Karlsruhe University demonstrates the extra cost of milling

Karlsruhe University demonstrates the extra cost of milling

Costs 45 per cent higher than EDM.

The question of the best manufacturing technology has been a matter of contentious debate for generations, in manufacturing on all levels, and from the shopfloor to the university chair. There probably never will be a universal answer.

Anyone attentively following the vari­ous articles and publications on this subject cannot help but notice a clear trend towards milling. In the field of hard machining in particular, there are repeated reports highlighting the possibility of replacing electrical discharge machining with hard milling. These articles suggest that EDM is on the retreat on the grounds that it doesn’t offer any scope for innovation. The reason for this is without doubt that technologi­cal advances extending to new spec­tac­ular materials and workpieces in the conventional machining sector are more memorable than incremental improvements in performance in the EDM sector.

Unlike conventional machining, the limits to the use of EDM are set by the physical properties of the workpiece and by the demanded level of produc­tivity. When it comes to volume per unit of time, conventional machining is vastly superior to EDM. If, however, complex geometries are involved, EDM has more to offer. Deep holes, thin and deep slits and sharp edges are the domain of EDM. Thanks to the latest cutting materials and coatings, the work­piece is becoming an increasingly important factor in conventional machining as well. But there is no let-up in development in EDM, with advanced generator technologies and efficient electrode materials with optimized parameters achieving significantly improved erosion rates and better surface quality at the same time. Major progress has been made in the last few years, particularly in wire erosion.

Institute of Materials and Processes

The Institute of Materials and Processes (IMP) of Karlsruhe University of Applied Sciences engages in the ongoing further development of manufacturing and production processes. In a research project funded by the German Ministry of Economics and Technology (BMWi), research and develop­ment activities are focused on wire erosion. Together with numerous partners in industry, the aim is to adapt wire erosion to highly complex component geometries – while sustaining high productivity. For this, a Mitsubishi FA20-S Advance wire-cut EDM system has been equipped with two extra rotational axes to facilitate the machining of metals in a complexity hitherto unachieved. By contrast with the current procedure, the components no longer have to be reclamped, thus permitting a further improvement in quality. These components are destined ultimately for use in the medical technology sector.

High-speed cutting (HSC) versus wire-cut EDM

To illustrate the state of performance achieved in the project so far, a sample component, a bevel wheel gear made of steel, was produced at the IMP. The production of individual gears is a key activity in the drive sector in mechanical and plant engineering. The machin­ing of individually adapted bevel wheels represents a special challenge in this connection. With the aid of this sample geometry, the IMP compared the production processes of high-speed cutting (HSC) and wire-cut EDM.

The two processes were demonstrated with modern machines; wire-cut EDM, as described, with the use of extra axes. The workpiece selected was a bevel wheel with a pitch circle diameter of 18 mm and 12 teeth. The dimensions of the bevel wheel make it relatively easy to produce, both on the milling machine and the wire erosion machine. The blank of 16MnCr5 (Mat. No. 1.7131) was prefabricated on a lathe.

Mitsubishi FA20-S Advance with ro­ta­tion­al and swivel axes.

Before: Blank component.

After: finished component.

Milling was divided into roughing and smoothing processes. Milling tools with diameters of 0.8 and 1.0 mm were employed. Milling was performed with five axes.

For wire-cut EDM, an FA20-S Advance from Mitsubishi Electric was employed with a rotary/swivel table from JauchSchmider. This makes it possible to produce virtually any contour described by the straight line of the erosion wire. By using the rotational and swivel axes, further process improvements were realized with constant flushing conditions. These are the result of the vertical position of the wire during ma­chin­ing, accompanied by higher pro­cess­­ing speed and precision. It is also possible to cut larger conical angles, even on workpieces of low height. A big advantage of the rotational and swivel axes is the reduction in reclamping frequency for the production of components with complex contours.

The lower cost of wire erosion

The criterion for the comparison of the techniques was cost-effectiveness. This encompasses both labour costs and machine and tool costs. Since production time has a direct effect on component cost, it is a crucial factor in this comparison. Also considered are the cost of tooling in the form of erosion wire and milling tools. The pie charts (below) show the breakdown of costs for the machining of the bevel wheel gear by HSC and wire-cut EDM.

Breakdown of costs for the two machining processes

Wire-cut EDM

HSC

Although machining by erosion took longer (34 minutes) than milling (30 min­utes), the erosion process is clearly less expensive. This is due to the lower cost of machine purchase and the lower cost of consumables (wire) compared to the much higher equivalent figures for milling. Cost savings of about 30 per cent were facilitated by the use of addi­tional axes on the wire-cut EDM machine for this sample geometry. The attainable surface quality and tolerances are the same as those for milling.

The tests have shown that wire-cut EDM has huge potential. In this exam­ple, it was possible to machine the gear at much lower cost. Highly intricate com­ponents ought therefore to be ma­chined in future by wire erosion. It offers engineers in development depart­ments totally new scope in component design. The productivity of wire erosion can also be significantly increased by reducing the frequency of reclamping. These issues will play a prominent role in the future of manufacturing engineering.

The IMP at Karlsruhe University presents the focuses of its research

The Institute of Materials and Processes (IMP) at Karlsruhe University of Applied Sciences concentrates on the development of new modelling and simu­la­tion techniques for virtual design, on the optimization of processes in production and manufacturing and on applications in fluid dynamics. The three departments are organized in a matrix form and are supported by cross-sec­tion­al technologies in the chararacterization of materials.

Research subjects

Materials modelling, simulation of mi­crostructure formations in multi-phase and polycrystalline grain structures, heat and mass diffusion in multi-component systems, elasticity, plasticity and magnetism

• Tool- and mouldmaking, medical technology, aerospace, HSC/HPC, EDM, tool machines
• Applied fluid dynamics and fan aero-acoustics
• Materials testing and structure characterization

Production and manufacture department

The production and manufacture de­part­ment is concerned with the opti­mi­zation and further development of conventional and nonconventional ma­­chining in production processes and particularly milling, turning, lasercutting and electrical discharge machining.

Focuses

• Cooling lubricant additives
• Low-temperature treatment of cutting tools to investigate wear behaviour
• Extension of the process boundaries of EDM drilling
• Improving process stability and achievable erosion results by using different oil- and water-based dielectrics
• Development of time prediction models for die-sink erosion
• Development of additional simultaneously controlled erosion axes for EDM cutting with a running wire electrode
• Nano-coatings for injection moulds to increase the stability under load of aluminium moulds
• Determination of energy conversion in machine tools
• Development of active clamping systems for conventional machining

The development and research activi­ties take place in close cooperation with German and international industrial partners, mainly small and mediumsize enterprises. With its state-of-the-art machine and equipment park, it yields results relevant to modern pro­cesses and ensures a high quality of teaching.