According to Bryan :
Well ... for a few guesses:
1) Hex has more surface area on the flats, and is harder to round
off with a wrench.
2) Hex uses less material relative to the major diameter than
3) Hex stock stores more compactly. No waste space.
4) It takes fewer cuts to machine the flats, if not working
from stock already that shape. This saves machining time.
5) Hex is easy to grip with a three-jaw chuck, which is the best
for quick gripping of round stock. Octagonal would require a
universal 4-jaw -- which does exist, but which is uncommon, and
a slight irregularity with 4 jaws means that one will not be
I'll leave others to deal with the history. I've just listed
the advantages to hex which come to mind readily. Besides -- it is time
to go to bed. :-)
First off, ( and most obvious ) if it had anything other than 6 faces,
wouldn't make much sense at all to call it a "hex"......
Eight points tend to strip too easily...
And at some point, needing to tighten fully to 90 degres in order to be able
to reposition a wrench was apparently too much for design engineers...
I kinda like the NAS style aircraft fasteners, personally.....
A guess: bolt heads were once square, requiring 90 degrees of
rotation before a simple wrench could be repositioned to grab another
bite. Machinery got more complex, spaces got tighter and access more
limited, hence hex requiring only 60 degrees of rotation between
bites before robust ratchet wrenches were available.
It's easier to strip the corners off a hex than a square, but the hex
is usually strong enough to shear a bolt before the corners fail.
Design engineers usually do what marketing directs because marketing
usually controls engineering funding.
Who do you suppose designed those fasteners? Might it have been
design engineers allowed to design for peformance?
Here is my answer:
Hex once were square (and not called hex :-). They used to much space
(distance bolt hole - housing). The next step would have been a
pentagon. One one huge pentagon was built (some in there are nuts), but
a pentagon is not something you could grap with a wrench. So a hex, the
next step, is the closest to a circle (least assembling space) and the
nearest to a square (best contact surface for a wrench).
Square is the first logical choice, and is still in use. No doubt that
was the start.
Next logical choice is double that or octagonal. Lots of work for those
First steps a labor unions or cost analysis? Hex.
Seems possible. Just guessing.
6) Because wrenches are harder than the fasteners, it's possible to
double the number of points on the wrench - from 6, to 12. 12 is
bigger than 8, and it's probably not possible to double from 8 to 12,
given the common metalurgy.
So by chosing hex heads, it increases the number of index points on the
fastener from 8 to 12, over octagonal.
Well, an even number of sides are needed to allow for open end
wrenches leaving out 3,5,7,...
4 is the first number that works but has a fairly large diameter and
only 4 wrenching positions. Worked great for farm implements. Still
a good choice at times. Still used.
6 is next. Must have been a good compromise on diameter relative to
bolt shank, 6 wrenching positions, and works with 12pt bo and sockets
to give 12 wrenching positions.
8 would have to be closer tolerance on bolt head and wrench. Likely
would not work well with open end wrenches or polish calipers.
Pentagons are used here as the utility companies standard for
shut-offs, especially below grade. Presumably to stop the
un-authorized actuation of these valves, either on or off.
Once I made a "hex" head bolt and nut from 2 inch brass and left it on my
desk at work as a paper weight. The bolt head was not really hex but had
seven faces rather than 6 and the thread was left-handed. Visitors would
invariably pick it up and play with it while talking or waiting for me to
get off the phone. Everyone would notice the left-hand thread, but almost
no one picked up on the seven-sided head until I asked, "Do you see anything
Doesn't work, you still wind up with an unused (0.14159 * radius)
A hexagon is a natural shape, unlike squares, octagons, etc. Ever seen a
beehive? A hexagon is stronger than a square or octagon and less likely
to round off than an octagon.
-- Regards, Curly
misc.survivalism removed from distribution.
Yes it does. The segments that comprise the circumfrence of
an inscribed hexagon are chords, not arcs. Each one is
(0.14159 * radius)/6 shorter than the arc they span.
Only six points of the circle lie on the hexagon, at the six
Thanks for making me think.