A knife represents a hand-held cutting tool with a cutting edge or blade. It may also have a handle. Originally made from rock, bone, flint, or obsidian, knife blades today are typically fashioned from iron, steel, ceramics, or titanium.
While knives may be used as a weapon, they are more commonly employed by people as useful tools in food preparation, dining, meat processing, hunting, construction, work projects, and hobbies for cutting or slicing an object. Many different types and designs of knives are known, but most of them share the trait of one or two sharpened blade edges.
But, over time, these sharpened edges of the knife blade will become dull or damaged. Blades are damaged usually by buckling due to compressive force arising from the user pressing the knife blade edge into a hard object like bone, ice, a hard cutting board, or other hard object, or simply by repetitive use. The cutting edge may also become bent from sideways pressure applied against the blade. Both of these forces tend to roll the knife blade's cutting edge due to the ductile characteristic of the metal material used in the blade. Moreover, tougher or abrasive materials will cause the blade to become dull more quickly.
Dull blades do not cut as easily or precisely, and can create a danger to the end user by requiring greater hand force to make a cut. Moreover, dulled blades can include burrs or ragged edges with regions along the cutting surface that are out of alignment with each other. Such misaligned blades can damage the material being cut, or produce an inferior cut by tearing or sawing the material being cut as opposed to a smooth, clean cut.
Therefore, such dulled knife blades must be periodically sharpened. This is a process in which the knife blade is ground against a hard, rough surface like a stone, or a soft surface containing hard particles. Metal can be removed from the knife blade in order to form a new edge along the blade. Typically, a grinding wheel or a whetstone is used. These sharpening stones come in varying grit degrees from very coarse to very fine, and can be described as hard or soft depending upon whether the grit comes free of the stone during the grinding operation. Ceramic hones are also commonly used, especially when fine grit size is desired. Coated hones with an abrasive diamond-based surface provide yet another option. Mineral oil often is used during the grinding application to separate the loosened grinding particles from the knife blade edge to prevent damage to the blade.
The cutting edges of the knife blade may also be straightened by a sharpening steel. The sharpening steel constitutes a hardened cylindrical, triangular or other shaped rod having a small diameter. This sharpening steel may have a smooth, polished exterior surface, or may be somewhat abrasive. It may also feature slight ridges or ribs running along the length of the rod. A butcher steel constitutes a round file with teeth running the long way, although it may also be smooth. As the knife blade with its cutting edge is swiped along the sharpening steel, the steel will exert high localized pressure against the cutting edge to straighten the turned edges of the cutting edge back into proper alignment. Unlike grinding, such steeling process does not usually remove metal from the blade edge.
Knives used by barbers are often stropped after steeling in order to polish the sharpened cutting edge. This is often done with a leather strap impregnated with an abrasive compound like chromium (III) oxide particles. This operation does not remove any metal material from the blade edge, but produces a very sharp edge.
However, grinding constitutes a precise operation in which the angle of the cutting edge of the blade must match the angle of the whetstone or grinding wheel surface. The smaller the angle between the blade and stone, the sharper the knife will be, but at the same time, the less side force is required to damage the knife blade by bending the cutting edge over or even chipping it off. The edge angle represents the angle between the blade and the stone. For symmetrical double-ground, wedge-shaped knife blades, the angle from one edge to the other edge of the blade will be twice the edge angle.
While steeling represents a less aggressive form of sharpening than grinding, it still is important to swipe the knife's blade at a proper angle with respect to the sharpening steel. Moreover, the two cutting edges of the knife must be swiped the same number of times against the steel or else the cutting edge will be pushed again out of alignment.
It is therefore easy to damage the cutting edge of the knife blade further if the sharpening exercise is performed poorly. Thus, most knife users need to send out their dulled knives to a professional sharpening service, or to replace the knife with a new knife. This can be time-consuming and expensive.
Some knife users employ the bottom of a ceramic coffee mug for sharpening the blade. The course ceramic particle surface can produce acceptable results, although a sharpening steel usually must then be used. But again, a proper angle must still be maintained during the sharpening swipes. Electric knife sharpeners are also available in the market.
U.S. Pat. No. 3,942,394 issued to Juranitch in 1976 is directed to a finishing sharpener device used in the field for sharpening a knife blade. It includes a handle having a pair of wings that fold out and extend at a 30 degree angle from the handle. Each of the wings constitutes a flat bar defining a sharpening edge that is arcuate in cross section and smooth. By drawing a dulled knife blade cutting edge across the sharpening edge of one of the wings at the proper angle, the cutting edge along one side of the knife blade may be restored to its sharpened configuration. The handle of the finishing sharpener serves as a rough visual guide for properly aligning the knife handle to draw the knife blade along the wing's sharpening edge. But, this process still requires some skill by the person sharpening the knife blade to ensure a proper match between the knife blade cutting edge angle and the angled surface of the sharpening edge of the wing. Moreover, the opposing cutting edges of the knife blade must be sharpened sequentially using first the one wing and then the other wing. Simultaneous sharpening of the opposed cutting edges of the knife blade is impossible. Furthermore, when the knife blade is swiped along one of the wings to sharpen it, the blade comes very close to the user's other hand on the handle, thereby producing a risk of injury. Finally, the arcuate cross-section surface of relatively small radial extent having a highly smooth configuration is insufficient for removing material from the cutting edge of the knife blade. This finishing sharpener may only therefore be used after the knife blade has been sharpened first on a hone or grinding wheel.
