The first version of this paper was received by the Editor on June 3, 2009;
The revised version was received on June 18, 2009.

Development of an Eco-friendly Electrical Discharge Machine (E-EDM) Using TRIZ Approach

V.S.Sreebalaji1, Dr.R.Saravanan2, Dr. J. Srinivas3

1Professor & Head, Department of Mechanical Engineering,
Einstein College of Engineering,Tirunelveli-12,Tamilnadu, India,
Mobile: 91-9894674531, Email : balajivsv@rediffmail.com

2Professor, Department of Mechanical Engineering,
Bannari institute of Technology, Satyamangalam, Tamilnadu, India,
Mobile:91-9443347210, Email : saradharani@hotmail.com


3Professor, Department of Mechanical Engineering,
Indian Institute of Science,IISC, Bangalore, India.
Email: srin07@yahoo.co.in

Abstract. Electrical Discharge Machine (EDM) is one of the non-traditional machining processes. A pulse discharge occurs in a small gap between the work piece and the electrode and removes the unwanted material from the parent metal through melting and vaporization. The electrode and the work piece must have an electrical conductivity in order to generate the spark. Dielectric fluid acts as a spark conductor, concentrating the energy to a very narrow region. There are various types of products can be produced and finished using EDM such as Moulds, Dies, Parts of Aerodynamics, Automotives and Surgical components. This research work reveals how an Eco friendly EDM (E-EDM) can be modeled to replace dielectric fluid and introducing ionized oxygen in to EDM to eliminate harmful effects generated while machining by using dielectric fluid and to make pollution free machining environment through a new design of E-EDM using TRIZ (a Russian acronym for Theory of Inventive Problem Solving) approach, since eco-friendly design is the need of the hour.
Key words: EDM, Dielectric, Ionized oxygen, model, eco-friendly, TRIZ.

1.Introduction To Electric Discharge Machining (EDM)
In 1970, the English scientist, Priestley, first detected the erosive effect of electrical discharges on metals. More recently, during research (to eliminate erosive effects on electrical contacts) the soviet scientists, Lazarenko and Lazarenko, decided to exploit the destructive effect of an electrical discharge and develop a controlled method of metal machining. In 1943, they announced the construction of the first spark erosion machining. The spark generator used in 1943, known as the Lazarenko circuit, has been employed over many years in power supplies for EDM machines and an improved form is being used in many current application[1]. The EDM process can be compared with the conventional cutting process, except that in this case, a suitably shaped tool electrode, with a precision controlled feed movement is employed in place of the cutting tool and the cutting energy is provided by means of short duration electrical impulses. EDM has found ready application in the machining of hard metals or alloys (necessarily electrically conductive) which cannot be machined easily by conventional methods. It thus plays a major role in the machining of dies, tools, etc., made of tungsten carbides, stellites or hard steels. Alloys used in the aeronautics industry, for example, hastalloy, nimonic, etc., could also be machined conveniently by this process. EDM is also used to machining of exotic materials, refractory metals and hard enable steels. This process has an added advantage of being capable of machining complicated components and making intricate shapes. Most of the surgical components (we all know how well the Surface Quality(SQ) is required, must for surgical components) are being machined by this process since EDM is one of the unconventional processes which can produce better surface quality.

1.2 Fundamentals of Electric Discharge Machining
Electric discharge machining is a thermo-electric non-traditional machining process. Material is removed from the work piece through localized melting and vaporization of material. Electric sparks are generated between two electrodes when the electrodes are held at a small distance from each other in a dielectric medium and a high potential difference is applied across them. Localized regions of high temperatures are formed due to the sparks occurring between the two electrode surfaces. Work piece material in this localized zone melts and vaporizes. Most of the molten and vaporized material is carried away from the inter-electrode gap by the dielectric flow in the form of debris particles. To prevent excessive heating, electric power is supplied in the form of short pulses. Spark occurs wherever the gap between the tool and the work piece surface is smallest. After material is removed due to a spark, this gap increases and the location of the next spark shifts to a different point on the work piece surface. In this way several sparks occur at various locations over the entire surface of the work piece corresponding to the work piece-tool gap. Because of the material removal due to sparks, after some time a uniform gap distance is formed throughout the gap between the tool and the work piece. Thus, a replica of the tool surface shape is formed on the work piece as shown in figure 1.2.1. If the tool is held stationary, machining would stop at this stage. However if the tool is fed continuously towards the work piece then the process is repeated and more material is removed. The tool is fed until the required depth of cut is achieved. Finally, a cavity corresponding to replica of the tool shape is formed on the work piece.


