Sharpening Part 12 – Skewampus Blades, Curved Cutting Edges, and Monkeyshines

Even monkeys fall from trees (猿も木から落ちる)

Japanese saying
A famous wood carving of 3 monkeys located at Nikko Toshogu Shrine post resconstruction that illustrates a famous saying originating in China that also works as a pun in the Japanese language. From right to left: See no evil; Speak no evil; Hear no evil (見ざる、聞かざる、言わざる).

Ideally, a tool blade will have absolutely uniform dimensions: the right thickness and taper, perfect cross-sections, uniform curvature, and straight edges and surfaces. However, professional grade Japanese tools are not made on CNC machines, but are hand forged, and have dimensional imperfections. Indeed, imperfections are part and parcel of all human endeavors. Most imperfections don’t matter; Sometimes they make the tool better; Other times they need to be remedied.

You, Gentle Reader, may not notice that the blade or cutting edge of one of your chisels or planes is “skewampus,” and consequently the cutting results are less than ideal. You may blame those poor results on your technique in using the tool or the irregular wood grain, when the real problem is the shape of the blade’s cross-section, or your unintentionally sharpening the blade with a skew. We will examine this problem in this post.

We will also look at the curved or “cambered” cutting edge profile in plane blades, the benefits and undesirable results it can produce, and how to incorporate this blade profile intelligently into your woodworking repertoire.

Many people, like monkeys in trees, learn bad habits from their friends and teachers. We hope this post will help you understand what is going on with your woodworking blades, and how to shape and sharpen them intelligently instead of just monkeying around. Please be sure to BYOB (bring your own bananas).

A serious craftsman doing Fine Woodworking in a Pixie-free workshop (notice the strategically-placed boots).

Dealing With Skewampus Blades

Skewampus is an interesting word I learned from my mother. I am told it is a combination of the word “Cattywampus” meaning “in disarray,” and “askew.” I think it is the perfect word for describing the ailments some blades suffer.

While less than ideal, it is not unusual for the thickness of a chisel blade’s cross section to vary slightly across its width, with one side being thicker than the other, forming an irregular quadrilateral cross section. This irregularity is found in plane blades too, but it is not typically a problem. Since there is more steel on the thicker side, the cutting edge will tend to develop a skew during sharpening.

Japanese plane and chisel blades are formed by laminating a layer of hard steel to a much softer body made of extremely low-carbon steel or iron. If the lamination exposed at the cutting edge is not uniform, the area of the blade with more hard steel touching the sharpening stone will abrade slower than areas with less exposed hard steel such that the cutting edge will tend to become skewed during sharpening. Perfection is not required, but the uniformity of the lamination is an important detail to observe when purchasing Japanese tools.

Likewise, Western plane and chisel blades that are not uniformly heat-treated, and that exhibit differential hardening across the bevel’s width, will also tend to become skewed during sharpening as one side of the bevel abrades quicker than the other. This problem is more common than you might imagine, especially in the case of inexpensive tools where appearance and low price are given higher priority than quality.

Anyone that has experience bidding high-dollar construction projects will understand the statement “the most profitable job may be the one you lose.” Cheap tools are much the same way: that low-cost chisel or plane may look good on paper, but if you count your time worth anything, if you dislike headaches, and real-world performance matters to your bottom line, then such a tool is often disastrous. Caveat emptor, baby.

A chisel or plane blade that has an irregular cross section or a skewed cutting edge may not be a problem for many cutting operations. However, when cutting mortises, a chisel blade with a skewed cutting edge or irregular cross section will tend to drift to the side gouging the mortise’s walls and ruining tolerances. If you find that your mortise walls are gouged, or that tolerances are poor, check your chisel blade’s shape, and correct any deformities.

Like all human work spaces, Japan’s smithies are not immune from pixie infestation despite annual blessings by Shinto priests and periodic offerings of rice, salt and wine to the spirits. In a previous post we discussed supernatural predators, so I will refer you to it for antidotes to pernicious pixie pox. But the deformities we are examining in this post are more often the natural result of the human eye misjudging hammer blows or non-judicious use of grinder wheels rather than precocious pixies at play.

If your blade’s deformity is not excessive, you can compensate by applying a little extra pressure on the blade’s thicker side while sharpening it. 

