The Japanese Gennou & Handle Part 14 – The Drawing: Part 3/6

Top: A 100monme (375gm) gennou head by Hiroki mated to a new Osage Orange handle. Bottom: A 60monme (225gm) Kosaburo classic ryouguchi head joined to a mellowed Osage Orange handle. The difference in the size of the eyes and weights of the heads combine to make handles of different dimensions. Therefore the design begins with the head.

“This life’s hard, but it’s harder if you’re stupid.” 

George V. Higgins, The Friends of Eddie Coyle

In the previous post we started the drawing of our gennou handle using the gennou head as a template. In this post we will determine critical design details including the overall length, butt dimensions, and typical cross sections.

But first, since we are talking about making a tool we must grip in the hand, let’s consider how to hold it. You would be surprised at how many people get this simple action wrong. It matters in the context of this article because we are designing a minimalist tool for which the smallest details matter.

The Grip is the Grip

There are many ways to hold a gennou handle. The hobbit-basher grip in the fist is popular among many hammer wigglers, even those with five fingers, but it provides both low power and poor control, good for Bagginses but not so much for woodworkers.

The most efficient grip is to orient the handle diagonally across the open palm, touching the heel of the palm at one end and the pad at the base of the index finger at the other. The index finger and other fingers then wrap around and under the handle on one side, with the thumb gripping the opposite side of the handle so its is pinched between the index finger and thumb. This is similar to the grip recommended by the famous golfer Ben Hogan.

This method of holding the handle provides significant advantages:

  1. The wrist is able to rotate more freely and to a greater degree;
  2. The length of the hand touching and motivating the handle is much longer than the Hobbit-basher fist grip providing more leverage (couple) with less effort and more control;
  3. The touchpoints at the thumb, index finger, and heel of the palm triangulate the handle in the hand and mind so the user always knows exactly where the gennou’s face is located and its angle with respect to the target without looking at the gennou or fiddling with it.
  4. The grip on the handle is more secure so the handle wiggles around less in the hand reducing blisters and stresses.

If you find this grip difficult, please stick with it until it becomes a good habit that displaces a bad habit.

Determine Overall Length

The next thing we need to add to our drawing begun in the previous post is the gennou’s overall length (OAL) measured from the handle’s butt (not including the dome) to the Vertical Centerline through the head’s face. This dimension comes from your body with some adjustment for how you grip and swing.

For most people, a good starting length can be measured by bending your hammer arm 90° at the elbow and laying a straightedge or folding ruler across your upward-facing palm and forearm. Touch one end of the straightedge against the tendon at your elbow and measure the distance to the base of your middle finger. In my case this 12 inches, but I’m not a big guy.

A classic Kosaburo gennou head mounted to a 12″ osage orange handle.

You may want to add an extra inch to your first handle. If you later decide the handle is too long, you can whittle it shorter.  Remember, the first handle you make will be experimental, and probably not a keeper. I suggest you plan on making at least three over a period of a few years and after sufficient testing to determine the ideal length for you.

As seen in the photo above, with the head held in a flat palm (not a fist), and the fingers wrapped over the head, the butt just touches my bicep tendon. In my case, by total coincidence, this is 12″. Add ¼” to the length for the domed butt; you will need it. At 5’8″ I am not a big man and have neither long arms nor big hands. Your handle may well be different.

Add this OAL dimension to your drawing.

Measure the Butt

So far we have focused on the head and overall length. The next details we need to determine are the dimensions of the butt, most importantly, its height.

The height of the butt is important not only because it determines how large the grip area’s circumference will be, but also because it partially controls the angle of the grip, and therefore the angle of the head and striking face at the time of impact.

Only you can decide what dimensions will work best for you, but since we need to start somewhere, I suggest you make the handle’s butt at least l” (25mm) wide (seen from above), and 1-⅜” (35mm) high. If you have large hands or prefer a large grip, or are using a heavier head, then the flat area should be wider to reduce pressure on the hand.

Reaction Forces

Next let’s consider the forces acting on the handle and the user’s hand, and their influence on the hammer’s performance and user’s comfort and endurance.

Commercial handles typically have a symmetrical oval cross-section that looks good, is easy to manufacture, and is consistent with the one-size-fits-nobody philosophy convenient for mass-production and mass-marketing. Sadly, this oval cross-section ignores the three reaction forces that act on the user’s hand. 

The first reaction force occurs when the user brings the hammer up in preparation for a strike. In this case, a force couple (matched forces) presses the handle into the first joints of the fingers and the heel of the hand (assuming, of course, one is gripping the handle as recommended above and not like a demented mountain troll).

The second reaction occurs when the hammer reaches the top of its swing and the user changes the direction of the hammer head towards nail or chisel. A force couple presses the handle against the first joint at the base of the user’s index finger, and the tips of the other fingers. The pinkie finger has a major role in retaining the user’s hold on the tool, you will notice.

The third reaction occurs when the speeding hammer head impacts nail or chisel and the handle kicks back at the user’s palm and fingers, just as Sir Isaac Newton predicted.  

If these reactions are balanced to the object being struck/cut by the weight and speed of the hammer, this cycle can be smooth and efficient and the kickback forces minimal. But if the hammer is too heavy, or too light, the forces the user must apply to the hammer and the force and pressure of the resulting reactions can be become tiring and even bruising. A good handle made to fit the head and the user’s body and hammering technique will improve efficiency and reduce wear and tear on the user’s body.

