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Sharpening

[shahr-puhn]

Sharpening is the process of creating or refining a sharp edge of appropriate shape on a tool or implement designed for cutting. Sharpening is done by grinding away material on the implement with an abrasive substance harder than the material of the implement, followed sometimes by processes to polish the sharp surface to increase smoothness and to correct small mechanical deformations without regrinding.

Tools and materials for sharpening

The substance on the sharpening surface must be harder (hardness is measured on the Mohs scale) than the material being sharpened; diamond is extremely hard, making diamond dust very effective for sharpening, though expensive; less costly, but less hard, abrasives are available.

Implements with essentially straight edges

Many implements have a cutting edge which is essentially straight. Knives, chisels, straight-edge razors, and scissors are examples. Sharpening a straight edge is relatively simple, and can be done by using either a simple sharpening device which is very easy to use but will not produce the best possible results, or by the skillful use of oil or water grinding stones, grinding wheels, hones, etc.

Sharpening these implements can be expressed as the creation of two intersecting planes which produce an edge that is sharp enough to cut through the target material. For example, the blade of a steel knife is ground to a bevel so that the two sides of the blade meet. This edge is then refined by honing until the blade is capable of cutting.

The extent to which this honing takes place depends upon the intended use of the tool or implement. For some applications an edge with a certain amount of jaggedness is acceptable, or even desirable, as this creates a serrated cutting edge. In other applications the edge must be as smooth as possible. Ultimately, the more closely matched the angles of the adjoining faces are, the sharper the finished edge will be. However, certain hunting and fishing blades will make use of a 'missed-matched' blade edge to increase the letting of blood. These blades are not used for finishing purposes.

Steeling

Sharpening straight edges (knives, chisels, etc.) by hand can be divided into phases. First the edge is sharpened with an abrasive sharpening stone, or a succession of increasingly fine stones, which shape the blade by removing material; the finer the abrasive the finer the finish. Then the edge may be honed or stropped by polishing the edge with a fine abrasive such as rouge or tripoli on a piece of stout leather or canvas. The edge may be steeled by passing the blade against a hard metal "steel" (which may be made of ceramic) which plastically deforms and straightens the material of the blade's edge which may have been rolled over irregularly in use, but not enough to need complete resharpening.

Other types of implements

Different techniques are required where the edges are not straight. Special tools and skills are more often required, and sharpening is often best done by a specialist rather than the user of the tool.

Examples include: