Don’t force it, get a bigger hammerArthur Bloch
The handle that is the focus of this series of posts is an interpretation of the gennou handle developed over several centuries by the anaya carpenters of Japan. In this post I would like to touch on some of their history and the ergonomic factors that drove their subtle innovations.
The word Anaya (穴屋) translates to “hole maker,” a type of carpenter that was common in Japan before the general availability of portable electrical mortisers. These craftsmen had their own guilds in major urban areas and specialized in cutting mortises in beams and columns for wooden structures. They didn’t do layout. They didn’t dimension timbers. They didn’t saw tenons. They didn’t do assembly or erection. Their only tools were the chisels and hammers they used from sunrise to sunset to cut mortises as quickly and accurately as they could.
Anaya did piecework, meaning they were paid according to the number of mortises they completed each day, not by the job or an hourly rate. Each individual Anaya was in direct competition with his fellows for speed and efficiency, so they were serious about the performance of their tools.
Consistent with the Japanese obsession with constantly making minor improvements to their tools, Anaya were forever asking blacksmiths to make them custom chisels and hammer heads reflecting their latest opinions. There are records of more than one chisel blacksmith, including the famous Chiyozuru Korehide, refusing to make chisels for Anaya because of their persistent, obsessive demands.
The gentleman that taught me how to make gennou handles 30 something years ago is now in his late 90’s. He was a young man back when the anaya trade in Tokyo was still burgeoning, and he learned from the best in the business.
Following are four ergonomic principles related to hammers in general and gennou in particular you should keep in mind when planning your handle. These principles are applicable to not just Japanese gennou, but to all varieties of hammers swung with a single hand. You need to understand them before you design your gennou handle.
- Handle Length: Every person’s combination of bones, tendons, muscles and work habits is different. Therefore one size of handle does not fit all; There is a handle length that best fits your body, the way you work, and the type of work you do.
- The Grip: For the reasons stated in No.1 above, one grip style does not fit all; There is a handle shape with dimensions that best fits your body, the way you work, and the type of work you do.
- The Knuckles: The human body operates a hammer or gennou most effectively when the plane of the head’s striking face at the instant of impact is oriented in line with the surface of the finger knuckles, particularly the pinkie finger, of the hand holding the hammer.
- Head Angle: When swinging a hammer, the hand naturally moves ahead of the hammer’s striking face. Therefore, instead of being in line with the arc of the swing, the head’s centerline will typically end up cocked out and away from the arc of the swing, assuming the handle is straight and hung (installed) with its centerline perpendicular to the head’s centerline. As a result:
- The hammer’s face is unlikely to strike the nail or chisel squarely;
- The center of mass of the head will most likely not be in alignment with the intended axis of travel of the nail or chisel on impact;
- The nail or chisel will therefore be kicked out of the desired axis of travel;
- Precision will suffer, and;
- Time and energy will be wasted.
Before you design your handle, I highly recommend you thoroughly understand these four essential principles. If you doubt their validity, investigate them yourself. Google will not suffice. There are a couple of tests described in future posts in this series you can perform to verify them. In the meantime, here is a homework assignment: Figure out a way to determine if your hammer’s face is striking the handle of your chisel squarely, or if it is cocked. Let me know your conclusions in the comments below.
The positive impact of incorporating these ergonomic principles into your handle design, as well as the negative impacts of ignoring them, can make a big difference in your performance and work efficiency. In future posts we will show you how to deal with these ergonomic factors to design and make a gennou handle perfectly suited to your body and the way you work.
But before our tumble ass-over-teakettle down this particular rabbit hole loses every semblance of dignity, in the next post in this series we need to examine a critical but oft-ignored part of any hammer : The Unblinking Eye.
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