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Edge Angle and Relation to Sharpness and Hardness

As mentioned in chapter I, the angle of the cutting edge has a major impact on both sharpness and edge retention. Comparing a blade with a narrower edge angle as in B1 with the wider angle in A1, the blade B1 can be obviously be pushed through food more easily, which is what we feel as sharpness.

In addition, even if the edge is slightly worn (indicated by the red line in fig. A2 / B2), due to the thinner edge at the more acute angle, blade B2 will still be able to cut, while A2 would be rather blunt, which means that on a blade with a narrower edge angle, the sharpness stays longer.

Graphic of knife blade with wide angle on cutting edge vs narrow acute edge angle

A1

B1

Graphics with thickness behind edge of knife blade wide edge angle vs narrow acute edge angle after edge is rounded and worn

A2

B2

Looking at the narrow edge in fig. B1, it’s also easy to understand you need a harder material to hold the edge without rolling.

 

There are two major insights:

  1. The harder the knife, the more acute an angle it can hold.

  2. The more acute the angle of the cutting edge is, the better sharpness and edge retention will be.

 

However, what was explained in chapter Blade about the sensitivity of a thinner blade geometry, naturally applies to the cutting edge as well. The thinner it is, the more sensitive it will be.

Western angle

In chapter 2, we also explained the differences between a Western and a Japanese blade geometry. Following the philosophy of a sturdy knife with softer steel, Western (German) blades are usually sharpened at a wider angle of 30-35° (included angle, 15 - 17.5° per side). As a result, knives will be less sharp and dull more quickly, but are less likely to be damaged / chip when subjected to abuse of some sort. The sharpening of Western knives usually is symmetric (fig C), which means having the same angle on each side.

Graphic of symmetric knife cutting edge with wider angle western style

C

Japanese angle and asymmetric edges

In keeping with the Japanese philosophy of thinness, Japanese knives are sharpened to more acute edge angles of around 20° (10° per side). So knives with a Japanese edge are sharper and stay sharp longer, but the edge is more prone to chipping when subjected to abuse.

The sharpening of Japanese knives usually is asymmetric (70/30), with a wider angle (shorter bevel) on the left and a narrower angle (wider bevel) on the right. A 70/30 edge means that, if the total angle is 20°, the angle on the left is 14° and on the right 6°.

graphic of asymmetric knife cutting edge wider angle left narrow angle right 70/30 style

D

What effect does an asymmetric edge have? In theory, an asymmetric edge (fig D) will make the knife steer to the right when cutting, as the wider angle on the left will offer more resistance during cutting and will push the blade to the right, which makes a straight cut difficult.

So why then do Japanese knives have asymmetric edges? There are different theories, one being that it is for compensating an asymmetric blade geometry (here thinner blade on the left), which is another feature of many Japanese knives, and thus straighten the cuts. So a thicker blade geometry on the right would cause a steer to the left, which is compensated with a wider angle on the left that will cause a steer to the right.

But many Japanese knives sold in international markets today are multi-layer. For those, a symmetric blade geometry is of essence for keeping the core material in the center of the blade. In such case, asymmetric edges make little sense.

Also, in sheer cutting performance (both initial sharpness and edge retention), there is no difference between symmetric and asymmetric edges. Hence the symmetry of the edge is not a quality criterion of a knife and can be changed to users liking when re-sharpening.

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