Water jet based tooling strategies for microproduction

The main objective of this contribution is to present a new tooling strategy based on the application of WJ machining, which would allow relatively quick and cost effective production of prototype micro components. In the first step the tool for MEDM is produced in copper by WJ technology. Then the copper tool is used by MEDM technology to produce the final tool in tool steel, which may be further used for replication processes such as hot embossing, pressure molding and others. The complete process chain is shown in Figure.

The proposed tooling strategy offers high flexibility and cost effectiveness. Additionally, it provides more freedom and testing opportunities during the development of new micro devices. The most common used tooling strategy is direct manufacturing of the tool by micro milling. When the features of the tool are rather ribs then grooves, the tooling strategy proposed in Figure has a great advantage over micro milling tool manufacturing, which is the most common used tooling strategy. In the latter case, an end-mill with a relatively small diameter has to remove relatively big volume of the tool which is time consuming and not cost effective.

The main application field of the proposed tooling strategy is the design and development of micro-fluidic devices. Typically, these devices require a well-controlled geometry and surface roughness. With this technology these devices can be manufactured relatively fast and in a cost effective way. Therefore many new concepts and designs can be experimentally validated during the development phase in order to improve the performance of the final product. In the actual context of R&D, flexibility in the manufacturing process enables variety and innovation in the design. The proposed tooling strategy consumes most of its machining time in MEDM machining while WJ machining accounts just for a small portion of the total machining time. However, facing this sequence of different processes, WJ machining of the MEDM tool has an important influence on the final result.

Figure: The process chain of micro-fluidic channel production

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On-line selection of the rough machining parameters upon the eroding surface size

The material removal rate and the surface roughness increase with increased power in the gap. In this way, rough and fine machining is distinguished. When rough machining is performed, the material removal rate should be as high as possible, while the achieved surface roughness does not play an important role.

The EDM process stability is determined by the proportion of harmful discharges in the gap between a workpiece and an electrode, i.e. arc and short-circuit discharges, which not only lower the material removal rate, but also increase the electrode wear. The process is more stable in the case of lower proportion of the harmful discharges. The main cause for unstable EDM process is the contamination of the gap with discharge products. But the surface power density in the gap also affects the process stability. To achieve the highest material removal rate, the roughing setup parameters should be tuned to the eroding surface size. The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction as shown in Figure. This was analyticaly prooved in the In general, the engaged surface is not plane and the eroding surface size changes with the depth of machining. To select the appropriate roughing setup parameters at any machining depth, the eroding surface size has to be determined on-line.

Figure: The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction.

Voltage and current in the gap define electric power in the gap (P=UI). There exists the optimal set of the setup parameters' values to obtain the certain power in the gap and the discharge voltage is nearly constant at all machining regimes, thus the power in the gap depends only on the current in the gap. In the literature, the boundary surface current density is given rather then boundary surface power density and it is stated that stable EDM process is achieved when the surface current density is less than 0.1 A. The relation between the surface current density and the material removal rate Vw is presented in Figure. At constant eroding surface size A1, the material removal rate increases with increased surface current density until the boundary surface current density is reached. Higher surface current density causes unstable machining process and the material removal rate decreases. When greater eroding surface is employed (A2), the higher current is needed to reach the boundary surface current density, thus the material removal rate is higher compared to the material removal rate at eroding surface A1.

To select the appropriate roughing setup parameters when eroding surface size varies during the machining, the eroding surface size has to be determined on-line.

