UltraSonic Machining

Ultrasonic processes

Index

• Ultrasonic processes ........................................................................................................................................ 1
• Introduction ................................................................................................................................................. 1
• Components ................................................................................................................................................. 2
• USM: process parameters ............................................................................................................................ 5
• Rotating Ultrasonic Machining ..................................................................................................................... 7
• USW .............................................................................................................................................................. 8

Introduction

Sound waves are mechanical vibrations in a solid or fluid. Ultrasound is the same, but at a frequency higher than the range audible to humans - the lowest ultrasonic frequency is taken as 20 kHz (20000 cycles per second). Typical amplitudes range from about 5 to 50 microns: the human body is not sensible to this low amplitude interval, making the process safe.

An ultrasonic system operating at 20 kHz and 50 microns is moving with a cyclic acceleration of 80000 g. The electric current feeds an electronic oscillator that transforms the low frequency into the high frequency (between 20 and 40 kHz). A magnetostrictive (typically Fe-Co) or piezoelectric transducer converts the direct current from the frequency in longitudinal vibration in equal frequency. Typical frequency to which the transducer is excited is 20-40 Hz. This generated vibration (amplitude 10^-6 - 10^-4 mm) is transmitted to the tool through a cone concentrator (horn) which has the task of giving the maximum amplitude of vibration at its tip (about 0.01 mm).

Material removal is achieved by the tool in contact with the flow of abrasive fed by a pump: the vertical movement of vibration added to the static force applied externally generates removal of material on the surface of the workpiece. The removal is due to 4 mechanisms:

• Free impact between abrasives and surface, with following fracture.
• Continuous localised hammering between grain surfaces and piece caused by sonotrode (principal contribution).
• Cavitation erosion, induced in the surface of the eroded piece.
• Chemical erosion caused by contact between surface and fluid.

The USM process has no thermal nor chemical alterations on the machined surface. The hammering effect of the abrasive particles on the workpiece surface induces compressive residual stresses which usually increase the fatigue resistance of the machined pieces. Materials do not have to be conductors; complex shapes and holes can be machined with good quality and tolerances. Common values of surface roughness: ≈ 0,5 − 0,76, but with highly accurate processes and low machining rates even much better ≈ 0,1 − 0,05): tolerances and roughnesses can be achieved (using abrasive particles #120-180 mesh it is possible to get tolerances up to ±0.08 mm, while with #240-320 mesh also of ±0.05 mm.

With US, welding can be achieved and the greatest advantage is the possibility to join different materials (almost impossible with traditional welding processes), without melting or vaporization, thus reducing

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Components

The principal components for the USM and USW are:

• The electronic oscillator, which converts the alternate current (50 - 60 Hz) into electric energy into ultrasonic frequency (typically 20 - 40 kHz). A peculiarity of the generator is the ability to keep the working parameters constant during the process, especially frequency and amplitude.
• The transducer, that can be divided into piezoelectric and magnetostrictive.

Piezoelectric

¹The piezoelectric effect is a reversible process: piezoelectric materials exhibiting the direct ²effect also exhibit the reverse piezoelectric effect ³. Max 1 kW of power is achieved and there is no need for chilling systems. However, a precise frequency is required to excite the material. Generally, a system contains more than one PZT crystal in order to work on more

¹ There are both natural crystals and synthetic ceramics.

² The internal generation of electrical charge resulting from an applied mechanical force.

³ Of course, the internal generation of a mechanical strain resulting from an applied electrical field.

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