A blunt knife serves its purpose only to a limited extent, because the proverbial sharpness rises and falls with this centuries old tool. And although everyone probably has a clear idea of what sharpness is, a technically reliable definition is hard to find.
A recognised industrial standard exists for sharpness only supplied with simultaneous assessment of the property of edge retention (DIN EN ISO 8442-5 cutting ability and edge resistance). Different subjective perceptions of sharpness can also be found again and again, And of subjective perception of sharpness by different assessors. All too often the sentence „But that‘s not very sharp! was the introduction to the onset of larger quantities of blood. For all important treatises on the subjects of steel, types of grinding, sharpening, leathering and whetting, the sharpness that is the goal of all these arguments and efforts, is rarely discussed. A uniform conception of the term is assumed. To clarify the understanding of all further topics would have to first be the following questions: What is sharpness? What factors does sharpness depend on? How can the sharpness of a blade be tested? What should sharpness do?
The phenomenon of sharpness is detached from the topic of edge retention - the ability of a blade to maintain its sharpness over a certain period of time. Complicated interactions between the steel quality, the grinding angle, the grit and the material to be cut are relevant. Here, however, we are only concerned with a definition and assessment of sharpness as a condition at a certain point in time. What is sharpness? Sharpness is the ability of a blade to separate material mechanically and in a controlled manner. What is separated is always a molecular connection. Non-mechanical and material cutting would be, for example, cutting with a flame cutter, a plasma cutter, laser or water jet cutting.
Non-controlled cutting, on the other hand, is equivalent to tearing. To try to use a technical measure of this, of how sharp a knife can generally become, in principle only the concept of the cut-off stump. The cutoff stump is the distance measured transversely to the cutting edge in the microscopic range - in fact, in the scanning electro-microscopic range - from which a blade can no longer be sharpened. We imagine a blade as an edge, in cross-section as a point. At the scanning electron microscope, however, a blade looks more like a rocky ridge or, in cross-section, a stump. The limit of the narrowness of this stump is given by the fineness of the steel structure. Steel has a crystalline lattice structure consisting of carbide nests. The blade cannot be narrower than such a carbide nest. With an impressive steel structure and careful grinding the width of the cut-off stump of the break can be less than a micron (also known as a micrometre and also written μm or My). A micron is the millionth part of a metre. For comparison, an average human hair has a thickness of about 60 μm, a fine dust particle has a maximum thickness of 10 μm.
What factors does sharpness depend on? The sharpness of a blade depends on three factors: Angle The grinding angle is specified differently by knife and grinding material manufacturers. either as an absolute angle between the two sides of the blade (for example as 40°) or as half of it, which is the angle between the grinding material and the centre of the blade (in this example 20°). The tendency for this influencing factor is, the flatter the angle, the sharper the edge. • Graining The graining, or grain for short, refers to the diameter of the grain of the grinding material - in manufacturing technology also referred to as abrasive materials - with which the blade was ground. The tendency here is, the finer the grain, the sharper the edge. A final leathering of the blade with paste (which contains an extremely fine, rolling grain and again minimally 332 Rasiermesser, vorgeschliffen auf Naniwa Chosera 1000. Copyright: Dr. Todd Simpson, University of Western Ontario Nanofabrication Facility. London, Ontario, Canada thins the edge again) or with untreated leather (which smoothes the edge once more without abrasion), can additionally increase the sharpness. Uniformity The factor of uniformity relates to both the angle and the grain. The angle must be maintained uniformly over the entire length of the blade. The abrasive material must have a homogeneous structure. Based on the manufacturer‘s experience, mixtures of grain sizes can also achieve a very good result. However, the quality and distribution of the abrasive grains is important, which must also be free of foreign inclusions.
How can you check the sharpness of a blade? A reliable and low-wear test for determining the sharpness of a knife is to rest the edge with its own weight, loosely on the thumbnail. By doing this, two different tests in succession of each other, different properties can be tested: • For the basic test place the blade across the thumbnail held at an angle. If the blade grips immediately and does not slip, it is sharpened. If it slips, the knife should be set at a steeper angle again. If it then grips, the blade is well honed and even, but at a relatively steep angle. The actual blade had no contact with the horn of the nail, but slipped off at the edge of the blade. Only when the blade slips at any other angle, it is blunt. The extended test is often seen with experienced professional grinders in order to check the uniformity and the grinding. In this test, the blade is drawn vertically over the tip of the thumbnail. In this way, you can immediately feel any small nicks and the grain immediately. Some experience and a sufficiently strong and a thumbnail not cut too short are a prerequisite for an injury-free performance! Both are largely wear-free methods for testing the sharpness. However, they are not suitable for razors, as the very fine edge of the razor can be damaged by the by the horn of the thumbnail, even with the utmost care.
Although the above methods for determining the sharpness are simple and do not require other tools, it is surprising that people perceive sharpness so differently and often use less appropriate and suitable methods to assess it. A common method is to try to check the sharpness with the tip of the thumb. It is almost impossible to feel the sharpness of a fine-grained and cleanly burr-free blade. Coarser grain or a burr, on the other hand, can be felt and are then often mistakenly taken as sharper. Another prominent example is the paper cutting test. Here it can be that the blade has been sharpened cleanly with fine grain but shows that the angle is too steep. The blade then acts like a wedge in the hard paper and prevents it from penetration into the cutting material. With the same blade hair from the forearm can be shaved in one stroke. The cutting test in paper should also be avoided because the blade can be severely damaged by the paper.
What should sharpness do? The demands on a blade can be many and varied, but in essence the work with a blade can be divided into two categories: With a PRESSURE CUT, the blade is pushed through the material to be cut. This type of cut is made, for example, with razors or knives for working leather (such as saddler‘s or furrier’s knives). The blades must be ground at a flat angle and with a very fine grain. The pressure cut usually requires the highest sharpness according to the factors mentioned above. With the PULL CUT, the blade is, as the name implies, pulled through the through the material to be cut. Typical examples of this type of cut are bread slicing or cutting a thick rope. Here a coarser grain, which has a certain sawing effect is advantageous. Blades that are exclusively designed for these two extreme examples, of course, usually have a special serrated edge. Which angle and which grain is optimal depends not only on the application and the use of the particular material to be cut, but also on personal preferences. In terms of grain, a chef‘s knife can be honed on a stone with a grain size of 8,000 JIS and then leathered with a little practice to a very fine sharpness. In a light bread crust or a ripe tomato, however, some may prefer the sharpness of a quick of a quick honing on a 400 stone. With the knowledge of the physics involved hopefully everyone can find their own optimum. Have fun with your sharp knives!