Ultrasonic assisted of die sinking electrical discharge machining using standard equipments and devices.

Turc, Cristian--Gheorghe ; Belgiu, George ; Pamintas, Eugen 等

1. INTRODUCTION

The electrical discharge machining (EDM) is suitable for economical machining of special hard metallic materials, or complex surfaces, that is too expensive or impossible to machine by classical cutting methods. There are a lot of machines for EDM, with massive electrode--the so-called die sinking EDM that is used to machine by the coping of the tool-electrode shape, or with wire electrode--the so-called wire EDM that is used for cutting out operations. Most of EDM machines are big, because they are used especially for the manufacturing of the dies.

The material erosion mechanism primarily makes use of electrical energy and turns it into thermal energy through a series of discrete electrical discharges occurring between the electrode and work piece immersed in a dielectric fluid. The thermal energy generates a channel of plasma between the cathode and anode at a temperature usually in the range of 8000 to 12,000 [degrees]C initializing a substantial amount of heating and melting of material at the surface of each electrode. When the pulsating direct current supply occurring at the rate of approximately 20-30 kHz is turned off, the plasma channel breaks down, causing a sudden reduction in the temperature allowing the circulating dielectric fluid to implore the plasma channel and flush the molten material from the electrode surfaces in the form of small, microscopic debris. The volume of material removed per discharge is typically in the range of [10.sup.-6]-[10.sup.-4] [mm.sup.3] and the material removal rate is usually between 2 and 400 [mm.sup.3]/min depending on specific application. Since the shaped electrode defines the area in which the spark erosion will occur, the accuracy of the part produced after EDM is fairly high (Ho & Newman, 2003).

The major disadvantage of the EDM consists on its small metal removal rate (MRR). According some authors, the time spent for a mold manufacturing is nearly 50% of the total manufacturing time, including mold assembling and probes (Witzak, 1997).

During the EDM process development, there were introduce some techniques in order to improve the MRR, such as (Abbas et. al., 2007):

* Rotating of the work piece or the tool-electrode;

* Improving of the dielectric liquid flushing;

* Using of CNC technology;

* Using of multi-electrode discharging system;

* Introducing of dry EDM;

* Process control improvements;

* Tool-electrode or work piece vibration.

2. ULTRASONIC ASSISTED EDM

Introduction of ultrasonic vibration to the electrode or work piece is one of the methods used to expand the application of EDM and to improve the machining performance on difficult to machine materials (Abbas et. al., 2007). The study of the effects on ultrasonic vibration of the tool-electrode on die sinking EDM has been undertaken since 1980s. The higher MRR gained by the employment of ultrasonic vibration is mainly attributed to the improvement in dielectric circulation which facilitates the debris removal and the creation of a large pressure change between the electrode and the work piece, as an enhancement of molten metal ejection from the surface of the work piece. Later, it was proposed spark erosion with ultrasonic frequency using a DC power supply instead of the usual EDM pulse power supply (Zhang et al., 1997). The pulse discharge is produced by the motion between the tool-electrode and work piece simplifying the equipment and reducing its cost. They have indicated that it is easy to produce a combined technology which benefits from the virtues of ultrasonic machining and EDM. Ghoreishi and Atkinson compared the effects of axial vibration of the tool-electrode, rotation of the tool-electrode and combinations of the methods (vibro-rotary) in respect of MRR, tool wear ratio and surface quality in EDM die sinking and found that vibro-rotary increases MRR by up to 35% compared with vibration EDM and by up to 100% compared with rotary EDM in semi finishing (Ghoreishi & Atkinson, 2002). Prihandana et al. have studied the effect of vibrated work piece, also. They have proven that when the vibration was introduced on the work piece the flushing effect increased, and that high amplitude combined with high frequency increase the MRR (Prihandana et al., 2006).

Most of the studies regarding ultrasonic assisted EDM (UAEDM) were carried out on experimental systems. This is the reason why the industrial applications of this new combined technology are not wide-spread yet, despite the good experimental results in improving the MRR.

