Why hosel coning is important
Dave Tutelman -- August 17, 2015
A casual round at an executive
course turned into a demonstration of why it is important to ream a
"cone" at the top of your hosel if you are building clubs with graphite
shafts. Here's why it's important. (Note: most clubheads made today are
already coned adequately. But it's worth checking; if they are not, the
results can be disastrous. And even if the head is already coned, it is
still incumbent on the clubmaker to fill the cone with epoxy to prevent
A few weeks ago, a friend and I were paired with a grandfather and his
grandson at a walk-on executive course. Tony was playing his
grandfather Ed's old Mizuno forged MX-23 clubs with Mizuno Exsar Blue
graphite shafts -- about twelve years old. Ed had his own much newer
clubs, and a properly fit set for Tony was on order. So the Mizunos for
Tony were a stopgap measure.
Tony was about 20, a strong, athletic kid still learning the game. He
had a big powerful swing, that sent the ball miles on a really nice
trajectory when he hit it on the face. As a beginner, he did not always
hit it on the face; his most frequent miss was fat. But he had lots of
clubhead speed, an aggressively steep angle of attack -- and the
frequent big divot.
On the twelfth hole, Tony's tee shot resulted in two projectiles
heading downrange. We didn't see where the ball went, but the
clubhead made it to the pond. Fortunately, we saw exactly where it went
in, and it was the work of a minute to fish it out. Unfortunately, the
scene was repeated a few holes later -- minus the pond. I heard Ed tell
Tony, "As soon as your new set arrives, we'll throw this set away." My
ears perked up. "If you're throwing them away, then perhaps..." Our
resulting agreement was when Tony's new set arrived Ed would give me
remained of the old Mizunos, for a post-mortem analysis and perhaps a
I already had a guess as to why the
clubs were breaking. I had looked at the broken shaft tips (picture at
left) and the clubheads. Every club snapped exactly (and remarkably
cleanly) at the top of the hosel. That
should not happen! Sure, the R-flex shafts were probably
too flexible for Tony. But they are flexible, not weak. They should not
break at the rate of two per round. (Ed said Tony broke two more from
the set the previous week.)
My guess was that the hosels had not been properly coned. Today, almost
every clubhead arrives with its hosel coned. But these old clubs date
back far enough that Mizuno would have had very little experience with
graphite shafts. Steel shafts are impervious to the sort of damage
caused by an unconed hosel bore. So Mizuno might not have discovered
the importance of coning at the time these clubs were manufactured.
The following week Tony had his new clubs, and I picked up four intact clubs,
four snapped shafts, and four clubheads with an inch and a quarter of shaft
still in them. Once I got the heads cleaned out, it was clear that the
hosel bores had never been coned. My initial guess had been absolutely correct.
Let's review what coning is and why it is important.
What is coning?
Here are two drawings of the top of a hosel. One is not coned, the
Without coning, there is a right angle where the flat hosel top meets
the cylindrical bore into which the shaft fits.
A coned hosel has a bevel cut where that right angle would be. It is as
if a small section of a cone had been removed from the hosel.
It is really easy to see coning in the engineering drawing above. You
have to look a little harder to see it in real life. Here are two clubs
from the Mizuno set.
The left one is a straight-bored hosel, which is how I found the heads
after I reamed and cleaned the old shafts out of them.
The one on the right looked the same, until I coned it. You can see the
difference coning makes if you look closely.
So how did I go about coning the hosel? And how would you do it?
Here is my drill press with a cone-shaped reamer mounted in it. I am
using a hosel-reaming fixture. But coning can be done just by clamping
the clubhead in a vise and mounting the cone reamer in a hand drill.
I've done it that way for years. Now, having built a fixture, I like
the additional control and smoothness. Which brings us to...
The fixture is easy to build. I have written a companion article about how
you can build your own for about $10 worth of aluminum.
Why is coning important?
Most clubmakers have long known that you have to cone the hosel for
graphite shafts. Not so essential for steel shafts. The story at the
top of this article is ample testimony to that belief. In fact, Mizuno
was a steel-shaft-only company for a long time, which is probably why
they didn't know about coning at the time these MX-23s were made.
