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The modern Kanna is made of essentially only 4 parts, the dai <dye> or body of the plane, most often made of shiragashi (Japanese white oak), the blade made of very hard steel laminated to a softer iron or steel backing, the osae-gane <oh-sah-ii ga-neh>(lit; control iron ~ chip breaker) and osae-bou chip breaker pin. While the number of parts is already low, the chip breaker and it's pin can be considered superfluous in a well tuned kanna reducing the number of parts to a scant two, blade and body. It has been said that "a plane is only a blade holder", and in the case of a kanna, it would appear that the saying has significant merit.
However simple a kanna may appear to be, it is very much a study of subtlety. The devil, they say, is in the details.
Starting with the wooden dai, chosen to be economical of material, economical of effort, easy to adjust as needed and replaced without emotion when required.
The blade, thick and solid to add mass to the blade and prevent any chance of chatter, laminated with a small section of hard steel and main section of soft iron to allow easy sharpening and hollowed on the back side so that little of that hard steel needs to be abraded when sharpening and also allow the shaving to pass by without undue hindrance.
The chip breaker, again elemental often being little more than a bent piece of steel or iron, also laminated in the case of better kanna, there are no frills.
The pin, no more than a steel rod, precisely positioned in the body.
The dai ~ Plane body.
The kanna-dai literally translates as 'plane base', and the word dai is also used to describe all manner of mountings and bases, including sharpening stone bases. The dai-atama, body-head is named so as kanna are pulled toward the user, not pushed. When looking at the plane in use, the orientation is such that the trailing end is 'the head' end. The shikomi-mizo, blade groove, is tapered in two directions, narrowing as it goes deeper into the body. This taper is matched by the blade, and tightens the grip on the blade, front to back, the further it is driven into the body. This groove must be matched to the blade, too large and the blade will be loose, too tight and the blade cannot be adjusted.
Some kanna have a small lip where the back side of the blade resides in the body called tsutsumi, and while it is often claimed that only the highest quality dai have this feature, presence or lack of it is no measure of quality, only that this area of the dai was created by human hand, as a machine cannot reproduce it. With the regular adjustment of the dai as is recommended, this feature is soon lacking on all kanna.
The kokera-kaheshi, mouth opening, is angled in the same direction as the blade rests, done so to minimize the opening becoming too large, too quickly with frequent sole adjustment.
The shita-ba, plane bottom, is not flat as might be found on other planes. The reasons for this is to reduce the effort required to pull the plane on the wood being planed and to allow the critical areas at the very front of the plane (in it's direction of travel) and the area just ahead of the mouth to be easily and quickly adjusted so they are 'in plane' with each other. Aside from these two critical areas, most kanna have the remaining area of their bottoms 'relieved' so they are not touching the wood being planed at all. There are exceptions to this rule of course, but the common dimensioning or smoothing plane only touches the wood being planed in these two relatively small areas.
The general shape of the dai is no more than a oblong block. Often criticized for it's lack of handles, a well adjusted kanna requires so little effort to pull that the added complexity and trouble of fitting handles or knobs is effort poorly spent.
(Of course, you may fit handles or adjust your dai to your own individual taste.)
The kanna-ba ~ plane blade.
The blade is comprised of a very thin layer of hard, high carbon (alloy) steel forge welded (or bonded in the case of impossible to weld tool steels such as High Speed Steel) to a large section of soft iron or soft, low carbon steel.
Kamaji, wrought iron, is a desirable material for this task as it is easy to work with when heated, easy to grind, inherently tough and in most cases, possesses a subtle attractiveness due it's method of manufacture and working, being repeatedly heated, hammered and shaped. Wrought iron is comprised of elemental iron, significant amounts of slag inclusions and very low carbon content. The low carbon content renders it impossible to be hardened, making it always malleable and soft. The slag inclusions give wrought iron a 'grain' similar to wood, and also allow the material to be easily ground away when sharpening. The best kanna-ba utilize wrought iron for the blade's bulk of mass, however suitable material is no longer manufactured. Supplies today are recovered from all manner of old-world iron items such as railway track, ship's anchor chain, boiler plate and structural sections from bridges and buildings. It also needs to be worked into a suitable shape for making a blade, causing plane blades that are made with it to be more expensive than they might otherwise be in both effort and monetary cost.
