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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 disaster.)

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 what remained of the old Mizunos, for a post-mortem analysis and perhaps a rebuild.

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 other is.

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.

Here 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.

The 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?

The 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!

The 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.

Now 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.

The 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 pressure times area, is still the same (in our example, 200 pounds). But the pressure is less, and pressure -- stress -- is what breaks the shaft.

For 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:

Elastic modulus of steel = 200 GPa
Elastic modulus of epoxy = 3.5 GPa

Compared with the steel of the hosel, epoxy is almost a big, fluffy pillow.

Conclusion

This 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.

Last updated - Aug 19, 2015