Figure 1.2.1: Tool shape and corresponding cavity formed on work piece after EDM Operation

The schematic of an EDM machine tool is shown in figure 1.2.2.The tool and the work piece form the two conductive electrodes in the electric circuit. Pulsed power is supplied to the electrodes from a separate power supply unit. The appropriate feed motion of the tool towards the work piece is generally provided for maintaining a constant gap distance between the tool and the work piece during machining. This is performed by either a servo motor control or stepper motor control of the tool holder. As material gets removed from the work piece, the tool is moved downward towards the work piece to maintain a constant inter-electrode gap. The tool and the work piece are plunged in a dielectric tank and flushing arrangements are made for the proper flow of dielectric in the inter-electrode gap. Typically in oil die-sinking EDM, pulsed DC power supply is used where the tool is connected to the negative terminal and the work piece is connected to the positive terminal. The pulse frequency may vary from a few kHz to several MHz. The inter electrode gap is in the range of a few tens of micro meter to a few hundred micro meter. Material removal rates of up to 300 cubic mm/min can be achieved during EDM.The surface finish (Ra value) can be as high as 50 ?m during rough machining and even less than 1 ?m during finish machining.


Figure 1.2.2: Schematic of an Electric Discharge Machining (EDM) machine tool

1.3 Limitations of EDM are:
There are quite a number of problems still to be solved to enable the process to be adopted on an extensive process.

  1. Lower Material Removal Rate (MRR) , Poor Surface Quality(SQ) are the real time EDM process limitations. In other words, maximizing the MRR, minimizing the surface roughness value [9] are the real time EDM process objectives.
  2. The wear rate on the electrode is considerably higher. Sometimes it may be necessary to use more than one electrode to finish the job.
  3. The work piece should be electrically conductive to be machined.
  4. The energy required for the operation is more than that of the conventional process and hence will be more expensive.
  5. Environmental concerns associated with the process have been a major drawback of EDM. The dielectric fluid used in EDM is the primary source of pollution from the process. Hydrocarbon based oils are the most commonly used EDM dielectric. Dielectric wastes generated after machining are very toxic and cannot be recycled. Also, toxic fumes are generated due to high temperature chemical breakdown of dielectric during machining. The use of oil as the dielectric fluid also makes it necessary to take extra precaution to prevent fire hazards. Since an environment friendly alternative for replacing the EDM process is not available, changing or totally eliminating the liquid dielectric medium provides a feasible solution.
  6. The ignition of spark discharge in a contaminated die electric fluid is an another real time process limitation since contamination of the dielectric fluid will affect machining accuracy. To start with, the dielectric is fresh, that is, it should be free from eroded particles, since eroded particles from the work piece contaminates the dielectric fluid. The dielectric should be filtered before reuse so that the contamination of the dielectric will not affect machining accuracy [1].
  7. The pollution caused by the use of liquid dielectric which leads to production of vapor during machining [2].

