Tempo is all, perfection unattainable,“The Golf Swing” by Roy McAvoy
as at the top of the swing… …there’s a hesitation, a little nod to the gods that he is fallible. That perfection is unattainable.
The Japanese gennou is outwardly the simplest of hammers comprised of just a differentially-hardened steel head attached to a wooden handle without wedges, pins, epoxy or rubber. But as simple as it is, there are several factors that drive this tool’s performance. One critical factor is its “eye.”
The Unblinking Eye
Since ancient times, hammer handles have ended in a “tenon” designed to fit inside a rectangular through-mortise hole in the hammer’s head called the “eye,” in English, and “hitsu” in Japanese. In Western hammers and the majority of Japanese hammers the interior walls of the eye are angled so that a wedge driven into the end of the handle will splay the tenon keeping it from slipping out of the eye. This connection works as well as can be expected, if the eye is deep and tolerances are acceptable, but Japanese gennou heads are compact and their eyes are not deep.
The downside to securing the head with a wedged tenon is that the wedge will frequently cause the handle to split weakening it considerably. Also, a wedged connection seldom has uniform contact and pressure inside the eye and may therefore loosen as the wood wears from impact forces.
A tight, high-friction, uniform-pressure fit between the eye’s walls and the tenon keeps the quality Japanese gennou head in place. Straight, parallel, square, clean walls are therefore critical.
Inexpensive mass-produced gennou and hammers typically have eyes with poor tolerances hidden by the handle tenon and sometimes concealed under resin caps. The heads and handles of such hammers and gennou seldom remain securely attached if used heavily long-term. Experienced Japanese professionals, therefore, have traditionally preferred to buy just the head without a handle so they can inspect the quality of the eye before laying down any money, and then make the handle themselves. This is consistent with the frugal craftsman ideology once common throughout the civilized world wherein the craftsman would make as many of his own tools as possible, often in imitation of his master’s tools and using metal components forged by the local blacksmith, all to be completed by the conclusion of his apprenticeship and graduation to “journeyman.”
This explains the demand in Japan for high-quality relatively expensive heads such as those hand-forged by Kosaburo or Hiroki. Not only are they properly shaped and consistently heat treated, but they have precisely dimensioned eyes that will not only hold onto the tenon a long time and reduce unwanted vibrations, but will save the owner a ton of effort both truing the eye and replacing failed handles later. And since the gennou is an heirloom tool, the extra cost of such a head is not wasted.
Since so much relies on the connection between the handle’s tenon and unseeing eye, let’s examine it and correct any deficiencies revealed.
Examining the Eye
The first step in inspecting a gennou head is to check the eye because if this is wrong, then the head will not only be difficult to make a handle for, but it will be unstable during the swing and skittish on impact, intolerable failings in a tool we need to use with speed and precision unconsciously.
Just sight down the length of the head’s body with the eye in view. The eye should be of uniform width over its entire length, and the sides parallel, of course. It should also be centered in the body and not skewed. The narrow end surfaces of the eye should also be straight and square to the sides. Mass-produced gennou heads typically fail this examination to some degree, and even some expensive handmade heads will too, sorry to say. Be sure to inspect both the top and bottom surfaces of the head.
Next you need to inspect inside the eye. Use a flashlight to check the interior walls are straight, parallel, square, free of twist, and without significant bumps, bulges or gouges. You may need to make a tiny square from wood or metal to perform these checks. There are special machinists squares and depth gauges that are ideal for this purpose. An accurate vernier caliper will prove useful.
Ofttimes the eye’s walls are intentionally sloped inwards from both ends so the center of the eye is narrower than either opening. This geometry is intended to compress the tenon as it is driven through the constriction locking the tenon into the eye. The crushed tenon is then supposed to expand afterwards, essentially relying on kigoroshi to bind the tenon in the eye. This geometry does work, kinda sorta, if a wedge is driven into a sawkerf cut into the tenon. Gennou heads with this style of eye are much easier to produce and are intended for mass-production beaters. Such head/handle combinations may exhibit strange harmonic vibration, which you may or may not be able to detect, and since pressure on the wooden tenon is not uniform, swelling/shrinking of the tenon with seasonal humidity changes will always cause the head to loosen over time.
If a gennou head is secured to the handle with wedges, it may be because the eye is sloppily made, or just because that is what amateurs are accustomed to seeing, but a quality gennou head fitted to a proper handle does not need wedges to secure it no matter what Fat Max says.
Correcting the Eye
As the erstwhile golf poet Tin Cup taught the World: “Perfection is Unattainable,” so I don’t encourage anyone to go OCD over the hole in a hammer head, but if you are patient, you can use small files to true the unseeing eye to the best of your patience and ability. This may be a tedious job because only small files can be used, and that within a narrow space both difficult to see into and with little room to develop leverage.
Please be careful when filing to avoid making things worse. Remember, flat, parallel walls free of twist with clean sharp corners are the goal. Once you have trued one poor quality eye you will understand the value of a high-quality premium gennou, which has nothing to do with finish or decoration.
If your gennou head has a constricted eye, please file all four walls straight.
Whatever you do, don’t leave the walls curved outwards so that the eye is wider at any point inside than at its openings, because the handle can never be properly fitted to such an eye and will always work loose.
Make a Layout Tenon
Once the eye of your gennou is true, cut and plane a piece of softwood that perfectly slip-fits into and through the eye with 3 or 4 inches protruding out both sides. Draw lines on the stick where it projects from both ends of the eye. We will call this stick the “layout tenon.” Save it to use later in the handle-making process.
In the next post in this series on making a handle for the Japanese gennou hammer we will look at selecting a gennou head.
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