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Blade steel gets most of the attention on a premium folder, but steel alone does not determine how a knife moves through material. Two Herman knives can use the same steel, similar hardness and the same blade-stock thickness while having different primary grinds, and that difference changes the cross-section behind the cutting edge.
Herman uses both flat and large-radius hollow grinds on selected builds. The grind is an individual configuration choice, not a permanent property of a model or steel. Recent documented examples include Slim, Sting, Ishtar and Dragonfly knives with large-radius hollow grinds, while other builds from the same model families use flat grinds.
The clearest comparison comes from the Ishtar. Two documented M390 builds use the same 0.130 in (3.3 mm) blade stock and 61 HRC hardness, yet one has a large-radius hollow grind and the other a flat grind.
The grind is set during blade production alongside heat treatment and finishing — see our guide to how Herman knives are made for where that fits in the wider sequence, and our knife grinds guide for flat, hollow, convex and other geometries in general. This article focuses specifically on the shallow large-radius hollow used on selected Herman configurations.
A large-radius hollow grind is a shallow concave primary grind formed on a large grinding radius. Because the radius is large, the curve can look almost flat to the eye. The difference appears in cross-section: a flat grind increases in thickness along a straight plane, while a hollow grind follows a concave path.
On a folding knife, a shallow hollow can be ground to keep the lower blade section relatively thin without removing as much supporting material as a much deeper hollow would. When that produces a thinner lower cross-section, less material has to open around the blade in a deep cut. In comparable geometries, the concave section also stays relatively thin farther above the original edge as repeated sharpening moves the apex upward.
None of those effects is automatic. Cutting behavior still depends on blade width, stock thickness, thickness behind the edge, edge angle, steel, hardness, surface finish and the material being cut.
The term hollow grind covers a wide range of geometries. A straight razor can have a pronounced hollow, while a premium folding knife may use only slight concavity.
Imagine a short section cut from two circles, one small and one very large. The section from the larger circle looks much closer to a straight line. The same geometric principle applies to a hollow grind.
A smaller grinding radius produces stronger curvature in the blade face. A larger radius produces a broader, shallower hollow. The latter moves closer to flat-grind behavior while still removing material from the lower blade section along a concave path.
That distinction matters because a deep hollow removes more material from the middle of the blade section. A large-radius hollow preserves more support farther above the edge when the other starting dimensions are comparable.
On a wide blade, shallow curvature can be visually deceptive. Stonewash, satin or mirror polish can make the primary face appear flat until the blade is tilted through reflected light or checked against a straight reference.
For that reason, photographs alone are not always reliable for identifying the grind on a Herman. The specification of the individual knife is more useful than trying to classify the geometry from one product image.
The standard description in our knife grinds guide — a hollow grind leaves a thin section behind the apex and gives up some lateral support — applies most clearly to pronounced hollow grinds such as razors, skinners and other deeply concave blades.
A large-radius hollow sits at the shallow end of the same family. With comparable starting dimensions, its slight concavity retains more supporting material than a deep hollow while still removing steel from the lower blade section along a curved path. It should not be treated as having the same cross-section, or the same degree of fragility, as a deeply hollow-ground blade.
The primary grind describes the shape of the blade face. Thickness behind the edge and the final sharpening bevel remain separate dimensions that the maker sets independently.
The phrase thin behind the edge is often confused with sharpening angle. They describe different parts of the blade.
The sharpening bevel ends at the apex, the actual cutting edge. Behind that bevel sits the primary blade geometry. Two knives can have the same edge angle while carrying very different amounts of steel immediately above it.
If both knives are equally sharp, the difference may be small when cutting thin paper. Push them into thick cardboard and more of the blade enters the material. The lower cross-section now affects how much the material has to open around the knife.
A sharpening angle mainly controls geometry at the apex. The primary grind controls a much larger section behind it. Good cutting performance depends on both.
This is also why steel comparisons need context. M398 can offer high wear resistance, but it does not determine how thick the finished blade is behind the edge. Our Herman blade steels and Damascus guide separates alloy properties from grind and heat treatment.