But why is coning necessary? The usual answer I hear is, "The sharp
edge of the hosel is cutting the shaft." I believed that explanation myself for years. If you want to go on believing
that, it probably doesn't hurt; you will do the right thing, even if it
is for the wrong reason. Well, you might -- but you also might get it a
little wrong if you don't really understand. For those interested in
the right reason, here is the explanation.
|There are forces and torques acting on the clubhead that the hosel bore transmits to the shaft. They are shown in this figure:
- Black - radial force: The side wall of the hosel bore presses against the shaft. No big problem here.
- Blue - axial force:
Force along the axis of the shaft, usually trying to pull the head off
the shaft. This is handled by the shear strength of the epoxy holding
the shaft on.
- Green - axial torque:
Twisting around the axis of the shaft. As with axial force, the shear
strength of the epoxy handles this. Theoretically, a shaft that is weak
in torque might break from this, but in practice I've never seen nor
heard of this happening. Just because a shaft is torque-flexible
doesn't mean it is torque-weak. It may twist easily, but it springs
back rather than breaking.
- Red - bending torque:
Here is where it gets interesting. This is handled by a pair of forces
the hosel applies to the shaft, one at the shaft tip and the other at
the top of the hosel.
are the two forces applied by bending torque. The one at the tip of the
shaft -- which is the bottom of the hosel bore -- is not much of a
problem. The shaft deflects enough to absorb the force.
force at the top of the hosel is a different story! All the force is
concentrated in a very small area, that 90-degree corner where the top
of the hosel meets the shaft.
Force alone will not break a
shaft, at least not forces of this magnitude (as much as a few thousand
pounds). What breaks the shaft is the stress, or pressure, produced by
the force. Force is measured in pounds, stress or pressure in pounds
per square inch. How much stress is involved here?
The force at
the top of the hosel could easily be a few hundred pounds; let's use
200 lb for a sample calculation. That whole force is concentrated in a
line at the top of the hosel. The line is perhaps 0.37" long and
probably not more than .005" wide, for an area of about .002 square
inches. That gives a stress of 200 pounds divided by .002, or 100,000
psi. That's a lot of stress, right up there around the breaking stress
of carbon fiber composites. Yeah, that could snap a shaft.
|What happens if we cone the hosel?
picture shows no advantage at all! We still have a very narrow line at
the bottom of the cone where the stress is being applied. So the shaft
is still likely to break, because there is no reason for the stress to
be any less than the no-coning case.
Physics note: the force may
actually be slightly higher with coning.The forces are closer together,
by an amount equal to the depth of the cone. Thus the lever arm for
applying the torque is shorter. That means that, in order to account
for the same torque, each force must be higher by that amount.
It appears we haven't accomplished anything. But let's see what happens when we do it right!
key to effective coning is filling the coned gap with epoxy (shown in
yellow in the diagram) when you epoxy the shaft to the head. The
epoxy provides a cushion that spreads the force over a larger area.
that the cone is filled with epoxy, the shaft cannot bend over the
corner without any push-back. Immediately above the corner there is a
very thin pad of epoxy, which provides some force against the shaft. A
little above that, there is a slightly thicker pad of epoxy, which
provides a little less force. And so on.
result is that the
force is spread over a larger area. Instead of all the force being
concentrated at the corner of steel, it gradually ramps down as there
is more epoxy and less steel absorbing it. The total force, which is
times area, is still the same (in our example, 200 pounds). But the
pressure is less, and pressure -- stress -- is what breaks the shaft.
instance, if an epoxy cushion widens the line of pressure from the
.005" it was before to .02", the stress concentration drops from
100,000psi to only 25,000psi.
For those who find it hard to think of cured epoxy as a "cushion", consider:
Compared with the steel of the hosel, epoxy is almost a big, fluffy pillow.
- Elastic modulus of steel = 200 GPa
- Elastic modulus of epoxy = 3.5 GPa
ConclusionThis article should not have been necessary. I
don't know of any clubheads made today that are not coned in the
factory. But my anecdote shows there are still respectable clubs out
there that are old enough to be before the coning era. If you ever need
to reshaft an old club, check to be sure the top of the hosel is coned
-- and cone it yourself if it is not.
And one thing that is very important: whenever you epoxy a clubhead to a graphite shaft, always be sure the cone cavity is filled with epoxy. Without it, the coning is not serving its intended purpose.
updated - Aug 19, 2015