The Japanese Gennou & Handle Part 2 – Ergonomics

A Kosaburo hand-forged gennou head hung on a Black Persimmon handle. This was the first professional-grade hand-forged gennou head I purchased many moons ago.

“We become what we behold. We shape our tools, and thereafter our tools shape us.” – 

Professor Marshall McLuhan

Marketing and mass-production have changed many things, but not how the human body works.

In this post we will examine some ergonomic factors of hammers Gentle Reader may find interesting, and ask some questions you may want to consider.

Ergonomic Factors

Making tools that fit the user’s body and way of working is an old idea. Here is an example.

Since the days when your humble servant was a scrawny kid with a Daisy BB gun, I have enjoyed making rifle stocks. As your humble servant matured, my efforts turned to marbled walnut stocks for bolt-action guns and curly maple for flintlock longrifles. But a custom gunstock is not just a chunk of beautiful wood, ooh no.

During my research into the art I learned how craftsmen have, for centuries, made custom shotgun and rifle stocks to fit each customer’s body. Indeed, unlike factory stocks, custom gunstocks are not straight, but are bent, twisted and offset in subtle ways to fit their user’s bodies and to provide a steadier hold, quicker target acquisition, and reduced recoil. These techniques work.

Indeed, like a bespoke suit or custom boots, there are a number of measurements that must be made of the user’s body and the rifle’s components, and numbers to crunch in advance of designing a custom rifle stock. I’m talking about a rifle made using thousands of dollars of wood and precision-machined steel, designed to fit a particular person’s body, and intended for a particular type of shooting, not a K-Mart blue-light-special killer of unsuspecting tin cans.

Through study, testing, trial and error and handwork I learned how employing these ergonomic principles could yield significant improvements in the performance of everything from reproduction flintlock longrifles to 1,000 yard target benchrest guns, and even .45 caliber bolt-action elephant rifles. When I heard that a group of specialist Japanese carpenters had, over centuries of experimentation, developed tool handle designs that applied similar principles, the pieces clicked together in my mind like a Purdy double-gun’s breech.

A hammer is not a complicated piece of precision machinery like a modern benchrest target rifle, so we tend to think of it as a stupid tool lacking finesse, but I disagree. Let us consider some of the challenges the lowly hammer is expected to meet.

The first challenge is air drag. The hammer is the most dynamic handtool a woodworker uses, moving relatively long distances at relatively high speeds. And during the swing the hammer pushes a lot of air aside creating drag and expending energy along the way. It adds up. This is just one reason why big-faced mallets are inefficient compared to a steel hammer. There are those who revel in their ignorance by disputing this fact, but to them I say: There is no medicinal cure for stupidity so I encourage them to learn some basic math. If you remember your freshman physics classes, you will recall that the formula for drag in a fluid (which includes air) is as follows:

F_{D}\,=\,{\tfrac {1}{2}}\,\rho \,v^{2}\,C_{D}\,A

where F D is the drag force, ρ is the density of the fluid, v is the speed of the object relative to the fluid, A is the cross sectional area, and C D is the drag coefficient, a dimensionless number.

The drag coefficient depends on the shape of the object and on the Reynolds number {\displaystyle Re={\frac {vD}{\nu }}},

You don’t need to input actual numbers into this formula to see that the two factors we, as consumers and craftsmen, can readily control are the area of the hammer (A) and its speed (v). The factor that we can manipulate to our benefit when designing our handle is the area (A), which includes not only the size of our hammer’s face but the width and length of its handle.

The second challenge the hammer must deal with is alignment during the swing because when using it we draw its head back beyond the range of our vision, and then, without looking back, swing it with great force to precisely hit targets as small as a nail head, while avoiding hitting our own head, ear and hand. If the hammer’s head naughtily wiggles out of proper alignment during the swing, a bent nail, headache or smashed finger may result, so we need a hammer head and handle combination that will be easy to instinctively keep in alignment during the swing without a thought.

The third challenge our hammer must overcome is the tendency of its striking face to impact the target with its center of mass misaligned with the centerline of the nail or chisel, or with the striking face canted forward, backward or to the side instead of square to the target’s centerline. Think about this next time you bend a nail or your chisel cuts in one direction when you intended it to cut in the opposite.

