Micro-edge drive when sharpening a knife
1. The latest buzz in sharpening
Nowadays in the knife world, there are many knife sharpening techniques that you can learn about from books or find on the internet. At the same time, each of the methods has its followers and specialists who completely reject one or another of them. Ironically each of them is convinced that they are absolutely right. In turn, we are confident that most sharpeners and advanced users are familiar with the concept of the micro bevel and know how to create it. Now we will analyze why you might need a micro bevel and how to create it.
2. The concept of the micro-bevel and blade structure
Micro bevel is a term for a bevel that forms a cutting edge and comes after the secondary bevel. If you consider knife sharpening in general, it seems quite obvious at first glance that the creation of a micro bevel takes off a great deal less steel than a complete sharpening of the bevels. However, everything should be examined step by step, so first we will take a look at the cross-sectional structure of the blade, regardless of the type of bevels.
2.1. How bevels affect the creation of secondary bevels
Bevels are one of the important parts in the blade structure and are directly involved in creation of cutting edges.
Image 1. The drawing of knife parts - thickness behind the edge and secondary bevels: 1 - blade width, 2 - thickness behind the edge, 3 - cutting edge, 4 - secondary bevels; A - sharpening angle of bevels.
The image 1 illustrates a cross-sectional view of the blade before you sharpen it. It is the bevels angle that determines the thickness behind the edge, which ultimately predetermines the possible angle of the secondary bevels of the blade and as a consequence the suitable micro bevels angle.
2.2. Section of a micro bevel
Image 2. The cross-section of a blade with micro bevels: 1 – bevel, 2 - secondary bevel, 3 - micro bevel, 4 – cutting edge
A is the bevel angle, which forms the thickness behind the edge in the form of a small trapezoid on the knife blank before you sharpen it, which is often less than 0.4 mm on many good knives.
B - angle of bevels which forms the cutting edge of the blade.
C - the full sharpening angle of the blade with a micro bevel
The main purpose of the micro bevel for this blade structure will be the strengthening of the cutting edge. The reason for the increase in resistance or strength on any steel is pretty clear - the angle of pressure from lateral impact on the bevel at the "B" angle is drastically different from the impact on the bevel at the "C" angle. The consequence of this is a higher level of cutting edge sharpness retention due to the micro-bevel at the "C" angle.
The mechanics of knives cutting involve a sharper, harder object penetrating the material due to high pressure within a very small contact spot. You may get the impression that the micro bevel has a larger angle and makes the blade duller, but that is not true.
There is a pattern here - the smaller the thickness behind the edge, the sharper the angle of secondary bevels can be, and the greater the difference with micro-bevels. With very little thickness behind the edge, and even with a significant difference in angle with a micro-bevel, knives will cut due to the geometry of the knife blade. First of all, this applies to kitchen knives edges.
If you consider the cross-section of knives with micro-bevels, you will notice that in some cases the cross-section partially resembles a multi-bevel compound grind, or even to some extent a variant of a full convex blade grind.
However, there are some significant differences - in the first two cases, as mentioned at the very beginning, we are dealing with different sharpening approaches, and the third case is a manufacturing process which aims at creating a certain type of knives bevels.
In this case, however, we focus specifically on sharpening processes that will allow us to sharpen and create micro bevels on knives.
3. Technical features of sharpening a micro-bevel knife.
When you sharpen knives, you want them to stay sharp as long as possible after one sharpening session, so the cornerstone here is the concept of sharpness retention of the cutting edge.
3.1. The difference of the blade grind with and without a micro-bevel
Although each blade's quality and steel hardness is different, you only need to take a look at the following image to see that there will be a significant difference in the amount of removed steel when you sharpen a knife with a micro-bevel, compared to sharpening a knife without one.
Image 3. What happens when you sharpen with and without a micro–bevel: 1 – chisel blade without a micro–bevel, 2 - approximate removal of steel over the entire bevel, 3 - removal of steel with creation of a micro-bevel.
If we look at the cross-section of the blade of a chisel, it becomes clear what a normal bevel looks like after you sharpen it without a micro-bevel (1). The second picture shows the approximate amount of material that gets removed when you sharpen the entire bevel area (2). The third picture shows the approximate amount of steel that gets removed when sharpening with creating a micro bevel (3).
Creating a micro bevel on a fully sharpened blade takes some extra effort and a certain level of precision. However, there will be much less material removal when sharpening the micro bevel again, which saves considerable effort and prolongs blade life.
If we consider the blunting of a cutting edge as a process of rounding of its apex (picture 3.2), then the purpose of sharpening is to remove this rounding by reshaping the bevels planes. Sharpening knives without a micro bevel removes more material and takes much longer time with harder steels.
This technique is used for knives with very small thicknesses behind the edges, on hard steels and steels with large carbides to avoid chipping, and to make the geometry of the blade work when cutting. This is even more relevant and useful for softer and more ductile steels, especially for European-type knives. They often recommend using a honing hand tool, the musat, for this purpose.
You can only get good quality micro bevels with high precision equipment that offers angle consistency.
With a steep angle of bevels and a large thickness behind the edge, the difference between the bevels and the micro bevels angle should be about 5 degrees per side.
It is best to keep the difference in the angles of the bevels and micro bevels within 5-10 degrees, and the width of micro bevels in tenths of a millimeter.
To get the best perspective, you should examine the cross-section of a chisel blade (instead of double sided blades), which is an example of a blade exposed to heavy loads, and which micro bevels continually prove to be effective,
Image 4. Schematic difference in the angles of a chisel with a micro bevel
The use of micro bevel is illustrated in image 4.
If you examine the area near the cutting edge of knives and the way you create a micro bevel on the knife blades, you will understand more about what happens when you sharpen knives.
Image 5. Knife micro bevels area: 1 - bevels angle, 2 - micro bevels angle, 3 - cutting edge without micro bevels, 4 - cutting edge with micro bevels.
The essential ingredient in creating a micro bevel is the use of a sufficiently fine abrasive with a medium to fairly high hardness.
A burr is inevitable when you sharpen knives with back and forth movements. The correct way to remove a burr is with light, only forward movements of the abrasive to avoid breaking the burr and creating a blunt apex with jagged, sharp edges. When you remove the burr this way, you already create a kind of micro bevel. So it makes sense to create a micro bevel the right way.
3.2. Industrial examples of micro bevels
The reason for mentioning blades with full convex or scandi grind with straight bevels is obvious. There are some industrial knife models, which come with micro bevels from the factory. A common example is MORA 510 with micro bevels, according to the manufacturer's information. This seems to contradict the concept of scandi bevels, but it is just a fact. As an example, there are many Japanese kitchen knives that come with micro bevel edges from the factory.
The creation of micro bevels is the final stage after you sharpen the secondary bevels. If you decide to create a micro bevel, you should use fine-grit abrasives and make forward movements without pressure on the sharpening stone. It is best to use high precision tools and equipment that offer consistency of the angles, as it is not easy to create knife blades with the proper micro bevel and sharp cutting edge with manual hand sharpening. A TSPROF sharpener is the best solution for this task.