Figure: Material removal rate Vw versus the surface current density


For on-line detection of the eroding surface size, it is necessary to monitor the appropriate process quantities z. Proper evaluation of the process quantities is the key to gain suitable process attributes x for the determination of the eroding surface size. The process attributes are the inputs into the model for the selection of the optimal rough machining parameters(see figure)

Figure: On-line selection of the roughing setup parameters of the EDM process

Design adaptation system for machining by EDM process

The EDM is still very often used, specially in tool engineering where tools for mass production are produced. The shape of the tool is a negative image of the product and it has to be easily and cheaply made. To adapt tool design, and at the same time also product design, for easier manufacture of the tool we distinguish two levels: design and manufacturing level. On the manufacturing level a manufacturing technology for tool machining is determined. There is always feedback information from manufacturing to design level to change the tool and the product design according to easier (cheaper) manufacture of the tool. The designer considers the suggestions of the technologist and together they find the best design by taking into account also the demands for the product and the tool.

A system for segmentation and determination of a proper machining process for machining each segment of the tool separately, has already been developed at the Faculty of mechanical engineering. A high speed milling (HSM) and the EDM process are considered as two machining processes for making each segment of the tool. In our work the system for adaptation of the product to easier tool manufacture with EDM process was developed. It is designed for designers to establish critical parts of the product from the point of view of machining the tool with EDM process. With these information the designer can adapt the critical parts of the product design without the tool engineer. By using the system it is possible to reduce number of information from manufacturing to design level and to reduce the time necessary to manufacture tools. In reality it is impossible to eliminate all information from manufacturing to design level or to replace the tool engineer with an expert system.

Figure: Scheme of the design adaptation system for EDM.

EDM controller

EDM process is very unstable, especially when working in fine regime. Often surface damage occurs by arc discharging. Unstable working is unavoidable, so the process is forced to work in liable, but effective region of working. The process is run by operator who overlook it and make a feedback control. Automation of process is the final goal of our research.

A strategy of controlling is a very important part of stable EDM action. It is complex and operator can master it after a long period of learning and collecting his own experience. Operator's knowledge and technological knowledge together with technological receipts can be added to adaptive controller which must be able to accept both of them. We are working on development of such a controller (Fig.). All basic function of controller (identification, reasoning and control) are computer made. Identification of process is well done by computer. The same is with reasoning, based on qualitative and probability assessment, which is usually reserved for human. Operator-to-computer communication is possible and controller can be improved. While the split between human way of reasoning and reasoning algorithms, a strategy of controller is developed by artificial intelligence (learning by example). Method FORS (First Order Regression System) is used. Rules IF-THEN-ELSE can be obtain by human demonstration of process leading. By this method, operator knowledge is transformed to algorithm used by computer. Rules IF-THEN-ELSE are human understandable, too.

Figure: Adaptive control system for EDM process control

EDM small hole machining

The use of EDM process is in parallel to development of new technologies. One of these new technologies is small hole drilling by EDM.

Small hole drilling (d <2mm,>h/d > 10) is a huge technological problem. By drilling problems arise with chip transport and heat deviating. Drilling tough and rigid materials on depth is also a problem. By other non-conventional processes like laser and electron beam small holes can also be made, but these processes are still to expensive for common use. EDM is the best choice for machining electric conductive materials, especially for holes of uncommon shapes.

Technological problems arise:

• precision of electrode machining
• electrode positioning
• high dielectric liquid pressure
• electrical parameters selecting
• electrode guide

Figure: EDM small hole sinking design

Figure: Small hole made by EDM machining

By research carried out and by our experience we obtain technological properties of the process and directions how to control it.

Electrical discharge machining (EDM)

Electrical discharge machining (EDM) is a non-conventional process of machining. the development of the EDM technology started in the forties. Since then, it is the most important machining process in tool engineering. General advantages to other machining processes are: accuracy, surface quality and the fact that hardness and stiffness of a workpiece material is not important for the material removal. The EDM has become mature technology but the researches and improvements of the process are still going on. The main reason why, is that there still does not exist a machining process, which could successfully replace the EDM.

Crater to pulse classification

By research on basic principles for material removal in EDM process we try to improve effectiveness of the process. A single discharge - an unit event of EDM process was researched (Fig. 1). The correlation between electrical parameters and surface give us important information about process.

Figure: Time dependence and surface crater of single discharge made by EDM machining