Hex key

How it works

The key is a straight hexagonal bar. Its end is inserted into a matching hexagonal recess in the screw, so that the six flat faces of the key seat against the six flat faces of the socket. CAM-TECH Industries describes this as producing the recommended tightening torque in a very small package.

The long arm of the L is the lever; TEKTON lengthens it in the larger sizes to increase leverage. Because the key works inside the fastener, it needs access to only a small area, where a conventional wrench needs room around the outside of the fastener (CAM-TECH).

The screw can then be sunk below the surface it is driven into. William G. Allen’s 1910 patent was written for “flush head” set screws whose heads are sunk even with the outer surface of the rotating part into which they are driven.

Anatomy

Short arm: The shorter leg of the L. TEKTON makes a short-arm form to work in tight spaces with greater control.
Long arm: The lever. TEKTON lengthens the long arm in the largest sizes to increase leverage.
Bend: The corner between the arms, formed from hexagonal bar.
Hex end: The straight hexagonal tip. TEKTON makes its keys at the large end of the applicable size range and chamfers the edges (except on the smallest sizes) for engagement.
Ball end: A rounded tip on one end that lets the key enter at an angle. TEKTON gives its ball end a 20 degree angle.

Variants

L-key (long arm and short arm): The standard form. Bondhus lists L-wrenches in ball-end and hex-end forms.
Ball-end key: One end rounded for angled entry; Bondhus lists stubby and double ball-end forms.
Stubby (short-arm) key: A shorter key for tight spaces; TEKTON compares its short-arm key with the long series in the ASME B18.3 length table.
T-handle and fold-up sets: Bondhus lists T-handles, loop T-handles and fold-up multi-pack hex tools.
Tamper-resistant hex key: Listed by Bondhus for security screws.
Hex bit: A hexagonal driving tip for a screwdriver handle or powered driver; see the screwdriver entry.

Specifications

Size across flats (inch series) · in: 0.050 to 3/8 (one maker’s series)TEKTON’s short-arm series runs from 0.050 in. through 3/8 in.; sizes are set by the screw socket.
Size across flats (metric series) · mm: TEKTON mentions metric sizes down to 1.3 mm; the standard size table has not been read.
Ball end angle · degrees: 20 (TEKTON)A maker’s figure.
Material and hardness · steel; HRC: 8660 steel, 54 to 58 HRC (TEKTON)A maker’s figure. No standard value has been read.

Made to

ASME B18.3
ISO 2936
DIN 911

Tips

Choose the short or long arm for the job
TEKTON says its short-arm key lets the user stay close to the fastener in tighter spaces and with greater control, and that the largest sizes are made longer to give more leverage where it matters.1
Use the ball end to enter at an angle
TEKTON states that a steep 20 degree ball angle increases the ability to reach fasteners, while keeping sufficient strength.1
A key made at the large end of the size range fits with less slop
TEKTON says it makes its hex geometry on the large end of the applicable range, which results in less slop in the fastener, and that it measures tolerance point-to-point and side-to-side.1
Seat the key fully and square
CAM-TECH explains that the six faces of the key seat against the six faces of the socket to deliver the recommended tightening torque. Allen’s 1914 advertisement made the same point from the screw side: an Allen screw has a hexagonal socket to the full depth, while a broached socket may use only part of the depth because chips accumulate.1 2
Set screws are meant to sit below the surface
The point of the hollow-head set screw is that it can be tightened into a recess without any head projecting, which is why it is used on pulleys, gears and shaft collars. Allen’s 1910 patent describes screws whose heads sink flush with the surface of the rotating part.1 2
The cup point on a set screw bites into the shaft
CAM-TECH says the cup point, a later refinement, penetrates the surface of the shaft more deeply than the original flat point, which only seated against the surface.1

History

Earliest documented

7 June 1910 (documentary, patent issued)

West Hartford, Connecticut

William G. Allen of West Hartford, Connecticut, applied on 9 January 1909 and was granted US Patent 960,244 on 7 June 1910 for a method of making flush-head set screws and machine screws by driving a blank through a die with a punch, preferably hexagonal, that leaves a socket of the same shape for a key. The patent says that hollow-head screws to receive a key had already been proposed to meet laws requiring projecting set-screw heads to be removed from rotating parts. An 1875 patent by Allan Cummings already describes a cavity in the screw head driven by a matching tool.1 2

The hex key exists because of a safety problem with the set screw. CAM-TECH Industries explains that early industrial machinery used square-headed set screws that stood out from rotating shafts and collars, where loose clothing, fingers or other objects could catch. The socket set screw emerged commercially in the United States around 1910 to 1911, and the socket let the screw sit flush or below the surface. Allen’s patent describes the same background: it says that numerous governments, state and national, had made it obligatory to eliminate the danger from projecting heads of set screws on rotating parts, either by a cover or guard or by sinking the heads flush, and that screws with hollow heads to receive a key for turning them had been proposed for the second purpose.