A flat grind and a large-radius hollow can both make excellent premium EDC blades. Their differences are geometric; neither label is a simple better-or-worse ranking.
| Factor | Large-radius hollow grind | Flat grind |
|---|---|---|
| Primary geometry | Shallow concave curve | Straight plane toward the edge |
| Visual appearance | Can appear nearly flat when the radius is large | Straight primary surface is usually easier to identify |
| Thickness above the edge | Maintains a relatively thin lower section when the hollow is ground thin behind the edge | Increases along the straight plane defined by grind height and stock thickness |
| Deep-cut resistance | Reduces wedging when the lower cross-section is thinner | Depends heavily on grind height, blade width and behind-edge thickness |
| Support above the edge | More support than a much deeper hollow when other dimensions are comparable | Provides substantial support depending on how thin the flat grind is taken |
| Repeated sharpening | Concavity preserves relatively thin geometry farther above the original edge in comparable blade sections | The edge progressively moves into thicker material unless the blade is thinned |
The table describes geometry, not guaranteed performance. A carefully ground flat blade can be thinner behind the edge than a conservative hollow grind, and either profile can be made too thin for its intended use.
For a broader comparison of Slim, Sting, Ishtar, Dragonfly and other Herman platforms by size and intended use, see our Herman Knives model guide.
There is no universal winner.
A large-radius hollow is useful when the goal is to keep the lower blade section relatively thin and manage resistance in deeper cuts. A flat grind offers simple planar geometry, predictable support and a more straightforward path for later thinning with flat abrasives.
The meaningful comparison is between specific blades. A very thin flat grind may cut more freely than a conservative hollow. A well-supported large-radius hollow may retain more strength than an aggressively thin flat blade.
If I were choosing between two otherwise comparable Herman builds, I would favor the large-radius hollow for a knife that will regularly cut cardboard and packaging. For a knife I expect to keep for decades and potentially have thinned later, I would favor the flat grind because planar geometry is simpler to restore with conventional abrasives. The hollow prioritizes cutting geometry in use; the flat grind simplifies later thinning. Neither choice is categorically better.
A large-radius hollow gives Bartosz Herman another way to tune cutting geometry without committing the blade to a visibly deep hollow. With comparable dimensions, the shallow concavity keeps the lower section thinner than a deeper cross-section would while retaining more material farther up the blade than a pronounced hollow.
Recent specifications show the geometry on both M390 and M398 knives, and on blade stock from 0.130 in (3.3 mm) to 0.157 in (4.0 mm). That range points to a tuning decision applied per build rather than a signature attached to a model line.
A documented Slim combines M398 at 62–63 HRC with 0.130 in (3.3 mm) blade stock and a large-radius hollow grind. Other M398 Herman builds use flat grinds. M390 likewise appears in both hollow-ground and flat-ground configurations.
Those combinations create different performance packages from the same steel. M398 can extend abrasive edge life, but it does not determine the blade cross-section. A hollow grind changes geometry, but it does not provide M398’s wear resistance.
Primary grind geometry becomes more important as more of the blade enters the material. When one blade has a thinner lower cross-section than another, it displaces less material and produces less wedging in a deep cut.
That does not mean a large-radius hollow will outperform every flat grind. Surface finish, blade width, edge angle, behind-edge thickness, material stiffness and adhesive buildup can all affect resistance.
Cardboard is a useful everyday example because the blade is often buried well beyond the sharpening bevel. Once several layers surround the blade, the material has to open around the primary cross-section. A relatively thin lower section reduces that displacement.
Steel still matters. Abrasive cardboard can dull the apex, so an M398 blade may retain its edge differently from M390 or MagnaCut. The grind and the steel are solving different problems: one shapes the blade passing through the cut, the other affects how the edge itself resists wear.
In a shallow draw cut, apex sharpness and edge finish can dominate. As thicker rope compresses around more of the blade, the primary geometry becomes more relevant. This is one reason rope tests are poor steel comparisons when the knives also use different grinds or edge geometries.
Rigid and semi-rigid plastics can bind around a thick blade as the cut deepens. A thinner lower section reduces that effect. Tape adds another variable because adhesive increases drag on the blade faces regardless of grind.
The large-radius hollow changes the cross-section; it does not eliminate friction from surface finish, residue or the material itself.
A knife can be extremely sharp at the apex and still feel inefficient in a deep cut. Once the blade passes beyond the edge bevel, resistance also comes from material displacement, compression, friction against the blade faces and binding.
That explains why a knife can shave hair yet feel resistant in thick cardboard or dense food. The apex may be sharp while the blade section behind it is comparatively thick.
A large-radius hollow addresses that cross-sectional part of the problem without requiring the pronounced concavity of a deep hollow grind.
