H.-P. Schulze

Measurement equipment for investigation of the influence of viscosity of dielectric working fluids on spark erosion

In this paper, measurement equipment for investigation of electrical discharges in dielectric working fluids is presented. Special features of these discharges are determined through a small distance between electrodes of 5 to 150 /spl mu/m and the ignition voltage up to 600 V. With the measuring system, a time-co-ordinated measurement of all electrical and optical processes of the discharging process can be accomplished. For using spark discharges is a machining process analyzing the influence of the viscosity of the work liquid on the ignition conditions is of great interest. Parameters measured at the plasma channel are starting parameters for a simulation of the thermal-affected zone of electrodes surfaces. The conditions for ignition and recovery strength of the work liquid are important characteristics for a machining process up to 4 million discharges per second.

Channel spreading during the spark discharge for selected conditions and working fluids

The paper presents the forms of the channel formation during spark erosion. The deviations from the cylindrical discharging channels leads to completely other removal craters and therefore for changed surface roughness. The different channel types are dependent on gap conditions and pulse parameters in the first place but too dependent on the compounds in the dielectric work liquid. For the projection the technological parameter like in particular roughness, are knowledge necessary via the channel spreading and channel form, for off-line process models. The used examination methods are High Speed Framing Camera (HSFC) for the optical observation and the Confocal Lasers Scanning a Microscopy (CLSM) for the determination of the craters topologies.

Gas bubble morphology in small working gaps at spark erosion [micromachining]

The morphology of the gas bubbles in the working gap determines the conditions for the electrical breakdown in spark erosion. Significant modification of the gas bubble structures can be proved by high speed recording if the pulses are changed only in a small parameter field. The investigations carried out are particularly important for the production of precise holes with the spark erosion process. In ED hole sinking, the work gap is smaller than 20 /spl mu/m so that the size and number of gas bubbles are dominant criteria for decontamination of the discharge gap. The gap cleaning effect can be optimized by the specific choice of the pulse parameters, which leads to an essential reduction of the processing time. Comparative investigations with a high speed framing camera (HSFC) indicate that also for single discharges, the gas bubbles are considerably longer than the pulse periods usually used in spark erosion.

Investigation of gas bubble formation at spark erosion in small working gaps

Gas bubble formation, especially in small working gaps, has an important influence on the spark erosion process. This paper demonstrates that the gas bubbles originate from electrical discharge in the dielectric. Investigations are carried out for two typical dielectrics: hydrocarbon (n-dodecane) and de-ionized water. From the experimental results obtained with single pulses conclusions are drawn about what happens at discharge sequences like in a micro-erosion process

Simulation of the discharge expansion of a spark discharge at small distances between electrodes

An important parameter for the expansion of the plasma bases at the electrodes is the expansion of plasma channel and gas bubble of the spark erosion. Simulations for the expansions on the basis of two different models are presented. The initial size of the discharge channel diameter is determined by the breakdown conditions in the narrow working gap (streamer entry). The expansion of the discharge channel parts can be simulated from the knowledge of the current and voltage curves at the gap and from the characteristics of the dielectric fluid which depends on temperature and pressure. In this context is compared a cylindrically symmetric layer model to an ellipsoidal layer model.

Propagation of Gas Bubbles at Spark Erosion in Small Working Gaps

For describing the propagation of gas bubbles we have to distinguish between internal and external states. At an example of spark erosion it is shown how the type of gas bubble propagation influence on the process of sequential discharges. Is the mean of gas bubbles the same over many sequential discharges than the removal process is running continuously with the same parameters. However, too much gas bubbles are undesired for an effective removal. To guarantee a high removal rate the pulse parameter have to be chosen depending on the gap geometry and the working fluid. To do this optimally, the knowledge of the gas bubble generation and their behavior is an indispensable need

Influence of the additives in dielectric working fluids in the case of single electrical discharges on ignition behaviour

The spark erosion metal working process demands, apart from working gaps from 10 to 100 /spl mu/m and a dielectric working fluid (deionized water or hydrocarbons) also electrical discharges (sparks) with a pulse operation frequency of 10 kHz to 1MHz. The course of this process is dependent on the physical and electrical properties of the working fluid, which can be influenced by adding certain additives. In this paper the influence of alicyclic and aromatic compounds for the ignition stage are presented. For these compounds, nonpolar and negative-inductive components were analyzed specifically in their ignition behavior.