An intelligent and automated system for EDM hole

An intelligent and automated system for EDM hole drilling of super alloys 1

Oguzhan Yilmaz , A. Tolga Bozdana, M. Ali Okka and I. Huseyin Filiz 1 Department of Mechanical Engineering, The University of Gaziantep, 27310, Gaziantep, Turkey

ABSTRACT This paper introduces an intelligent and automated system proposed for EDM hole drilling of super-alloys in manufacturing and re-manufacturing stages of products used in aerospace, die/mold and medical surgery. The system is a user-friendly computer-based program, which guides the end-user in order to perform sustainable and effective process. To accomplish the goals of the work, a well-planned workshop tests have been performed via drilling of micro and macro-scale holes (0.4-3mm) on Ti-6Al-4V and Inconel 718 alloys, which are commonly used in aerospace industry and medical sector for manufacturing of highly critical components. The experimental data was refined and analysed using ANOVA to obtain parameter’s relations and mathematical modelling for optimisation. ANFIS has been adopted to correlate the EDM hole drilling parameters (i.e. pulse current, pulse duration/interval, capacitance, material removal rate, electrode wear and surface roughness) for fulfilling reliable, cost/time effective and efficient hole making operations. Key words: EDM hole drilling, Titanium, Inconel 1 INTRODUCTION Electrical Discharge Machining (EDM) hole drilling is a different variation of EDM processes, i.e. die sink EDM and wire EDM. Although the EDM hole drilling shown in Figure 1 uses the same principles as other EDM methods, a constantly rotated hollow electrode and pumping of dielectric fluid through the electrode tube are the two distinct features. The deionised water is preferred as dielectric instead of petroleum-based fluids due to environmental issues. The rotating electrode helps in producing concentricity, causing even wear, and helps in the flushing process. The high flushing pressure through the center of the electrode tends to stiffen it. Also, the dielectric being forced out of the hole produces a centering effect upon the electrode. With the aid of the electrode guide and the flushing effects on the electrode, EDM drilling can penetrate much deeper in higher rates than almost any other drilling

method [1]. Drilling rates up to 1mm/second can be achieved and the hole size is generally between 0.15 to 3 mm, with a length-to-diameter ratio of over 150:1 [2]. This process has been alternatively used for making holes in turbine blades, fuel injectors, medical equipments, cutting tool coolant holes, hardened punch ejectors, plastic mold vent holes and wire EDM starter holes. Figure 2 shows some of the applications of EDM hole drilling. Drilling of macro and micro holes in super alloys is beyond the capabilities of conventional twist drilling process [3].

Figure 1. A hole drilling machine. Besides using this technology in drilling of micro and macro scale holes of medical equipments and highly precision components, manufacturing and repair of cooling holes on aeroengine components have been utilized for many years using the same technology. For instance, high value engine components such as combustors, nozzle guide vanes, blades, and other transition parts are regularly refurbished and put back into service. Leading engine manufacturers are increasingly adding new components to the repair list concerning with cost, time and manufacturing difficulties. A significant part of an engine overhaul involves refurbishing airflow holes (cooling holes) through engine components. Large combustor components contain thousands of small holes of various sizes and shapes. The holes maximizes engine thrust by selectively cooling critical components, and precision drilling using the most secure and safe method of EDM in carefully plotted patterns over a part’s contoured surfaces. The challenge with refurbishing the holes in these complex parts is that they must re-drilled in the same orientation and with identical precision and shape as the original. Re-manufacturing of precision holes in a used part is often more difficult than generating new ones.

