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side by side knife structure

10 Knife Sharpening Things To Know

Anyone new to knife sharpening has a lot of questions that are sometimes quite difficult to answer on their own. To answer them, you can attend various thematic seminars, read specialized literature or post on forums on the Internet, which are devoted to various details. It is always a good idea to look for information from various sources and to systematize your knowledge.

Knife purpose

Before beginning to understand the terminology of knife sharpening, one very important question should logically arise about the purpose of this or that knife. It is no secret that there are a large number of different knives: kitchen, hiking, hunting, and other knives. Why? The answer is very simple - each individual knife has its own purpose. If any knife is sharpened properly and according to its purpose, then it will be comfortable to work with and it will serve the longest possible time before it has to be sharpened again. Sometimes it happens that the knife is sharpened well, but not enough to suit the intended purpose. The result can be a bad cut or a rapid blunting. Both options are unpleasant, especially if you do not have another knife nearby or have no way to sharpen it.

Knife structure

For a general understanding, some technical concepts defining the components and parts of a knife should be highlighted:

outdoor kitchen
1  point tip
2  tip belly
3  spine, back cutting edge
4  heel spine, back
5  butt, pommel  handle scale
6  belly  pin, riverts
7  bevel, grind  heel
8  cutting edge  bolster
9  choil  butt, pommel
10  ricaso
11  finger groove
12  guard, quillon
13  lanyard hole
14  fuller
15  cheek
16  bevel line
17  handle scale
18  jimping
19  thumb ramp
20  screw


Depending on the purpose, some design details vary slightly. For example, some tourist and hunting knives sometimes have other distinctive features depending on the purpose and design of the knife. 

Blade geometry and cross-sectional structure of the blade

The geometry of the blade determines the overall quality of the knife's cutting and its ability to cope with a particular task. A wider blade with a certain spine thickness ensures durability. A narrow blade with a thin spine provides greater biting into the material. The strength of a knife and its cutting qualities are revealed in most cases by opposing concepts.

Therefore, a universal knife that can do all tasks in the same way does not exist. There are many different knives that are designed for very specific jobs. That's why you often have to find a compromise or have a range of knives.

The cross-sectional structure of the blade

When considering blade geometry, it is necessary to take a deeper look at what cross-sections of the blade exist depending on their use. There are several of the most common geometries, as well as their various variants:

Structure of the blade
Blade sectionsBlade angles

Image bevels structure 1: 1 - blade width, 2 – spine, 3 – plane, 4 – bevels, 5 – secondary bevels, 6 - TBE (thickness behind the edge).

Image bevels structure 2: 1 - spine, 2 – sharpening angle, 3 – bevels, 4 – secondary bevel, 5 – cutting edge

Concave (hollow) bevels (1)

These bevels can rightfully be called the best in terms of cutting quality among the sharpest blades. The most common use of this geometry is for razor blades. This geometry can also be found on carving knives. Besides, it is common to find such bevels on hunting, hiking, and pocket folding knives. One of the methods of making a blade with this kind of bevels is to use a grinder's contact wheel.  

The good thing about using a knife with such bevels is that it will last longer before secondary bevels begin to increase in size. The good thing about using a knife with such bevels is that it will last longer before secondary bevels begin to increase in size. The high level of sharpness also suggests that such knives should only be used for slicing food and cutting thin materials. Dangerous razors are the clearest proof of the capabilities of blades with such bevels.

In some cases, deep cuts with such knives are not entirely comfortable because the blade can get stuck. The cutting edge of the blade with this geometry is more sensitive to lateral stresses compared to bevels from the spine. In turn, both geometries strongly lose in terms of strength of the bevels to the blade with convex secondary bevels and convex primary bevels, i.e., full convex. 

Bevels from the spine with secondary bevels (Flat grind) (2)

With this type of bevels, the name speaks for itself. The bevels start from the spine and form a plane across the entire width of the blade. 

Bevels from the spine are a good compromise in comparison to concave and convex bevels and the provide high cutting efficiency with minimal effort.

The disadvantage of this geometry compared to concave bevels is that it is less aggressive when slicing or cutting thin materials. Compared to convex bevels, it will be a bit more difficult to cut stronger or coarser materials.

These bevels are made on a belt grinder or a belt sander on the plane of the thrust plate. Most often, knives with bevels from the spine with secondary bevels are made for kitchen, hunting, and hiking purposes.

Convex bevels (3)

This geometry is based on a smooth curve of the bevels line from the bevel to the cutting edge, with no secondary bevels. Depending on the manufacturer, they may have a slightly different design. Such a shape of the bevels is very well suited to tourist knives due to the nature of their use. Lately, this technical solution has been used more and more often for kitchen knives.

The obvious advantage of such a blade will be the high strength of the cutting edge and the high performance in cutting hard materials. When slicing food, such knives are even more efficient than all other kitchen knives.  Sometimes convex secondary bevels are made on a blade with bevels from the spine, rather than plane secondary bevels. This solution gives excellent cutting performance and significantly increases the resistance of the cutting edge to side stresses.

