What is a Convex Edge?

Description

A convex edge (also called hamaguri-ba / 蛤刃 in Japanese, meaning 'clam-shell blade') is a blade edge ground to a smooth, curved profile. Unlike a flat bevel which meets the blade at an angle, a convex edge tapers gradually from spine to cutting point with no visible shoulder or transition line.

What is a Convex Edge?

A convex edge (also called hamaguri-ba / 蛤刃 in Japanese, meaning “clam-shell blade”) is a blade edge ground to a smooth, curved profile. Unlike a flat bevel which meets the blade at an angle, a convex edge tapers gradually from spine to cutting point with no visible shoulder or transition line.

Why It Matters for Scissors

The convex edge produces the smoothest cut with the least resistance of any blade geometry. Hair slides along the curved surface rather than catching on an edge shoulder, making it the ideal profile for slide cutting, point cutting, and precision bob work.

In practice, a convex-edge scissor requires roughly 30-40% less hand force to close through the same section of hair compared to an equivalent beveled-edge scissor. This reduction in effort matters enormously over a full day of cutting — a stylist performing 200+ cuts per day will experience significantly less hand fatigue.

However, convex edges carry trade-offs. They are harder to sharpen (specialist equipment is required), more prone to damage if dropped, and more expensive to manufacture. The edge geometry demands precise grinding that only experienced craftspeople or advanced CNC equipment can achieve consistently.

Technical Detail
Hikari Scissors (株式会社ヒカリ) of Niigata Prefecture is credited with pioneering the application of hamaguri-ba to hair scissors in 1967. The company's founder drew direct inspiration from Japanese sword (日本刀) design, where the hamaguri-ba grind has been used for centuries to create blades that cut cleanly while resisting chipping. Hikari holds the first Japanese patent on the specific blade angle and sharpening method for convex-edge hair scissors. This innovation was endorsed by Vidal Sassoon and Paul Mitchell during their visits to Japan in the 1970s, establishing convex edges as the dominant premium blade profile worldwide. Before Hikari's innovation, virtually all Western hair scissors used beveled edges. The term "hamaguri" (蛤) literally means "clam" — the cross-section of the blade edge resembles a clamshell in profile. When viewed under magnification, the edge curves smoothly outward on both sides before meeting at the cutting point, with no flat facets or abrupt transitions. There are three main sub-types of convex edge used in modern scissors: full convex (smooth curve from spine to edge), semi-convex (convex near the edge with a slight flat section mid-blade), and sword-edge (asymmetric convex with a steeper curve on the inner face). Full convex is preferred for slide cutting, while sword-edge is common in dry-cutting scissors. Sharpening a convex edge requires a rotating leather or felt wheel loaded with abrasive compound, or a specialized convex honing system. Flat stones will destroy the curved geometry and convert it to a beveled edge — a common mistake made by untrained sharpeners.

Sources

Frequently Asked Questions

Not recommended. Convex edges require specialized equipment and technique. Standard sharpening stones can flatten the convex profile, ruining the edge geometry.

Convex edges taper smoothly like a clamshell for effortless cutting. Beveled edges have a flat angle creating a shoulder that grips hair — better for beginners but more cutting resistance.

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