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Piezo actuators: good vibrations for die-sinking EDM

10 August 2014

Actuators based on the piezo effect move heavy loads with a resolution in the sub-nanometer range and with response times of less than one millisecond, enabling dynamic operation at high scanning frequencies.

Piezo-ceramic actuators are advancing technology in diverse applications, ranging from medical and metering devices, to photonics and semiconductor manufacturing. For example, when die-sinking electrical discharge machining (EDM) is used to produce precision components, these actuators can help to shorten the production time - often substantially so.

As well as industry’s desire for faster production throughput, the increasing complexity of products and processes demands uncompromised precision and repeatability. And as there is also a trend towards miniaturization, die-sinking EDM is now quite widely deployed in the manufacture of micro-structured precision components - often in large piece quantities. Examples include the manufacture of filter elements or injection nozzles for the automotive industry.

The quest for faster die-sinking EDM using vibratory spindles has been taken up by the Institute for Microtechnology in Mainz (ICT-IMM), which became part of the Fraunhofer group earlier this year. Its Sonodrive 300 is a serial-production vibratory spindle, which, in high-precision micro-drilling operations, is claimed to cut machining times by up to 60 percent, compared with standard equipment.

The spindle rotates and vibrates simultaneously, preventing the particles produced during the EDM process from contaminating the workpiece, and thereby eliminating the need for secondary machining operations. Conventionally, this is unavoidable, because the small electrode distances do not allow any flushing in the micrometre range. Depending on the material to be machined and the job at hand, the vibration path can be set and adjusted at any time during the process.

Compared with conventional methods, this delivers substantial speed benefits, accelerating the entire production process. With a 0.2mm diameter blind hole in a 1.0mm thick VA material, for example, the machining time in a test on a EA12 die-sinking EDM machine from Mitsubishi Electric was reduced from 200 seconds to just under 80 seconds. On an Agie Compact die-sinking EDM machine, the 0.2mm diameter through-hole in a 0.4mm thick VA material was also obtained 60 percent faster through the use of a vibratory spindle.

The new spindle principle combines high concentricity tolerance of 1 to 2µm absolute at up to 3,500rpm with a vibration frequency no greater than 300Hz and a stroke of up to 15µm. Moreover, the spindle fits with all commercially available die-sinking EDM machines in a ‘plug-and-play’ manner. The matching miniaturised wire pay-off device, likewise developed by ICT-IMM, can also be easily integrated. This allows the concentricity tolerance to be reduced to 1µm absolute.

Such a technological leap in micro-structuring has been achieved with the aid of a piezo actuator supplied by PhysikInstrumente (PI), which provides the vibration element of the spindle operation. Piezo actuators convert voltage directly into mechanical displacement, achieving typical travel ranges of a few hundred micrometres and high dynamics with frequencies of up to several hundred Hertz. 

Their fast response times also benefit such vibration drive applications. With a height of 25mm, a diameter of 50mm and an inner aperture of 2 mm, the actuator could also easily be integrated in ICT-IMM’s vibratory spindle.

Since piezo actuators are also capable of shifting large loads, the permanent motion of the spindle, whose weight varies between about 250 and 450g, depending on the electrode, was no problem for this small drive. Indeed, it can lift more than one kilogram.

Electromechanical components for vibration generation were not an option for this application, as their larger dimensions would have precluded integration into the miniaturised Sonodrive 300 spindle.

Since the motion of a piezo-ceramic actuator is based on crystalline solid-state effects, there is no danger of abrasion with this technology and there are no cogwheels, bearings or other mechanical parts that might otherwise be subject to wear.

This means the piezo actuator is virtually maintenance-free - an important characteristic, given that the piezo actuator operates throughout the entire machining operation. The actuator supplied by PI has already proven its reliability in prototypes of the vibratory spindle, which have been tested by ICT-IMM over a period of some four years now – the equivalent of approximately 100 million trouble-free operating cycles.

The piezo actuator is driven by an efficient, pulse-width-modulated amplifier, also supplied by PI. This amplifier is especially designed for the demands of low-voltage piezo actuators; at a peak output power of 280W and an average output power of 100W, it can supply a peak current of up to 2,000mA. This allows dynamic operation of piezo actuators of high capacity at a bandwidth in the kiloHertz range - more than is required for micro EDM.

The piezo actuators have also been tested and proven in a vibration-supported electrode chuck (the Microvibe 300), which is another suitable ‘plug-and-play’ drop-in for all commercially available die-sinking EDM machines and clamping systems, thanks to its compact dimensions (80 x 80 x 150mm).

Here, too, the 300Hz vibrating drive with an adjustable stroke of up to 15µm contributes to much faster production cycles. As an example, a long-term experiment with a hard-metal electrode (0.2 x 5mm) and a sinking depth of 7mm resulted in a production time reduction from 17 hours and 20 minutes to just 5 hours and 15 minutes.

This has the potential to benefit many areas of precision manufacturing, including micro-machining, tool-making and mould-making and the manufacture of medical and metrology devices and precision machine parts.

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