The Japanese Gennou & Handle Part 2 – Ergonomics

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

Professor Marshall McLuhan
A Kosaburo hand-forged gennou head on a Black Persimmon handle

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 you 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 time I was a boy with a Daisy BB gun, I have enjoyed making beautiful rifle stocks using marbled walnut for my bolt-action guns and curly maple for flintlock longrifles. But a custom gunstock is not just a chunk of beautiful wood.

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 to provide a steadier hold, quicker target acquisition, and reduced recoil. These techniques work.

Indeed, there’s a surprising number of calculations one must crunch, measurements that must be made of the rifle’s components, and details of the user’s body that must be determined 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 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 1000 yard target bench 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 the hammer as a stupid tool lacking finesse, but I disagree. Let us consider some of the challenges the lowly hammer is expected to meet that an ergonomic design can help it overcome.

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. It adds up. This is just one reason why big-faced mallets are inefficient compared to a steel hammer. There are those who will revel in their ignorance by disputing this fact, but to them I say: There is no medicinal cure for stupidity so 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 in this equation we 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.

Second, when using our hammer we draw its head back beyond the range of our vision, and then, without looking, swing it with great force to precisely hit targets as small as a chisel handle or 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 headache or smashed finger may result, so we need a hammer head and handle combination that will be easy to keep in alignment during the swing without giving it a lot of 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 or 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 wanted it to cut in the opposite direction.

A person proficient in using mass-produced hammers must train their eye and body to match the hammer they are using at the moment. Of course, this can be done, but it is inefficient. What I am proposing instead is to design our hammer handles so they match our individual bodies and the work we need it to perform instead of being forced to adjust our grip and swing to fit standard one-size-fits-nobody design parameters.

A lot of blowhards and marketing departments give lip-service to so-called ergonomics, but not here at C&S Tools, madame. 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.

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. My gennou are tools that please 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 hammer ever could.

For years I have encouraged people to ask themselves three questions on the subject of hammers. So I pose them to you now, Gentle Reader.

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 become an heirloom appreciated by your descendants, or will it end its days 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 posts.

In the next post 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.

YMHOS

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Previous Posts in The Japanese Gennou & Handle Series

Part 1 – Introduction

Part 3 – What is a Gennou?

Part 4 – Varieties of Gennou

Part 5 – Kigoroshi