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 Wrist: 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 bottom edge of the wrist 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 Part 13 of 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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The difference between something good and something great is attention to detail.
Charles R. Swindoll
In previous articles in this series about the Japanese hammer known as the gennou, we examined the background, history and general varieties commonly available nowadays. In this article, we will expand our analysis of the gennou to include a function not well known outside Japan. We hope our Beloved Customers and Gentle Readers find it amusing.
As mentioned in Part 4 of this series, the standard ryoguchi gennou hammer has a flat striking face on one end and a domed striking face on the opposite end. The flat face is well suited to striking chisels, driving nails and the ceremonial wacking of thumbs, while the domed striking face excels at setting nails below the surface of a wooden board, just as Western hammers are. It can also be used for a task called “kigoroshi.” Indeed, this is a technique that can be employed with any hammer having a domed face, although the domed face on many Western claw hammers may be too drastic in some cases. It is a technique worth knowing.
The term Kigoroshi (木殺し）translates to “wood killing” meaning to use a hammer to temporarily crush wood cells. It is achieved by judiciously striking the wood with the hammer or gennou’s domed face. Easy peezy.
When a piece of wood is subjected to successful kigoroshi, the wood cells are deformed reducing their internal volume, but if the pressure is later relieved and some moisture added, over time the cells of many (but not all) species of wood will swell back to near their original volume.
So how is kigoroshi used? For instance, in the case of a mortise and tenon joint, the tenon is cut oversized, and then struck with the convex face of a gennou to deform the wood cells to the point the tenon will fit into the mortise. With time, the tenon absorbs moisture from its surroundings and naturally tries to swell back close to its original size locking it tightly into the mortise. I’m sure you can see the possibilities.
Another application of kigoroshi is seen in traditional Japanese boat building where the edge joints between planks forming the hull are hammered, effectively making the planks narrower. After the planks are attached to the ship’s ribs, their crushed cells gradually swell and attempt to return to their original volume, tightly pressing the planks against each other and closing any gaps to create a waterproof joint. In this way, a joint that might otherwise loosen with time and changes in moisture content can be made to remain tight and waterproof. This boat building technique is not unique to Japan, of course.
One more example. When making a rectangular wooden cask or bathtub from hinoki-wood boards (not staves) in the Japanese style, grooves are cut in the bottom board to receive tongues from the vertical side boards. If these tongues are planed oversize and then their sides are pounded judiciously with a hammer with a slightly rounded face like that of a ryouguchi gennou to reduce their thickness to fit into the groove, when assembled and then wet with water the crushed wood cells in the tongue will rebound and will expand to close its original thickness not only locking the tongue and groove tightly together, but also creating a watertight connection. If done properly, the joint will remain tight even after all the boards are no longer wet, same as the ship’s planking mentioned above.
Many people’s understanding of kigoroshi is too shallow to use the technique effectively and consistently without some practical experience. The opinions of inexperienced people therefore should be scrupulously ignored, but the Beloved Customer of C&S Tools are expected to meet a higher standard of woodworking, so I share this advanced technique with you.
There are a few points you should be aware of before attempting kigoroshi in a professional situation, in other words, a situation where cost, schedule, or reputation are at risk.
First, please remember that if the flat face of the genno is used for kigoroshi, or the domed face is cocked so its corners dig in too far, or is used with too much force, the striking face’s perimeter edges may crush cells and sever fibers permanently so that they cannot return to anywhere near their original volume thereby defeating the purpose of kigoroshi and simply weakening the wood. That’s not good.
Second, be aware that if used in fine cabinetry and joinery work, kigoroshi can create unpredictable tolerance shifts at joints, making, for instance what should be a flush joint offset, so caution and experimentation may be necessary to avoid embarrassing snafus.
And third, kigorishi does not work well with some woods, especially hard, stiff woods, and can cause permanent damage in some cases. We will discuss this further below. But first, let’s examine the mechanics of kigoroshi.
Nuts and Bolts
Most commercial varieties of wood grow in climates with seasonal changes of winter and summer. A tree is essentially a big water pump that pulls (not pushes) water and some nutrients up from the ground through the pressure differential created by water evaporation at its leaves. The highest volume of water pumped, and cellular growth, occurs when the weather is warm, water is moving, and the sun is shining. Without water, sunlight, and functioning leaves, the pump stops. In the case of freezing weather, evergreen trees stop pumping water to prevent freezing and the resulting expansion that would destroy the tree.
During the colder months, beginning when leaves fall and the sun fades in Autumn, the pump as well as the tree’s growth slows and then stops. The pump starts up again during the spring thaw when water moves, the sun again shines, and leaves bud.
