MYTHS ABOUT KNIFE EDGES

[KnifeGrinders]

Common Misconceptions About Knife Edges



Myth – The best edge angle is 40-50° for chopping, 30-40° for cutting, and 20° included for slicing


Source: Tormek handbook

The truth is that the best is the thinnest possible edge for the cutting task.
The antique wootz swords in my collection that still have a battle-ready sharp edge show around 18-20 degrees included on a laser protractor.
Japanese katanas edge is 16-18 degrees included.
For the best performance the knife edge must have the lowest angle its steel can hold; give your edge the least possible angle not over-thinning the edge past the angle at which it deforms in cutting the stuff you intend to cut.
This has been proved by Cliff Stamp (http://cliffstamp.com/knives), by our BESS-SET experiments on edge rolling in mainstream knives (http://knifeGrinders.com.au), and by CATRA tests on high-end steels done by Larrin Thomas (http://knifesteelnerds.com).

Myth - Obtuse edge angle has longer edge retention
Some have also speculated that lower angle edges start out sharper but a more obtuse edge lasts longer, and that reducing edge angles decreases the cutting lifetime.

Reality is that smaller angle stays considerably sharper and the difference holds to the end of the CATRA and BESS-SET tests on mainstream and super steels.
Take for example well-hardened felling axes – they are sharpened at 25° included and sharp enough to shave with. The only situation where the obtuse angle wins is when the knife is used to open tin-cans.

Myth – Too fine an edge dulls quicker
A common misbelief is that sharpening past a certain point is useless since the edge that is too sharp will dull fast.
 
Truth – For better cutting performance, the edge should always be sharpened to the sharpest apex you can.
Experiments have shown that the best cutting performance through the test have knives with the best initial sharpness.
Sharp apex does not predispose the edge to rolling - over-thinning the edge past the angle at which it deforms in cutting certain stuff does.
Try to visualize the rolled edge with your mental vision, see this edge bend and the sharp apex at the bend end - the stuff it cuts wraps over the bend until it contacts the edge apex where it is severed; when the apex is sharper even the rolled edge cuts better. In other words, the sharper edge mitigates edge rolling.

Myth - Coarse edge cuts better



The "micro-serrations" concept contributes to the confusion as people tend to generalise properties of the serrated edge onto the microscopic level of a ragged coarse edge.
A coarser edge does slice better initially due to "micro-serrations", however the coarse finish is associated with increased grip force, cutting effort and cutting time, and worse edge retention – the knife dulls faster as the "teeth" get broken off or bent to one side or the other. In precise cutting and carving unpolished bevels cause traction in cuts and lower accuracy and quality of cut.

Myth – Convex grind aids in edge retention
Reasoning is that the edge apex needs enough "meat" to last, and the edge will dull quicker where there is not enough metal behind it to give it strength.


Source: Bark River Knife Collector's Association

Truth - Stump splitting and bone chopping are the only tasks where the convex edge is advantageous. Other than for meat cleavers, splitting axes and machetes and larger camping knives used for splitting, the optimal edge is not convex. Those blades don't work as well with a hollow grind because the shape of the bevel "stops" the force of the stroke, and it also does not provide the same support for the edge subjected to a serious impact.


For those kind of blades it is the go, but for most other every day applications the convex edge is less than ideal. Yet "survivalists" would argue that if left with only one knife, the convex edge is superior - if I were left to survive in the woods, I’d rather have a hollow-grind knife and a hatchet.

Myth - Super steels hold super-sharp edge



Truth - The initial blunting rate is relatively rapid regardless of steel. The high-end wear-resistant edge sharpness quickly moves beyond the shaving range to just sharp. Though wear-resistant edges win in the long run, they lose their initial keenness almost at the same rate as a mainstream knife. High-end knives win over mainstream as stayers, but they are equal sprinters.
We’ve seen this in both in our BESS-SET tests and CATRA tests done by Larrin Thomas, let alone the real life.

Myth – knife manufacturers sharpen at an angle optimized for the best performance


Source: Zwilling J.A. Henckels

Actually, where the knife comes with a warranty, the manufacturer applies the edge angle which will make repair returns as unlikely as possible - in other words the factory edge is optimised for abuse, not cutting.
Where there is no clear replacement policy, the edge angle can be anything no matter what the knife purpose is. We’ve checked edge angles on slicing, filleting, carving, sticking and boning knives made by leading professional knife manufacturers – any of these knives could have any angle: 30, 32, 34, 36, 38, or 40° included, seemingly regardless to the knife function; the voluntary ignorance we’ve seen is almost flabbergasting.
It is normal for knives with distal taper to have edge angle slightly increasing from the heel to the tip to maintain a uniform bevel - however hardly we can call it "normal" when we see an edge angle on one side higher than on the other on a production knife. In the few manufacturers’ videos on YouTube and Instagram knives are sharpened freehand, with no angle guides.

Myth - Cheap knives are easier to sharpen


By courtesy of Lewis Hilsenteger, Canada

Every so often I run across a customer asking for a discount on sharpening his junk knives, motivating that the knives cost him less than would sharpening. Video gamers "collectible" knives are also in this category.
True that those inferior steels are easier to grind, but that shiny stainless steel is a pain to deburr - getting rid of the gnarly burr these cheapo knives usually produce is so difficult that you can clean the edge apex only at the cost of rounding it.
In response to your efforts the highly malleable burr just keeps bending back and forth and bending and bending like a foil leaf. To get rid of this crud of metal you take a higher and higher angle, gradually realizing that in the process of removing the existing burr you're also drawing new metal out past the apex and creating a new burr – the reason people call it mutant burr and burrZilla.
There are times when you think you’ve succeeded, yet under a good light you still can see remnants of the burr. Overall, sharpening junk knives can be extremely frustrating unless you know a method to abrade the burr away faster than a new burr is created.

Myth - Japanese knives must be sharpened on Japanese waterstones



The demand for natural Japanese sharpening stones comes mainly from the traditional sword polishers, while chefs prefer synthetic stones. The Japanese synthetic stones are made of the same common abrasives used elsewhere outside Japan like aluminium oxide, silicon carbide and chromium oxide; what makes them Japanese waterstones is the technology of fusing the abrasive particles in a porous matrix – but even this is becoming history. Nowadays less and less Japanese synthetic stones require soaking in water for use, and more and more resemble non-Japanese sharpening bench stones that should be stored dry and used with just a splash of water on the surface.

Myth – Finer abrasive grit correlates  with sharper edges



There is no clear correlation between the abrasive grit rating and apex width you get on it. For example, you can produce an edge apex in the safety razor range of 0.1 micron on a 300 grit abrasive, though with a significant variation in the apex width along the edge.
Factory edges never come polished, and when they are sharp they are the best proof that edges don't need a fine scratch pattern, let alone polished finish, to be scary sharp.
However, in refining the sharp edge the honing compound grit size and the scratch size are related by about an order of magnitude, e.g. honing/stropping with a 0.5 micron abrasive refines your edge finish to 0.05 micron.
Fine grits are used is to improve the edge finish and keenness for effortless cutting, but not the sharpness as such; a more refined edge also holds up better.

Myth – Mirror edge is not worth the effort
Someone called mirror finish "a labor of love". One look is worth a thousand words -


Milla Jovovich photo by Chris Floyd


When I sharpened this Santoku for an Australian knifemaker Snake & Rabbit Knives I mirror polished the 8 dps edge to match the rest of the blade. The photo is unedited, taken in daylight - the mirror finish is so perfect that the blade blends with the reflection like a stealth.