The combination of laser ablation and micro-EDM is a recent development for the quick finishing of products with small details in hard materials, such as nozzles for diesel engines. EDM is used to remove larger volumes, while laser ablation is used for the fine details.

Laser ablation combined with micro-EDM is a hybrid surface modification technique and is suitable for the machining of hardened steel and carbide. The process has already been valorised in the lab environment.

This combination of ablation and spark erosion consists of a sequence of machining processes integrated on a single machining platform. EDM is typically used to remove bulk material (due to the higher removal rate compared to laser ablation) and to machine geometries with a large length to height ratio. Laser ablation in turn is used to create fine details, to make small superficial holes and for finishes. Mainly nanosecond and picosecond lasers are used for ablation.

Set-up of laser ablation in combination with EDM

A few figures showing the performance of this hybrid process:

• Ablation rate: 1 x 10-5 mm3/s
• EDM speed: 6,2 x 10-3mm3/s
• Maximum undulation of the soil: 2 µm

Applications and benefits

Applications for this combined technique include making holes in nozzles for diesel injection (with a diameter of up to 140 µm), microforms and moulds.

Detail machined by EDM followed by laser ablation

The technique offers a number of interesting advantages:

• A wide range of materials can be machined, even metals and ceramics that are difficult to machine (due to the absence of contact forces and the fact that the process is independent of the hardness of the workpiece).
• The production time to achieve the same specification is shorter due to the combination of the two processes. EDM is mainly used to remove larger volumes, while laser ablation is used for the fine details. In the case of fuel injectors, the starting hole is made by laser ablation, followed by an EDM operation to achieve the final size and precision.
• The combination makes it possible to manufacture components with a large length to height ratio (up to 100) and small dimensions (up to a few micrometres, limited to the diameter of the laser focus).

Process improvements

Knowledge and control of the machining depth are essential for the adequate operation of both processes. Deviations from the machining depth occur due to electrode wear (EDM) and varying absorption properties (laser ablation). Steps have been taken to increase the controllability of the machining depth. For micro-EDM machining, a confocal white light sensor was integrated into the hybrid machining platform to measure the wavelength after each machining step. Adjusting the electrode wear compensation strategy results in a flatter bottom surface.

Geometrical deviation due to wear and tear

The depth control for laser ablation is based on the monitoring of airborne noise during ablation. Based on the amplitude and frequency of the signal, the focal position in the plane can be determined and from there the required machining depth per layer can be achieved. However, these developments are still at an early stage.

Sources

• L. Li, C. Diver, J. Atkinson, R. Giedl-Wagner, H.J. Helml, Sequential laser and EDM micro drilling for the next generation fuel injection nozzle manufacture, 2006, Annals of the CIRP, Vol. 55/1
• V. Schulze, P. Weber, C. Ruhs, Optical and acoustical process control in hybrid micromachining using a picosecond pulsed UV-laser and micro-EDM-milling, 2011, Proceedings of the 8th International Conference on Multi-Material Micro Manufacture, p. 83-86
• J. Fleischer, J. Schmidt, S. Haupt, Combination of electric discharge machining and laser ablation in microstructuring of hardened steels, 2006, Microsystems Technology, Vol. 12, p. 697-701