The stained cross-section of oak below is an excellent illustration of this point. The photo is bifurcated by a a nearly solid band of tight fibers bordered above and below by larger cells, some are rather large white voids. This nearly solid band of cells forms during late Autumn and early spring and is called “late wood” or “Autumn wood.” The areas of less density and larger voids is formed during warmer months of high-growth and is called “early Wood or “Spring wood.” These voids form branching and merging tubes leading from the tree’s roots to the tiny holes in the leaves where the water they carry evaporates powering the pump.
The difference in appearance between these bands of cells (aka growth rings”) can be seen on the surface of a board as its “grain.”
Every type of wood, indeed every piece of wood, is different and will react differently to kigoroshi attempts. Let’s review the physical properties of wood relevant to kigoroshi by examining a cross-section of a tree. For instance summer wood is carefully designed to transmit large amounts of water and nutrients, and so is comprised of large cells with thin walls. After the tree is felled and as the moisture content of the wood decreases, the cells shrink, the cell walls become thinner, harder, stronger and wrinkled and crinkled.
Winter wood in most commercial varieties is designed less to transmit water and nutrients and more to resist wind and winter storms. It is comprised of much smaller cells with thicker, stronger walls.
Effective kigoroshi temporarily squashes the cells of summer wood in what is called elastic deformation, meaning the deformation is temporary so that the cells rebounds to near their original volume when the moisture content is increased depending on the nature of the wood and the elapsed time.
The cell walls of winter wood, on the other hand, instead of squashing and then rebounding, are often shattered by kigoroshi in many cases and will rebound little. This is called plastic deformation.
Why does this matter? Consider a cube of quartersawn Douglas fir, a wood with very soft summer wood, and very strong winter wood. If we strike this cube perpendicular to the parallel rings, the larger, weaker cells of summer wood will squash down while the harder lines of winter wood will just be pressed closer together as the layer of summer wood squashes. An application of moisture to this block of wood will cause the summer wood to return to near its original volume and the cube of wood may retain any apparent damage.
Now what happens when we wack an identical cube in-line with the layer of harder winter wood? Some of the winter wood cells are squashed elastically and will rebound. But the rebound will be less and some of deformation will be permanent.
The oak, on the other hand is more dense and the cell walls are stiffer than a softwood like pine, so crushing the cells in kigoroshi will result in even less rebound, and may greatly weaken the wood permanently.
The point is to be aware of the nature of the wood you plan to do kigoroshi to beforehand.
Kigoroshi for Gennou Tenons, and Chisel Handles
There are those who advocate using a hammer to perform kigoroshi on the tenon of gennou handles, the idea being that an oversized tenon can then be crushed a little allowing it to fit into the eye, and that the wood will rebound later locking it into the eye tightly. This sounds like a great idea, but it has problems that stem from the fact that gennou handles are typically made of dense hardwoods like white oak, and not softwoods like cedar.
We need the extra toughness and density that hardwoods provide when making a gennou handle because tenons cut in softer woods will loosen over time. Hard woods like white oak, for instance, do not submit well to kigoroshi because the more rigid cell walls are broken in plastic deformation instead of elastic deformation and won’t rebound enough. In other words, kigoroshi on hardwoods like oak, hickory or persimmon may decrease the cellular volume, but it will also physically weaken the wood. Why would you want to do that?
Instead of kigoroshi, a better solution is to use a good dense hardwood and to precisely cut the tenon just enough oversize so that a lot of force is required to insert it fully into the eye. In this way, you will have a tight tenon without compromising it’s cellular strength, a better long-term solution and a more craftsman-like technique.
Another option especially effective when making a gennou handle in humid months is to cut the tenon oversized and shrink it by removing water from the cells using gradual heat. Placing the handle in a more-or-less sealed container with a dry heat source such as an incandescent light bulb will do the job. Silica gel desiccant is another method, but slower. I do not recommend putting the handle in an oven of any kind to accomplish this, however. You have been warned.
Still others advocate performing kigoroshi on the ends of chisel handles to make the crown (hoop) fit better. They then say one must soak the end of the handle in water to make it swell back to shape and lock the crown in place. While popular, this is poppycock which wastes your time and weakens the handle. Please do not do this with C&S Tool’s chisels.
If the handle is in fact too big to accept the crown (unlikely if you purchased the chisel with a handle and crown already attached), please shave or file the end of the handle down to a dimension where it takes a number of hard hammer blows from a steel hammer to drive the crown onto the handle. The crown will thereby automatically perform all the kigoroshi necessary. This method is more professional and will provide better service.
Kigoroshi is a useful technique in some applications and with some types of wood. You may not need it but it’s worth understanding, especially if you have a gennou.
In the next post in this series we will examine the ancient ergonomic roots of the gennou handle we advocate and the unusual Japanese carpentry guild that codified them.
If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google or incompetent facebook and so won’t sell, share, or profitably “misplace” your information. Cross my heart.
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