The Japanese Gennou & Handle Part 17 – The Drawing Part 6/6

Dom Campbell’s in-progress atedai, and a few of the tools he used, including a most excellent gennou with a Kosaburo head and a handle he made himself.

Used to be that bats had thick handles and a big barrel. Then they found it’s not the size of the bat that gets the home run – it’s the speed with which you can swing it.
Stan Musial

In the previous five posts in this sub-series about making a drawing for a high-performance custom gennou handle, we measured various dimensions and incorporated them into our drawing. In this post we will bring everything together, and then discuss some of the strange features of this design, including a physics equation that drives it.

During this phase of the handle-making process you will have another opportunity to express the minimalist artiste lounging stylishly within your soul, probably wearing a cravat and sipping espresso languidly. C’est Magnifique!

The Sides

Let’s start work by drawing the butt’s width, determined in the previous post, on the plan view (upper half of the drawing above) centered on the handle’s centerline.

Next, once again in the plan view (upper half of the drawing above), draw two lines from where the tenon exits the eye to the butt.

Beginning at the grip area, draw curves from these two lines to to the right and left sides of the butt in a smooth transition, gradually expanding in width. The curvature/flair you produce will depend on the size of your grip, the width of the butt, and your sense of what makes a beautiful line. Feeling artistic yet? More espresso please.

Whither the Bulge?

Gentle Readers will have noticed from the drawings and photographs seen in this series so far that, unlike commercial handles, the handle we are designing does NOT exhibit a cancerous swell just below the eye. Make no mistake, this is not an error of omission nor the would-be supermodel in me seeking skinny expression! Remember, we are making a lean, mean, racing machine, not a nail bender with a sloppy eye and mass-produced tenon that needs wedges to hold it together.

This shape, with its narrow neck, flare towards the butt, and lack of the typical bulge below the eye strikes most people as strange, so an explanation may be useful.

To begin with, Japanese gennou heads of the quality assumed in this article are not secured with wedges, but by an extremely tight fit between the wooden tenon of the handle and the surfaces inside the precision-forged eye. Indeed, the fit should be so tight that, if by accident or some terrible oversight, one attempts to drive the tenon of a handle made from a wood too weak for the job into the head’s eye, the handle and/or tenon will fracture. Your humble servant has done this several times. Such a tight fit does not occur by accident. Wedges could only weaken the tenon and handle causing early failure.

To ensure this fit is indeed tight and secure, Gentle Readers must prepare the eye if the head is of lessor quality than the Hiroki or Kosaburo heads we carry, as described in previous posts in this series. If that work was necessary, we will assume it has already been completed.

Gentle Readers must use a strong, tough wood suitable to the task. The selection of wood will be the subject of the next chapter in this series.

They will also need to create a properly sized tenon on the handle’s end. These are critical points.

Because it is a craftsman’s hammer, not a wood butcher’s maul, there are no wedges to split the handle, and therefore no need for a tumor below the eye to both reinforce the sloppily-made handle and to keep the wedge from pushing the head down the handle.

Indeed, without the cancerous bulge, if the handle loosens sometime in the future, tapping the handle further into the eye will tighten it up, something a bulge would make impossible. Best to eliminate unsightly, unnecessary bulges entirely.

Air Resistance

Your humble servant has previously suggested that the handle we are designing will be a “high-performance tool,” indeed a “racing machine.” While not in the same class as a Formula-1 race car, air resistance is definitely a factor affecting performance, one impeded by an unnecessarily large hammer face, a thick, obese handle, and a bulge below the eye as is typical for commercial handles.

Why is air resistance an issue, you say? Of all the hand tools used in woodworking, aside from the long-handled axe, the hammer is the one that moves the fastest, and since air resistance varies with the square of the object’s velocity, hammers and axes are impacted by air resistance more than any other hand tools. Remember, we are talking about energy created by your body that is wasted by pushing air around unnecessarily.

For those Gentle Readers that enjoy math, the formula for calculating air resistance includes the area of the object, a drag coefficient specific to the object’s shape, and the object’s velocity squared.

F = Force due to air resistance, or drag (N)

k = A constant that combines the effects of density, drag, and area (kg/m)

v = The velocity of the moving object (m/s)

ρ = The density of the air the object moves through (kg/m3)

C_D = The drag coefficient, includes hard-to-measure effects (unit-less)

A = The area of the object the air presses on (m2)

We can’t control air density.

The total C_D drag coefficient is a combination of the C_Dhead of the head and the C_Dhandle of the handle. We can reduce this combined Total C_D by using a more aerodynamic steel hammer head instead of a huge, silly mallet, and by reducing the area of the handle pushing the air aside during the swing. I haven’t made the calculations, but the energy squandered by the excess drag of an obese handle over thousands of swings during a day’s work is not insignificant.

Another advantage of a slender neck on a hammer handle is the surprising reduction in vibration transmitted to the user’s hand, saving wear and tear on joints. This alone makes it a worthwhile improvement in my experience.

But all is not blue bunnies and fairy farts, I fear, for there are two downsides to a skinny super-model neck on a handle. First, if you tend to miss a lot when driving nails, errant nail heads may chew up the handle in an area where there is not much material to spare. Gentle Readers would be fully justified in blaming this damage on the malfeasance of malicious pixies, or the luck of Murphy, but my advice is: don’t miss.

The second downside to a slender neck in a gennou handle is that it’s inconvenient for choking-up on. But on second thought, that’s not a disadvantage to anyone except mountain trolls and grannies, bless their fluffy-white souls. The solution? Don’t choke up on the handle; The grip is the grip.

All Choked Up

Commercial hammer handles are a one-size-fits-nobody design, intended to accommodate many grip styles, apparently by many species, along most of the handle’s length. Holding the hammer like a hungry troll tenderizing a dwarf for the stewpot (perhaps with a delicate sprinkle of sage or a more bold glob of “floater” spice), and choking up on the grip like a near-sighted grandmother is the lowest-common-denominator design standard for commercial hammers, a crude detail simply not to be borne by C&S Tool’s Beloved Customers and the exceedingly refined Gentle Readers of this blog.

“They should be grilled and *sautéed* with a sprinkle of sage”

The ultimate goal of this exercise is to produce a hammer that fits Gentle Reader’s body perfectly, not every Tom, Burt or William that staggers into The Home Despot from the Ettenmoors. It will fit your arm, and your hand, and your grip without choking-up on it.

What’s wrong with choking up on the handle? What’s that? Did I just hear you say: “If it’s good enough for Granny it’s good enough for me?” If so we may need to procure more of the salve Mifune Toshiro lamented not having.

Choking up on the handle is inefficient for two reasons. First, because it changes the balance of the hammer and your working rhythm (pendulum physics). This is bad.

Second, when you choke up on the handle, for at least a couple of strikes you lose the sense of the distance from your hand to the striking face’s center, reducing both your precision and confidence, and the energy imparted to the chisel or nail. Reestablishing the correct distance in your mind requires a glance at the hammer, an adjustment in your head, and an interruption in your hammering rhythm. All this nonsense is easily avoided by gripping the handle in the same location every time.

You have basically designed most of the grip’s details when you set the butt’s shape and dimensions, and the location of your palm’s heel, index finger, and pinkie finger. I suggest you leave well-enough alone for now, and, assuming this is your first custom handle, make it a tad oversized at first, and then whittle, shave, and sand it as you use it until it fits you perfectly.

In the next post in this series we will select a piece of wood from which to make our craftsman’s gennou handle. Soon we will be making sawdust… how exciting!

YMHOS

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