It is interesting how a little off-center pressure on a blade being sharpened over many strokes can change its shape. Many people unintentionally deform their cutting edges by not paying attention to the amount and location of the pressure their fingers apply. A word to the wise.

Another potential solution is to skew the blade in relation to the direction of travel when sharpening the bevel. This works because the leading corner of a skewed blade is abraded quicker than the trailing corner. But once again, inattention causes many people to skew their blades when moving them around on their sharpening stones unintentionally creating, instead of intentionally correcting, skewed cutting edges. There is nothing wrong with skewing the blade when sharpening so long as you are aware of the distortion this practice can produce and compensate accordingly. Another word to the wise.

If these methods don’t compensate adequately, you may want to grind and lap a chisel blade to a more uniform cross-sectional shape. A chemical bluing solution used afterwards will help conceal the shiny metal exposed by this operation if your chisel objects to the shiny spots. Some of them can be quite vain, you know.

A chisel with a an adequately uniform lamination and cross-section, and nice polish.

Cutting Edge Profiles

Many people have access to electrical jointers and planers, but relatively few have industrial equipment with the capacity to dimension wide boards such as tabletops. And of course architectural beams and columns are typically too long or too heavy to dimension with most stationary electrical equipment.

The choices available to most people for dimensioning such materials therefore are either handheld electrical power planers and/or sanders, or axes, adzes and hand planes. Powerplaners, sanders, axes and adzes are beyond the scope of this article, but we will look at hand planes.

I need more than one plane? You can’t be serious!

Although the very idea gives some woodworkers vapors (I don’t mean gas), an efficient craftsman will have multiple planes with cutting edges honed to profiles matched to specific operations.

Everyone that dimensions larger pieces of lumber by hand needs a plane with a wide mouth and a curved or “cambered,” cutting edge called a “scrub plane” in the West, and “arashiko kanna” in Japan.

This variety of plane excels at hogging a lot of wood quickly when the craftsman needs to significantly reduce the thickness of his lumber.  If the blade is narrow and curvature is deep, this plane will hog wood quickly, but leave a deeply rippled surface, often with bad tearout.

One might also have a second arashiko, or jack plane with a wider blade with a shallower curvature for the next steps in the dimensioning process. Such a plane will not hog wood as quickly, but it will produce a surface that is closer to flat and smooth and with less tearout. You can see the advantage of having two arashiko planes, or a scrub plane and a jack plane, with different cutting edge profiles when dimensioning lumber.

Many Gentle Readers use electrical-powered planes to dimension lumber before turning it into furniture, doors, chairs, or sawdust, etc. and are aware that planers always leave tiny ripple-like scallop cuts on the wood’s surface, along with some tearout. This will not do as a final surface. A hand-plane finish is far superior, but it doesn’t make sense to remove any more than the bare minimum of wood necessary to remove the washboard.

A finish plane is the perfect tool for this job on condition that it is sharp, set to a fine cut, the chipbreaker is tuned and set properly, the blade profile is appropriate for the width of the wood to be finished, and the wood does not have too many large knots. In one or two passes such a plane can easily remove the ripples and leave the wood clean and shiny without changing its dimensions much at all.

Assuming the wood is cooperative and one knows how to sharpen and setup their plane properly, blade profile frequently remains a key factor many fail to grasp. Obviously, the curved cutting edge of a scrub plane cannot produce the perfectly flat surfaces required for joining two pieces of wood together. On the other hand, the corners of a perfectly straight blade will leave clearly visible steps or unsightly tracks on the surface of a board wider than the blade, which is not a problem when rough dimensioning a board, but is painful to see if the board’s surface is to be left with just a planed finish.

So how do we solve this conundrum? When finish planing, the professional approach is to use two planes each with a different cutting edge profile. The first type of finish plane has a perfectly straight cutting edge used to plane pieces narrower than the blade’s width. Since the blade’s corners are not riding on the wood while cutting it, they won’t leave tracks and ridges.

