screws with this head were manufactured by Reed & Prince, and are sometimes referred to by that name.
In the United States, Frearson screws are mainly found as the marine bronze wood screws used in boat building.
Note the difference in points: Frearson has sharper V (75°).
Any Frearson driver fits all Frearson screws.
Identified in ANSI standards as type IA. As it doesn't cam out, great torque can be applied. Pozidriv screws can be turned by Phillips screwdrivers, but Pozidriv drivers won't turn Phillips screws.
|Size||Wood screws||Machine and tapping screws|
|#0||#0, #1||#0, #1|
|#1||#2, #3, #4||#2, #3, #4|
|#2||#5–#9||#5 – #10|
|#3||#10–#16||#12 and ¼″, 5⁄16″ in some head styles|
|#4||#18–#24||5⁄16″ to ½″|
Supadrive drivers will turn Pozidrive heads.
Screws with triangular recesses are found in some consumer appliances. Unlike a screw with a square recess, these cannot be turned with a slotted screwdriver, and so the use of these screws discourages do-it-yourself tampering. The bits are sized by the distance from a corner to the midpoint of the opposite side (the altitude of the equilateral triangle, for those who took geometry). They are available in at least 4 sizes (TA18, 0.079 inch; TA20, 0.091″; TA23, 0.106″; TA27, 0.126″) but are uncommon. Try www.mcmaster.com.
Triangular heads are used on fire hydrants and similar devices to prevent ordinary, parallel-jaw wrenches from turning the head. The example at right is from Austria.
A triangular tamperproof screw was patented, but we do not know if it was ever actually produced. The patent was assigned to Lockheed.
Square nuts and four-sided heads are now mainly found in farm equipment and on lag screws.
A square recess design was invented by P. Lymburner Robertson in 1908. Its advantages are great resistance to camout and 4 possible positions for the driver. Henry Ford used such screws in the Model A, but dropped it when Robertson refused to give him exclusive rights. Robertson also refused to license other fastener manufacturers, so the design spread very slowly. Many recreational vehicles built in the 1950s use these screws. In Canada, most wood screws have square recess heads.
|Size||Color||Fits these screws|
|1||green||#5, #6, #7|
|4||black||#16 and larger|
Scrulox fits square recesses, made in four sizes: double square Stanley??
Five-sided heads are used for caps and valves of fire hydrants, and in other situations in which a fastener that cannot be removed by commonly available wrenches (most of which have parallel jaws) is needed.
Courtesy LoggerHead Tools
Probably the most common of all fastener heads, hex heads are also very old. Fasteners with hexagonal heads were used to hold armor together in the 15th century.
To find the size of wrench needed to turn a hex head (or hex recess), measure from flat to flat, not from point to point. Socket wrench sizes for some common machine screws and bolts are given in the table.
Sizes are the flat-to-flat dimension.
Inch sizes are: .050″, 1⁄16″, 5⁄64″, 3⁄32″, 7⁄64″, 1⁄8″, 9⁄64″, 5⁄32″, 3⁄16″, 7⁄32″, 1⁄4″, 5⁄16″, 3⁄8″.
Metric sizes are 1.27 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, and 6 mm.
Originated by United Screw and Bolt. The recess in clutch heads looks like a bowtie. In a pinch, a clutch head screw can be driven by a slotted screwdriver. A worn tip on a driver can easily be restored by grinding off the end. Clutch head screws were popular in mobile home construction and electric motors. The size is the maximum diameter in inches of the bit point: 1⁄8″, 5⁄32″, 3⁄16″, 1⁄4″, 5⁄16″.
Originated by the Bristol Co. A recess with 6 flutes (except for 2 sizes that have 4 flutes). Sized in inches: .048, .060, .069 (4 flute), .072, .076 (4 flute), .096, .111, .145, .183
The Torx system was introduced in 1965 by Camcar, and patented in 1971. Its great popularity, however, really began in the 1980's, when it became very common in trucks and automobiles.
Both internal and external versions are made. The walls of the recess are not tapered, so camout is absent. Drivers greatly outlast similar hex head drivers.
The same system of sizes is used for both metric and inch fasteners. To match drivers to fastener sizes, see the table below, under Torx Plus.
Driver sizes for Torx recesses begin with a T. The dimensions below are measured on the driver bit, from point to opposite point
|Size||point to point,
The sizes of the external drivers, which are less common, begin with an E.
|Socket Size||Point to point, internally
Tamperproof Torx heads are the same as the internal recess heads, but have a post in the center which prevents ordinary Torx drivers from entering the recess. TT-7, TT-8, TT-9, TT-10, TT-15, TT-20, TT-25, TT-27, TT-30.
From U.S. patent 5207132.
The Torx Plus system was introduced by Camcar in 1991. Like Torx, it is 6-lobed and has straight walls. The improvement is in the design of the lobes, which were changed to elliptical (from circular in Torx). The result is that the force imparted by turning the driver is perpendicular to a radius of the fastener.
A Torx driver can drive or remove a Torx Plus screw, but the added benefits of the Torx Plus design are lost and wear is increased.
External Torx drivers (i.e., sockets) do not fit external Torx Plus fasteners.
|pan head||flat head||socket head||socket button||truss head||fillister|
|100IP||—||—||—||—||7/16, 1||M22, M24||—||—||—||—||—|
Tamper-resistant Torx Plus recesses differ from other Torx Plus recesses in having 5 lobes instead of 6. It also has a center post.
"Tamperproof" screws are used by manufacturers to prevent consumers from opening things they shouldn't. The usual strategy for creating such screws is to add a post in the center of the recess in the screw head, and a central hole in the driver. The post prevents the ordinary driver from entering the recess. The TT Torx screws are an example of such a system. However, today anyone can buy drivers to fit such security screws, often in the local hardware store, if not on the web. They are thus not as tamperproof as originally intended.
Torx Plus tackles the weak point in this situation by restricting the sale of the driver bits to equipment manufacturers and their authorized repair services. Those who can produce the necessary credentials can purchase tamper-resistant Torx Plus bits from www.wihatools.com. The identifier is IPR. They come in sizes IPR-8, IPR-10, IPR-15, IPR-20, IPR-25, IPR-27, IPR-30 and IPR-40 (fasteners from M2.5 to M25; #3 to 1 inch).
A Japanese system found, for example, in the IBM PS/2 computers and Nintendo games.
internal: ALR2, ALR3, ALR4, ALR5, ALR6
external: ALH2, ALH3, ALH4, ALH5, ALH6
internal tamper resistant: ALR3T, ALR4T, ALR5T, ALR6T
Line head bits can be difficult to locate. A source is www.etool.ca/RENDER/1/26/235/11267.html
Fasteners are often made with heads that combine two systems, usually so that service people in the field will be able to disassemble the product with tools in a different system from the one the factory uses. Examples include hex head cross drive, slotted internal Torx, hex head internal Torx, and so on.
Wooden Boat forums. The boat builders' discussion of the comparative merits of slotted, Robertson and Frearson screws is especially interesting.
Assembly Technology Buyer's Guide
Wheeling, IL.: Hitchcock Publishing, annual.
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Last revised: 30 December 2010.