Frearson (Reed & Prince)
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 screws 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 obsolete. 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.
|Tip width||Tip thickness|
|1⁄8 inch||0.012, 0.020|
|3⁄16 inch||0.031, 0.037|
|¼ inch||0.030, 0.039, 0.042|
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.
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. Consumers are likely to think that any screw head with a cross drive recess is a Phillips, which can lead to problems.
|Size||Fits these wood screws||Fits these machine & tapping screws|
|#0||#0, #1||#0 and #1|
|#1||#2 #4||#2, #3, #4|
|#2||#5, #6, #7, #8, #9, some #10||#5 #10|
|#3||some #10, #11 #16||#12, ¼ inch, 5⁄16 inch if roundhead|
|#4||#18, #20, #24||3⁄8 inch, 9⁄16 inch, plus 5⁄16 inch flathead|
|#5||||5⁄8 inch, 3⁄4 inch|
A later cross drive system referred to in ANSI standards as type II recess. Originated by Reed & Prince, it is sometimes referred to by that name. Now found mostly on marine hardware. 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|
|#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 http://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.
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. L. 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 to its use. 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.
Scrulox fits square recesses, made in four sizes:
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.
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 fifteenth 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. 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. This style of head is now very common in trucks and automobiles. The walls of the recess are not tapered. Drivers greatly outlast similar hex head drivers. Torx driver sizes for Torx recesses begin with a T. The sizes of the external drivers, which are less-common, begin with an E. T-6, T-7, T-8, T-9, T-10, T-15, T-20, T-25, T-27, T-30, T-40, T-45, T-50
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
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/1/26/170/4301.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.
Assembly Technology Buyer's Guide
Wheeling, IL.: Hitchcock Publishing, annual.
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Last revised: 6 September 2008.