Life On The Edge

Life On The Edge

Life On The Edge

How To Knife, Part 3

 

Now that you know what you are doing, let's talk about how to do it, and that's where the edge comes in; the part of the knife that actually does the work.

You want an edge that is fine enough to cut smoothly and with minimal resistance, but stout enough to not roll or chip during the activity you are engaged in, which depend on edge geometry and stability, respectively.

Edge stability is, to a relative approximation (i.e. this steel is better than that steel), a combination of toughness and hardness, but misleading in absolute terms because toughness varies far more than hardness, and the actual mode of rolling vs chipping is different, so a direct comparison of which is more important will very much depend on the task.

Edge geometry includes various grinds, or bevels, such as Flat, Chisel, Scandi, and Hollow grinds, and the sharpening angle. Flat ground blades have no bevel, so the edge is flat all the way to the spine of the blade. The Chisel grind is only sharpened on one side, leaving one side of the blade perfectly straight. The Scandi grind is a simple bevel on each side, while the Hollow grind is concave, the edge itself is actually curved on each side. There are other grinds, such as Convex, Saber, and Compound, but these are less common in kitchen, pocket, or even hunting or camping knives than they are in axes, swords, or specialty blades.

Sharpening angle depends on the intended use and the properties of the steel; measured in DPS, degrees per side, 10-25DPS is the normal range for various knives, with 16DPS being an inflection point; lower than that, and resistance (i.e. wear) goes down dramatically; above that, edge stability increases significantly.

In practical terms, this means that a stronger (tougher and harder) blade can take a finer edge which will cut more smoothly and therefore wear more slowly than a blade made from a high wear resistance alloy that needs a broader edge to avoid chipping. This is relative to the task, of course; a high wear resistance alloy with a fine edge used carefully (i.e. always cutting straight onto a cutting surface) will hold its sharpness for a very long time, indeed.

For harder, less stable, or inexpert use, however, erring on the side of a tougher alloy with a finer edge is generally, if nothing else, easier to fix by sharpening or reprofiling than a chip in a super-steel.

 

 

Sharpening

 

This could be an entire article, or book, on its own (and it is), but the basic idea remains the same: If there are places on the edge of a blade that could be sharper, you can abrade material away to create a finer edge.

 

The devil, of course, is in the details, but here are some general guidelines:

 

  1. Use an appropriate stone; a natural stone, usually made of quartz, is fine for Carbon steel blades, but even regular stainless steels will be more easily sharpened by a ceramic stone, like Aluminum Oxide or Silicon Carbide, and super-steels with harder carbides may be almost impossible to sharpen without a diamond stone.

     

  2. Wet the stone. Ceramic stones use water, but most natural stones need some kind of oil; mineral oil is best, but baby oil, motor oil, even vegetable oil (although it will go rancid if left on) will work in a pinch. Do not wet diamond stones.

     

  3. Watch your angle. There are different ways to do this: One is by using a felt-tipped pen or marker to cover the old edge, and make sure that you are removing all of the ink while you sharpen, which will keep you at roughly the same angle as the old edge; there are wedges with different angles which you can hold between the knife and the stone; or there are sharpening systems which let you set the angle for a consistent edge. The pull-through systems are not recommended. The ink method is also useful for determining the existing edge angle.

     

  4. "Chase the burr." There is some difference of opinion on this subject, but once you understand what is happening, the answer is simple. Tests will show that "pushing" the blade towards the edge across the stone will result in a finer edge... in a given number of strokes, but it is just removing material more quickly, at the risk of damage to both the edge and the stone. Some water-cooled grindstones with firm angle guides are intended to work this way, but it is not the correct technique for any other sharpening method. A sweeping motion, down the edge (opposite from the cutting direction) and up the base to the tip will develop a fine burr.

     

  5. Strop. Pull the edge lightly across either a stropping block (wood with stropping compound on it) or a piece of leather (the back of a full-grain leather belt in a pinch), several times, alternating sides. This will bend the burr back and forth until it comes loose from the edge, leaving a hard, straight cutting line.

 

Tools

 

Here are some examples of different sharpening stones and systems:

 

Intelitopia Sharpening Set: https://amzn.to/46qlgEa

Two double-sided whetstones with a base, angle guide, and a strop; a solid, basic kit.

 

Ruixin Pro RX-009: https://amzn.to/3R7xLiH

This is the system I use on my own knives; convenient, portable, easy to setup, the only issue is trying to set very fine edges on small (narrow) blades, as the clamp can get in the way of the stone.

 

Work Sharp Professional: https://amzn.to/46mYld4

Less portable with more features, such as preset angles (you have to use a separate angle finder on the Ruixin, although every smart phone has one built in), this is definitely a nicer piece of equipment for set use in a shop.

 

Tormek T4 Water-Cooled Power Sharpener: https://amzn.to/3G686Sc

Got a lot of knives to sharpen? Or one, great big one? Take the effort out of it and let the machine do the work for you!

 

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