2. Current Research Trends in EDM Development
There are two kinds of research trends are carried out by present researchers as for as EDM is concern. One is called Modeling technique and other one is called Novel technique[2]. Modeling technique deals with Mathematical modeling, Artificial Intelligence and Optimization techniques like Regression analysis, Artificial neural network, Genetic algorithm etc., respectively are used to validate the efforts of input parameters on output parameters since EDM is a complicated process of more controllable input parameters like machining depth, tool radius, pulse on time, pulse off time, discharge current, orbital radius, radial step, offset depth and output parameters like Material Removal Rate (MRR) and Surface Quality(SQ)[2]. Novel techniques deals how other (unconventional) machining principle such as ultrasonic can be incorporated into EDM to improve efficiency of machining process by get better material removal rate, surface quality. Novel techniques have been introduced in EDM research since 1996 [2]. This research work is a revelation to introduce TRIZ in EDM novel research trend.

3. Problem Context of EDM Research
To increase the machining efficiency of Electrical Discharge Machining (EDM) in terms of improving the Material Removal Rate(MRR).

3.1 Discussion of Problem
This is an excellent example of a non-trivial (and hence interesting) problem. There is a desirable output characteristic that we need (in this case material removal rate) that unfortunately appears to be associated with something costly or unwanted (in this case, increase in machining time). In the language of TRIZ it is called a technical contradiction [10]. As we try to improve one parameter (material removal rate) the other (machining time) becomes worse, and vice versa.

3.2 Tackling the Problem
In this case, it has been decided to use one of the classic tools from the TRIZ toolkit for solving technical contradictions: The Contradiction Matrix [10]. To resolving contradictions, the method is a three-step systematic process [10]:

Step 1: Identifying the Contradiction(s)
It is necessary to map the specific problem parameters (material removal rate and machining time) onto the generic parameters used by the Contradiction Matrix. In this particular case, an appropriate mapping was arrived as:

The full description of each of these parameter is given below[10] : Parameter 26 Amount of substance: The number or amount of a system’s materials, substances, parts or subsystems which might be changed fully or partially, permanently or temporarily. Parameter 25 Loss of time: Time is the duration of an activity. Improving the loss of time means reducing the time taken for the activity. Cycle time reduction is a common term.

Step 2: Using Contradiction Matrix
The Contradiction Matrix is based upon a statistical analysis of a very large number of existing patents. The matrix maps the inventive principles according to the generic contradictions (defined by opposing engineering parameters) that were most frequently solved by them. This contradiction is used with the Contradiction Matrix to obtain suggested Inventive Principles. Contradiction to improve the Amount of substance i.e. Material Removal Rate(MRR)

Improving Parameter Worsening Parameter (i.e. Undesired)
Amount of substance (MRR) Loss of time(Increase in Machining time)

These two parameters are used to cross-index the Contradiction Matrix to obtain the following four Inventive Principles that (statistically) have been found to be the most successful ways of obtaining better material removal rate without more machining time.

Step 3 : Inventive Principles
The following inventive principles are suggested by contradiction matrix between the parameters Amount of substance and Loss of time: # 18 (Mechanical vibration), # 38 (Strong oxidants), # 16 (Partial or excessive action), # 35 (Parameter Change).

3.3 Application of suggested inventive principle(s)
Having derived some suggested Inventive Principles, to apply the principles to this particular problem. This is the part of a recurring theme in TRIZ that the sequence follows a chain like Specific Problem-General Problem - General Solution-Specialized Solution to Specific Problem. The next step is, to understand the suggested inventive principles to appreciate the full definition of each principle before attempting to apply it. In this case, the suggested four generic principles’ definition with some examples are given below for better reference to understand, validate the research findings and to propose a new design.