The tool followed the screw. A hexagon socket needs a hexagonal key, and the key needs very little room: CAM-TECH notes that a conventional wrench needs room around the outside of the fastener while a hex key needs access to only a small area. Allen’s own first product, according to Alloy Artifacts, paired the safety set screw with a hexagonal L-handle. The L shape gives a short leg for the screw and a long leg for leverage, and the modern short- and long-arm series, ball ends and T-handles are variations on the same bar.

What was new in 1910 was how to make the socket cheaply. Allen’s patent is a manufacturing patent: a blank is upset to form a head and driven through a die by a punch, preferably hexagonal, which leaves a socket of the punch’s shape. The patent says this makes screws as cheap as, in fact cheaper than, the common headed set screws. Alloy Artifacts adds that competitors soon offered broached hexagonal sockets and that Allen’s 1914 advertisement argued the cold-formed socket ran to full depth while a broached one did not. Holo-Krome’s history repeats the pattern: Making Places reports that William Purtell, a former Allen salesman, built a business in 1929 on a process for making socket screws by cold fabrication instead of machining.

Priority is not settled by the documents read here. Cummings’s 1875 patent already put a cavity in the head of a screw to receive a shaped driver, and Allen’s patent says that hollow-head screws for keys had been proposed before. CAM-TECH credits both Allen and the Standard Pressed Steel Company with developing hex socket systems at about the same time, SPS independently around 1911. The Bristol Company says it entered socket cap screw manufacture in 1913. Allen’s patent and advertisements from 1910 are the earliest documents located here for a hex-socket screw sold with a hex key.

The name followed the market. CAM-TECH states that “Allen screw” is a commercial or trade name for the hex socket drive associated with Allen Manufacturing. O*NET’s Tools Used table lists hex keys in 9 of the 24 occupations mapped to this archive’s trades.

Timeline

1875
Allan Cummings of New York is granted US Patent 161,390 for a wood screw with a cavity (in one figure a triangle) in the head, driven by a driver of matching shape, so that the driver does not slip.1
1909
William G. Allen applies for his patent on the manufacture of flush-head set screws with a hexagonal socket (9 January).1
1910
Allen’s patent 960,244 issues on 7 June. The Allen Manufacturing Company is founded in Hartford, Connecticut; its first product is a safety set screw with an internal hexagonal socket and a hexagonal L-handle to drive it. An advertisement for the screws appears in the 1910 proceedings of the International Association of Factory Inspectors.1 2
c. 1911
The Standard Pressed Steel Company develops a hexagon socket screw independently, according to the company founder’s account as reported by CAM-TECH; its screws are sold as Unbrako.1
1913
The Bristol Company becomes engaged in socket cap screw manufacturing; the company’s history says its line of hex socket set and cap screws was added in 1940.1
1914
Allen advertises its patented screws against competitors’ broached hexagonal-socket set screws (American Machinist, 31 December).1
1922
Allen sells socket sets under the Bay State name and makes the sockets by its cold-forming process (Motor Age notice, 9 March); Alloy Artifacts says the Bay State Pump Company had just changed to machined and broached sockets with hexagonal L-handles.1
1929
The Holo-Krome Screw Corporation is organized in Hartford by William A. Purtell, a salesman at Allen Manufacturing; it later develops a method of making socket screws by cold fabrication rather than machining.1 2

Where the sources disagree

Who first made the hexagon socket screw and its key?

William G. Allen, whose patent 960,244 was issued in 1910 and whose company advertised the Allen safety set screw from 1910.1 2
US Patent 960,244; Alloy Artifacts
The Standard Pressed Steel Company developed a hex socket drive in house, independently of Allen, around 1911, according to its founder Howard T. Hallowell Sr.’s account.1
CAM-TECH Industries, reporting the SPS claim
Screws with hollow heads to receive a key had already been proposed for safety before 1909.1
Allen’s own patent specification

When was Allen’s patent granted and what did it cover?

Filed 9 January 1909 and issued 7 June 1910, a method of making flush-head screws by forcing a blank through a die with a hexagonal punch.1 2
US Patent 960,244; Alloy Artifacts
A method of forming a hexagonal socket into the body of the set screw, patented in 1909.1
CAM-TECH Industries

Gaps in the record (7)

Howard T. Hallowell Sr.’s account of the Standard Pressed Steel Company is known here only through CAM-TECH’s summary; the autobiography has not been located.
The Bristol Company’s early sockets are described on a local history page that does not say what drive they used; the “Bristol wrench” and multiple-spline drive named in the research leads were not found in a source.
No source has been found for the form of Allen’s original hex key beyond Alloy Artifacts’ description of it as a hexagonal L-handle.
The ISO 2936 and ASME B18.3 size tables have not been read, so metric and inch size specs are not given.
Care and safety have no sourced entries; no manufacturer instruction for hex keys has been located.
Production of Allen-brand keys by Apex Tool Group, which reportedly ended in 2017, rests only on a secondary encyclopedia entry and is left out.
No independent test data has been found, so buying is null.