Urasuki — The Inner Hollow Grind

Urasuki (裏スキ) is the concave channel ground into the inner flat face of a professional Japanese blade. Paired with the outer Hamaguri-ba convex curve and the ride line at the pivot, it completes the three-element system that makes Japanese convex shears mechanically distinct from European beveled shears.

Why It Matters

Without urasuki, the two inner blade surfaces contact each other across the full length of the blade on every close. That full-surface contact generates significant friction, accelerates wear on the cutting edge, and traps hair fragments in the pivot area. Urasuki solves all three problems in one geometry: by scooping out the inner face, only the cutting edge and the ride line ever touch, which drops friction to near-zero and gives hair fragments somewhere to go.

The Three-Element System

Japanese convex shears work as an integrated system, not as isolated features:

• Outer curve: Hamaguri-ba — the sweeping clam-shell convex on the outer face
• Inner scoop: Urasuki — the concave channel that reduces inner-face contact
• Apex reinforcement: Uraoshi — the thin flat rim around the urasuki that strengthens the cutting edge

All three must be present and in the correct proportion for the blade to cut as designed. A blade with Hamaguri-ba outer curve but no urasuki cuts like a regular convex and dulls quickly. A blade with urasuki but too much uraoshi loses the acute cutting angle. The ratio is what skilled Japanese smiths train for years to achieve.

Trade-offs

• Upside: Self-cleaning friction reduction at the mechanical level; no moving parts, no maintenance overhead during normal use. When it works, it just works for the life of the blade.
• Downside: Specialist-only territory. Improper grinding is catastrophic — a single pass with the wrong sharpener destroys the blade permanently. Most general sharpeners cannot handle urasuki restoration and will decline to touch the inner face, limiting the blade’s total service life to whatever the original factory grind provides.

Maintenance Considerations

Do not attempt urasuki maintenance without a specialist. Daily care is the same as any Japanese convex blade: wipe clean after each client, oil the pivot, store dry. If the blade starts feeling gritty during close — that’s a ride-line issue first, not an urasuki issue, and that should be diagnosed and addressed before any deeper work. True urasuki restoration happens maybe twice in the blade’s service life, typically at the 10-year and 20-year marks on a premium shear, and it’s done by the original manufacturer’s authorized technician or a master sharpener trained specifically on Japanese convex geometry.

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Frequently Asked Questions

What is urasuki? +

Urasuki (裏スキ) is the engineered concave hollow grind on the inner face of a Japanese convex blade. Literally translated, urasuki means 'back scooping' — the name describes how the inner surface is scooped out to create a small gap between the two blades everywhere except at the cutting edge and ride line.

Why do Japanese blades need a hollow grind? +

Urasuki serves two purposes: it reduces friction during closure by ensuring the two blades only touch at the intended contact points (cutting edge and ride line), and it creates a self-cleaning effect as hair fragments that enter between the blades are pushed out by the next close rather than getting stuck.

Can urasuki be restored if it wears away? +

Urasuki restoration is some of the most difficult work in scissor maintenance. Improper grinding of the inner hollow permanently ruins the mechanical integrity of the blade — there is no recovery path. Only specialist Japanese-trained sharpeners should touch the urasuki, and most sharpening runs avoid it entirely, focusing on the outer edge and ride line.