Let's Explore How To Prevent It Happening
Those who are passionate about knives are aware of the fact that most factory knife edges are not up to par when compared to edges created and finished by a sharpener who knows what they're doing, in terms of sharpness and durability. Consequently, a large number of knife owners opt to sharpen their new knives right off the bat, with some even suggesting that multiple sharpenings are necessary before an accurate assessment of a factory knife's performance can be made. Why multiple sharpenings? Do Power Sharpeners Harm Knife Edges?
The BESS sharpness scale addresses this factory "dullness", noting that typical NEW high-end kitchen cutlery tests out on the Edge On UP! sharpness tester at 250-350 grams of force to cut the calibrated test media, while a kitchen knife done by a proficient sharpener can easily be in the 150-175 gram range.
In addition, knives produced in factories may have uneven edges as a result of the grinding process, which may involve the use of rapidly moving belt grinders or wheels, sometimes even grinding free hand. It has been observed that factory knives may exhibit discrepancies in edge angle of up to ten degrees in bevels between the two sides of the edge and seven degrees from the heel to the tip. Please keep in mind that while this information is pertinent, it is not the primary focus of today's discussion. The focus today is on discussing the potential for harm to a knife edge, that may arise from power grinding the apex or edge of a knife. When steel is ground with abrasive wheels or a belt grinder, the abrasives create friction which in turn generates heat. What we sometimes don't consider is that if this heat exceeds the original tempering temperature of the steel, there is a very real possibility of over-tempering the apex, which can cause the steel to become softer. This can have a negative impact on the knife's durability and performance. If the heat is allowed to continue increasing, it can cause the edge to re-harden and become untempered martensite again, thereby further compromising the overall quality of the knife, by causing the apex to become brittle. It's of critical importance to approach knife grinding with utmost care and precision to ensure that the best possible outcome is achieved. Bear in mind that the apex of a knife is micron-thin and as such, heat build-up can be rapid and often catastrophic if not managed properly.
When grinding the blade of a knife, a moderate amount of heat is acceptable in the thicker regions, like toward the spine. This is because the steel volume of the blade functions as a heat sink and assists in the distribution of heat before higher temperatures can damage the blade. However, the scenario is different for the microscopically thin edge of the blade. A well-honed kitchen knife generally has an edge width of a half micron or less. (A micron is about .00004") This size is so minute that it can only be observed clearly with a scanning electron microscope. This minuscule area is of utmost importance and is highly susceptible to damage that can be caused by heat. It is, after all, the BUSINESS part of the knife. Do power grinders harm knife edges? Making knives requires the steel to be heated to high temperatures, quenched quickly, or cooled down to room temperature, and then tempered at a lower temperature to reduce internal stresses. Typically, the tempering temperature is around 400°F (200°C ) or less. If a knife being sharpened is heated to a temperature lower than that tempering threshold, it's unlikely to cause any damage at all. However, the temperature can easily exceed that limit in the sub-micron area of the edge's apex within a second or less, resulting in a compromised "heat-affected zone". In this zone, grain structure, hardness and temper will likely be affected. Edge stability and edge retention will be adversely affected. The heat-affected zone (HAZ) is an area of the knife edge that has not been melted but has undergone changes in properties as a result of being exposed to relatively high temperatures. The HAZ is located from the apex back through the blade to the unaffected base material. This HAZ may be only a couple of thousandths of an inch back from the apex, but if it is there, your edge is degraded. Tempering color or "heat tint" is a useless barometer for checking for HAZ. If you're grinding fast and dry, HAZ cannot be avoided. Your knife won't hold an edge as well as it SHOULD. So is it best to avoid the situation altogether? I believe it is, and there are ways to mitigate the creation of this heat-affected zone (HAZ). Let's go over a few:
1) Only Hand Sharpen Knives-- This isn't a viable option for professional knife makers and sharpeners. Still, it could be the answer for the home sharpener who only wants to keep personal knives keen and ready. Hand-sharpened edges haven't been exposed to the potential heat caused by grinding with belts or wheels.
2) Grind wet-- Add some water or a Kool Mist to your setup. It's messy, but it saves edges! Some sharpening machines come ready for wet grinding as well. Make sure your abrasive belts are the "wet/dry" variety. A belt not rated for wet use will disintegrate rapidly.
3) Grind Slowly-- Slow down the speed of your machine. A powered knife grinder, whether using belts or wheels, should be a variable-speed grinder, period. Does your machine have only one speed? Then don't grind knives on it. You cannot control the heat. If you're ruining your customers' knives or even your own, why are you doing that? Light pressure and quick passes can't stop the edge, the APEX, from being overheated.
4) Use Structured Abrasives-- Structured abrasive belts such as 3M Trizact generate far less heat than other types of belts, while still moving material at an acceptable rate.
5) Dip The Blade-- Dipping the blade in cold water after each pass on your grinder will help prevent the build-up of excessive heat, but it WILL NOT save the apex of the edge. That heat and over-tempering does happen, in milliseconds.
6) Hand Sharpen After Belt Grinding-- I do this for every knife I sharpen. ALL knives get sharpened by hand after a touch-up, edge thinning, or reprofile on the belt sander. This helps remove the possibility of HAZ. At Keith Nix Knives, we use all the other precautions when belt sharpening, thinning edges, or repairing chips, cracks, or broken tips. Then as an added precaution, all finish work, or the final sharpening, is done by hand. Hand sharpening knives ensures we deliver the finest edges possible while minimizing the chance for a HAZ (heat affected zone) that may ruin your blades.
SUMMARY: It is common for knife edges to overheat during power sharpening operations, as the small volume of steel at the blade's apex makes it prone to this issue. To minimize the chance of overheating, it is advisable to grind wet, grind slowly, use structured abrasives, and dip the blade in water. Hand sharpening ONLY is an effective way to eliminate or minimize this issue. Hand sharpening a small micro-bevel after grinding can also help. Exercising great care while sharpening a fine knife is essential, as it can be ruined in mere seconds if not appropriately handled.
As always, I'm grateful to the Knife Steel Nerd in charge, Dr Larrin Thomas. I reference his article and study for much of this. If this subject interests you, I suggest you read Dr Thomas' article. It can be found here: "Does Sharpening With A Grinder Ruin Your Edge?"
-By Dr. Larrin Thomas
Other sharpening posts and articles at Keith Nix Knives: Local Knife Sharpening in Black Mountain
Thanks for reading,