Optical Monitoring During Sharpening

Optical monitoring plays an important role in sharpening operations. Even during the stage of learning to sharpen, when the sharpener has not yet became skilled, he already needs to have a clear idea of what a burr is, what are the scratches, left by various abrasives on the secondary bevel, what are the unwanted scratches, how should the properly polished secondary bevel look like and so on. A clear understanding of the complex aspects makes it possible to move on, increasing the level of skill.

By using optical monitoring, the sharpener can solve a number of problems:

1. Verify that you are getting the desired sharpening angle. This is especially important when changing abrasives if they are of different thicknesses and the sharpener forgets to use the abrasive thickness compensator.

2. Control of processing of the cutting edge, which will be carried out by the look of the burr. It is very important to visually monitor its presence along the entire length of the cutting edge and the absence of unsharpened areas. The look of the burr belongs to this point as well, thin and flexible (foiled) burrs may require methods such as "step sharpening" or "technological barrier" for removal.  It is also important to check the complete absence of burr on the cutting edge visually after deburring.   

3. Control of evenness of sharpening marks and cleanliness of the surface. This is a very important criteria, according to which it is possible to determine the appropriateness of the use of specific abrasives on a particular steel and the duration of work with them.  It is especially important for sharpening knives with complex blade geometry (tanto, recurve, changeable geometry, etc.).

4. Detection of stone clogging, when grains of coarser abrasives get on the soft bundle of fine-grained abrasives and get stuck there, leaving scratches on the surface of the secondary bevel. 

For optical monitoring during sharpening you can use special devices from other spheres of human activity (medicine, repair of high-precision equipment, jewelry industry). These are magnifying glasses (loupes) and various microscopes.  

Loupe (magnifying glass) is an optical system that consists of one or more lenses and is designed to observe small objects. Its main parameters are diameter, magnification, field of view and focal length. Usually magnifiers with focal length of ~2-20 cm and magnification up to 40x are used to monitor the sharpening process.

The degree of magnification, or multiplicity, is the difference in size between the image of an object seen through the lens and its actual size. And the higher the magnification, the shorter the focal length. For example, a lens with a focal length of 2.5 cm will provide a clear image at 2.5 cm from your eyes: the object at this distance will be as visible as if you were looking at it at 25 cm. The magnification of such a magnifier will be 10x. The magnification is calculated using a special formula: MP = 250/FL, where MP is the magnification and FL is the focal length in millimeters. When buying a magnifier for sharpening purposes, it is necessary to remember that the depth and sharpness of the shown object decreases with increasing magnification power. That is, the stronger this device magnifies, the harder it is to catch sharpness while holding it in your hand without using additional holders. In addition, the higher the magnification of the lens on the magnifying glass, the smaller its diameter, which means a smaller section of the cutting edge can be covered in one glance, without moving the magnifier. And as the diameter of the lens decreases, so does its luminosity. Therefore, the best magnifiers have a fairly small diameter and obligatory additional illumination.

Glass, optical polymer or acrylic plastic are the materials used to manufacture modern magnifying lenses. A glass lens has a substantial weight, forms a clear image and is resistant to minor damage. Optical polymer is twice as light as glass, but is quite expensive and prone to various damages. Acrylic plastic is the cheapest option. It is lighter than glass and more resistant to destruction than optical polymer. The disadvantage of acrylic is blurriness and lack of sharpness of the image, especially at the edges of the lens.

Although the magnification is always less than that of a microscope, it has a number of advantages in optical monitoring. They include an enlarged field of view, the ability to control light sources from either side and to view both the secondary bevel and the cutting edge at any angle. In addition, the magnifying glass allows you to quickly monitor the sharpening process without removing the knife from the clamps. Thanks to the magnifier you can see the result of work with each abrasive, the location and type of sharpening marks, and the consistency of raising a burr in real time. Its use makes it possible to quickly correct the abrasives used. For example, if a diamond stone creates chips, you can continue sharpening with ceramic-bonded aluminum oxide stones. Or if you notice that a certain area of the secondary bevel is not properly sharpened, you can go back to the previous abrasive and correct the flaw. Quick use, lightness and accessibility make the magnifying glass an indispensable tool for sharpeners.

A microscope (Greek μικρός "small" + σκοπέω "looking") is an instrument designed to obtain magnified images as well as to measure objects or structural details that are invisible to the naked eye. Its key quality is the resolution - the ability to produce a clear separate image of two closely spaced points of an object. This characteristic is determined primarily by the wavelength of the rays used in microscopy. It can be visible, ultraviolet or X-ray. Microscopes with visible rays are used for sharpening purposes. Most often, electronic devices with a connection to a computer (digital) or optical medical instruments are used. For sharpening purposes, a microscope up to 100x is sufficient. Microscopes can be as small as possible and, like magnifying glasses, can be used for quick monitoring. However, such a microscope does not have much power and a significant advantage over a good magnifying glass.

A powerful microscope is most often a stationary device, which has a sufficiently large weight and is placed in a specially designated place for it. In this case, the object in question is placed on the slide. That is, to use most microscopes in sharpening, you need to take the knife out of the clamps. This significantly limits its use during quick monitoring. Therefore, sharpeners most often use it to examine the results of their work. In particular, thanks to the microscope, it is possible to form an opinion on the cleanliness of the secondary bevel achieved after polishing, as well as to consider difficult points that may be encountered by the sharpener. For example, the microscope allows you to see the nature of a persistent foil burr and understand why it could not be properly removed. It allows you to look at the sharpening mark, in particular at the unwanted mark formed by the finishing abrasive, and to think of ways to remove it. In addition, this device can be used to find unwanted inclusions on the surface of the sharpening stone and to take measures to remove them.

Optical monitoring during sharpening is one of the most important methods for understanding all the processes that take place when abrasives work on the surface of steel. Its use significantly speeds up the formation of sharpening skills and abilities.