A person proficient in using mass-produced hammers has no choice but to make their eye and body match the hammer they are using at the moment. Of course, this can be done, but it is inefficient, uncertain, and frankly, bass-ackwards. What I am proposing in this article is that we design our hammer handles so they match our individual bodies and the work we need it to perform instead of forcing ourselves to adjust our grip and swing to fit standard one-size-fits-nobody hammers.

A lot of well-spoken, well-dressed and intelligent-looking blowhards in marketing departments give lip-service to so-called ergonomics, but here at C&S Tools ergonomics really matter for we strive to always hit the nail or chisel squarely instead of just meeting quarterly sales goals for tool-like products. Indeed, in future posts in this series we will discuss in great detail a number of ergonomic factors our Beloved Customers should include in their gennou design specific to their individual bodies and style of work, including the length of the hammer handle, twist and offset, grip location and shape, handle details to help the gennou index automatically in their hand without having to actually look it, and of course, the angle of the head.

We will both explain why and show you how to design, draft, and make a hammer handle suited to overcome these challenges while in your hand.

Three Questions

I am not fond of gaudy, decorated tools, but that does not mean my tools are plain as mud.

As you may be able to tell from the photographs of one of my favorite gennou in this article, I enjoy subtle details that give them a unique attractive appearance, especially if those details improve their performance while pleasing both my eyes and hands. I don’t know if they have shaped me, as Professor McLuhan suggests, but they certainly give me more confidence and joy in my work than a run-of-the-mill rubber-handled blister-maker ever could.

For years I have encouraged people to ask themselves three questions on the subject of hammers. So I pose them to Beloved Customers and Gentle Readers now.

First, does your hammer and its handle fit your body and style of work, or is it a “one-size-fits-nobody” product made by a conglomerate that knows everything about selling hammers but nothing about using them?

Second, is your hammer aesthetically pleasing to your eye and an extension of your hand, or is it like every other hammer that ever fell off the hardware store’s rack?

And finally, is your hammer likely to be a trusty partner in your woodworking projects, maybe even becoming an heirloom appreciated by your descendants, or will it end its days rusting away sad and lonely in a landfill?

If you answered nay to any of these questions, I promise you will find something of value in this series of articles.

In the next document in this series on designing and making gennou handles, we will examine some history and the ergonomic factors that resulted in the design that is the subject of this series. Until then, I have the honor to remain


The following link is to a folder containing pricelists and photos of most of our products. If you have questions or would like to learn more, please use the form located immediately below titled “Contact Us.”

Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google, facist facebook, thuggish Twitter, or a US Congressman’s Chinese f*k buddy and so won’t sell, share, or profitably “misplace” your information. May dogs and puppies forever hate me if I lie.

Previous Posts in The Japanese Gennou & Handle Series

6 thoughts on “The Japanese Gennou & Handle Part 2 – Ergonomics

  1. I’m looking forward to your thoughts on how to shape the curve of the handle. I have a gennou that I bought years ago that has a straight handle. I love the weight and balance and use it all the time but I dislike having to check which face to use every time I pick it up. For now I have just stuck a piece of blue tape on one side. Not acceptable.

    I’m happy to make a new handle for it but I’m awaiting guidance.


    1. Gary: You definitely qualify as a Beloved Customer, so I will send you the entire article with drawings and photos to your email account. If you follow the guidelines written therein the problem of (1) How to determine the handle’s curvature; and (2) How to make the handle so you can orient it properly in your hand (not only the correct face, but the same grip on the same location on the handle’s length) without looking at it every time or being careful how you set it down will be as clear as the sun. A lot of other questions you didn’t mention will also be answered. Please take a look at it. If you have questions after reading it, please let me know. Stan


  2. Well, all I can add is that I got the file you sent and it answered my first question and my next two questions, and two more questions I hadn’t thought of yet. But I haven’t thought of all the of the questions that I haven’t thought of.

    Many thanks, Stan.


    1. Gary: Glad it helped. There’s a lot to consider when making a lowly hammer handle isn’t there. A few details, along with the analysis and explanations, are mine, but the heart of the design is by the Anaya carpenter guild of Tokyo. Those old boys knew what they were doing.


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