Advice and Theory on Sharpening Angles for Knives and Tools
The most important consideration when sharpening your knife or other edged tool is selecting the proper angle for your edge. The following information will take the mystery out of determining which angle will be most efficient and why.
Before we can talk about sharpening, we must have a conversation about how we discuss and consider the angles on a knife or tool. Most knives have a bevel on both sides. When we talk about a 20 degree angle on a knife or tool, we are talking about each side of the edge being sharpened to 20 degrees. This creates a total angle of 40 degrees (see diagram above). So the angle on the edge of your knife, is the same as the angle at which you hold the knife to your sharpening stone.
Some specialty Knives and tools are not double the angle that you sharpen each side of your knife, like a chisel. Some traditional Asian knives are only beveled on one side. In this example, one side may be sharpened to 20 degrees while the other side is at 0 degrees for a total angle of 20. While the vast majority of Asian knives sold in the United States are not single bevel, it is important to determine this before you put your knife or tool to a stone. If you’re not sure, it is generally safe to assume that your knife has a bevel on both sides. You can determine this by laying your blade on a straight edge or other flat surface like a pane of glass. If one side of the blade contacts the flat surface completely, this would be the 0 degree edge of your knife and the opposite edge is the one that would require sharpening. Most Asian knives do have a slightly lower angle and both sides are sharpened to roughly 17 degrees.
Choosing an angle to sharpen your knife is essentially a compromise between the sharpness and the durability of an edge. The most important factor when determining the angle comes down to how you will be using your knife. Will you be shaving your face, filleting a fish, cutting vegetables, carving or chopping wood? From these examples, it is easy to see how each case requires a different edge.
Toughness vs. Hardness
Not all steels are created equal. Bladesmiths and toolmakers must choose from a large selection of currently available general purpose and specialty steels. One of the biggest considerations of any blade or tool is the balance between toughness and hardness. Hardness in a blade ensures the blade can hold a sharp edge, yet makes the blade more susceptible to breakage. Toughness in metallurgy is the ability of the material to resist fracturing or a lack of brittleness. Glass is a material that is very hard, yet not very tough. Rubber is a material that is very tough, yet not very hard. When making a knife, the Bladesmith must consider the hardness and toughness depending on the intended purpose of the finished product. A blade for shaving will require a very sharp edge that will last and therefore must be on the harder side. An axe used for cutting and splitting wood must be tough, yet not very sharp as it will be swung with great force and requires it be more on the tougher side. The level of hardness is achieved through a process called Tempering, and that would be a discussion all of it’s own
The hardness of steel is very easy to understand and is measured on a scale called the Rockwell C Scale.
Axes: about HRC 45-55 Rockwell C scale Hardness
Chisels and high quality knife blades: average HRC 55–66 Rockwell C scale Hardness
(Hardened High Speed Carbon and Tool Steels such as M2, W2, O1, CPM-M4, and D2, as well as many of the newer powder metallurgy Stainless Steels such as S30V, 154CM, ZDP-189, etc.)
Under 10 Degree Angles
The lowest angles are reserved for edges that are typically cutting softer materials. In this case, the edges are not subject to abuse so the lower angle can be maintained without damage or edge failure. The lowest angles that we typically see are on straight edge razors. These are sharpened to an angle, which is roughly 7 to 8 degrees (although the back of the blade is used as a guide so knowing the angle isn’t important and nor is it adjustable). A straight razor has a very delicate edge that is very easy to damage. In proper usage, a straight razor would never see the type of use that would damage the edge.
10 to 17 Degrees Angles
A sharpening angle of 10 to 17 degrees is still quite low for most knives. With a total angle of 20 to 34 degrees, this is still a very fine edge. This edge is typically too weak for any knife that might be used in any type of chopping motion. Also consider that harder steels are also more susceptible to impact damage because they are more brittle. If your knife is used for cutting soft items or slicing meats, this lower angle can hold up and provide a very smooth cutting action.
17 to 22 Degree Angles
A 17 to 20 degree angle covers most kitchen knives. Some knives (typically Japanese manufacturers) will sharpen their knives to roughly 17 degrees. Most western knives are roughly 20 degrees. It is our experience that kitchen knives sharpened to 15 to 20 degrees cut very well and are still durable. These angles are still not highly durable as a total angle under 40 degrees will not respond well to rougher treatment in harder materials.
22 to 30 Degree Angles
In this range, the knives edges are considerably more durable. A pocketknife or a hunting knife will inevitably see abuse not seen by knives meant primarily for slicing or chopping softer materials. While the edge may not ultimately be cut as well (but you may not notice a difference) it will be considerably more durable.
Over 30 Degrees Angles
Any edged tool or knife that is sharpened past 30 degrees will be very durable. Its cutting ability will be noticeably reduced. This durability has an advantage because more force can be used to make the cut. While the majority of knives won’t benefit from this sharpening angle, an edged tool like a machete, cleaver or axe must be durable as the typical cutting action of these tools would damage other edges.