Blade Edge Angle and Polishing: Designing the Sharp Edge that Balances Cutting Performance and Durability in Nihonto

What is Edge Angle? — The Concept of "Blade Edge Angle" in Japanese Swords

Edge angle (hasaki kakudo) refers to the angle formed by the two faces of the blade in a cross-section of the sword's cutting edge. Professionally, it is also called "blade angle (hakaku)" or "edge-mounting angle."

While the concept of blade angle exists in the world of ordinary knives and kitchen knives, in Japanese swords the issue of edge angle involves three complex interrelated factors—"sword body shape," "steel structure," and "polishing process"—so its depth cannot be conveyed by simple angle measurement alone.

Let us first organize the role of edge angle. The smaller the edge angle (acute), the better the sharpness; however, as the edge becomes thinner, the risk of chipping and edge damage increases. Conversely, the larger the edge angle (obtuse), the greater the blade strength; however, sharpness declines. This simple relationship holds true for Japanese swords as well, but due to the unique structure and intended use of Japanese swords, the design of optimal blade angle requires a different approach than modern cutting implements.

The Relationship Between Sword Body Structure and Edge Angle — The Design Philosophy of Shinogizukuri

The most popular sword body structure in Japanese swords is "shinogizukuri." It features a ridge line called shinogi on the side of the blade, and is composed of two surfaces: the "edge meat (haniku)" on the edge side (edge-tip side) from the shinogi, and the "shinogi-ji" on the spine side (mune side) from the shinogi.

The design of edge angle in shinogizukuri is determined by a combination of "the cross-sectional shape of the blade (width of shinogi-ji, thickness of edge meat, thinness of edge)." At the forging stage, the swordsmith designs this blade cross-sectional shape using the term "nikuoki (meat placement)." Nikuoki refers to the distribution of metal (thickness) at each part of the blade, and the manner of gradual reduction in thickness from the base (moto) to the tip (saki), and from the spine side to the cutting edge, directly affects the overall sharpness, strength, and weight balance of the sword.

The ideal cutting edge is in a state of being "as thin as possible, yet reinforced by hardened cutting steel created through the structural changes of hardening." To realize this "thin yet strong" cutting edge, the swordsmith precisely controls the blade's cross-sectional shape through the forging and hardening processes.

Hardening and Edge Hardness — Control of Martensitic Transformation

The greatest factor enabling Japanese swords to achieve both sharpness and durability is the change in steel structure brought about by "hardening (yakiire)."

The area near the cutting edge becomes high-carbon martensitic structure through hardening, possessing characteristics of high hardness (around HRC60) and high brittleness. Meanwhile, the center of the blade (shin-tetsu) and spine side remain as low-carbon soft steel, retaining toughness.

This coexistence of "cutting edge hardness (= maintenance of sharpness)" and "body toughness (= resistance to breaking)" is the structural elegance of the Japanese sword. In the design of edge angle as well, determining "how thin the hardened cutting steel should be finished" becomes a critical decision.

If the edge is honed thin to the point where the hardened layer (yakiba) is extremely thin, the soft part outside the hardened steel (yakiba) becomes exposed at the edge, resulting in good sharpness but an edge prone to chipping. Conversely, if the edge is polished to remain in the relatively thicker part of the hardened layer, durability is high but sharpness is somewhat reduced.

This judgment is within the professional domain of the sword polisher (togishi), made after discerning the sword's intended use, storage condition, and the health of the hardened steel.

The Role of the Sword Polisher — The Final Arbiter of Sharpness and Edge Angle

It is important to accurately understand the division of roles between the swordsmith and the sword polisher regarding the sharpness of Japanese swords.

The swordsmith handles the process of determining "the sword's potential sharpness" — steel selection, forging, and hardening. The sword polisher handles the process of "drawing out that potential sharpness as an actual blade."

Even with the same sword, the final edge angle and finish change depending on the sword polisher's skill and approach, and the perceived sharpness changes as well. Some sword polishers take "art polishing" (polishing for appreciation, not making the edge excessively thin) as their foundation, while others apply "practical polishing" according to the sword's intended use.

The primary process for the sword polisher to adjust edge angle is "edge shaping (hatsuke)." This process, which uses whetstones to adjust the thinness of the edge, directly determines the sword's final sharpness and risk of edge chipping. The factors affecting the realization of edge angle are as follows:

• Whetstone grit (coarseness)
• Application method
• Number of polishings
• Choice of whetstone for final finishing

Measured Edge Angles of Japanese Swords — Comparison with Other Cutting Implements

Japanese sword edge angles vary depending on intended use, era, and school, but generally a single-edged blade is reported to range from approximately 10 to 25 degrees, while the combined blade angle of both sides ranges from 20 to 50 degrees.

For comparison, the edge angles of other cutting implements are summarized as follows:

Japanese kitchen knives are extremely acute and designed with sharpness as the top priority. Chisels, meanwhile, are thicker, and cutting implements designed for bone cutting or hard ingredients tend to have wider blade angles.

Japanese swords range widely in quality from mass-produced practical swords to masterwork art swords, and their edge angle design philosophies are not uniform. It is natural that the edge angle design and polishing finish differ between swords as weapons intended for actual use and modern art swords or display swords.

Aging and Re-polishing — Maintenance of Edge Angle

Swords that have been used or that develop chips during storage have their edges restored through re-polishing. However, because metal is removed from the blade with each re-polishing, repeated re-polishing can reduce blade thickness and alter the edge angle design itself.

This is why "over-polishing" is considered a problem in the preservation of art swords. The concept of "conservation polishing"—maintaining blade condition with minimal polishing and prioritizing the preservation of shape, thickness, and signature (mei)—has become the fundamental approach in modern sword preservation.

Conclusion — Design Wisdom for Balancing Sharpness and Durability

The issue of edge angle in Japanese swords is produced as a result of the overlapping knowledge and skill of three specialties: the swordsmith's forging design, hardening control, and the sword polisher's polishing technique.

It is not simply a matter of "sharpen thin and sharp to cut." Rather, the optimal edge angle and finish are selected after comprehensively judging the sword's steel structure, the health of the hardened steel, intended use, and storage condition. This multi-layered design philosophy underpins the functional beauty of Japanese swords and continues to live as wisdom referenced in modern cutting implement design.

Those interested in the technical and artistic aspects of Japanese swords are invited to view the swords featured in TOUKENZA's online catalog. Please feel free to contact us via the form.