Ultrasonic machining, Thermal modeling, Plasma technology

Ultrasonic Machining (USM)

Sound waves are mechanical vibrations in a solid or in a fluid. Ultrasounds are the same, but at a frequency higher than the range audible to humans; the lowest ultrasonic frequency is normally taken as 20 kHz (20000 cycles per second).

The tool tip vibrates in the vertical direction very fast at a frequency > 20 kHz. It must be small to avoid too high inertia. Typical amplitudes at the tool tip range from about 15 to 50 microns (low amplitude, fast oscillation). An ultrasonic system operating at 20 kHz and 50 microns is moving with a cyclic acceleration of 80000 g.

Ultrasonic can be used in

• machining
• inspection
• welding
• cleaning

Process

• The electric current feeds an electronic oscillator that transforms the low frequency into the high frequency (between 20 and 40 kHz).
• A magnetostrictive or piezoelectric transducer converts the direct current from the frequency into longitudinal vibration of equal frequency.
• The vibration generated (amplitude 10^-5-10^-6 mm) is transmitted to the tool through a concentrator (HORN) which has the task of giving the maximum amplitude of vibration at its tip.
• The tool is in contact with the flow of abrasive fed by a pump and the vertical movement of vibration, added to the static force applied externally generates removal of material on the surface of the workpiece.

Ultrasonic Machining (USM)

Sound waves are mechanical vibrations in a solid or in a fluid.

Ultrasounds are the same, but at a frequency higher than the range audible to humans; the lowest ultrasonic frequency is normally taken as 20 KHz (20000 cycles per second).

The tool tip vibrates in the vertical direction very fast, at a frequency > 20 KHz.It must be small to avoid too high inertia.Typical amplitudes at the tool tip range from about 15 to 50 microns (low amplitude, fast oscillation).An ultrasonic system operating at 20 KHz and 50 microns is moving with a cyclic acceleration of 80000 g.

Ultrasonic can be used in

• machining
• inspection
• welding
• cleaning

Process

• The electric current feeds an electronic oscillator that transforms the low frequency into the high frequency (between 20 and 40 KHz).
• A magnetostrictive or piezoelectric transducer converts the direct current from the frequency into longitudinal vibration in equal frequency.
• The vibration generated (amplitude 10^-5-10^-6 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.
• The tool is in contact with the flow of abrasive fed by a pump and the vertical movement of vibration, added to the static force applied externally generates removal of material on the surface of the workpiece.

Material Removal Mechanism

The removal is due to several mechanisms:

1. Free Impact: at high speed between the abrasive grains and the workpiece, causing fragile fractures. The abrasive is accelerated, hits the material and create fractures.
2. Localized Hammering: the grain is compressed into the material by the tool that is acting like a hammer.
3. Sliding Erosion: just on the vertical surfaces (not dominant mechanism).
4. Cavitation and Chemical Erosion: due to the cutting fluid acting on the work surface (not dominant).

The tool doesn't hit the workpiece, so fragile materials can be worked by ultrasonic machining.

The material of the workpiece is fractured, but also the abrasive grains will be fractured, so the slurry must be renewable to have a refresh of grains, that are usually oxides or carbides (hard phases).

Also a vertical feed movement is needed to go deeper into the workpiece, and it is obtained by simply applying a vertical force (no force control).

The problem is that the removal mechanisms and the MRR are different between horizontal surfaces and vertical surfaces.

SYSTEM

PIEZOELECTRIC TRANSDUCER

We want to convert alternate current at a given frequency into mechanical vibrations.

The piezoelectric effect is the interaction (linear electro-mechanical interaction) between the mechanical one and the electrical side in some crystalline materials.

It is a reversible process. Piezoelectric materials that exhibit the direct effect (linear mechanical charge as the result of an applied mechanical force) also exhibit the reverse piezoelectric effect (inverse generation of a mechanical strain as the result of an applied electrical field).

Piezoelectric materials are: natural crystals, synthetic ceramics, ...

The amplitude of vibration generated is not enough: we need to amplify it wi