Principle 18.Mechanical vibration

  1. Cause an object to Oscillate or Vibrate.
  2. Increase its frequency even up to the Ultrasonic. E.g. Distribute powder with vibration.
  3. Use an object‘s resonant frequency.
  4. Use piezoelectric vibrators instead of mechanical ones.
  5. Use combined ultrasonic and electromagnetic field oscillations.
Principle 38: Strong Oxidants
  1. Replace common air with Oxygen - Enriched air.
  2. Replace enriched air with pure Oxygen.
  3. Expose air or oxygen to ionizing radiation.
  4. Use ionized oxygen. E.g. Ionize air to trap pollutants in an air cleaner
  5. Replace ozonized (or ionized) oxygen with ozone. E.g. Speed up chemical reaction by ionizing the gas before use.
Principle 16: Partial or Excessive Action (Application of Principle 3 and 9)
  1. Change an object’s structure
  2. Make each part of an object function in condition most suitable or its operation.
  3. Make each part of an object fulfill a different and useful function. E.g. Multi function tool.
  4. Replace with anti- actions to control harmful effects. E.g. Buffer the solution to prevent harm from extreme of PH.
  5. Create before hand stresses in an object to oppose undesirable working stresses.
Principle 35: Parameter Change
  1. Change an object’s physical state.
  2. Change the concentration or consistency.
  3. Change the degree of flexibility.
  4. Change the temperature.
At this point in the problem solving process, appropriate problem and technological domain knowledge is important. The final solution derived from these principles were used to validate the previous research findings and to justify how well this novel approach TRIZ can be used in emerging engineering and technological research domain to reduce the research time, cost and to give right direction, solution to research.

4. Validation of TRIZ with Experimental Research Findings on following Suggested Matrix Principles.

Principle # 18 Mechanical Vibration:
Rajurkar[3] has indicated some future trends activities in EDM machining on advanced materials, mirror surface finish using Powder Additives and introduction of Ultrasonic Vibration to the electrode is one of the methods used to expand the application of EDM and to improve the machining performance on difficult to machine materials. Guo et al[4] has proposed the higher efficiency gained by the employment of Ultrasonic Vibration is mainly attributed to the improvement in dielectric circulation which facilitates the debris removal and creation of large pressure change between the electrode and the work piece, as an enhancement of molten metal ejection from the surface of the work piece. Ogawa et al [5] proved that the depth of micro holes by EDM with Ultrasonic Vibration becomes about two times as without ultrasonic vibration and machining rate increased. Kunieda et al [6] introduced an improvement of dry EDM characteristics using Piezoelectric Actuators to help in controlling the gap length to elucidate the effects of the piezoelectric actuator an EDM performance simulator developed to evaluate the machining stability and MRR of dry EDM.

Principle # 16 Partial or Excessive action:
This principle is an application of Principle #03 Local Quality and Principle #09 Preliminary Anti-Action. Principle # 3 suggests change an object structure, Q. H. Zhang et al[8] has proved this suggested principle by introducing an investigation of Ultrasonic-Assisted Electrical Discharge Machining in gas with thin walled pipe electrode i.e. Tube Electrode. Principle #9 suggests that do an action with harmful effects, for example, buffer a solution to prevent harm. Q. H. Zhang et al [8] has proved and validated this suggested principle by introducing gas in Ultrasonic Electrical Discharge Machining (UEDM) by avoiding using dielectric fluid.

Principle # 35 Parameter Change:
Zhang et al.[7] studied the ultrasonic EDM in gas. The gas is applied through the internal hole of a Thin Walled Pipe Electrode.

5. New Design of E-EDM using TRIZ for better Material Removal Rate (MRR) in EDM
Principle # 38 Strong Oxidants suggests replace common air with Oxygen - Enriched air, Replace enriched air with pure Oxygen, Expose air or oxygen to ionizing radiation, Use ionized oxygen, ozonized (or ionized) oxygen with ozone, e.g. speed up chemical reaction by ionizing the gas before use. As per the principle # 38 recommendation, we propose a new design E-EDM by introducing ionized , ozonised oxygen into EDM process by replacing dielectric fluid from the existing EDM process, since EDM current research already started introducing dry EDM (i.e. introducing gas medium) by removing dielectric fluid, it validates our findings that dielectric can be eliminated from the existing design of EDM. As per our team’s recommendation that by introducing ionized, ozonised oxygen into EDM by eliminating dielectric fluid, it can speed up material removal rate at the cost of introducing an eco friendly design of EDM. Since the principle # 38, recommends both ionized and ozonised oxygen. The following research literatures, statements have been found and the same validate our recommendations in terms of implementing in real time EDM design with respect to ionization and ozonisation. The following findings on ionization and ozonisation were presented below in detail for better understanding. Schematic of new experimental design of E-EDM using TRIZ is shown in Fig.5.1.