Low carbon iron is used for lower cost planes. It is readily acquired as it is still being made today, can be made to a specific shape and is quite easy to work with under hammer and grind stone. It does not share the attractiveness, strength or easy grinding of wrought iron, but can still produce a very serviceable blade.
The kanna-ba has many particular features on it, and this is the only area where the plane maker can execute any individualism or flair. When the blade is in the body, we see the ura, the back, of the blade. The lower area is where the hard steel is located. This area is hollowed and usually bright and clean. The upper area is where the maker will stamp or engrave the mei, which literally translates as "epitah, inscription, motto or signature". Here you will usually find the name of the plane as well as who made it. Some blades have a simple 'brand name' on them, others may have colourful names. More still may include a description of the blade in abbreviated form. Regardless of what is inscribed, this is the area where a kanna may be identified from. This is area is also where any decoration will be found. This may be a repeating pattern, a special treatment or in some cases the blade will be etched to show a curious wood grain like pattern known as mokume, lit.; wood look.
As with the dai, the top of the blade is the atama or head and again, is a place for the blade maker to exercise some artistic flair.
The measurement of kanna width is performed at the point where the upper part of the blade exits the body, which is also where the hard steel in the blade ends called the kae-saki. Whatever that measurement may be, the actual cutting width is less due to the taper of the blade across the width.
You may notice a curious word there, kou meaning 'shell'. It does not mean that there is a shell in the blade, only that the shape of the blade represents a kind of shell. Contrary to it's appearance, there are very few straight lines in a kanna-ba, and most prominent is the slight curve across the width of the blade matched by the blade bed as well as the distinct hollow on the blade's back.
The very slight (and occasionally non-existent) width wise curve only really warrants acknowledgement here than an explanation. Regardless, the curve of the blade must be matched by the dai in the blade's bedding area as closely as possible to ensure good, vibration absorbing contact between the blade and body.
What is important (and requires more than a tip of the hat to) is the significant hollow in the blade back, called the ura, lit; back. It has been often repeated that this hollow, which is also present in Japanese chisels, is primarily there to allow the very hard steel to be sharpened easily, and this is quite true. The minimalist area of the back means that you only need pay attention to a very small sliver of steel in order to make the blade's back sharp and 'flat'. But there is more to it than that, and in the modern plane with it's chip breaker this secondary point of value is often overlooked, or at least reduced to non-effectiveness.
Originally, as has been mentioned, kanna only had a body and a blade, and in this configuration the blade made contact with the shaving for only a very small distance. Once the shaving reached the hollow, it was floating in open space and not requiring any more muscular effort of the craftsperson other than to actually create the shaving. This very small reduction in drag is another part of the great efforts made to reduce the pull of the kanna. Regrettably, the addition of the chip breaker nullifies this effect, and actually creates more drag than there might otherwise be, but the chip breaker can make the plane easier to use and solves some planing problems. You win some, you lose some.
The ura itself may take on 2 distinct shapes. That of a bowl shape, with a thin, flat area running down either edge to a flat at the very tip of the blade, the ha-saki or more of a very shallow valley with only a flat area at the blade tip.
The 'bowl' shape on the blade's back is very much traditional, and is still the most common shape. Great care should be taken to ensure that the side ashi or legs are as thin as possible, and that the flat at the tip is also kept only large enough to produce an edge. With use, there will be a natural tendency for these areas to grow as you sharpen, so periodically the bevel side of the blade may require 'tapping out' where the soft iron or steel is hit with a hammer to produce a slight bend at the tip of the blade toward the hard steel side. This is not an easy skill to master, and mistakes such as errant blows can be costly.
The 'valley' style, known as uradashi-fuyoh (back-hollow-not-needed) is a recent development, and championed by Tsunesaburo as part of their commitment to making their tools easy to use. This blade type with it's thin 'back' present only at the very tip requires no maintenance, as the small area is constantly renewed at each sharpening.
