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The Mechanicsof KnifeSharpening

You’ll have an easier time properly maintaining your knife edges if you understand the basic mechanics of sharpening.

The apex of a knife blade is the very tip or point where the two sides of the bevel meet. The apex radius refers to the radius of the curve at that point. A smaller apex radius means a sharper, more acute point, while a larger apex radius means a more rounded, less sharp point.

Imagine a very sharp needle. The point is extremely fine and tapers to a tiny, almost invisible point—that’s like a knife with a small apex radius. It's incredibly sharp and can easily penetrate materials, but it's also more delicate and prone to damage.

Now imagine a dull pencil. The point is more rounded and less defined, like a knife with a large apex radius. It's not as sharp and may require more force to cut through materials, but it's also more durable and less likely to chip or roll over.

Ideal Apex

The ideal apex radius for a knife depends on its intended use. For delicate tasks like skinning big game or slicing tomatoes, a smaller apex radius provides cleaner, more precise cuts. For tougher tasks like batoning through wood or cutting rope, a larger apex radius will offer greater strength and durability.

When sharpening a knife, you're essentially shaping and refining the apex radius to achieve the desired sharpness. Different sharpening methods and techniques will produce different apex radii, so it's important to choose the right approach for your specific needs.

Understanding knife sharpening angles is crucial when learning how to sharpen a knife. The aim here is to create a new bevel on the blade that meets the sharpening surface at a specific angle.

The Right Sharpening Angle

Each knife comes with a factory-set edge angle, and sharpening to that angle will preserve the knife’s original design and performance characteristics. Grinding to a new angle can be time-consuming and may alter the blade’s functionality, but it’s an option if you want to sharpen the knife for a specific application. 

Here’s a quick guide to choosing the right sharpening angle for the job:

  • Everyday use: A good sharpening angle for everyday carry (EDC) knives typically falls between 17 and 25 degrees per side. 17-20 is good for tasks like opening packages or cutting food. For heavier tasks like cutting rope or wood, a higher angle (22-25 degrees) offers greater durability and resistance to chipping.

  • Food prep: Sharpen to a lower angle (15-20 degrees) for slicing, dicing, and chopping. A low angle gives the blade the razor-sharp edge it needs to glide through ingredients.

  • Outdoor use: Outdoor and survival knives are often made from tougher steels to handle rugged tasks. Sharpening to a higher angle (20-30 degrees) will improve durability and chip resistance.

Here’s a quick tip: To visualize the angle, imagine a line extending from the blade’s spine down to its edge. The angle between that line and the sharpening surface is your sharpening angle. One of the best knife-sharpening tools for beginners is a blade-mounted angle guide.

As you sharpen a knife, a burr will form on the opposite side of the edge you're working on. This burr is a thin wire of metal that indicates you've successfully sharpened that side of the blade.

Checking for Burrs

To check for a burr, gently run your fingertip perpendicular to the edge (never along it!). You'll feel a slight roughness or raised edge on the side opposite to the one you’ve been sharpening. Once a burr forms, switch to the other side of the blade and repeat the sharpening process until a burr forms on the original side.

After achieving and removing the burr on both sides, switch to a finer grit abrasive and repeat the process. Alternating with progressively finer grits will refine the edge, leaving it smooth and sharp. Finish things off with a strop to gently realign and polish the edge for optimal sharpness.

Next: Knife Sharpening MethodsThe CRKT Knife Sharpening Guide