The second type of finish plane found in the professional’s toolkit has a curved cutting edge, or more correctly, curved just at the corners to prevent it from leaving tracks and ridges when planing boards wider than the blade. Nearly all the edge is left straight, but creating this tiny amount of curvature at the right and left corners causes it to smoothly disappear into the plane’s mouth so no tracks are made and any ridges are nearly impossible to see or feel. In other words, the corners of the cutting edge never touch the surface of the board, and so don’t leave discernible tracks or ridges. The finer the cut made the smaller any ridges created will be. Indeed, where a high-quality surface is required, the final cut with the finish plane will produce shavings thin enough to see one’s fingerprints through.

You may want to reread the previous two paragraphs to make sure you understand what these two cutting edge profiles are and what they can accomplish before you read further.

Naturally, a professional doing high-quality work needs at least two finish planes, one with a straight cutting edge used to produce flat, precisely-dimensioned surfaces on wood narrower than the blade’s width, and another finish plane with a cutting edge very slightly curved at the corners used to finish wider surfaces.

There are those that advocate using a curved blade, sometimes dramatically “cambered” as some call them, for all applications. Those who teach this sloppy technique twist themselves into knots justifying tricks to approximate flat surfaces using such blades. I have no doubt this is an ancient technique, but I think it is a sad practice that sprung from the carelessness of some craftsmen in flattening their sharpening stones, and with time this bad habit became a tradition in some quarters. I strongly suspect fans of this strange way of doing business habitually sand all visible surfaces anyway so tracks and ridges are not a problem for them. But the fact remains that perfectly flat, track/ridge-free surfaces work best for joinery.

Tradition and “monkey see monkey do” are a useful place to start, but as his skill level increases, the thoughtful and efficient craftsman will eventually seek to confirm the validity of the traditions he has been taught. I urge you to get started early.

Sadly, too many people never notice the strange instruction label pasted to their boot’s sole, nor that smelly stuff sloshing around inside.(ツ)

monkey-see-monkey-do
Mommy monkey teaching baby monkey bad habits. When will they ever learn?

Conclusion

As we come to the end of this post, my advice to you, Gentle Reader, is to learn two bedrock basic skills to perfection. First, learn how to keep your sharpening stones flat; And second, learn how to sharpen your blades to have a straight cutting edge. Everything else will flow naturally from these skills. Your blades deserve it. We will talk more about these subjects in the future.

In this post, we have discussed 12 serious points about plane and chisel blades and how to use and improve them all but a few woodworkers in the West are unaware of, or ignore, but which are common knowledge among professional Japanese woodworkers in advanced trades. While condensed, it is enough information to fill a book, but we are giving it to you for the price of bananas (BYOB, remember?). We hope you picked up on each point, and test those that are new to you.

The next installment in this simian soap opera of sharpening will focus less on monkeyshines, and more on stones and techniques. Please stay tuned.

YMHOS

I can’t wait to read the next post!!

Please share your insights and comments with everyone in the comments section below. If you have questions or would like to learn more about our tools, please use the questions form located immediately below.

3 thoughts on “Sharpening Part 12 – Skewampus Blades, Curved Cutting Edges, and Monkeyshines

  1. As always, an enjoyable and amusing read, Stan. I don’t need any more reason to buy another finishing plane but i’ll sure take it!

    Quick question/thought.. If the blade is sharpened with a flat bevel and the same goes for the chipbreaker, when the chipbreaker is inserted, assuming the chipbreaker ‘bar’ is only making contact and putting pressure on the center of the chipbreaker, in theory would this put just enough pressure from the chipbreaker towards the center of the cutting edge compared to the outside of the cutting edge, in turn flexing the blade edge ever so slightly in the center compared to the edges creating a sort of ‘temporary’ camber? I hope that makes sense…

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    1. I suppose you are correct, Rhys.

      However, there are a few details we must consider.

      First, the back of the plane blade (opposite the ura) is supported by the wooden body, so the only significant deflection of the blade will occur over the distance of the bevel’s short direction (in the long direction of the blade). Therefore the lever arm is short and the bending moment acting on the blade which would cause it to deflect and create the camber you mentioned is small. Of course, that does not mean there is no deflection/camber created.

