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Screw Drive Systems
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”).
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SLOTED DRIVE SYSTEM
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:
- most people have a screwdriver that fits them (sort of)
- worn drivers are easily reground
- a new slot can easily be cut with a hacksaw.
Otherwise the slotted head is the worst screw drive system, and is generally obsolete. Some of its deficiencies:
- A screwdriver does not automatically line up with the slot; it is easy to get off center. The user must keep the axis of the screwdriver aligned with the axis of the screw.
- The driver can engage the head in only two possible positions, at 180° to each other.
- The sides of most slotted screwdriver bits are tapered. When the driver is turned it tends to be pushed up and out of the screw head. This is called “camout”.
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 |
3 ⁄ 32 inch |
- |
1 ⁄ 8 inch |
0.012, 0.020 |
3 ⁄ 16 inch |
0.031, 0.037 |
¼ inch |
0.030, 0.039, 0.042 |
5 ⁄ 16 inch |
0.039 |
3 ⁄ 8 inch |
0.055 |
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.
CROSS DRIVE SYSTEM
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. This 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 |
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.
| #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.
POLYGONS DRIVE SYSTEM
Used on fire hydrants and similar devices to prevent ordinary, parallel-jaw wrenches from turning the head.
Square nuts and four-sided heads are 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.
- #0,#1 fits screw gage 6
- #2 fits screw gages 8 and 10
- #3
Scrulox fits square recesses, made in four sizes:
- #0 fits screw gages 3 and 4
- #1 fits 5, 6 and 7
- #2 fits 8, 9 and 10
- #3 fits 12 and 14.
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. | |
Allen (Hex Recess)
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 diameter in inches of the bit point. 1 ⁄ 8 ″, 5 ⁄ 32 ″, 3 ⁄ 16 ″, 1 ⁄ 4 ″, 5 ⁄ 16 ″ .
BRISTOL SPLINE DRIVE
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 less-common external drivers 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.
Combination Head
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.
ANSI B18.6.1
