Ultrasonic machining, also known as ultrasonic impact grinding, is a machining operation in which a vibrating tool oscillating at ultrasonic frequencies is used to remove material from the workpiece, aided by an abrasive slurry that flows freely between the workpiece and the tool. It differs from most other machining operations because very little heat is produced. The tool never contacts the workpiece and as a result the grinding pressure is rarely more than 2 pounds, which makes this operation perfect for machining extremely hard and brittle materials, such as glass, sapphire, ruby, diamond, and ceramics.
The tool that does the cutting is made of a softer material than the workpiece. Commonly used tool materials are nickel and soft steels. As the tool vibrates, it pushes down on the abrasive slurry, a liquid containing abrasive grains, until the grains impact the workpiece. Because of the brittleness of the workpiece, under the impact of the abrasive particles its surface abrades, while the softer tool material simply deforms slightly. The advantages of ultrasonic machining over ordinary machining:
- Virtually any hard material can be machined
- Little heat is produced
- Since the motion of the tool is up-and-down vibration rather than rotation as in traditional machining, the holes cut are not limited to circular shapes but can be any shape. Custom "cookie cutter" tools can cut complicated shapes
- good surface finish and higher structural integrity is obtained
Machine time depends upon the frequency at which the tool is vibrating, the grain size and hardness (which must be equal or greater than the hardness of the workpiece), and the viscosity of the slurry fluid. Common grain materials used are silicon carbide and boron carbide, because of their hardness. The less viscous the slurry fluid, the faster it can carry away used abrasive.
Mechanics of ultrasonic machining
The physics of the ultrasonic machining process are not fully understood, but the material removal is believed to be due to some combination of:
- The hammering of the abrasive particles on the work surface by the tool.
- The impact of the free abrasive particles on the work surface.
- The speed of the vibrating tool.
- The erosion due to cavitation, and
- The chemical action associated with the fluid used.