      Don’t forget that contact between the blade and dai at the blade’s back should be just a kiss and the resulting pressure very low. Remember that, unlike Western wooden-bodied planes, it is the wedging action and resulting friction of the blade in the slots at the dai’s sides, not pressure on the blade’s back, that keeps the blade in position. Too much pressure on the blade’s back is a rookie mistake that can lead to mental instability and alcoholism.

      Second, some people like the rod (osaebo) to apply a lot of pressure on the chipbreaker to make sure it stays in-place. I understand this idea. It is simple, straightforward and unsubtly “carpenterish.” (ツ)Not a bad thing, but a bit ham-handed. And it sometimes creates problems.

      Remember that it is not pressure but friction between the chipbreaker and rod, and chipbreaker and blade, that keeps the chipbreaker (uragane or uchiba) in-place. The pressure from the rod is just needed to create this friction. Remember, related to what I mentioned above, unlike Western wooden-bodied planes Japanese planes do not rely on the wedging action of the chipbreaker or another wedge to keep the blade locked into position, because the blade itself is a wedge. So keep the pressure between rod and chipbreaker a friendly hug instead of a death crush.

      It doesn’t take much friction to keep the chipbreaker in position if the chipbreaker and rod are tuned properly. Too much pressure tends to distort the dai (wooden body). Indeed, if pressure from the rod is too high the chipbreaker will be difficult to adjust. The chipbreaker should settle in place with just a couple of light hammer taps (I tap the chipbreaker into position and adjust it using the butt of a wooden mallet’s handle since I hate scoring and dinging the blade with a steel hammer). More pressure is unnecessary and may be detrimental.

      Third, the pressure from the rod should be approximately uniform across the chipbreaker’s width, so it locks the chipbreaker into place securely without a high point for it to rotate on. Remember that the rod flexes too. The plane’s owner needs to adjust and tune both chipbreaker and rod to achieve this friendly, intimate fit.

      Indeed, the very best dai don’t even have a continuous rod across the mouth opening, but just a couple of steel nubs projecting from the dai’s sides to secure the chipbreaker in-place. See the photo of the elegant dai made by Ito-san with the blade by Usui Kengo in the post on jigane below.
      https://covingtonandsons.com/2019/09/14/sharpening-part-8-soft-steel/

      This point is intimately related to the next point.

      Fourth (and last, I promise), if the itoura (land immediately at the chipbreaker’s cutting edge) is perfectly flat initially, over multiple sharpenings of the blade and wear on the chipbreaker (yes, chipbreakers do wear), a gap will sometimes develop between blade and chipbreaker that shavings will get jammed in making a terrible mess both of the plane’s mouth and the wood being planed. Can’t have that happening.

      The solution to this tendency, and to keep the blade/chipbreaker in close contact for a long time, is to have the itoura of the chipbreaker actually arched a tiny bit. And by tiny I mean too narrow for your thinnest feeler gauge to measure.

      You can detect this arch by removing the blade and chipbreaker from the dai, clamping the chipbreaker into its proper position on top of the blade with your fingers, and peering at a light source shining between the blade and chipbreaker from the direction of the blade’s head. What we want to see is solid contact at the chipbreaker’s corners, and just a tiny bit of light showing through at the center. When we press down with our fingers on the center of the chipbreaker, this tiny gap should close up nicely. Notice I said finger pressure alone.

      The result of this type of setup is that the chipbreaker is easily and quickly installed, it is easy to make fine adjustments to, it is stable, meaning it doesn’t tend to twist, bind, shift position or loosen once installed, there is no gap between blade and chipbreaker the full length of the chipbreaker’s width, and in direct answer to your question, pressure on the blade is not large, and that pressure is focused primarily at the chipbreaker’s corners, not the center of the chipbreaker/blade assembly.

      I cannot say the blade does not deflect/camber somewhat, but I have never had it become a problem.

      Does this make sense?

      If this was reply was part of a course in Japanese Planes, it would be designated around 306. Definitely not first year stuff, so I don’t expect readers that have not put a lot hours into fettling and using Japanese hiraganna to follow most of this.

      I intend to jump headfirst down the rabbit hole that is Japanese planes in future posts. Probably wearing HALO gear and screaming “KAWABUNGA!!!” into my oxygen mask as I bounce off the hole’s walls. There may be injuries, but it will be about as exciting as planes can get.

      Regards,

      Stan

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