The grind name alone does not determine strength. A deep hollow and a shallow large-radius hollow can leave very different amounts of material behind the edge.
Durability depends on the complete blade specification:
| Factor | Why it matters |
|---|---|
| Steel and heat treatment | Influence toughness, strength, wear resistance and edge stability |
| Thickness behind the edge | Directly affects how much material supports the apex |
| Edge angle | More acute edges generally have less material supporting the apex |
| Hollow radius | Controls how aggressively material is removed from the primary section |
| Blade stock and width | Set the starting dimensions available for the primary grind |
| Use | Controlled slicing and lateral prying place very different loads on the blade |
In practice, the geometry itself is not the limiting factor on a Herman. Steel, heat treatment, edge angle and thickness behind the edge set the working limits, and all of those are specified per build rather than implied by the word “hollow.”
A large-radius hollow is a cutting-oriented geometry. It makes sense for controlled EDC work such as cardboard, packaging, rope, plastic and similar utility slicing. It is not a reason to use a premium folder for lateral abuse.
Prying, twisting a buried blade, batoning and chopping place very different loads on the edge, blade, pivot, stop system and liner lock. If the intended work routinely involves impact or heavy lateral force, the better solution is a knife designed for that job, not a different Herman grind.
Hard inclusions matter as well. Staples, grit, metal clips, ceramic debris or glass particles can damage an edge regardless of primary geometry. High wear resistance helps against abrasion; it does not make an edge immune to impact damage.
Every sharpening removes steel from the cutting edge. Over time, the apex moves higher into the blade, where the cross-section is usually thicker.
On a flat grind, thickness increases along a straight plane. On a comparable hollow grind, the concave primary surface leaves a relatively thin section above the original edge. Repeated sharpening therefore moves the apex through that thinner section for longer before behind-edge thickness becomes a major limitation.
The effect is gradual. A hollow-ground blade does not remain thin indefinitely.
If the sharpening angle remains the same while the blade becomes thicker behind the apex, more steel has to be removed to reach the new edge. The visible sharpening bevel tends to become wider.
A wider bevel is not automatically a defect, but it can accompany a change in cutting geometry. More useful signs appear in use: the knife still takes a sharp apex but begins to require more force once the cut moves into thicker material.
At that stage, the problem may be geometry rather than sharpness.
Sharpening restores the cutting apex. Thinning removes material behind the edge to change the blade cross-section. Our knife sharpening angle guide explains how to identify and maintain the existing edge bevel.
Routine maintenance of a Herman large-radius hollow should normally address the secondary edge bevel, not the hollow primary grind.
There is no reason to grind the concave blade face every time the knife becomes dull. Working the primary hollow unnecessarily removes far more material than normal edge maintenance and can alter the radius, grind line, symmetry and blade finish.
Match the existing edge geometry unless you have a specific reason to reprofile it. Herman builds vary, so a universal factory angle should not be assumed. A marker can show whether the abrasive is following the existing bevel.
Yes. A guided system works on the secondary edge bevel, so the hollow primary grind does not prevent normal sharpening.
The main issue is secure clamping. A hollow-ground blade may offer less flat surface for a clamp than a broad flat grind. On polished or patterned blades, protect contact areas from scratches and verify that the knife cannot shift before sharpening.
Abrasive choice depends more on the steel than on the primary grind. M398, M390 and MagnaCut do not remove material at the same rate even when the blade geometry is similar.
There is no fixed number of sharpenings. Light touch-ups remove little steel, while repeated reprofiling or repair of damaged edges can move the apex upward much faster.
Thinning becomes relevant when restoring a clean apex no longer restores the cutting behavior you want. Possible signs include a much wider edge bevel, increased resistance in deep cuts and a knife that tests sharp on paper but feels less efficient once more of the blade enters the material.
Lowering the sharpening angle is not the same as thinning the primary geometry. If the blade has become too thick behind the edge, the section behind the bevel has to be addressed.
A flat stone cannot reproduce a concave radius. Removing enough material from a hollow blade with flat abrasives gradually changes the original geometry toward a flatter profile.
Preserving the large-radius hollow requires a regrinding process capable of controlling radius, grind height, symmetry, heat and thickness behind the edge. On a polished, stonewashed or patterned Herman blade, the surface finish also has to be restored.
