Designing the new chef’s knife sharpener – Diamond Pearl

When we started designing a new chef’s knife sharpener, we assumed the answer was obvious. It wasn’t.

The work actually began last October, when Björn and I tried to settle a long-standing dispute about which sharpening method gives the best results in real-world use.
For a cross-type quick sharpener, we decided to settle this once and for all. We ended up with three competing concepts:

1. Spring-loaded with variable angle
   A spring-loaded system that aims to follow the existing grinding angle of the blade.
   In theory, this should be the most flexible and best solution. If it hits the edge exactly, it should produce the sharpest edge very quickly, since very little material needs to be removed while retaining the original angle. Our favourite — in theory.
2. Crossed wheels
   The same idea, but with curved grinding surfaces.
   This was Björn’s idea, and he swears it produces the sharpest edges he has ever encountered in a quick sharpener, and that it should hit any angle precisely even though the wheels are fixed. It sounded too good to be true, but we decided to give it a try.
3. Spring loaded with fixed angles
   A fixed sharpening angle that allows the exact contact point on the edge to move slightly up and down during sharpening.
   This approach is interesting because, in theory, it should introduce a small amount of micro-serration — something I’m a strong advocate of for food-cutting knives. It also works well in most applications, except for razors and wood carving tools, where a polished edge is preferred.

The first to go into idea’s wastebin was #2, the Crossed wheels. They didn’t do anything better then straight ones did, except that Björns test knives got exceptionally sharp because he happened to find that exact grinding point more or less by accident.
No further work was done on this concept.

That left the variable angle vs fixed angle (#1 vs #3).
We thought this would be an easy win for #1, but to our surprise the #3 fixed angle gave a noticably sharper edge!
However we tried and whatever knives we put trough the test-sharpeners the #3 gave either an equal or better result… and did it quicker!
The quicker part wasn’t hard to understand, because obviously sharpening in two dimensions vs one will be quicker (and give more long lasting sharpeners) but why was the edge coming out sharper when we used a conservative 22° sharpening angle on knives that were often down to 15° originally? This is where the variable angle should shine and use the original angle of say 15° and simply work with that!

The answer didn’t come in the microscope because we couldn’t really spot the difference, but after hours of testing.

The only time the sharpness came close was when we managed to be super-consistant of applying the same pressure on the knife on every stroke trough the sharpener. Meaning that when we didn’t do it the angle varies between strokes. This is of course a big nono since it in effect creates a rounded edge.
So basically the prpblem isn’t the sharpening angle intself but rather achieving consistency. The diagrams below show why:

The conclusion was rather obvious:
To achieve real world sharpness we had to compromise on sharpening angle and use a slightly blunter then knives normally have to allow all knives to be sharpened efficiciently. 22° was chosen and used in solution #3 and this results in a really solid sharpener that both sharpens quickly, gives micro serration (which we love) and a really sharp edge which will just be better over time because it adjusts to the sharpeners fixed angle and gets easier and easier to sharpen. Below is video showing off the result and final design of the sharpener based in the #3 findings.

To sum things up:

1. Spring-loaded with variable angle – great idea in theory, not very accurate in practice. Impossible to maintain the sharpening angle due to variable pressure.
2. Crossed wheels – “feels” good and like it could hit any angle preciesly but in reality it’s a single angle with no advantage over a straight sharpener with fixed angle.
3. Spring loaded with fixed angles – kicks ass!