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.
Surface finish 
The surface finish of ultrasonic machining depends upon the hardness of the workpiece/tool and the average diameter of the abrasive grain used. Up close, this process simply utilizes the plastic deformation of metal for the tool and the brittleness of the workpiece. As the tool vibrates, it pushes down on the abrasive slurry (containing many grains) until the grains impact the brittle workpiece. The workpiece is broken down while the tool bends very slightly. Commonly used tool material consist of nickel and soft steels. advantage 1-any material can be machined regardless of their electrical conductivity 2-good surface finish obtained and higher structure integrity
Machine time 
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.