3. UAEDM DEVELOPMENT

In our opinion, the future development of the ultrasonic assisted EDM technology is possible through standardization, applying specific equipments and devices that are already in the industrial use. Thus, our research is focused on the aspect of linking the existing solutions using a minimum set of adapting devices. Figure 1 presents a possible combined set of devices, specific to both ultrasonic and EDM technologies. Thus, the ultrasonic stack, consisting of the transducer (converter) 4, the booster 6 and the sonotrode (horn) 7 is attached to the clamping journal 2 through a pair of dedicated parts: the ultrasonic stack housing 5 and the custom pallet 3. The whole assembly is then mounted into the tool clamper 1, which is attached to the EDM machine tailstock. The set-up also needs an ultrasonic generator that supplies the transducer. In order to avoid the undesirable interferences, the pallet 3 should be made of an insulating material. Except the ultrasonic stack housing, the pallet and the sonotrode 7, which is designed according to the cavity to machine and can be manufactured through rapid prototyping technique (Cosma et. al., 2006), all the set-up components should be standard supplied. Thus, the tool clamper (figure 2) and the clamping journal (figure 3.a) are standard solutions in EDM fixturing systems. The insulating pallet should be designed according to the fixturing system specifications, as shown in figure 3.b (Hirschmann, 2009).

[FIGURE 1 OMITTED]

The ultrasonic stack type depends on the specific process needs. A wide range of typo-dimensional ultrasonic stacks are delivered by a lot of manufacturers. Figure 4 presents an ultrasonic booster and transducer design that is suitable for averages UAEDM operations (Krell Engineering, 2009).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. FURTHER RESEARCH

The further researches will be focused on the development of a methodology in choosing the right ultrasonic stack (type, power, frequency), and in developing of a parametric design for the ultrasonic stack housing and pallet that are necessary to assembly the ultrasonic stack into the specific fixturing system.

5. CONCLUSION

The present study presents some elements on an easier way to implement ultrasonic assisted die sinking EDM operations by using on a large scale of the existing tooling solutions. This approach will lead to a significantly development of UAEDM.

6. REFERENCES

Abbas, N. M.; Solomon D. G. & Bahari, Md. F. (2007). A review on current research trends in electrical discharge machining. International Journal of Machine Tools & Manufacture, Vol. 47 (2007), pg. 1214-1228, ISSN: 08906955

Cosma, C.; Iclanzan, T.; Dume, A. I. & Tulcan, A. (2006). Rapid prototyping for reverse engineering. Academic Journal of Manufacturing Engineering, Vol. 4, Nr. 2, pg. 17-22, ISSN: 1583-7904

Ghoreishi, M. & Atkinson, J. (2002). A comparative experimental study of machining characteristics in vibratory. Journal of Materials Processing Technology, Vol. 120 (2002), pg. 374-384, ISSN: 0924-0136

Ho, K.H. & Newman, S.T. (2003). State of the art electrical discharge machining (EDM). International Journal of Machine Tools & Manufacture, Vol. 43 (2003), pg. 12871300, ISSN: 0890-6955

Prihandana, G. S.; Hamdi, M.; Wong, Y.S. & Mitsui, K. (2006). Effect of vibrated electrode in electrical discharge machining. Proceedings of the First International Conference and Seventh AUN/SEED-Net Fieldwise Seminar on Manufacturing and Material Processing, pp. 133-138, ISBN: 983-42876-0-7, Kuala Lumpur, March 2006

Witzak, M. P. (1997). Improving of the process control system of the die sinking machines using fuzzy technologies. Dissertation, Universitat der Bundeswehr, Hamburg. Available from: http://erosion.de/ Wissenswertes/inhaltmwk.html Accessed: 2009-01-10

Zhang, J.H.; Lee, T.C.; Lau, W.S. & Ai, X. (1997). Spark erosion with ultrasonic frequency, Journal of Materials Processing Technology, Vol. 68 (1997), pg. 83-88, ISSN: 0924-0136

*** (2009). http://www.hirschmanngmbh.de/english/sys5000 .htm--Hirschmann Fixturing System 5000 for Sinking EDM, Accesed on: 2009-03-13

*** (2009). http://www.krell-engineering.com/fea/industr/ industrial_resonators.htm--Industrial Resonators, Accesed on: 2009-03-10