Spark erosion-based machining involving wires on CNC machines has been used successfully in industry for years to produce blanking punches, clipping beds, electrodes used in die-sinking, and precision parts – especially when it is not worth carrying out precision stamping in small batches. But to produce highly sophisticated or even almost polished surfaces, parts have to be reworked many times to remove rough edges left by previous processes. This takes time and, depending on the material, it can also result in corrosion or pitting because the dielectric needed to carry out the necessary eroding consists of deionized water. The Steinbeis Transfer Center for Process Development has developed a hybrid wire erosion process that sidesteps this issue. The material surfaces it produces are smooth, shiny, and free from corrosion.
Spark-erosion cutting using wires on CNC machines – sometimes referred to as wire-cutting or wire-cut EDM in workshops – is an electrothermal removal method that works with deionized water. The erosion wire is tensed between an upper and lower wire guide and moved at a constant wire-sweep velocity. The work piece, which is electroconductive, is moved along a programmed, defined path. This path is always relative to the movement of the erosion wire.
The machining speed is dictated by the power of the generator, the thickness of the work piece and the nature of the material. A distinction is made between different sequences within the process – the rough cut, the split skim cut and the fine cut. To achieve ultimate precision down to the micrometer and produce highly sophisticated surfaces, it generally requires several rounds of cutting during the fine cut phase. The previously cut contour has to be reworked with a reduced level of sparking along an adjusted cutting path so that only the rough peaks or edges left behind by the previous process are removed. This is extremely time-consuming and despite using deionized water, it is impossible to avoid electrochemical side effects. Visible distortions may be left behind, as well as erosion. Depending on the material, metals may even start to form alloys and there can even be hydrogen embrittlement.
The aim of the process developed by the experts at the Steinbeis Transfer Center was to eliminate these problems and produce a shiny surface on materials after only one fine cut. To do this, they worked through a systematic matrix of possible measures, including physical processes on the impulse generator and chemical processes in the dielectric. Power pulses were adjusted, not only according to flank angles as the current rises and drops, but also to accommodate bi-polar functions. The deionized water, which acts as an active medium and typically only has a conductivity of a few microsiemens, had radical-forming substances added to it. This resulted in electrochemical removal processes in a thin layer on the surface of the material. For the process to work, the work piece had to have positive polarity. To support the chemical and physical effect of the process, a high-frequency pulsed direct current was superimposed. The method that has subsequently been developed by the experts is a genuine hybrid process: It involves two completely different processes running not consecutively, but simultaneously.
The results speak for themselves. The kinds of microstructures that were previously produced on spherical balls, using spark erosion, have been replaced by steel surfaces which are smooth and shiny – without negative intercrystalline grain boundary effects resulting from the electrochemical process. As a result, extremely high surface qualities can be achieved, even with wire erosion.
Prof. Karl Schekulin
Steinbeis Transfer Center Process Development (Reutlingen)
su0076@stw.de