By optimising the chip-forming process during grinding, energy consumption or energy conversion are reduced and the thermal effect and deformation of the workpiece are also reduced. Our research shows that ultrasonic-assisted grinding (UAG) can contribute significantly to optimising the chip-forming process in grinding. The principle of this technique consists of adding high-frequency (16-40 kHz) vibrations with amplitudes (2-30 μm) in the feed direction or transverse to the feed direction to the tool or the workpiece. Compared with conventional grinding, UAG is a hybrid process. By using ultrasonic-assisted machining, a significant reduction in feed force, chip size, tool wear, heat development and also an improvement in surface quality have been achieved. Zhang et al [Zhan01] came to the conclusion, both theoretically and in experiments, that an optimum state of vibration can reduce the feed force and the torque in cutting. Onikura et al [Onik98] found that the use of ultrasonic vibrations reduces the friction between the chip and the tool face of the inserts, which leads to thinner chips and, as a result, a reduction in cutting forces. Azarhoushang and Akbari [Azar07] have achieved significant improvements in terms of roundness, cylindricity and surface roughness in drilling through the use of ultrasonic vibrations. Mult [Mult96] and Uhlmann [Uhlm98] have established that ultrasonic-assisted grinding can be used as an efficient production method for ceramic materials and that ultrasonic-assisted deep grinding offers an enormous reduction in normal forces with slightly greater wheel wear and surface roughness. Tawakoli et al [Tawa07] have shown that with the ultrasonic-assisted dressing of CBN wheels a reduction in grinding forces and dresser wear can be obtained. For our investigation an ultrasonic-assisted grinding system was developed and used. In the grinding of 100Cr6, both with a coolant and in dry grinding, by superposing ultrasonic vibrations an improvement in surface roughness Rz and also a reduction in normal grinding force was achieved. The effect of vibration amplitudes, feed and depth of cut on surface roughness and normal grinding force was also investigated. Set-up of the ultrasonic-assisted grinding system To investigate UAG, a driven workpiece holder device was developed. The ultrasonic vibration chain consists of a piezoelectric converter, a booster,

a sonotrode and a special workpiece chucking device. The ultrasonic generator converts the electric current at about 50 Hz into high-frequency impulses at 21 kHz. These high-frequency electrical impulses reach a piezoelectric converter and by the piezoelectric effect are converted into mechanical vibrations with an ultrasonic frequency (21 kHz). The sound amplitude is amplified first by the booster and then by the sonotrode and is transferred to the workpiece, which is connected to the sonotrode. The resulting vibrations of the workpiece in the tool holder reach an amplitude of 10-30 μm at a frequency of about 21 kHz. The vibrations in the workpiece were tested both transversely to the feed direction and also in the feed direction. Fig 1 shows the test rig for the ultrasonic vibration chain and the workpiece holder.

Grinding is a manufacturing method that is in widespread use for fine machining. Because of the undefined cutting edges and the many factors involved in the process, grinding is very complex and difficult to control. Optimisation of the chip-forming process is the primary objective of research and development in grinding technology. Paper by T. Tawakoli, B. Azarhoushang and M. Rabiey.

Kinematics of ultrasonic-assisted grinding

The oscillating movement of the workpiece (ultrasonic vibrations) transverse to the feed direction leads to wave shaped (sinusoidal) movements of the abrasive grits on the surface of the workpiece. In broad terms the cross section of a ground surface (grinding traces) can be shown by peaks and valleys. In ultrasonic-assisted grinding the abrasive grits engage the protruding grinding traces (peaks) laterally. These grinding traces (peaks) have no lateral support and can be shifted and removed relatively easily. In conventional grinding, the active abrasive grits have to shift and remove the material which is supported by the masses of material in front of it during the chip-forming process itself. Therefore, in this case more work has to be performed (plastic deformation of the masses of material around the chip) in order to remove chips. Fig 14a shows the path of an abrasive grit in ultrasonic-assisted grinding. Fig 14b shows the path of a number of grits that engage the surface of the workpiece simultaneously or one after the other. In conventional grinding the feed is rectilinear. With small feed distances the movement of a grit can also be shown as being rectilinear. In nature, however, movements are rarely rectilinear. If one observes radiation in nature (electromagnetic radiation such as light, a laser beam, radio waves, etc., but also sound waves), it can be seen that the radiation spreads in the form of waves. This law of nature is used in ultrasonic-assisted grinding.

Summary

Hitherto it was generally thought that ultrasonic-assisted cutting could only be used for the machining of brittle-hard materials. In a large number of tests the KSF has investigated the use of UAG for grinding using coolant and also in dry grinding. The material used was 100Cr6 in the soft state. The main results obtained with ultrasonic-assisted grinding are:

The grinding forces are 30-50% less using UAG compared with conventional grinding.

The surface roughness values (in particular Rz) are also less by about 30-50% compared with conventional grinding.

The structure of the surface has a different appearance with a greater ratio of contact area, which is to be studied in further depth.

 In ultrasonic-assisted grinding the abrasive grits move in the form of waves (sinusoidal). This wave-like movement of the path of the grit leads to an interrupted cut, which results in a reduction in grinding forces and surface roughness values.