Three Grinds, One Onion: Flat, Convex, and Wide Bevel

You can sharpen two knives to the same angle and still have one that cuts better. The difference is often the grind — the shape of the blade behind the edge. Today we compare three common grinds on the same onion and notice what each one does.

The three shapes

Flat grind (平刃（ひらば）) — the blade tapers in a straight line from the spine to the edge. Think of a simple wedge. It is easy to make and easy to maintain.

Convex grind (凸刃（とつば）) — the blade curves outward, like a lens. The edge is strong because there is more metal just behind it, and the curve helps push food away.

Wide bevel (広い刃（ひろいは）) — a large, flat bevel that runs high up the blade, common on single-bevel knives like deba and yanagiba. The bevel is the whole side of the blade, not just a thin strip at the edge.

The onion test

Take a halved onion and cut it into thin slices, using the same pressure each time.

With a flat grind, the blade goes in cleanly at first, but as you get deeper, the straight sides start to grip. The onion slice tends to stick to the blade. You feel resistance, and the slice clings.

With a convex grind, the curved sides push the slice away as you cut. The onion releases more cleanly. The trade-off is that a convex edge can feel slightly less aggressive at the start of the cut.

With a wide bevel, the large flat surface acts like a plane. On a firm onion, it can wedge — the blade splits the onion before the edge has finished slicing. The cut feels powerful but less precise. This is why wide-bevel knives are loved for fish and hateful for delicate vegetable work.

Why this matters at home

If you mostly chop vegetables, a convex or a thin flat grind will feel smoother. If you break down fish or squash, a wide bevel gives you control and strength. The grind also affects how you sharpen: a wide bevel needs the whole face on the stone, while a convex edge is best maintained with a slight rocking motion.

Yesterday we looked at the lamination line on san-mai blades; that line often hints at where the grind begins. But the grind itself is something you can see by holding the blade up to a light and looking at the profile.

Next time you cut an onion in your city, pay attention to whether the slices stick or slide off. That tells you more about your knife than any spec sheet.

A note on maintenance

A flat grind is the easiest to keep true on a stone. A convex grind can round over if you sharpen with too much pressure in one spot. A wide bevel shows every mistake — but also rewards patience with a beautiful, even finish.

If you are in your country and buying your first Japanese knife, ask the seller what grind it has. It will change how you cook.