The key idea to remember here is that the heart of a kanna is the blade. The dai is mostly a refined device to holding the blade in the proper orientation to the work surface, but the blade is where it all happens, where the kanna is identified and where most of the effort is focused when a kanna is made.
The osae-gani ~ control iron/chip breaker.
This small, blade looking feature is a recent addition to kanna having only really appeared in the aftermath of World War II. With many of the highly skilled shokunin (tradesmen, i.e; carpenters, cabinetmakers, etc.) having been displaced, permanently maimed or killed by the war, the only workers that were left to "rebuild the country" were usually of moderate skill, and unable to extract the best from their tools. Also, with the large volumes of wooden material required for rebuilding, the quality of lumber was lower and more difficult to plane properly as well. The highly tuned kanna in the hands of a highly skilled shokunin could easily deal with the situation, but as they were few and far between, an idea was borrowed from the mainland (Korea and China) as well as the West, that being a chip breaker, or more correctly in the case of kanna, a osae-gani/control iron.
When planing, the blade cuts the wood just below the exposed surface, removing a small layer of wood that exits the plane as a thin shaving. When the wood is straight grained and planed correctly, there is no trouble leaving a smooth, glossy surface on the wood. When the wood's grain is not so straight, then there is the potential for 'tear out' where the shaving begins to separate from the bulk of the material before the blade arrives to cut it. The end result is not a cut, smooth surface but a torn, uneven surface. If the blade is not sharp, similar effects can be seen as the blade is not cutting but 'levering out' the wood. This problem is made more likely by the typically low cutting angle of kanna.
The control iron serves to break up these 'chips' so that the leverage is reduced to the smallest amount, and the resulting, damaging chips are as small as possible. Perhaps the resulting surface is not so smooth, but generally acceptable and is more easily mastered by lesser skilled shokunin.
The osae-gani is little more than a piece of steel that contacts the blade's back very close to the cutting edge, held in place by a wedging action as a function of it's shape. It appears to be a smaller blade, and is often called a 'sub blade', and can be constructed in a similar manner of a thin layer of hard steel forge welded to a softer iron body, or may be a single, homogeneous piece of softer steel.
The ha-saki (tip) is prepared in a similar fashion to the blade, having it's bevel and back honed to a sharp edge. The small hollow in the back allows easy preparation of this 'edge' as it does in the main blade. The mimi (ears) give the control iron a wedge like profile, and this is how it is held in place, being wedged between the blade on one side and the osae-bou (pin) on the other. Adjustment of pressure and where on the main blade's back it rests is fashioned by adjustment of these mimi by adding or reducing their protrusion by hammer blows, stoning or filing.
While this osae-gani is a very common feature in most modern kanna, it is not required to be in place for the plane to be used. As part of this fact, it is not a wedge for holding the blade in place as it might superficially appear to be, but only an aid when planing is difficult and some assistance in reducing tear out is required.
The osae-bou (chip breaker/control iron pin).
This is little more than a steel cross pin used with the control iron to act as the fulcrum for the wedging action it requires to function correctly. Precisely positioned, and small in dimension, it has no other purpose than to allow the control iron to be used.
The kanna as a plane, while appearing to be very simple and uncomplicated is in essence, a highly evolved tool designed to offer the maximum performance in every area, and also to allow that performance to be accessed easily. In use, every effort is made to allow the kanna to plane effectively with it's heavy blade that resists chattering, solid contact with he vibration absorbent wood dai, a low cutting angle and easy passage of the shaving past the blade and out of the plane itself, reducing the effort required to pull it across the wood. This planning effort is further reduced by the minimalist contact area with the work and even to the design of the blade's back. The wood body allows easy adjustment with common woodworking tools for both adjustment of the blade's receptacle, adjustment of the sole and the areas where the shaving passes through.
The apparently simple kanna is more complex than a simple glance might allow, and every part of it is designed to do one thing.
And at that single task, it's ultimate performance is without peer.
I hope that this rather lengthy essay allows a better understanding of kanna, and the next time you encounter one, you might find that they are not at all intimidating.
After all, hundreds of years of evolution and refinement has been put into them for you to benefit from.