screw drive systems

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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”).

screw and screwdriver drawing

Slottedhead screws

drawing of a slotted head

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.

Tip width Tip thickness
332 inch  
18 inch 0.012, 0.020
316 inch 0.031, 0.037
¼ inch 0.030, 0.039,
0.042
516 inch 0.039 
38 inch 0.055
 
machine
screw
size
slot width,
inches
min max
0000 0.005 0.008
000 0.008 0.012
00 0.010 0.017
0 0.016 0.023
1 0.019 0.023
2 0.023 0.031
3 0.027 0.035
4 0.031 0.039
5 0.035 0.043
6 0.039 0.048
7    
8 0.045 0.054
9
10 0.050 0.060
12 0.056 0.067
¼ 0.064 0.075
5/16 0.072 0.084
3/8 0.081 0.094
7/16 0.081 0.094
½ 0.091 0.106
9/16 0.102 0.118
5/8 0.116 0.133
¾ 0.131 0.149

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.

screw and screwdriver drawing

Cross Drive Systems

drawing of frearson point

From Frearson's 1873 U.S. patent.

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.

Phillips screw heads

drawing of a Phillips head

patent drawing

From Phillips 1936 patent.

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. 

Size Fits these
wood screws
Fits these machine
 & tapping screws
#0 #0, #1 #0 and #1
#1 #2 – #4 #2, #3, #4
#2 #5 – #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

Consumers are likely to think that any screw head with a cross drive recess is a Phillips, which can lead to problems.

Frearson screw heads

drawing comparing Phillips and Frearson screwdriver points

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.

ISO metric cross drive screw head

 

Pozidrive® screw heads

drawing of Pozidriv head

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″,  516″ in some head styles
#4 #18–#24 516″ to ½″

Supadrive screw heads

drawing of Supadrive head

Supadrive drivers will turn Pozidrive heads.

screw and screwdriver drawing

Regular polygons

Triangular recesses and heads

photo of screwdriver bit with triangular tip

photo of screw with triangular recess

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.

fire hydrant

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 recesses and heads

drawing of head with square recess

Square nuts and four-sided heads are now mainly found in farm equipment and on lag screws.

Robertson head 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
00 orange #1, #2
0 yellow #3, #4
1 green #5, #6, #7
2 red #8, #10
3 black #12, #14
4 black #16 and larger

Scrulox fits square recesses, made in four sizes: double square Stanley??

 

Five-sidedscrew heads

drawing of 5-sided head

photo of 5-sided head on fire hydrant

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.

photo of wrench for 5-sided heads

Courtesy LoggerHead Tools

Hexagonal heads and recesses

drawing of hex head

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.

drawing showing head dimension to be measured

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.

Allen (hex recess)

Sizes are the flat-to-flat dimension.

Inch sizes are: .050″, 116″, 564″, 332″, 764″, 18″, 964″, 532″, 316″, 732″, 14″, 516″, 38″.

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.

screw and screwdriver drawing

Clutch head

drawing of clutch head

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″.

screw and screwdriver drawing

Bristol Spline

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

screw and screwdriver drawing

Torx®

drawing of Torx head

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.

Internal Torx

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,
millimeters
T1 0.81
T2 0.93
T3 1.10
T4 1.28
T5 1.42
T6 1.70
T7 1.99
T8 2.31
T9 2.50
T10 2.74
T15 3.27
T20 3.86
T25 4.43
T27 4.99
T30 5.52
T40 6.65
T45 7.82
T50 8.83
T55 11.22
T60 13.25
T70 15.51
T80 17.54
T90 19.92
T100 22.13

External Torx

The sizes of the external drivers, which are less common, begin with an E.

Socket Size Point to point, internally
milimeters
E4 3.8
E5 4.7
E6 5.6
E7 6.1
E8 7.4
E10 9.3
E12 11.1
E14 12.8
E16 14.7
E18 16.6
E20 18.4
E24 22.1

Tamperproof Torx

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.

screw and screwdriver drawing

Torx Plus®

patent drawing for Torx Plus

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.

driver
size
pan head flat head socket head socket button truss head fillister
inch metric inch metric inch metric inch metric inch metric inch
1IP M0.9 M0.9,
M1
M0.9
2IP M1 M1.2 M1
3IP M1.2 #0 M1.4 M1.2
5IP #0 M1.4, M1.6 #1 M1.6,
M1.8
M1.4, M1.6
6IP #1 M2 #2 M2 #0 M2 #2 #2 #1
7IP #2 #3 #1 #3 #3 #2
8IP #3 M2.5 #4 M2.5 #2, #3 M2.5 #4 M3 #4 #3
10IP #5 M3 #5,
#6
M3 #4, #5 M3 #6 M3.5 #5,
#6
M3.5 #4, #5
15IP #6 M3.5 #8 M4 #6 M3.5 #8 M4 #8 M4 #6
20IP #8 M4 #10 M5 #8 M4 #10 #10 M5 #8
25IP #10 M5 #12 #10 M4.5 M5 #12 #10
27IP #12 M6 M5 M6 ¼ #12
30IP ¼ M6 ¼ M8 ¼ M6 ¼ 5/16 M6 ¼
40IP 5/16 5/16 M7 5/16 M8 3/8 M8 5/16
45IP 3/8 M8 3/8,
7/16
5/16 M8 3/8 M10 7/16 3/8
50IP 7/16 M10 ½ M10 3/8 M10 ½ M10 7/16
55IP ½ M12 9/16,
5/8
M12 7/16, ½ M12 ½ M12,
M14
9/16,
5/8
M12 ½, 9/16
60IP ¾ 9/16 M14 5/8 M16 ¾ 5/8
70IP 5/8 M16 3/4
80IP ¾ M18
90IP M20
100IP 7/16, 1 M22, M24

Tamper-resistant Torx Plus

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).

Line head screws

drawing of ALR heads

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


Combination heads

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.

resources

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.

ANSI B18.6.1

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