Sharpening a Knife with a Convex Grind: A Complete Guide
What is a convex grind?
Take an apple seed and look at it from the edge. Both sides smoothly curve outward, meeting at a fine point. This is the convex grind — also called lens-shaped or hamaguri-ba (Japanese for "clam shell").
Let's compare it with two common alternatives:
· Flat (Scandi) grind: each side is an even slope, like a curb ramp. It is simple and predictable.
· There is also a flat (Scandi) grind with a convex cutting edge: each side is an even slope that smoothly transitions into a convex point.
· Concave (hollow) grind: each side curves inward, like the inside of a teaspoon. The cutting edge ends up very thin, but fragile.
How it looks in practice: if you place a knife with a convex grind on a flat sharpening stone, only the very tip will touch the stone. The rest of the blade curves away from the surface. This exact fact is the source of all this geometry's advantages and the main reason why it is harder to sharpen.
The cutting edge itself — the cutting point — can be sharpened to the same sharp angle as any other knife. The difference lies in the metal behind this edge. With a convex grind, the blade gradually thickens as you move away from the cutting edge — in a smooth curve, without a flat slope and without a concave dip.
The physics of cutting: why it works
Imagine driving a wedge into a wooden block. A flat wedge splits wood well — but as soon as it goes deeper, its flat sides start rubbing against the walls of the cut and create resistance. Now imagine a wedge with convex sides. When entering the material, these walls push the wood to the sides and forward, rather than letting it press back against the blade.
The convex surface works as a deflector. The material being cut is pushed apart and guided away from the blade. This is why knives with a convex grind:
· Cut thick material with less sticking. A flat slope gets "pinched" in a thick piece of meat or wood — the material grips the flat sides. A convex grind glides through it much cleaner.
· Feel aggressive at the start of the cut. The tip remains sharp. The convexity helps after the blade enters the material, not before.
· Are resistant to deformation upon impact. The smooth thickening of metal behind the cutting edge means the tip is supported by a significantly larger amount of metal — compared to a concave or thin flat grind. The physics here is simple: a more massive supporting geometry absorbs the impact load, distributing it across a larger volume of steel.
Main advantages
1. Cutting edge durability — it does not chip on impact
On a knife with a concave grind, not only the very tip is thin, but also the metal one or two millimeters behind it. A strike against a bone, frozen food, or a hard knot in wood can cause a microcrack or chip in this area. With a convex grind, the metal right behind the cutting tip is thicker. It has better support. The impact load is distributed across a wide cross-section of steel, rather than being concentrated exactly where the blade is thinnest.
In practice: axes, cleavers, and bushcraft knives with a convex grind used for batoning (splitting wood by striking the spine) hold their edge longer than their concave counterparts.
2. Deep cutting
Knives with a convex grind are much more effective at deep cutting compared to knives with a V-shaped grind (assuming similar blade thickness) — this is due to less "sticking" to the material.
3. Long edge retention
Edge retention is determined by two factors: steel hardness and how well the metal supports the cutting edge. A thin, unsupported edge — even perfectly sharpened — bends and deforms under lateral load. Convex geometry creates a continuously thickening "spine" of steel behind the tip. The edge has something to "lean on". During chopping and heavy cutting, this means the knife stays sharp longer before needing to be resharpened.
A full convex grind also allows you to use the knife longer. As a regular blade is sharpened, metal is gradually removed and the thickness of the cutting edge increases, which greatly reduces cutting efficiency. When sharpening a full convex blade, metal is removed from the entire surface of the blade, so the thickness of the cutting edge always remains the same. A knife with a full convex grind will cut just as well as a new one after several years of use. This is especially important for those whose knives are a primary tool for work or survival, as their tools require constant maintenance.
4. Efficiency when working with hard materials
When working with wood, bone, frozen foods, or in any task where the blade must repeatedly penetrate dense material:
· The edge withstands impacts without chipping;
· The convex surface pushes the cut material to the sides, rather than holding it close to the blade;
· Chopping with a convex grind offers less resistance mid-strike than a flat grind of the same geometry.
That is exactly why almost all high-quality factory-made axes intended for serious work — felling axes, mauls, Scandinavian hand axes — come with a convex grind.
Disadvantages and limitations
1. Difficulty of home sharpening
This is the main cost of a convex grind, and it is significant. A standard V-shaped sharpener, pull-through ceramic sharpeners, or a flat whetstone used at a fixed angle — none of these will properly sharpen a convex grind. Here is why:
· When you press a flat stone against a convex blade at a constant angle, you are only sharpening the very peak of the curve.
· You are not reproducing the convex shape — you are slowly destroying it, grinding it down into a flat slope.
· Repeat this long enough, and the geometry will be irreparably ruined.
To properly sharpen a convex grind, you must follow the curve — meaning the angle of the blade to the abrasive must continuously change during each stroke, following the arc of the grind. This requires either advanced freehand sharpening skills or sharpening systems with a special attachment (for example, TSPROF Kadet Pro, K4, Profile K03, and Pioneer sharpeners working in conjunction with Convex attachments, 6 mm and Convex attachments, 8 mm).
2. Not the best choice for thin, delicate slicing
A soft ripe tomato, thinly sliced salmon, neat carrot julienne — a thin knife with a flat or concave grind will handle these tasks better. The reason is the opposite of its advantage: the same convexity that pushes material away during a deep cut creates a slight lateral pressure on thin, soft foods at the very beginning of the cut. This pressure can crush or tear, rather than cut.
Professional kitchen knives — the Japanese gyuto, the German chef's knife, a slicer — are almost never made with a convex grind precisely because kitchen work requires precision on soft foods. The convex grind belongs to the world of extreme loads and field use, not the cutting board.
Where is it used?
· Axes and hand axes — almost universally feature a convex grind. The blade must withstand repeated strikes across the wood grain without chipping.
· Bushcraft and survival knives — knives designed for batoning, wood carving, and camp chores, where thin slicing is secondary and durability is key.
· Japanese katana — hamaguri-ba geometry — the traditional Japanese grind on a sword is convex, called hamaguri-ba ("clam shell"). Master weaponsmiths found through experience that this geometry better resists edge deformation in battle while maintaining enough sharpness to cut. The same logic applies today.
· Expensive hunting knives — custom and production hunting knives designed for field dressing and deboning carcasses are often made with a convex grind. The edge can withstand contact with bone without needing to sharpen the knife between carcasses.
· Mauls and cleavers — any blade whose main task is to split, not cut. The convex curve actively pushes the material apart as the blade enters.
Quick summary table
A convex grind is a geometry for work, not for a display case. It lasts longer under load, withstands harsher operating conditions, and better resists impacts than most alternatives — at the cost of higher demands on sharpening skills and tools. Understand this compromise, and you will understand everything you need to know about this grind.
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