Sharpening Part 5 – The Sharp Edge

The gem cannot be polished without friction, nor man perfected without trials.” 

Confucius

This post may not be as entertaining as my previous ones on the subject of sharpening Japanese woodworking tool blades: No swords or artwork or handsome Hollywood philosophers, I’m sorry to say. But with this post we will roll up our sleeves and dig into unartistic nitty gritty. I pray tender sensibilities are not offended. Many of my Gentle Readers already know most of what I will present in this post, but it is my fervent hope that one or two useful gems are hidden among the gritty.

You know the difference between the quality of work a sharp edge performs compared to that of a dull edge. Cuts are clean and finished surfaces are smooth, maybe even shimmering. Your tools are happy, singing and chirping as they cut away. But have you given thought to what a sharp edge really is?

Since the purpose of sharpening is to produce this condition in a blade, a clear understanding is useful. We will consider the basics in this post.

We shall also examine the naughty cutting edge that seems sharp but suddenly and unexpectedly dulls after just a little use. Would it be useful to know how to detect such a cutting edge before it fails wasting your time and money?

Let’s begin with bedrock basics.

The Basics

A cutting tool is essentially a wedge, with two flat sides meeting at an angle. Applying force causes it to sever materials, be it wood, metal, meat or mushrooms.

The geometry of this wedge is critical to its performance. At one extreme, the angle could be 90°. It won’t be sharp, it will be hard to push, and it will crush and tear wood instead of cutting it cleanly, but it will be durable.

At the other extreme, the wedge might be made more acute, say 3°. It could be extremely sharp indeed, but it would be too fragile to cut anything but whip cream for long. The point is that the sharp edge is a compromise, acute enough to cut well, but not so acute that cutting pressure and friction will make it dent, roll, wear away, crack or chip easily.

The effective blade must have a bevel angle that cuts the intended material well for a relatively long time. The words “well” and “long” in the previous sentence are where the magic lies. We will examine these important points in future posts in this series.

Germ’s Eye View

The extreme edge of the ideal metal tool’s extreme cutting should be perfectly smooth and only a single molecule thick. In the real world, cutting edges are rougher and wider, but still manage to cut pretty well.

Examine a sharp cutting edge under a microscope, and you will see imperfections. A dull blade will look even worse of course, showing dents, rips, and even cracks. 

knife edge_microscope800
The edge created by an 800 grit stone
Still sharp but starting to wear
A dulled and dented knife blade

Using a blade wears away and damages the cutting edge rounding and flattening it, destroying the geometry that makes it an effective wedge. Sharpening is the process of (1) restoring the intended wedge geometry; and (2) removing defects from the meeting of the wedge’s sides by abrading metal from one or both sides down past any damage, leaving a relatively clean, uniform wedge with minimal defects. This is the sharp edge. It is what the wood experiences. It requires effort to achieve, but it ain’t rocket surgery.

The most difficult part of achieving the two objectives listed above is making nothing from something, in a place that cannot be seen. Now that’s a Zen koan.

Building confidence in one’s ability to achieve results at the microscopic level is not easy. The key is to understand the goal, and to consistently follow reliable procedures. I will describe those goals and procedures in future posts in this series.

Edge Failure

The ideal cutting edge is uniformly sharp, but few edges in the real world meet these severe criteria at the microscopic level where it matters most. A blade may be sharp in some places, and dull in others. Likewise, a blade may cut well for a while and then dull quickly and suddenly. We have all experienced these irritating failures.

One common cause of these inconsistencies and failures is that the edge is sharp only because it has a defect called a burr. Burrs by themselves can be sharp indeed, but they are fragile and can bend, roll over, or break off at the root suddenly and unpredictably creating a nasty dull edge in an instant. A truly sharp edge will not just feel sharp, but will stay sharp for a relatively long time because it is properly shaped and well supported, instead of being only temporarily sharp because of an irregular and fragile burr.

I call burrs a “defect” because they are, but creating a burr is an important step in making a sharp edge. The trick is to continue to refine the wedge after the burr is created until it is gone and the edge is as perfect as we can reasonably expect to make it. Stop the refinement work too soon, or fail to do it completely, and all or part of that unreliable burr may survive to cause trouble.

So how does one tell if an edge is properly sharp and free of deceptive burrs without using a scanning electron microscope?

Do you remember ‘Nando’s philosophy described in my previous post? One must use reverse logic from our latin lover. Don’t rely on mahvelous appearance. Don’t rely on bar room tricks like shaving arm hair or cutting strips of paper. Develop skills and train your senses other than eyesight to detect the shape of steel at the microscopic level. This may sound strange but it is possible because your nerve endings are microscopic and can sense the difference between a burr and a truly sharp edge.

I will save the explanation of detailed techniques for a future post, but for now, here are two essential skills: Use your fingerprints to detect the presence and size of burrs. Use you fingernails to check the condition of the burr and determine when the blade is ready to move onto the next stone in the sharpening process. Please don’t cut yourself.

In the meantime, let’s have some pleasure before pain. Prepare to be amazed, Ladies and Germs, because in Part 6, coming soon, The Mystery of Steel will unfold before your very eyes! There will be marble relief carvings, bronze statues, oil paintings, gods and demons, death and destruction, and even a pagan soap opera about forbidden love. Oh my! We’re in negotiations for the movie rights now ♫꒰・‿・๑꒱ and need someone to play Vulcan. If anyone knows Spiderman’s agent, please have his people contact my people right away.

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 question form located immediately below.

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