Fig.5.1. New experimental design of E-EDM using TRIZ.

5.1. Advantages of ionization in new design of E-EDM
Ionization is part of a healthy atmosphere. It is one of nature's tools for maintaining and cleaning the air. Ionized gas molecules make up more than a very small percentage of the atmosphere. However, despite their small numbers, these molecules play a profound role in maintaining the health of the atmosphere by removing particulate and chemical pollutants. In the indoor environment, ionization provides an even wider spectrum of benefits, including the destruction of bacteria and elimination of odors. In particular, negative ionization has the effect of cleaning the atmosphere by removing particulate and chemical pollutants. Ionized gas molecules will help eliminate chemical and particulate pollutants from the indoor air, however, the greater the volume of pollutants the more the ionization is reduced [11-13]. Negative ionization is generally considered to be beneficial, positive air ions appear to cause harmful effects. The acceptable minimum concentration of negative ions for indoor air is 200-300 ions per cubic cm. The optimal level is 1000-1500 negative ions per cubic cm.

5.2. Advantages of ozonisation in new design of E-EDM
In term of introducing ozonised oxygen, the new design will have the following eco friendly advantages. Ozonised oxygen is nothing but a molecule composed of three oxygen atoms, it is highly reactive state of oxygen and Ozone degrades to oxygen and only oxygen. Ozone (O3) is a high energy form of oxygen, so it is quite ready to revert to the lower energy, more stable form of molecular oxygen (O2). Once the oxygen atoms are ionized, they enter into a series of very rapid reactions, including recombinations with electrons to become neutral species again. They do not "remain ionized" much at all. But there is a continuing pool of ionized oxygen atoms because new ones are being formed all the time.

5.3. Environmental acceptability and safety of introducing ozonised oxygen
Ozonisation is proven technology that has been used for years in the industrial purpose like food, waste water purification, process industry and in many engineering applications. No harmful residuals are reported that should be treated or removed. Ozonisation also increases the amount of dissolved oxygen, which is a benefit in many industrial applications. It is also a greenhouse gas. Recommendations on the usage of ozonised oxygen based on safety at normal atmospheric conditions are discussed here. As far as EDM design utilization is concern, the entire machine unit will be kept under closed environment with at most safety [14-15]. In case of manual interventions needed during machining process for process parameter selection, the following guidelines may be taken into account .According to Andersen et al. (2001), the Permissible Exposure Level (PEL) or time weighted concentration for ozone to which workers may be exposed is an average of 0.1 ppm (parts per million) over 8 hours and 5 days a week. The short-term exposure limit is an average of 0.3 ppm over 15 minutes. A concentration of 10 ppm in the air is generally accepted as immediately dangerous to life or Health. When setting the threshold limit for ozone, the vital issues are the effects of ozone towards the respiratory ducts and lungs.The health protection threshold limit for 8hours according to the EY Directive 92/72/EEC is 0.055 ppm (EY, 1992). The detrimental effects of ozone may occur if the concentration in the air exceeds 0.05 ppm in exposure over eight hours and 0.2 ppm in exposure over 15 minutes (FIOH, 2001). By the Degree on Concentrations Known to be Hazardous (109/2005), the Ministry of Social Affairs and Health in Finland has confirmed a list of concentrations of impurities in workplace air known to be hazardous Human Toxic Potential (i.e. HTP values). The HTP values for ozone has been defined as 0.05 ppm (0.1 mg/m3) in exposure over eight hours and 0.2 ppm (0.4 mg/m3) in exposure over 15 minutes.In human health risk assessment, Environmental Protection Agency (EPA) assigns a health risk to every hourly average concentration above 0.04 ppm. The Occupational Safety and Health Administration (OSHA) in the U.S.A has determined the maximum ozone concentration levels in the atmosphere of work places to 0.1 ppm in exposure over eight hours and 0.05 ppm over a 24 h exposure. U.S.A EPA believes that natural background levels of ozone range from 0.03 to 0.05 ppm

5.4. Added Advantages of new design E EDM are:

  1. Flushing is not required to circulate liquid dielectric since dielectric is replaced by ionized oxygen..
  2. It is a power saving new EDM design since dielectric circulating pump, filter have been removed from the circuit.

