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Ultrasonic machining is a method of grinding that uses an abrasive liquid rather than direct tool contact. Most grinding processes involve a work tool making direct contact with a work piece in order to gouge material away. In ultrasonic machining, a liquid filled with abrasive material flows through over the work piece, and the work tool vibrates against the abrasives. The abrasive materials affect the work piece and remove material. Since the tool doesn’t directly touch the work piece, the pressure and tool materials used in ultrasonic machining are often very different from those used in more common machining techniques.
The key to an ultrasonic machining process is the abrasive liquid. This material, called slurry, is a mixture of a free-flowing liquid and one or more types of solid abrasive. The liquid part of the slurry is generally water. For some jobs benzene, glycerol or oil may be used instead, but increasing the viscosity of the liquid will often lead to a slower process.
Since the abrasive used in ultrasonic machining slurry needs to be harder than the machined material, a wide range of abrasives are common. The basic abrasives are often silicon carbide or boron carbide, mostly due to their hardness and low cost. Occasionally, diamond dust is used to work the hardest materials.
The work tool used in ultrasonic machining is different from the ones used in a standard process. The tool is often made of a softer material with a high plasticity. This allows the abrasives to impact the tool, but not damage it the way it does the worked material. These tools are often far too soft for standard machining jobs; they would deform as soon as they touch the work piece.
The process of machining a piece ultrasonically looks similar to a normal process, but is actually quite different. The slurry flows over the work area, creating a connection between the work piece and work tool. The tool vibrates, which causes the abrasives to bounce back and forth between the piece and tool. Since the tool deforms, it absorbs the impacts of the abrasives while the work piece develops small cracks. The cracks eventually cause small pieces to break off until the machined area of the work piece matches the shape of the work tool.
The most common reason to use ultrasonic machining is when a work piece is very brittle. On a brittle substance, a standard machine process will cause the material to crack and break. This will generally result in a ruined final product. Ultrasonic machining uses thousands of tiny impacts and very little pressure to move material away from a substance. This rarely results in a break, even in very brittle materials.