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(b)

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Figure 2. Applications of EDM hole drilling, (a) aerospace; (b) tool-die; (c) medical

Although much work has been reported in EDMing of different materials, there are still few works on EDM fast hole drilling of aerospace alloys. Jeswani [4] drilled small holes ranging from 0.19-0.71 mm diameter in high carbon steel using copper wires. Wang et.al. [5] and Mohan et.al. [6] studied EDM hole drilling of aluminium alloys. Moreover, the effect of EDM parameters on deep hole drilling of titanium alloy using copper electrode were analysed by Asokan et.al. [7]. Soni et.al. [8] made an experimental investigations which were carried on electro-discharge machining of titanium in respect to surface finish, out-of-roundness, surface roughness and overcut using rotating copper-tungsten tool electrode. Leao et.al. [2] studied on the optimisation of fast hole drilling through evaluation of dielectric and electrode materials. Kuppan et.al. [3] investigated the influence of EDM process parameters in deep hole drilling of Inconel 718. This paper introduces a proposed intelligent and automation system, which interactively guides the end-user in order to perform a reliable, optimum, efficient, and time-cost effective EDM hole drilling of machine-to-difficult materials, Inconel 718 and Ti-6Al-4V. 2 AUTOMATED AND INTERACTIVE EDM HOLE DRILLING The proposed system is an integration of two embedded modules, which use the same data-base obtained from the experimental works. The experimental works were designed using design of experiment (DOE) and conducted in the workshop with precise equipments. Figure 3 shows the general structure of the system.

Figure 3. System structure. 2.1 Automation The first module of the system is the parameter optimisation. In this module, the operational parameters used for EDM hole drilling are optimised and monitored according to the desired conditions. These conditions are determined prior to the operation and considered as the system inputs: hole diameter, hole length, material, desired surface roughness. Simply, these input parameters are the characterisation of the hole on a part. In other words, these are typical request of engineers from EDM operators. In the automation stage, the main task is to optimise the EDM hole drilling process parameters and guide the EDM operator. Guiding is such that the all

necessary technical and operational details of the EDM hole drilling operation are calculated and monitored. The output results will not only be some numerical results but also they will be additional details of the optimised values. For instance, according to the desired hole characteristics, the system will display the optimum diameter of the EDM electrode, proper electrode material, optimised EDM conditions (discharge current, pulse-on/off time, capacitance). Thus, the EDM operator is guided to use the calculated electrode diameter, electrode material and EDM conditions for reliable and efficient operations. Moreover, the EDM user is also be informed of total drilling time, electrode length needed for the given hole length and expected surface roughness after drilling operation. 2.2 Intelligent parameter selection The second module of the system is the intelligent parameter selection for EDM hole drilling. This module correlates the EDM input and output parameters using artificial intelligence methods, i.e. artificial neural network, fuzzy logic and expert system. The basic EDM input parameters are discharge current, pulse-on time, pulse-off time and capacitance. Output parameters are material removal rate, electrode wear and surface roughness. In EDM operations input and output parameters have direct effect to each other in an uncontrollable manner. This system uses ANFIS (adoptive neuro-fuzzy inference system). The ANFIS model is structured and trained using the verified experimental results in which the expert rules are also extracted from the experimental works. Thus, the complicated interrelation between these parameters is simplified using developed ANFIS model. The relations of the inputs and outputs for different operational conditions can be modelled and the effects are to be controlled without having an expert EDM operator. The other beneficial effect of this module is a training tool for EDM operators in order to test variation in operational conditions. 3 CONCLUSIONS An intelligent and automated approach for EDM hole drilling of super alloys is presented in this paper. The system is being developed for the application of making precision holes in aerospace and medical parts, which use almost similar alloys, i.e. Inconel 718 and Ti-6Al-4V. Although EDM hole drilling is a variation of EDM process, it does not use oil-based dielectric and do not leave toxins. Therefore, EDM hole drilling machines are environmentally-friendly machines, since the dielectric is commonly pure (deionised) water. Moreover, the automation of EDM hole drilling saves time and material particularly for critical applications. The proposed approach can also be applicable for both manufacturing and repair stages of aeroengine components in order to increase productivity and improve sustainability. ACKNOWLEDGMENTS The authors would like to thank The Scientific and Technological Research Council of Turkey (TUBITAK) for fully-financially supporting this research work under 1001 Scientific and Technological Research Project Scheme, Grant No. 108M022.

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