5.5. Limitations of new design E EDM are:

  1. Ozone is unstable at atmospheric pressure and thus it must be generated onsite, this may cause on increase in investment and it is toxic in high concentrations which should be taken care with at most care. The limitation on toxic in concentration may be overcome by adhering strict guidelines issued by OSHA and EPA.

Conclusion This research work reveals how TRIZ has been used in EDM novel research toovercome the limitations of existing EDM process and to a new design of Eco-friendly Electrical Discharge Machine. TRIZ is an vibrant tool in all means with its powerful search features and offers maximum results with minimal effort. As EDM novel research is concern, works have been started during 1996 and till research continues on improving the machining performance. In this work, using TRIZ, how novelty in EDM research could be achieved with maximum results with minimal effort through a new design of E-EDM. This novel tool can be used to extend in EDM research further.

References:

  1. P.C.Pandey, H.S.Shan, Modern Machining Processes,2003, Tata Mcgraw-Hill Publishing company limited.
  2. Norliana Mohd Abbas, Darius G.Solomon, Md.Faud Bahari, A review on current research trends in EDM machining, International Journal of machine tool and manufacture,47 (2007) 1214-1228.
  3. K.P.Rajurkar, Handbook of design, Manufacturing and Automation, Chapter 13:Nontraditional manufacturing processes,wiley,USA,1994.
  4. Z.N.Guo,T.C.Lee,T.M.Yue,W.S.Lau, A study of ultrasonic aided wire electrical discharge machining, Journal of material processing technology 63 (1997) 823-828.
  5. Ogava Hitoshi,Nogami Teuruo,Marimoto iwao, Study of micro machining of metals by EDM with high frequency vibration, Takushima perfectual industrial technology centre,1999-2001.
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  10. G. S. Altshuller, The algorithm of Inventing, Moscow, 1973 (in Russian).
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  12. Jokl, Miloslav V., Microenvironment, The Theory and Practice of Indoor Climate, Thomas, Illinois, 1989
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  15. Jukka Sassi & Jorma Rytkönen, Experiments with ultraviolet light, ultrasound and ozone technologies for onboard ballast water treatment VTT Industrial Systems,Satu ViitasaloFinnish Institute of Marine Research Erkki LeppäkoskiÅbo Akademi University.


Editor's notes: Section 4 of the paper fails to mention that principle #38 has been also used to resolve the contradiction lying at the heart of the problem. Indeed, Zhang et al. [7-8] "introduced" gas between tool and working piece. Apparently they "introduced" it so that to replace dielectric fluid. Moreover, I conducted some search and figured out that in 1999 Adachi et al. published a paper titled "Enhancement of accuracy of a second cut by Electric Dischanrge Machining conducted in athmosphere". (It is mentioned in US patent 6744002). Moreover, US patent 6903297 explicitly says that dielectric fluid can be replaced by a pressure gas.

The authors relate such solutions to Principles 16 and 35. But there are patents where air and "gas" were replaced by oxygen. For example, the US patent 6903297 by Kunieda et al. mentions replacing dielectric fluid by pressure gas and oxygen. This is already Principle #38 in its pure form.

The authors were aware of this work, which is evident from the paper they published in TRIZ-journal this month, which mentions that Kunieda et al. used oxygen in EDM. From this prospective replacing oxygen by ionized oxygen and by ozonized oxygen does not require starting solving the contradiction from scratch with the help of contradiction matrix. One has to only recognize prinicple #38 in the known solutions and improve them in accordance with the principle's recommendations.