Steffen Arnold and Frank Neumann explain how microstructuring is benefiting from the latest breed of piezo actuators
The increasing complexity of products and processes requires production processes to constantly increase throughput, precision, geometric diversity and accurate repeatability. At the same time, the trend in automation is toward miniaturisation. So it is hardly surprising that in the field of die-sinking EDM, microstructured precision components are now manufactured, often in large piece quantities. Cost and resource efficiency also play important roles in this context.
Faster die-sinking EDM operations are being enabled via a vibratory spindle. This topic has been taken up by the Institute for Microtechnology in Mainz (ICT-IMM), Germany. The Sonodrive 300 is a serial-production vibratory spindle, which, in high-precision microdrilling operations, can cut machining times by up to 60%, compared with standard equipment. It employs a patented process. The spindle rotates and vibrates simultaneously, preventing the particles produced in EDM from being deposited in the hole, thereby eliminating the need to machine them again. In conventional methods, this is unavoidable, because the small electrode distances do not allow any flushing in the micrometre range.
Compared with conventional methods, this delivers substantial speed benefits. The new spindle principle combines high concentricity tolerance of 1 to 2µm absolute at a revolution of up to 3,500 min-1 with a high-frequency vibration of max. 300Hz and a stroke of up to 15µm.
Piezo actuator ensures vibration
A piezo actuator from Physik Instrumente (PI), ensures vibration. The creation of vibrations virtually is a classic piezo application, because the piezo element starts to oscillate when an AC voltage is applied. This means that piezo actuators convert voltage directly into mechanical displacement. They achieve typical travel ranges of a few hundred micrometres and high dynamics with frequencies of up to several hundred Hz. The short response times of the piezos naturally also benefits the application as a vibration drive. With its height of 25mm at a diameter of 50mm and its inner aperture of 25mm, the selected actuator could also easily be integrated in the vibratory spindle. Since piezos are also suitable for large loads, the permanent motion of the spindle, whose weight varies between about 250 and 450g, depending on the electrode, was no problem for the small drive.
Since the motion of piezos is based on crystalline solid-state effects, there is no danger of abrasion with this technology. This makes the piezo actuator maintenance-free; which is an important characteristic, given that the piezo actuator operates throughout the entire machining operation.
Piezo actuators have also been tested and proven in a vibration-supported electrode chuck, which is also suitable as a ‘plug and play’ solution for all commercially available die-sinking EDM machines and clamping systems. Here, too, the 300Hz vibrations at an adjustable stroke of up to 15µm accelerate the production process considerably. An impressive illustration of this was found in a long-term experiment with a hard-metal electrode (0.2x5mm) and a sinking depth of 7mm, which resulted in a time saving of 70%.
Piezo actuators used as vibration generators have therefore made a substantial contribution to advancing the technology of die-sinking EDM to the lowest micrometre range.
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Steffen Arnold is with Physik Instrumente (PI) and Frank Neumann is with the Fraunhofer ICT-IMM in Germany.