Sharpening Part 13 – Nitty Gritty

“The true mystery of the world is the visible, not the invisible.” 

Oscar Wilde

In this post we will dig into a few important nitty gritty points about sharpening stones everyone needs to know. Perhaps you already know all these points, but please ready your shovel because there may be at least one buried surprise.

A Flea’s-Eye View

When seen under high-magnification, the surface of a sharpening stone looks like millions of densely-packed stones embedded in a flat field. The smaller the stones, the finer the grit.

As the blade is pushed and pulled over these stones, they scratch and tear metal from the blade’s surface leaving behind scratches corresponding to the size of these small stones. This violence continues until the blade’s ura and bevel form a clean intersection of two planes.

A view of a blade sharpened with 1200 grit diamond plate showing the furrows left by individual pieces of grit

Seen under high-magnification, the cutting edge is jagged where these furrow-like scratches terminate at the cutting edge. To some degree, it may even look like a serrated sawblade. Some blades, like kitchen knives and swords, are used in a slicing motion to cut soft materials like meat and vegetables and enemy arms, and their performance benefits from a serrated cutting edge more than a highly-polished edge, and so do not need to be highly polished on fine-grit sharpening stones. 

Plane and chisel blades, however, are used to cut wood, a material typically harder than foodstuffs, in a straight-on direction, not in a slicing motion, for the most part. In this situation, a rough, serrated cutting edge is weaker than a highly polished edge because the jagged edges are projecting out into space like the teeth of a handsaw blade, and are relatively unsupported and more easily damaged than a highly-polished blade with smaller, more uniform scratches terminating more cleanly at the cutting edge. 

Therefore, in order to produce a sharp durable blade, we must make the microscopic cutting edge smoother and more uniform by using progressively finer grit stones to produce shallower and narrower scratches, and a thin, uniform cutting edge.

But how fine is fine enough? There is a curious phenomenon related to friction that is applicable to cutting edges, and is useful to understand. 

The Friction Paradox

Imagine a cube of heavy stone with its downward flat face resting on the level, flat surface of a larger slab of similar stone. Let’s say it takes some specific measure of force pushing horizontally on the stone cube to overcome the static force of friction between the two stone surfaces in order to make the cube start moving. 

If we gradually increase the degree of polish between the two contact surfaces and measure the force required to start the cube moving at each progressively higher level of polish, we will find the force decreases with each increment of increased polish, for a time. This is at least partially because the irregularities between the two surfaces (asperities) do not interlock as deeply when the surfaces become more polished. 

However, at some point, more polishing brings the surfaces of the two stones into such intimate contact that the molecular attraction between them, and therefore the force necessary to move the cube, actually increases. 

The Inflection Point

The same phenomenon occurs with tool blades. If you sharpen and polish your blades past a particular point, the friction and heat produced between blade and wood will increase, as will the energy that must be expended, while the resulting quality of the cut and durability of the cutting edge will not improve significantly. Of course, the time and money invested in stones spent sharpening past this point will be mostly wasted.

The inflection point where additional polishing yields increased friction with little improvement in cut quality will depend on your tool and the wood you are cutting, but you can gain a pretty good idea of where it is if you pay attention over time. While the sharpening stone manufacturers hate my saying it, in my well-informed opinion there is little practical gain, beyond self-satisfaction, to be had from sharpening chisels or planes past 6,000~8,000 grit, making this range of grit an inflection point in my mind. What about you?

Conclusion

I encourage you to conduct your own experiments to determine the inflection point in the case of your planes and wood you cut. Many who figure this out save themselves significant amounts of time and money sharpening over the long-term.

To those of our Gentle Readers that love sharpening more than woodworking, and enjoy putting money in the pockets of sharpening stone manufacturers more than keeping it for themselves, I apologize for pointing out the floater in the punch bowl. But you probably would have it noticed it eventually anyway, if only from the taste difference.

I will touch more on this important point in the next exciting installment in this scientificish adventure.

YMHOS

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 below.

7 thoughts on “Sharpening Part 13 – Nitty Gritty

  1. Are you referring to my astounding bravery in the face of pixie infestations or my willingness to write the obvious truth about sharpening stones? Sometimes I think the world needs a support organization for sharpening stone addicts, and perhaps jail time for pushers. It’s almost criminal.

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    1. There was a widely held belief in 19th c. in the existence of the so-called luminiferous aether (or ether), theorised to be a medium whose existence was required for the propagation of light. In one of her books, the Jungian analyst Marie-Louise von Franz recalls how a physicist presented his findings rebuking the aether hypothesis in Congress. An old man stood up and exclaimed, in a quavering voice, that if there is no ether, there is nothing else! From psychoanalytical perspective, the old man in this story subconsciously projected his idea of God into ether. To quote von Franz:

      ‘It is a question of belief, not of science, and therefore something which cannot be discussed, and people get excited and fanatical if you present them with a fact which does not fit the frame.’

      Or, to use a different allegory, it takes a whole lot of courage to cry out, ‘the King is naked!’ (especially where King is God).

      On a personal note, I finish on naturals and therefore try not to get too caught up in actual grit Nos beyond the medium stones. This is an exceptionally convenient curtain for me to hide behind since my artistic proclivities far outweigh any technical sense for these matters (haha!). I do examine scratch patterns and condition of the edge under magnification, though, which serves as a visual reference and allows me to grade stones mentally and match to different steel types.

      Thank you for thought provoking and informative blog posts.

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  2. For me it depends on the type of work I do. For paring with an usunomi I go up to 10-12k in grit. A super duper fine edge is especially beneficial when I’m cutting end grain. When I’m pounding on my atsunomis 6k is just fine. I finish my oirenomis around 8k as these serve a more all round function of some light chopping and some fine paring. The hardness of the hagane is also important here. The harder the steel, the shallower the scratch pattern becomes. A reasonably fine aoto for example will yield a different result on a 66Hrc blade than a 63Hrc blade.

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  3. And speaking of friction, remember the Stanley planes with the corrugated soles? People always assumed this would reduce friction but tests have shown this is not the case.

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