It takes time to get used to using a knife with convex bevels, because controlling the exact cutting angle is not easy. When sharpening the blade, there may be large scratches which spoil the appearance. On the other hand, manual finishing of such blades in the field terms is simple, since the bevel itself is the guiding element for holding the angle.  

In some ways, tourist knives with convex bevels and with Scandi bevels are similar in use, but they also have their own peculiarities. 

Scandi bevels (4)

Scandi bevels is a distinctive feature of most Scandinavian knives for nature, tourism and hunting. Quite often the geometry of the blade is shaped by the transition of the bevels from the plane at 50-75% of the blade width directly into the cutting edge. When honing, the angle of such a blade does not change. Such knives are convenient for whittling or woodcarving.  Cutting hard objects with such knife is not very convenient, but quite possible. The durability of the cutting edge mostly depends on the initial sharpening angle. 

The advantage of this type of bevels is that it is easy to control the angle when honing manually. However, with large bevels, this is both a plus and a minus, because a significant amount of metal must be removed from both sides of the blade. Of course, this is easier to do with blades made of soft steel. Powder steel with a high hardness is extremely difficult or almost impossible to sharpen manually.

Single bevel (5)

This kind of bevels are most common on woodworking tools and leatherworking knives. A straight edge is formed on one side of the blade, sometimes with a slight secondary bevel. On the other side of the blade, only a slight finish is made and the plane remains flat. Due to the growing popularity of Japanese knives, one-sided kitchen knives have emerged. They are intended for precise long cuts of raw fish and slicing vegetables.

These knives are simple to sharpen manually, because it is easy to control the sharpening angle and the cutting edge turns out neat and very sharp. 

It requires a certain skill to work with such knives, because the blade can move sideways. In addition, different knives are used for right-handed and left-handed people.

Sharpening angle

Cutting efficiency depends directly on the quality of the cutting edge, which is defined by the angle of convergence of the planes of the bevels or secondary bevels. The correct sharpening angle depends on each knife individually, depending on its purpose and geometry of the blade. There are some general recommendations regarding the honing angle, and there are no strict guidelines. A lot depends on the personal taste of the owner.

Basically, you can rely on the following general recommended angles, depending on the purpose of the knife:

  • Household kitchen knives with a priority for wear resistance — 35-40 degrees
  • Professional and specialized kitchen knives — 25-35 degrees
  • Professional chef’s knives 20 – 25 degrees
  • Tourist and hunting knives with a priority for sharpness — 30 – 35 degrees
  • Tourist and hunting knives with a priority for wear resistance — 40 – 45 degrees
  • Pocket folding knives with a priority for wear resistance — 40 – 45 degrees

In each individual case, the angle values may differ, which depends on the quality of the steel. The methods of keeping and controlling the angle are directly related to the concept of the sharpening angle, and they differ significantly for manual sharpening and for honing with various devices.

Steel

In addition to all the factors that ensure high cutting performance, the steel gives the cutting edge certain characteristics and qualities. The majority of knives are manufactured using a grade of steel which in its normal condition has a needle-like structure called martensite. 

Once quenched, the steel forms a structure in which the basic component is a solid solution oversaturated with ordered carbon in the iron. The carbide phase also affects the formation of martensite in steel, and the carbides themselves act as cutting elements and provide a high level of biting to the cutting edge.

When the cutting edge with projecting carbides comes into contact with the material, a stress concentration occurs which results in the material being cut. 

In classical metallurgical production, various alloying additives are introduced for controlled carbide growth, which increase the main properties such as hardness, strength, wear resistance, corrosion resistance and ultimately affect the retention of sharpness of the cutting edge.

If you divide knife steels into types, you will find three main groups: tool steels, carbon steels and stainless steels.

In today's knife community there is a constant debate about which steel is most acceptable for this or that knife. No matter what brand of steel is chosen, you will always have to decide between the characteristics. Hard steels are more brittle but resistant to abrasion, while mild steels are more ductile and easier to sharpen. Carbon steels have a low threshold of corrosion resistance, but are more aggressive to cut. In contrast, stainless steels have a high threshold of corrosion resistance, but a less aggressive cut than carbon steels.

It is often said that a good knife with quality steel is a compromise between such characteristics and therefore there is no single ideal steel grade. An extensive list of knife steel grades with their detailed characteristics is quite easy to find on the Internet and in specialized literature. 

Sharpening stones and bars: shapes and sizes

According to their size, sharpening stones are divided into standard bench stones and combined stones,  which consist of two glued stones. Both types of stones have approximately the same dimensions of about 20 cm x 5 cm x 2.5 cm (7.9x2x1" in). As a separate type, we can distinguish natural stones, which often have only one working side. Such stones come in a variety of sizes, but they are not used very often.

Generally speaking, large stones are used for sharpening long and wide knives to increase efficiency, accuracy, and convenience of manual honing.

Apex hand sharpeners use a standalone type of stones of size of 152x25x6mm (6"x1"x0.25") which is much smaller. This size allows precise control of the sharpening process and a very high quality finishing of the bevels.

