Sharpening Part 13 – Nitty Gritty – Covington & Sons Tools

“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 Beloved Customer already knows all these points, but please ready your shovel because there may be at least one buried surprise.

A Wood Shavings-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 needs to continue 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, mostly in a straight-on approach, not in a slicing motion. 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, polished 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 top stone cube to overcome the static force of friction between the two stone surfaces in order to get the cube moving.

If we gradually increase the degree of polish between the two contact faces and measure the force required to start the top cube moving at each progressively higher level of polish, we will find the force decreases with each increment of increased polish, at least 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 same phenomenon occurs with tool blades. If you sharpen and polish your blades past a particular point, the friction and heat produced during the cut 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

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 get a pretty good idea of where it is if you pay attention over time. While the sharpening stone manufacturers turn red in the face and salesmen froth at the mouth and spray spittle in anger when I say 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 Beloved Customer to conduct your own experiments to determine the practical 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 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 icky 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

If you have questions or would like to learn more about our tools, please click the “Pricelist” link here or at the top of the page and use the “Contact Us” 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, fascist facebook, or thuggish Twitter and so won’t sell, share, or profitably “misplace” your information. If I lie may frogs infest my boots.