Knife sharpening is the process of making a knife or similar tool sharp by grinding against a hard, rough surface, typically a stone, or a flexible surface with hard particles, such as sandpaper. Additionally, a leather razor strop, or strop, is often used to straighten and polish an edge. The smaller the angle between the blade and stone, the sharper the knife will be, but the less side force is needed to bend the edge over or chip it off. The angle between the blade and the stone is the edge angle – the angle from the vertical to one of the knife edges, and equals the angle at which the blade is held. The total angle from one side to the other is called the included angle – on a symmetric double-ground edge (a wedge shape), the angle from one edge to the other is thus twice the edge angle. Typical edge angles are about 20° (making the included angle 40° on a double-ground edge). The edge angle for very sharp knives can be as little as 10 degrees (for a 20° included angle). Knives that require a tough edge (such as those that chop) may sharpen at 25° or more. Different knives are sharpened differently according to grind (edge geometry) and application. For example, surgical scalpels are extremely sharp but fragile, and are generally disposed of, rather than sharpened, after use. Straight razors used for shaving must cut with minimal pressure, and thus must be very sharp with a small angle and often a hollow grind. Typically these are stropped daily or more often. Kitchen knives are less sharp, and generally cut by slicing rather than just pressing, and are steeled daily. At the other extreme, an axe for chopping wood will be less sharp still, and is primarily used to split wood by chopping, not by slicing, and may be reground but will not be sharpened daily. In general, but not always, the harder the material to be cut, the higher (duller) the angle of the edge. It can also be divided into diamond knife sharpener, ceramic knife sharpener and carbide knife sharpener according to the material. Manual knife sharpeners that are considered to be countertop utensils are also available that are more detailed than the Sharpening Steels. Tools that are built with grinding devices built into plastic or metal cases are often small in size but able to simplify the sharpening activities considerably. Often, the Knife Sharpener will contain a diamond coated abrasive or a tungsten carbide grinding stone for sharpening. A knife is drawn light through the grinding stone at an angle that is preset with the use of guides that keep the knife angle fairly consistent as it is drawn across the sharpening stone. Four to six draws of the blade is usually sufficient to keep the blade well honed. A guard is commonly attached to the sharpener so hands are kept safetly away from the knife blades. Electric knife sharpeners are another alternative for shapening knifes and are a type of utensil that often simplifies the sharpening process, particularly if the knives are being used frequently for various food cutting activities. Electric sharpeners are designed with basic features for home or moderate use as well as sharpeners built for higher volume usage, such as commercial sharpeners. When selecting consider the size desired, the type of grinding material on the stones, the types of blades that can be sharpened with the device, the safety features, and the ease of cleaning to remove small bits of edge grindings. Lower priced models will contain only one sharpening slot that offer single sharpening features while other models will have sharpening sections for different purposes such as pre-sharpening to remove burrs and old edges before placing the knife in the sharpening or honing section to finish the process. If a knife is to be used only periodically with a minimal amount of cutting, select the lower priced models. However, if the slicing, dicing and cutting chores are frequent, the higher priced or more heavy duty models may be a much better option for keeping all types of knives well sharpened. And there are also scissor knife sharpener and pocket knife sharpener. The pocket knife sharpener is a compact, packable knife sharpener designed for sharpening on the go. The diamond sharpening plate and ceramic honing rod use built-in angle guides to create a sharp edge anytime, anywhere. A honing steel, sometimes referred to as sharpening steel, whet steel, sharpening stick, sharpening rod, butcher's steel, and chef's steel, is a steel rod, ceramic sharpening rod, or diamond sharpening rod used to restore the sharpness of dulled edges. They are flat, oval, or round in cross-section and up to 30 centimetres (1 ft) long. The steel and ceramic honing steels may have longitudinal ridges, whereas the diamond-coated steels are smooth but embedded with abrasive diamond particles. Non-abrasive honing rods such as smooth ceramic or ribbed steel are able to remove small amounts of metal via adhesive wear. In normal use, the rod is applied to the blade at a slightly higher angle than that of the bevel, resulting in the formation of a micro-bevel. Sharpening stones, or whetstones, are used to sharpen the edges of steel tools and implements, such as knives, scissors, scythes, razors, chisels, hand scrapers, and plane blades, through grinding and honing. Such stones come in a wide range of shapes, sizes, and material compositions. They may be flat, for working flat edges, or shaped for more complex edges, such as those associated with some wood carving or woodturning tools. They may be composed of natural quarried material or from man-made material. They come in various grades, which refer to the grit size of the abrasive particles in the stone. (Grit size is given as a number, which indicates the spatial density of the particles; a higher number denotes a higher density and therefore smaller particles, which give a finer finish to the surface of the sharpened object.) Stones intended for use on a workbench are called bench stones, while small, portable ones, whose size makes it hard to draw large blades uniformly over them, especially “in the field,” are called pocket stones. It can be divided into diamond sharpening stone, ceramic sharpening stone and flattening stone, etc. Also, there are a raft of sharpening accessories, such as knife sharpening guide, sharpening oil and head of electric knife sharpener.