Knife makers make a lot of sharpness claims, but what does it mean?

Sharpness has no technical meaning and because of this, knife manufacturers can make outrageous claims without being sued by the FTC.  Complicating the issue is that customers think “sharp” should mean “effortless cutting.”  Cutting effort is determined by three factors: 1) the knife edge angle and profile; 2) the width of the blade and; 3) the coefficient of dynamic friction between the blade and the substance being cut.  As an expert sharpener, and engineer we wondered if we couldn’t quantify aspects of the cutting edge as a way to measure sharpness.  We were more successful than we expected. 

We all know that a sharp knife cuts easier than a dull knife so we decided to find out why.  We put all our Rocket Scientists into one room and the rest of us brought in samples of various dull and sharp tools.  When we looked at these objects under high-power magnification, it became clear that the sharper the tool, the more planar the edge.  As you can see from the pictures below (which show cutting edges on the lower part of the frame), the planarity of the edge of a razor blade is at least 100 times better than a brand new Wüsthof chef’s knife. Note how jagged the Wüsthof edge looks compared to a razor blade.  The variations are 1-3 microns deep and 10-50 microns wide.  Using this system we can monitor the quality of our sharpening service and at the same time, quantify at least one definition of sharpness.  We call it the Holley Edge Metrology System. 

Razor blade edge in profile                                                 
Wüsthof edge in profile

If you’ve followed this so far, you are probably wondering why knife makers don’t make razor-like edges despite making razor-like claims.  One of the reasons is that it probably isn’t needed.  The Wüsthof knife above is extremely sharp - they don’t need to make any apologies.  One micron is about 1/100 the width of a human hair and that is as deep as the variations get.  The other reason is that razor blades are sharpened to a more acute angle which makes planarization easier.

Our sharpening service aims to keep the edge of our customer’s knives planar to no more than one micron.