a tool, generally positioned according to a vertical axis called “sonotrode”, reproduces its own shape while it is penetrating the piece to be machined. This is due to abrasive particles movements (boron carbide, silicon carbide or diamond of about ten µm diameter) carried away between the piece and the sonotrode (gap) because of the vibration of the latter which is excited on its resonance frequency (20kHz). Hard particles are thrown onto the piece surface, penetrate it and cause a deformation and then a matter removal in the form of micro-shavings. The abrasive is in suspension in water which is a good ultrasonic frequencies transmitter. This machining is particularly adapted to hard and fragile materials (as quartz crystal) which do not withstand high thermal gradients. This process depends on a lot of parameters such as: vibration, static pressure, size and abrasive particles nature, penetration depth, and piece shape and nature... Thus, machining parameters are to be determined for each application.
Achieved results: A good knowledge about abrasive actions enables to get millimetre order dimensions machining. When the depth is too important, it is necessary to use several sonotrodes in order to keep a good precision (about 3-4µm) on the whole height. For the last sonotrode, a smaller particles diameter can be eventually used to improve the surface condition (< 20µm). In this case, cavities smaller than 2 µm deep (less than a tenth of a particle diameter) will appear on the surface. In such conditions, 300µm diameter several millimetres deep holes have already been performed in quartz crystal.