Accelerated EDM Microstructuring Aided by Electroceramic Transducers

Precision Robotics and Automation: Hexapods Advance Production Processes

Piezoelectric high-speed motion improves performance of die-sinking EDM in the micron realm.

The basic principle of the machining process, now known as electrical discharge machining (EDM), or die sinking, was discovered in the 1940’s by accident, when Russian scientists were trying to prevent the erosion of electrical contacts caused by sparking. Today, EDM is well established for precision manufacturing of tools and lately, micro-structured components, even in large quantities. The process requires high voltage run between two electrodes causing the removal of metal from the workpiece by the discharge of electric current.

Adding Vibration to Speed-Up Die-Sinking EDM

The Institute for Microtechnology in Mainz (ICT-IMM), Germany put a new spin on this old technology. The Sonodrive 300 vibratory spindle uses a patented process to cut machining times, in high-precision micro-drilling operations, by up to 60%. The spindle rotates and vibrates simultaneously, preventing the particles produced by EDM from being deposited in the hole, eliminating the need for additional machining time.

Pictured left,the Sonodrive 300 serial-production vibratory spindle can cut machining times by up to 60%, in high-precision micro-drilling operations, employing a patented process. (Image: ICT-IMM)

Animation of the solid-state effects inside piezoceramic materials when the electric field is changed (Image: PI)

Shown to the left,an animation of piezo motion in PICMA multilayer low-voltage piezo stack: When an A/C voltage is applied to a piezo transducer, it will begin to oscillate, according to the applied voltage and frequency. Note: Typical amplitudes are on the order of 0.1% of the stack’s length. PICMA® piezo actuators survived 100 billion cycles of life-testing by NASA/JPL without failures (Image PI)

Compared with conventional methods, this process delivers substantial speed benefits because the small electrode distances do not allow any flushing in the micron range. The new spindle principle combines high concentricity tolerance of 1 to 2µm at speeds of up to 3,500 rpm with an oscillation of max. 300Hz at amplitudes up to 15 microns.

(left) Ø64µm hole produced with a vibratory-spindle equipped die-sinking EDM machine shows extremely good concentricity and cylindricity with deviation of only 1 micrometer (Image: ICT-IMM) (right) A Ø20µm hole, produced with the same process (Image: ICT-IMM)

A piezoelectric transducer / actuator, designed by Physik Instrumente (PI), generates the vibratory motion. Piezo transducers are active ceramic elements that convert electric energy into mechanical energy. When an AC voltage is applied, the transducer starts to oscillate. The high stiffness and short response times of the electro-ceramic material (down to microseconds are feasible) benefits applications with fast motion requirements. Piezo actuated direct drives typically achieve displacements up to few hundred micrometers and high dynamics (flexure motion amplifiers are used for longer travels). With its low profile of only 25mm and Ø50mm OD / Ø25mm ID, the selected actuator facilitates integration in the vibratory spindle.

(top) Compact piezo transducer: Ø50x25mm with Ø25mm aperture for easy integration. The piezo transducer produces vibration up to 300Hz with amplitudes up to 15µm. (Image: PI) (bottom) Piezo transducer inside the vibratory spindle (image: ICT-IMM)

Piezoelectric motion is entirely based on solid-state effects; there is no wear and tear, no particulate generation and abrasion making the technology maintenance-free. In fact, NASA/JPL tested PI piezo actuators for the Mars mission, where they passed 100 billion cycles of life testing with zero failures.

Replacement Electrode Chuck for Commercial Die-Sinking EDM Machines

Piezoelectric transducers were also integrated in a vibration-supported electrode chuck, suitable as a “Plug & Play” solution for commercially available die-sinking EDM machines and clamping systems. A long-term experiment with a hard-metal electrode (0.2 x 5 mm) and a sinking depth of 7mm showed time savings of 70%. Likewise, the chuck generated vibrations of 300Hz with variable amplitudes up to 15 micrometers:

(left) Microvibe 300 electrode chuck for micro EDM with integrated piezo transducer; a “Plug & Play” solution for all commercially available die-sinking EDM machines (Image: ICT-IMM) (right) Time reduction for producing Ø200µm blind holes in 1mm thick stainless steel: from 200sec to <80sec. Tested on Mitsubishi Electric EA12 die-sinking EDM machine. (Image: ICT-IMM)

Authors: Frank Neumann, Department Head of Production Processes at Fraunhofer ICT-IMM, Steffen Arnold, Physik Instrumente (PI) GmbH & Co. KG, Stefan Vorndran, PI (Physik Instrumente) LP.