What is Hollow Grinding?

Description

Hollow grinding creates a concave surface on a blade by grinding against the circumference of a wheel. In scissors, the most important hollow grind is ura-suki on the inner blade face. The term means something different in scissors versus knife terminology.

What is Hollow Grinding?

Hollow grinding is the process of creating a concave surface on a blade by grinding it against the outside circumference of a grinding wheel. The curvature of the wheel transfers to the steel, producing a concave profile. In scissors, hollow grinding serves different purposes depending on where it is applied — the inner blade face, the outer blade face, or the edge bevel each benefit differently from concave geometry.

Why It Matters for Scissors

The most important application of hollow grinding in scissors is ura-suki (裏スキ) — the concave hollow on the inner face of each blade. This hollow, typically 0.05-0.15mm deep, means the two blade faces do not contact each other across their full width. Instead, contact occurs only at the ride line (near the edge) and at the spine. This dramatically reduces friction during cutting — a flat-faced scissor requires significantly more hand force to operate than one with proper ura-suki.

The hollow also creates a functional benefit: hair or material being cut is less likely to be pinched between the blades away from the cutting edge. Without ura-suki, material can wedge between the flat faces and cause the scissors to bind or push hair instead of cutting cleanly.

On the edge bevel itself, hollow grinding produces a thinner geometry near the cutting edge while maintaining thickness further back. A hollow-ground edge on a 6-inch professional scissor might be 0.1mm thick at the very edge but 0.5mm thick at 2mm back from the edge. This creates a keener cutting edge with less resistance, though it is more fragile than a convex edge of the same nominal angle.

Wheel diameter matters: a larger wheel produces a shallower concavity, a smaller wheel produces a deeper one. Japanese manufacturers typically use wheels of 150-300mm diameter for ura-suki grinding, carefully selected to produce the correct depth for each scissor model.

Technical Detail
The geometry of hollow grinding is defined by the grinding wheel diameter and the depth of cut. The resulting concave surface follows an arc whose radius equals that of the grinding wheel. This has important implications: **Ura-suki grinding specifics:** The inner blade face is pressed against the side or circumference of a rotating wheel. The wheel diameter determines the hollow profile: - 150mm wheel: produces a more pronounced concavity, suitable for shorter blades (4.5-5.5 inch) - 200mm wheel: moderate concavity, standard for 5.5-6.5 inch scissors - 300mm wheel: shallow concavity, used for longer blades (7+ inch) where excessive hollow would weaken the blade The depth of the hollow is controlled by how far the blade is pressed against the wheel. Typical ura-suki depth ranges: - Light: 0.03-0.05mm — common in mass-production scissors - Standard: 0.05-0.10mm — typical for professional-grade scissors - Deep: 0.10-0.15mm — premium Japanese scissors where maximum friction reduction is desired Deeper ura-suki reduces cutting resistance but also reduces blade rigidity. The blade becomes more flexible, which can be desirable for slide cutting but problematic for blunt cutting where blade stability matters. This is why different scissor models intended for different cutting techniques receive different ura-suki depths. **Edge hollow grinding:** When the cutting edge itself is hollow ground, the geometry creates a very acute effective edge angle with minimal material behind the edge. The mathematical relationship: - A hollow-ground bevel with 25° included angle has less steel supporting the edge than a flat-ground bevel at the same angle - This means easier initial cutting but faster edge degradation under impact - Hollow-ground edges are preferred for push cutting and fine detail work - Convex edges are preferred for heavy-duty cutting and slide cutting **Distinction from knife terminology:** In the Western knife world, "hollow ground" almost always refers to the main blade bevel being concave — as on a traditional straight razor. In Japanese scissor terminology, hollow grinding primarily refers to ura-suki, and the edge may independently be convex (most common on premium scissors), flat, or hollow ground. **CNC hollow grinding** uses programmed wheel paths to create consistent hollow profiles. The wheel is dressed (shaped) to the correct diameter and profile, and the CNC program controls the contact pressure and dwell time. Some premium manufacturers still hand-grind ura-suki because the craftsperson can vary the hollow depth along the blade length — slightly deeper near the pivot (where cutting force is highest) and shallower near the tip (where the blade is thinnest and needs maximum rigidity).

Sources

Frequently Asked Questions

In knife terminology, 'hollow ground' typically refers to the edge bevel being concave — ground on a wheel to create a thin, acute cutting edge. In scissors, the most critical hollow grind is ura-suki, the concave surface on the inner (flat) face of the blade, which reduces friction and controls blade contact. The edge grind type is a separate consideration.

No. Budget and mid-range scissors often have flat inner faces with no ura-suki. The edge bevel may be flat ground or convex rather than hollow. Hollow grinding on the inner face is primarily a feature of Japanese-style professional scissors and some premium German scissors.

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