Razor Edge Systems of Ely, Minn. has commercialized a knife sharpening device referred to as MOUSETRAP STEEL that is further disclosed in U.S. Pat. No. 5,655,959 issued to Juranitch in 1997. It has been used to sharpen the cutting edges of a knife blade in the meat processing industry, but also for any other end-use application where there is a need to maintain a sharp knife edge. It constitutes a bench-top mounted vertical base member having a vertical slot partially bisecting the base member from its top edge. Pivotably mounted to the base member are two counterweights having equal masses. A pair of upwardly curved sharpening steels are connected to the upper and inner ends of the counterweights and extend toward each other in a crossed relationship, intersecting at and along the slot. A pair of guard rods are needed to protect the sharpening steels from outside damaging force, further aided by guard blocks mounted to the side edge of the base member. As a knife blade is pushed down through the slot, it engages the sharpening steels at this intersection point and pushes the steels inwardly, sharpening the opposing cutting edges of the knife blade simultaneously as the knife blade is swiped along the sharpening steels. A pivotably-mounted wiper wing under the influence of its own counterweight polishes the sharpened blade. A pair of cams that are eccentrically mounted to the base member act to arrest the lateral movement of the steels during the knife sharpening operation, as well as to define the downward resting point of the counterweights when the sharpening steels are in their standby position when the knife blade is disengaged. However, these cams must be carefully adjusted in their eccentric positions along the base member prior to the knife sharpening operation to define how high or low the crossed intersection point of the steels will be situated over the slot. A higher position requires greater force applied to the knife blade during the sharpening operation resulting in this higher intersection point of the steels producing a less sharp cutting angle along the knife blade. A lower intersection position on the other hand requires less force applied to the knife blade during the sharpening operation resulting in a sharper cutting angle produced by the steels along the knife blade. Thus, the MOUSETRAP STEEL sharpener requires the user to know in advance the angle of the cutting edges that must be produced along the sharpened knife blade, and to precisely adjust in advance the cams' positions to achieve this desired angle. This requires skill and patience by the user. Yet over time, the significant weight of the heavy, 20-ounce counterweights will cause the cams to move from their intended position, thereby making repeated sharpening of knives with the same cutting edge angle impossible without further precise adjustment of the position of the cams. Furthermore, these heavy counterweights make the MOUSETRAP STEEL device heavy and non-portable. The large number of parts mounted to the base member and the bolts and nuts used to mount them also produce environments for bacterial growth which makes it difficult to keep the device clean and sanitary.
U.S. Pat. No. 4,934,110 issued to Juranitch back in 1990 discloses an edge sharpening apparatus having a panel with a slot and two pivotably curved sharpening steels attached to counterweights similar to U.S. Pat. No. 5,655,959. It also discloses, however, another embodiment in which the counterweights are replaced by coil springs that are connected between the pivotable plates attached to the sharpening steels and the panel member of the sharpening device. These coil springs act to pull the pivotable plates to impede the sharpening steels from pivoting under the force of the knife blade that is being sharpened. However, it was found in actual practice that these coil springs failed to provide enough tension and counterforce on the sharpening steels to allow them to provide sufficient pressure against the knife blade to sharpen its cutting edges. Moreover, the coil springs had a tendency to become separated from the knife sharpener device during operation, and contaminate meat that was being processed on an industrial line, or damage surrounding equipment like grinders or conveyor belts. For these reasons, this product design with coil springs failed in the commercial field, and springs were abandoned in favor of counterweights.
Therefore, it would be very advantageous to provide a sharpening apparatus that may be used by a relatively unskilled person to simultaneously sharpen the two opposed cutting edges of a knife blade by hand and to maintain a sharpened cutting edge along the blade with minimal effort and training. Such an apparatus should be small and portable so that the person may easily take it out to sharpen the cutting edges of the knife blade when needed, and small enough to be mounted in a stationary location where working space is limited and tight. Moreover, the device should enable the sharpening of the blade's cutting edges without having to maintain a specific swiping angle, or precisely adjust the position of cams to enable the sharpening steels to produce the desired cutting edge angle along the knife blade. Furthermore, the sharpening device should feature a relatively small number of parts without bolts and nuts to reduce surfaces on which bacteria can grow. Such a sharpening apparatus can be used to maintain an extremely sharp cutting edge for precise cutting of a material without crushing or other damage with significantly reduced physical force and strain upon the user.