Blade construction: GEOMETRY

japonski noži - choil shot

When we talk about the shape of a blade, its profile is usually the first thing to spring to mind. The blade, however, has two other very important characteristics: the shape of its cross-section – the grind – and the ground angle and shape of its cutting edge – the bevel.

🦉  If we look at the knife straight from the handle towards the tip, we see the cross-section of its profile or the angles at which the blade surfaces have been sharpened from its spine to the cutting edge. The grind is therefore how the blade is thinned to reveal the cutting edge.  

This is a rather complex topic, so we’ll try to simplify it here for the sake of providing a clear and concise explanation. Having said that, every category has a few additional variations, such as an asymmetrical or combined grind (the possible variations are almost endless). 

The cross-section is a combination of surfaces sharpened at different angles, i.e., the primary and secondary bevel, which together form the final shape of the blade.

A blade’s cross-section determines the thickness of the blade, its shape − which lends itself to either delicate tasks or cutting through tougher foods −, the amount of strength and pressure required for cutting and the manner in which the knife pushes away the chopped food. 

When the blacksmiths decide on a blade’s geometry, the first two most important factors that they take into account are: What will the knife be used for and what steel (strength of material) is it made of? In general, the thinner the blade, the better it will cut through food and the least drag it will create. A thin blade, however, is also more vulnerable, thus the type of blade’s cross-sections should be adjusted to knife’s main purpose. Just like a more durable knife with a thicker blade is usually less sharp, a thinner and finely shaped grind ensures superb sharpness, yet decreases a knife’s ability to withstand deformation or damage. Each grind excels at a specific cutting task, so check and see which grind and thickness would be the perfect options for you. 

Profiling or grinding is the process of creating the cross-section of a blade’s profile. A great amount of material is shaved off during this process, determining the thickness and geometry of the blade and in turn defining knife’s cutting characteristics. The grind is a determining factor in how long a blade will maintain its edge and how it can be sharpened. 

With regular sharpening a profile cross-section “thickens” and, in order to ensure its intended use, we have to adjust and correct it by thinning it, so we can improve its cutting performance (reduce the strength needed to cut through food).

blade grinding

 

Single bevel blade

 ➨ Chisels
 ➨ Traditional Japanese kitchen knives: yanagiba, deba, usuba, kiritsuke
 ➨ Scissors

With these single bevel, or, alternatively, chisel bevel blades, we can make precise cuts and they are easy to resharpen. One side is flat and the sharp side tapers in a straight line to the end. Thus, depending on which side the blade is ground, these knives are intended either only for right- or left-handed users.  

Japanese kitchen knives: urasuki grind

Urasuki refers to a slightly concave lower part or the backside of single bevel knives. Traditional Japanese single bevel knives are knives with a bevel on the front – shinogi – side and a concave back – uraoshi – side. Uraoshi is the thin, flat rim that surrounds the urasuki which serves to optimize performance and enhance strength of the blade. The urasuki grind can be resharpened very quickly and easily, reduces the effect of food sticking to the blade and lends itself well to certain cutting techniques (e.g., katsumaruki or art of peeling a white radish (daikon)). New single bevel knives have a concave primary angle (the surface below the shinogi line), which, depending on the manner of sharpening, changes into a flat or hamaguri grind during use and sharpening. We cannot recreate a concave surface with whetstones, but, on the other hand, they help us create a more durable, resilient blade. 

 ➨ For deba, we recommend a hamaguri grind, while for usuba and yanagiba a flat cross-section is most desirable.

Double bevel blade 

A double bevel blade consists of four surfaces: two parallel surfaces at the upper part of the blade and two surfaces that meet at the blade’s edge. Double bevel blades can usually be used in either hand, whereas if they’re sharpened asymmetrically, they can be used in either right or left hand. 

The transition between the angles can be on various parts of the blade, thus we grouped them in three categories:

HIGH PROFILEBurja, knives from the Tojiro Atelier series
MEDIUM PROFILE:
most widely used
LOW PROFILE:
ZDP-189 knives

A transition line above the cutting edge where the angle of the blade changes is called a shinogi line. It’s typical of single and double bevel Japanese knives. It isn’t a literal line, it’s just where the angle of the surface changes and what separates the flat surface of the blade from the primary bevel. Because the blade consists of two different planes on either side, they can be turned into a canvas for two different looks on one blade: for example, a kurouchi finish on the upper part of the blade and a kasumi finish on the lower part of the blade.

Western knives are usually double-beveled and the angle doesn’t change along the sides of the blade (a flat, convex grind all the way from the spine to the cutting edge). They usually fall into the category of a V-grind with a cross-section at 30 degrees and a grind at 15 degrees.

Double bevel knives can be further grouped according to their cross-section and the location of the shinogi line (where the angle of the surface changes):

CONCAVE
CONVEX
FLAT

Concave profile or hollow grind

The part of the blade below the shinogi line is of concave shape. Hollow grinds finish in a very thin and extremely sharp point, the thinnest among double bevel knives. The blade remains sharp for a very long time despite frequent sharpening. This thin profile gives the impression that a knife is sharp even though it’s already ripe for sharpening. 

Convex profile or hamaguri 

Convex grinds are the most difficult to produce owing to the outward bowing of these edges. Such a cross section is also called hamaguri, which translates to a clam-shaped edge, and derives from the blade on katanas. It allows superior sharpness that requires delicate handling and helps to prevent food sticking to the blade. Such a shape keeps a lot of metal behind the edge and makes for a stronger edge, yet it still allows a good degree of sharpness. The downside to a convex grind is that it is very hard to reproduce by anyone other than a very experienced sharpener.

Flat profile or V-edge, scandi or sabre

The blade tapers all the way from the spine to the edge on both sides at a common angle and there are several types depending on where the shinogi line is located. Flat grinds are very common on Japanese knives, as well as most other kitchen, hunting and tactical knives. They can produce very sharp edges and are wonderful at making very thin slices. They offer good balance between the ease of cutting and robustness. As the edge finishes at a microscopic point, flat edges will dull more quickly than other edges. It is also somewhat thicker (in comparison to a concave grind) and can turn even more so with frequent sharpening. 

Distal taper

This is a concept developed by the smiths and is used to describe the thinning of the blade from the heel of the blade toward its tip. A knife is heaver and stronger at the handle (we can exert more pressure with the heel) and thinner and more precise at the tip. This shape of blade allows for smooth and precise cuts. It is easily maneuverable and nicely balanced.

← Older Post Newer Post →