Joško Valentinčič

Percentage of harmful discharges for surface current density monitoring in electrical discharge machining process

Abstract Electrical discharge machining (EDM) is performed in the gap between the workpiece (usually the anode) and the electrode (usually the cathode). The gap is filled with a dielectric and the pulses generated by a special generator cause an electrical breakdown in the gap. A breakdown discharge occurs which removes material from the workpiece. The EDM process has been used for more than 50 years, but the influence of the surface current density on the process stability is not well understood. The surface current density is very important in the case of the machining of rough surfaces where the highest material removal rate (MRR) is preferred. For a given eroding surface there exists an optimal electrical current at which the highest MRR is achieved. It is reported in the literature that the percentage of the discharges that are harmful to the EDM process depends on the surface current density in the gap. Thus, experiments were performed to find out the possibility of using the percentage of harmful discharges to monitor the surface current density of the EDM process.

Assessment of Electrode Wear Measurement in Micro EDM Milling

In micro electrical discharge milling (micro EDM milling), the tool electrode kinematics is the same as in conventional milling, but the material removal is obtained by the heat and electrical forces in the gap between the tool electrode and the workpiece. The electrode wear in micro EDM milling is one of the main problems to be solved in order to improve machining accuracy. Most common, the electrode linear wear is measured on machine by touching the reference point with the electrode after machining a certain number of layers. The reduction of the electrode length due to the wear is used to modify the electrode trajectory. This paper presents an assessment of the electrode wear on micro EDM milling machine. The experiments were performed on state-of-the-art micro EDM equipment. The electrode wear was measured on the machine in a conventional manner as already described. Additionally, a Laser Scan Micrometer (LSM), implemented on machine as well, was used in two ways, namely to detect only the coordinates of the electrode tip before and after machining, and by acquiring the electrode profile. Experiments were performed by machining micro features in steel using several layer thicknesses and various process parameters. The results show that the accuracy of the electrode length measurement by touching the reference point is enough accurate for the estimation of the electrode linear wear. But to accurately measure the electrode volumetric wear, it needs to be calculated from electrode profiles acquired before and after machining.

A comparative study of ultrasound, microwave and microreactor-assisted imidazolium-based ionic liquid synthesis

Abstract Synthesis of ionic liquid 1-heptyl-2,3-dimethylimidazolium bromide was accomplished with the assistance of ultrasound, microwave irradiation, and a continuously operated microreactor and was compared with a conventional laboratory scale process applying magnetic stirring and water-bath heating. Results were compared with respect to process productivity, energy consumption, and product colourisation as an indicator of its purity. By using nonconventional technologies, volumetric productivity was 10- to 30-fold superior, while energy consumption was reduced by 45%–65%. Among the alternatives tested, ultrasound-assisted synthesis was shown as the most efficient one in terms of volumetric productivity (4.40 mol l-1 h-1) and specific power consumption (909.1 W h mol-1), while microwave-assisted process was the least favourable. However, only a microreactor system enabled the synthesis of a noncoloured product resulting from very efficient mixing and temperature control. Due to significant process intensification along with high product quality and superior industrial perspectives, a continuous quaternisation within microchannels could be selected as the most promising green approach among the alternatives tested in this study. Integration of ultrasound and microreactor technology including miniaturised heat exchanger is foreseen for process intensification.

A novel approach to DFM in toolmaking: a case study

A novel approach to avoid the knowledge gap between design and production is presented in this paper. The main idea is to build a system in the form of a computer program whose core is the manufacturing expert systems to be used by the product designer. The system reveals critical features of the designed product from the manufacturing point of view and points them out to the product designer. The product designer can decide whether to change the critical part of the product or not. The system presented in this paper is prepared for the toolmaking, where a lot of relatively cheap products are made by one relatively expensive tool. Since the shape of the cavity in the tool is the negative shape of the product, small changes in the product design can significantly reduce the manufacturing costs of the tool.

Water jet machining of MEDM tools

This contribution presents an investigation about the possibilities of using Water Jet (WJ) technology in combination with Micro Electrical Discharge Machining (MEDM) for tooling production in micro manufacturing. In the first phase the tool copper used in MEDM is produced by WJ machining. Afterwards, the final tool in steel is produced by MEDM. Such kinds of tools intend to be used in processes like hot embossing, molding, and other replication technologies in the field of micro manufacturing. The first results are very promising and the proposed tooling strategy, which involves besides MEDM also WJ technology, shows a lot of potential especially in the design and developing phase of micro-fluidic devices.

Preliminary Study on Staggered Herringbone Micromixer Design Suitable for Micro EDM Milling

This paper discusses interplay between the design of a Staggered Herringbone Micromixer (SHM) and micro Electrical Discharge Machining (EDM) milling technology. SHM consists of a main channel and many herringbone lookalike grooves at the bottom, which enhance mixing. Firstly, a brief overview on how the technological model of micro EDM milling was constructed is presented. Anticorrosive stainless steel was used as a workpiece material on the basis of which the database for the technological model was established. In the second part the technological model was used for micromixer design optimization. Different SHM designs that can be machined in the same amount of time, were compered via their mixing efficiency. Mixing efficiency was estimated by performing Computational Fluid Dynamics (CFD) simulations. The results show, that smaller and shallower grooves are not efficient in the terms of mixing. An important design parameter is also the orientation of the grooves. In the end, a favorable SHM design is presented.

Repeatability and Limitations of Water Jet based Micro-tooling Process Chain

Microproduction is one of the fastest growing fields of industry with increasing demands from the market. A key factor in microproduction is micro-tooling. There are various micro-tooling techniques, such as LIGA, electroplating, etc., which are very accurate but on the other hand also very expensive and time consuming. An alternative process chain is applied consisting of producing the electrode for Die-Sinking Micro Electrical Discharge Machining (MEDM) by Water Jet (WJ) technology, which can not compete with above-mentioned technologies regarding the accuracy, but it is time efficient and cost effective. The final tool is to be used for replication processes and is produced by MEDM. The results present the process chain in view of repeatability and limitations of producing micro-channel structures through statistical analysis. The analysis shows good repeatability of WJ machined features on the MEDM electrode which indicates the appropriateness of the chosen machining process. Due to the specific characteristics of involved technologies the smallest channel width is limited to 100 μm.

Micro EDM parameters optimisation

Abstract Electrical discharge machining (EDM) is an important process in the field of micro machining. However, a number of issues remain to be solved in order to successfully implement it in an industrial environment. One of these issues is the processing time. This paper investigates the optimisation of machining parameters for rough and fine machining in micro EDM. In one case, the parameters are selected to achieve the highest material removal rate (MRR). In the other case, the best surface roughness is targeted. Some of the main difficulties linked with micro EDM are caused by the high wear occurring on the electrode. The study focuses on a specific combination of electrode and workpiece material and proposes a typical method for micro EDM process optimisation.