What is this wear and tear you say? All the aforementioned reaction forces cause the handle to kick the muscles, tendons and bones in the user’s hand, and can cause bruising. Even if it doesn’t bruise, after a long day of chiseling the hand wielding the hammer will be more worn and tender than when the day started.

Even if you never grow tired and pain means nothing to you, Oh Master Blaster, these reaction forces tend to push the handle out of alignment in the user’s hand shifting the next hammer stroke slightly off-line, and twisting the hammer’s face slightly out of ideal alignment. Have you ever noticed how the hammer becomes wobbly after a couple of blows in quick succession? Guess why that happened.

Back-edge/Top Cross-section: Flat

In the case of this Yamakichi gennou by Hiroki, the striking face is is towards the right side of the photo. Please notice the shape of the handle with respect to the striking face.

The solution to these troublesome physics problems is simply to make the top/back edge of the hammer in the grip area more-or-less flat instead of round or oval, a modification that will spread the reaction forces more evenly across your hand, preventing bruising and reducing fatigue. It will also stabilize the handle in your hand improving unconscious alignment and reducing twisting.

You have probably never seen a handle shaped like this much less used one. It may look uncomfortable in your mind’s eye or on paper, but it is comfortable in-use. Try it before you dismiss it.

Another big advantage of the flat on a hammer handle is one you have probably never thought about, but the lack of which has wasted much of your time while using hammers. When doing woodworking especially, we tend to set our hammers down while we change position, move the workpiece, or remove chips. Often, we don’t recall the angle or direction we set the hammer down, so when we pick it back up we must double check two things by eye: First, where our hand is located on the grip, which distance of course determines how far the center of the striking face is from our hand; and second which face/ claw of the hammer is facing which direction.

I don’t know about you, but I want my hand to learn this information and get the hammer oriented properly without my having to take my eyes or attention away from the work at hand. Anything else is stupid.

The flat immediately tells your hand which direction the striking face is oriented, and where your hand is located on the handle, all without using your eyes or giving the matter a single thought. It’s a psychic handle, sorta (ツ)

Front-edge Cross-sectional Shape

While the top/back of the handle is more or less flat, as seen in the photo above, the lower/leading edge (surface facing the chisel or nail) should be circular at the butt, gradually transitioning to a flat surface as it nears the eye. Some people like this surface to be egg-shaped. I prefer a simple circular radius. For your first gennou handle, I recommend you begin by making it a simple circular radius and shave and whittle it to fit your hand more precisely later.

The reasons for making this surface rounded are simply to maximize the surface area of contact between hand and handle, and to make the grip comfortable.

Put all this into a simple sketch and transfer it to the drawing. At this point the end view of the butt should be shaped like a pregnant letter  “D” with the curved surface oriented downwards towards the bottom of the page of the profile drawing.

Side Surface Cross Section: Flat and Parallel

So far, we have determined the back/top edge will be mostly-sorta-kinda flat, and the front edge will be more-or-less rounded. Next, let’s examine the two side surfaces.

Looking at the butt in cross-section, the handle’s right and left sides should be perpendicular to the flat on the handle’s back/top edge (plan view). These curved-plane surfaces begin at the head’s eyes, and curve down the handle all the way to the butt with some radiusing and softening of course.

Please note that, unlike commercial wooden hammer and axe handles that need a goofy flair below the eye to keep steel wedges from splitting the poorly-made handle, this minimalist gennou handle does not. It is an elegant, handmade craftsman’s tool, not a nail-bender or cockroach killer, and simply doesn’t need wedges.

Instead of a cancerous bulge, the handle remains a few shavings thicker than the eye for some distance until it smoothly flairs into the grip area. This design has serious advantages we have discussed in earlier posts in another series about what hammers to use with chisels, which I will briefly summarize.

  1. A minimalist, thin handle has less air resistance wasting less energy, a serious concern in a tool moved as quickly as a gennou;
  2. A thin handle transits fewer shocks and vibrations to the user’s hand;
  3. A thin handle is easier to pinch between index finger and thumb providing maximum control in the Ben Hogan golf club grip;
  4. If and when the handles loosens (it’s only wood after all, not eternal Chinese-made plastic-covered bent sheet metal), the user can tap the handle further into the eye quickly tightening the handle without pausing work. Note that this is not necessarily the ideal permanent fix, although it certainly can be.

Some will look at the thin handle and fear it will break. This is always a possibility with any wooden handle, but your humble servant has never broken a properly-made gennou handle, although I have broken plenty of commercial hammer handles. The key is to make your handle from the right wood, with proper grain orientation and minimal runout (we’ll talk more about these details in future posts), and to always use the head, not the handle, to drive nails and stuff. Duh.

We will talk about these specifications and details in future posts in this story of crime and passion.

In the next post we will determine the angle between the head and the handle. If you think this angle is unimportant then you’re in for a big surprise.

YMHOS

PS: We will provide a comprehensive document covering all the steps of making a gennou handle to our Beloved Customers (may the hair on their toes never fall out) upon request.

A confused, lost Hobbit basher. And just look at that dried-out skin! Won’t you please help him find his way back to a dark, damp forum somewhere? A stop-off for a pedicure and a moisturizing treatment would not go amiss.

If you have questions or would like to learn more about our tools, please use the form located immediately below titled “Contact Us.” Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google, incompetent facebook, thuggish Twitter, or an IT manager for the US Congress and so won’t sell, share, or profitably “misplace” your information. May I swallow a thousand needles if I lie.

Previous Posts in The Japanese Gennou & Handle Series

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