For that reason, substantial thinning of an expensive Herman is better treated as professional regrinding work, not normal edge maintenance.
| After repeated sharpening | Large-radius hollow | Flat grind |
|---|---|---|
| Edge moves upward | Into a concave section that may remain relatively thin for some distance | Along a straight section that becomes progressively thicker |
| Routine sharpening | Maintain the secondary bevel and leave the primary hollow intact | Maintain the secondary bevel while the primary flat remains unchanged |
| Major thinning | Original curvature requires controlled regrinding to preserve | Flat abrasives can follow the planar geometry more naturally |
| Long-term geometry | Delays significant thickening behind the edge relative to comparable flat geometry | May require thinning once sharpening reaches substantially thicker material |
For platform-level differences between Slim, Sting, Ishtar, Dragonfly and other models, see the Herman Knives model guide. Recent large-radius hollow examples include:
| Documented Herman build | Steel | Blade stock | Hardness | Primary grind |
|---|---|---|---|---|
| Slim #1992 | M398 | 0.130 in (3.3 mm) | 62–63 HRC | Large-radius hollow |
| Ishtar M390 Rose | M390 | 0.130 in (3.3 mm) | 61 HRC | Large-radius hollow |
| Sting M390 Green Snail | M390 | 0.130 in (3.3 mm) | 62–63 HRC | Large-radius hollow |
| Dragonfly M390 reviewed example | M390 | 0.157 in (4.0 mm) | 62 HRC | Large-radius hollow |
Availability and individual specifications change, so this table documents examples; it is not a permanent model rule. Browse the Herman Knives category for current individual specifications.
Two documented M390 Ishtar builds provide a useful comparison. The Rose-engraved example uses M390 at 61 HRC, 0.130 in (3.3 mm) stock and a large-radius hollow grind. Another M390 Ishtar uses the same hardness and stock thickness but a flat grind.
| Specification | Ishtar M390 Rose | Ishtar M390 Titanium |
|---|---|---|
| Steel | M390 | M390 |
| Hardness | 61 HRC | 61 HRC |
| Blade stock | 0.130 in (3.3 mm) | 0.130 in (3.3 mm) |
| Primary grind | Large-radius hollow | Flat grind |
The comparison does not prove that one knife cuts better than the other because behind-edge thickness and edge angle are not controlled measurements here. It does prove a narrower and useful point: the primary grind is an independent build specification, not something determined by the steel, hardness or model name.
It gives the maker a shallow concave primary geometry that keeps the working section thin without creating the pronounced hollow of a razor-style grind. Herman uses it on selected builds rather than across every knife.
Not simply because it is hollow-ground. A shallow large-radius hollow retains more supporting material than a deep hollow with comparable starting dimensions. Edge strength still depends on steel, heat treatment, behind-edge thickness, edge angle and how the knife is used.
As the apex moves upward, it enters a thicker part of any blade. Compared with similar flat geometry, a concave primary section delays that increase, but it does not prevent it indefinitely. Eventually the knife may need thinning or professional regrinding.
Maintain the secondary edge bevel and leave the primary hollow alone during routine sharpening. Follow the existing bevel unless you intentionally want to reprofile it, and avoid removing more steel than necessary from a collector-grade blade.
Yes. The system sharpens the secondary bevel, not the hollow itself. Stable clamping and finish protection matter because a shallow concave blade face may provide less flat clamping area than a broad flat grind.
Documented examples include Slim, Sting, Ishtar and Dragonfly knives. These examples establish that Herman uses the geometry across several platforms, but they do not make it a permanent specification for every knife in those families.
Both M390 and M398 are documented. A Slim in M398 uses 0.130 in (3.3 mm) blade stock at 62–63 HRC with a large-radius hollow, while documented Sting, Ishtar and Dragonfly builds pair M390 with the same general geometry.
They control different variables. Steel and heat treatment affect wear resistance, toughness and corrosion behavior; the grind determines the cross-section that moves through the material. A useful comparison needs both.
The large-radius hollow is a subtle geometry, not a dramatic visual feature. Its value lies in controlling the lower blade cross-section: selected Herman builds remain relatively thin behind the working edge without using the pronounced concavity of a deep hollow.
That matters in cardboard, packaging, rope, plastics and other cuts where a substantial portion of the blade enters the material. It also affects long-term maintenance because the concave primary geometry delays the point at which repeated sharpening reaches substantially thicker steel.
For a buyer, the useful question is not whether a hollow grind is universally better, but whether the combination of steel, hardness, stock thickness, primary grind and edge geometry suits the intended use.
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