Abrasive material and binding substance

When it comes to the abrasive material of sharpening stones and bars, there is one important rule - a hard stone sharpens soft steel, and soft stone sharpens hard steel.

Among all abrasives there are two main groups: synthetic and natural. 

The main synthetic abrasive materials include: 

  • Aluminum oxide is, from the chemical point of view, a compound of aluminum and oxygen, which is made from bauxite clay. These abrasives work with water unless otherwise specified by the manufacturer. Such abrasives can be used on various steels with a hardness up to 60 HRC.
  • Silicon carbide is a powder made from quartz sand. The hardness of silicon carbide is second only to CBN and diamond. There are two varieties, green (harder) and black. The powder is sintered into bars using a ceramic bond. It is considered a water abrasive and is suitable for steels with a hardness up to 64 HRC.
  • Diamond abrasives consist of a mixture of diamonds and several binding substances such as organic, galvanic, magnesia and polymeric. Depending on the bonding substance, they have different durability and are used on sufficiently hard steels with a hardness up to 64 HRC. They are mainly used with water or water-soap solution. Except the galvanic binded stones, they all tend to get clogged quickly if you do not wet them.
  • CBN is a super hard abrasive material with the full name - cubic boron nitride, which is composed of nitrogen and boron atoms. It is second only to diamond in hardness, has a more uniform grain structure and therefore leaves cleaner sharpening marks on the secondary bevel. 

Natural abrasives are mainly slate rocks, followed by micro-quartzites and sandstones. It is not as important to know the chemical composition of each individual natural bar as the fact that they all do not have a homogeneous structure - grains may be different in size and there may be different inclusions. All bars from one layer and one quarry will be different.

A Natural stone has a denser and finer structure. Due to these properties, the treated metal surface turns out cleaner. Compared with synthetic abrasives, such a stone work much slower, but as a result the cutting edge becomes more resistant.

According to the binding agent that is used for synthetic stones there are the following types: 

  • Ceramic bonding agent - due to sintering is used mainly for the production of silicon carbide bars, and the melting method is used to produce wear-resistant aluminum oxide bars
  • magnesia bonding agent - often used in the production of Japanese water stones based on aluminum oxide, which have a very fine structure and high density
  • Electroplated bonding agent - used for the production of diamond plates
  • metal bonding agent - aluminum-zinc and copper-tin agents are used for the production of CBN and diamond-based bars
  • Organic bonding agent - also used for diamond stones
  • Polymer bonding agent - used for diamond stones, but quite rarely because of the high cost

All bonding agents are designed for processing hard steels, have different hardness, provide different performance in combination with abrasive grit and have different service life. You should choose the sharpening bar (synthetic or natural) depending on the intended use of the knife and the amount of metal to be removed. 

Just as there is no all-purpose blade, there is no one-size-fits-all abrasive. You should know that grinding efficiency is provided by fairly aggressive hard abrasives, and precision by thin and soft ones. 

The concept of abrasive grit is also essential. Sometimes a coarse abrasive can be quite hard, and sometimes it can quickly crumble. Therefore, it is necessary to understand the task.

Grit size of sharpening stones and bars

The grit size of a sharpening stone or bar is commonly understood as the grain size of the abrasive material in the binding agent. The larger the grain, the more metal the abrasive will remove. The finer the grain, the more accurate and clean the result will be. There are a number of standards and regulations around the world, but each manufacturer has its own specific way of labeling these parameters. Summary tables of standards can be found on the Internet. Synthetic abrasives fall under certain standards; the grit size designations of natural stones, due to their heterogeneity, should be understood as a relative indicator.

Cooling liquids

Cooling liquid performs a very important role in sharpening - it washes out metal particles, crumbling grains of abrasive material and does not let the bar get clogged. When honing a knife, due to friction, heat energy is released. That's why coolants are essential.

The concept of the blade steel, the abrasive material of the stone, its bonding agent and the coolant are inseparable from one another. There are two main coolants - water and oil. There are also different emulsions based on water. Oil-based coolants can be made from plant, synthetic or mineral oils.

Selection of coolant comes down to the fact that abrasives are conditionally divided into 3 types: only work with water, work both with water and oil and those that work only with oil.

Quality control of sharpening

If all of the previous points have been taken into account, the last step in the sharpening process is a visual inspection and a practical test of the cutting edge to check the finishing quality of secondary bevels.

A visual inspection can be made with a powerful magnifying glass, a regular microscope or an electronic microscope. In any case it is necessary to have a good lightening to check the condition of the secondary bevels and the cutting edge. It is essential to check the presence of the burr, because if it remains, it can lead to undesirable consequences of chipping of the cutting edge and in general significantly affects the quality of the cut and should be removed with the last finishing stone. 

Conclusion

As mentioned above, the accuracy and efficiency of a knife's cut directly depends on the quality of the cutting edge, which is determined by the angle of convergence of the planes of the bevels or secondary bevels. If you clearly  understand the purpose of the knife, properly select the sharpening angle, including the method of keeping the angle, choose the necessary sharpening stone of different grit for a particular steel and the coolant, you will get a knife with a professionally sharpened blade.

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