Stationary Sharpening Stones

The most frequently used sharpening equipment consists of a block or stick of abrasive material called stone. The stone is supported on a firm surface, and the instrument is oriented and held by hand while being stroked against the stone surface. Stationary stones are often called oilstones because of the common practice of applying a coating of oil on them as an aid to the sharpening process. Sharpening stones are available in a variety of grits, shapes, and materials.

Stationary oilstones are available in coarse, medium, and fine grits. Only a fine-grit stone is suitable for the final sharpening of dental instruments to be used for tooth preparation. Coarse and medium grits may be used for initial reshaping of a badly damaged instrument or for sharpening other dental equipment such as bench knives. Coarser stones cut more rapidly but produce a rougher surface. If the use of two or more grits is required, the coarser one is used as little as needed for reshaping, and then the final sharpening is done with a fine stone.

Stationary stones can be obtained in various shapes, including flat, grooved, cylindrical, and tapered. Flat stones are preferred for sharpening all instruments with straight cutting edges; other shapes are most useful for sharpening instruments with curved cutting edges. Cylindrical stones are used for sharpening instruments with concave edges, and tapered stones permit the use of a portion of the stone with a curvature matching that of the instrument.

Sharpening stones are made from any of several natural or synthetic materials. The normal manufacturing process for the synthetic materials involves pressing carefully sized particles of an abrasive into the desired shape and heating to form a solid. To maintain sharp edges on the particles, the process must result in a porous material. The properties of the stone depend on the volume and size of the pores and on the composition and size of the abrasive. Four types of materials are in common use for sharpening stones: Arkansas stone, silicon carbide (SiC), aluminum oxide, and diamond.

Arkansas stone is a naturally occurring mineral containing microcrystalline quartz and traditionally has been the preferred material for fine sharpening stones. It is semi-translucent, white or gray in color, and hard enough to sharpen steel, but not carbide instruments. Arkansas stones are available in hard and soft varieties. The hard stone, although it may cut more slowly, is preferable because the soft stone scratches and grooves easily, rendering it useless. These stones should be lubricated with light machine oil before being used. This assists in the fineness of sharpening, prevents clogging of the stone pores, and avoids the creation of heat, which alters the temper of the steel blade. An Arkansas stone should be covered with a thin film of oil when stored. During the sharpening of an instrument, the fine steel cuttings remain on the stone and tend to fill up the pores of the stone; when the stone appears dirty, it should be wiped with a clean woolen cloth soaked in oil. If the stone is extremely dirty or difficult to clean, it may be wiped with a cloth soaked in alcohol.

SiC is widely used as an industrial abrasive. It is the most commonly used material for grinding wheels and “sandpapers” and for sharpening stones. It is hard enough to cut steel effectively, but not hard enough to sharpen carbide instruments. SiC stones are available in many shapes in coarse and medium grits, but not in fine grits. As a result, they are not as suitable as other materials for the final sharpening of dental instruments. SiC stones are normally of a dark color, often black or greenish black. These stones are moderately porous and require lubrication with a light oil to prevent clogging.

Aluminum oxide is increasingly used to manufacture sharpening stones. Aluminum oxide stones commonly are produced in various textures from different particle sizes of abrasive. Coarse and medium grit stones generally appear as speckled tan or brownish in color. Fine-grit stones are usually white, have superior properties, and are less porous so that they require less lubrication during use. Either water or light oil is adequate as a lubricant.

Diamond is the hardest available abrasive and is most effective for cutting and shaping hard materials. It is the only material routinely capable of sharpening carbide and steel instruments. Diamond hones are small blocks of metal with fine diamond particles impregnated in the surface. The diamonds are held in place by an electroplated layer of corrosion-resistant metal. Most hones include grooved and rounded surfaces and a straight surface and are adaptable for sharpening instruments with curved blades. These hones are nonporous, but the use of a lubricant extends the life of the hones. They may be cleaned with a mild detergent and a medium-bristle brush.

Mechanical Sharpeners

As high-speed rotary cutting instruments have been improved and their use has increased, the use of hand cutting instruments and the need for re-sharpening has decreased. As a result, some dental office personnel do not do enough hand sharpening to remain confident of their proficiency. Under such circumstances, the use of a powered mechanical sharpener is beneficial.

The Rx Honing Machine (Rx Honing Machine Corp, Mishawaka, IN) is an example of a mechanical sharpener (Online Fig. 23-1). This instrument moves a hone in a reciprocating motion at a slow speed, while the instrument is held at the appropriate angulation and supported by a rest. This is much easier than holding the instrument at the proper angulation while moving it relative to the hone. Interchangeable aluminum oxide hones of different shapes and coarseness are available to accommodate the various instrument sizes, shapes, and degrees of dullness. Restoration of the cutting edge is accomplished more easily and in less time than by other sharpening methods. This type of sharpener is also very versatile and, with available accessories, can fill almost all instrument sharpening needs.

Handpiece Sharpening Stones

Mounted SiC and aluminum oxide stones for use with straight and angle handpieces are available in various sizes and shapes (see the section on other abrasive instruments). Those intended for use in straight handpieces, particularly the cylindrical instruments with straight-sided silhouettes, are more useful for sharpening hand instruments than are the smaller points intended for intraoral use in the angle handpieces. Because of their curved periphery, it is difficult to produce a flat surface using any of these instruments. These stones also may produce inconsistent results because of the speed variables and the usual lack of a rest or guide for the instrument. Satisfactory results can be obtained, however, with minimal practice, especially on instruments with curved blades.

Principles of Sharpening

Most operative hand cutting instruments can be sharpened successfully on either a stationary stone or the mechanical sharpener. The secret to easy and successful sharpening is to sharpen the instrument at the first sign of dullness and not wait until the edge is completely lost. If this procedure is followed, a fine cutting edge is restored with a few strokes on a stationary stone or a light touch to the mechanical sharpener. At the same time, operating efficiency is not reduced by attempting to use an instrument that is getting progressively duller.

The choice of equipment used for sharpening is up to the dentist. In the use of any equipment, several basic principles of sharpening should be followed: