Editor | On 07, Jun 2010

By Professor V.S. Sreebalaji and Dr. R. Saravanan

Abstract

Electrical discharge machining (EDM) is a non-traditional process based on thermoelectric energy between a work piece and an electrode. A pulse discharge occurs in a small gap between a work piece and an 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 a spark. Dielectric fluid acts as a spark conductor, concentrating the energy to a narrow region. There are various types of products that can be produced and finished using EDM such as moulds, dyes, aerodynamic parts, automotive and surgical components. The following research work reveals how ultrasonic vibration can be incorporated into EDM through modeling an advanced design called ultrasonic-aided electrical discharge machining (UEDM) with the Theory of Inventive Problem Solving (TRIZ).

Introduction to Electric Discharge Machining (EDM)

The history of EDM techniques goes as far back as the 1770s when it was discovered by English scientist, Joseph Priestly. Although it was originally observed by Priestly, EDM was imprecise and riddled with failures. During research to eliminate erosive effects on electrical contacts the Soviet scientists, Dr. B. R. Lazarenko and Dr. N. I. 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 for several years in power supplies for EDM machines and an improved form is used in many applications.¹ Commercially developed EDM began to be a viable technique in the mid 1970s that helped shape the metal working industry. In the mid 1980s, the EDM techniques were transferred to a machine tool. This migration made EDM more widely available and a more appealing choice over traditional machining processes.

Current Research Trends in EDM Development

There are two kinds of research trends that are carried out by researchers as far as EDM is concerned. One is called the modeling technique and the other is called the novel technique. Modeling technique deals with:

• Mathematical modeling
  
• Artificial intelligence
  
• Optimization techniques such as regression analysis, artificial neural network, genetic algorithm, etc.

The modeling techniques are used to validate the efforts of input parameters on output parameters since EDM is a complicated process of more controlled input parameters such as:

• Machining depth
  
• Tool radius
  
• Pulse on time
  
• Pulse off time
  
• Discharge current
  
• Orbital radius
  
• Radial step
  
• Offset depth
  
• Output parameters like material removal rate and surface quality

Novel techniques deal with how other machining principles are conventional or unconventional such as ultrasonic can be incorporated into EDM to improve efficiency of machining processes to get better material removal rate and surface quality. Novel techniques have been introduced in EDM research since 1996.²

Introduction to Conventional – Trial and Error Approach

Blind commitment to a theory is not an intellectual virtue it is an intellectual crime.- Imre Lakatos, 1973

Trial and error (or trial by error) is a conventional method of problem solving for fixing things or for obtaining knowledge. Learning does not happen from failure but rather from analyzing the failure, making a change and then trying again.³

Trial and error approach has a number of features:

• Solution-oriented: trial and error makes no attempt to discover why a solution works, merely that it is a solution.
  
• Problem-specific: trial and error makes no attempt to generalize a solution to other problems.
  
• Non-optimal: trial and error is generally an attempt to find a solution, not all solutions and not the best solution.

Trial and error can proceed where there is little or no knowledge of the subject. It is possible to use trial and error to find all solutions or the best solution, when a testable finite number of possible solutions exist.

The Use of Trial and Error Approach to Solve Problems

Some complex problems can be solved by a technique that is called trial and error. Trial and error is typically good for problems where there are multiple chances to get to the correct solution. This is not, however, a good technique for problems that do not give multiple chances to find a solution. In addition to this, trial and error is also a great way to gain knowledge. There are a number of important factors that make trial and error a good tool to use for problem solving. The purpose of trial and error is not to find out why a problem was solved. It is primarily used to solve the problem. While this may be good in some fields, it may not work so well in others. For example, while trial and error may be excellent in finding solutions to mechanical or engineering problems, it may not be good for certain fields that ask “why” a solution works. Trial and error is primarily used to find a single solution to a single problem. One of the most powerful advantages to this technique is that it does not require an individual to have a lot of knowledge.

Limitations of Trial and Error Approach

Trial and error is not a method of finding the best solution. Nor is it a method of finding all solutions. It is a problem solving technique that is simply used to find a solution. It may require a large amount of patience. Trial and error is tedious and monotonous. Also, it is time-consuming. Trial and error is inefficient in situations where causing an error can lead to serious injury or death. A good example of this would be the attempts that were made by aircraft engineers to break the sound barrier. Though they succeeded, many pilots died because they used the trial and error process. In this situation trial and error would often lead to the plane crashing. Figure 1 shows the difference of the conventional design approach and TRIZ.

Figure 1: Difference of Conventional Design Approach and TRIZ

Introduction to TRIZ

Theory of Inventive Problem Solving (TRIZ) is a method based on logic and data. It accelerates an individual’s ability to research and creatively solve daily problems including impossible-to-solve problems. Theory of Inventive Problem Solving also provides repeatability, predictability and reliability due to its structure and algorithmic approach. Genrich Altshuller, the founder of TRIZ, and his colleagues developed the method between 1946 and 1985. Altshuller discovered that the evolution of a technical system follows predictable patterns. Inventiveness and creativity can be taught. The Theory of Inventive Problem Solving has helped solve thousands of difficult technical problems and several Fortune 500 companies are successfully using TRIZ. Given a good inventive technique, the impossible becomes the possible. The TRIZ approach is included in a rigorous step-by-step process, somewhat similar to an algorithm, although not as rigorous as a formal mathematical algorithm. This problem solving algorithm is called the algorithm for inventive problem solving (ARIZ). For more than 60 years the research on the algorithm has unfolded in several stages.

The three primary findings of this research includes:

1. Problems and solutions that are repeated across industries and sciences. The classification of the contradictions in each problem predicts the creative solutions to that problem.
   
2. Patterns of technical evolution that are repeated across industries and sciences.
   
3. Creative innovations use scientific effects outside the field(s) where they were developed.^4,5

As an international science of creativity, TRIZ, relies on the study of the patterns of problems and solutions. Not, however, on the spontaneous and intuitive creativity of individuals or groups. More than four million patents have been analyzed to discover the patterns that predict breakthrough solutions to problems. Research for TRIZ began with the hypothesis that there are universal principles of creativity and they are the basis for creative innovations for advancing technology. If these principles could be identified and codified they could be taught to people to make the process of creativity more predictable.

The idea is, somebody, someplace has already solved this problem (or one similar to it). Creativity is finding that solution and adapting it to this particular problem. Unified Structured Inventive Thinking (USIT) is an Americanized TRIZ term. It is a simplified methodology developed in the United States. It recommends a simple, yet powerful process of problem solving. It is composed of three stages:⁶

1. Problem definition
   
2. Problem analysis
   
3. Concept generation

In Japan, TRIZ has been introduced and promoted by the Nikkei Mechanical Journal, Mitsubishi Research Institute, Sanno Institute of Management and by professor Yotaro Hatamura’s group at the University of Tokyo and in various pioneering Japanese industries.

The Essence of TRIZ

There are several important principles and methods in TRIZ including:

1. Altshuller’s 39 generic engineering parameters
   
2. The contradiction matrix
   
3. The 40 inventive principles
   
4. The 76 standard solutions
   
5. Trends of evolution
   
6. Algorithm for inventive problem solving (ARIZ)

This research work involves the first three steps of the TRIZ methodology to match the problem context of EDM research.

Problem Context of Electrical Discharge Machining (EDM) Research

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

1. Lower material removal rate (MRR) and poor surface quality (SQ) are the real time EDM process limitations. In other words, maximizing the MRR and minimizing the surface roughness value are the real time EDM process objectives.⁷
   
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. The additional time and cost used for creating electrodes for ram / sinker EDM.
   
3. Reproducing sharp corners on the work piece is difficult due to electrode wear.
   
4. The work piece should be electrically conductive to be machined.
   
5. Specific power consumption is high and the energy required for the operation is more than that of the conventional process and, therefore, more expensive.

This work is focused on the real time EDM process objectives (increase the machining efficiency of EDM in terms of improving the MRR in the existing EDM design since MRR is one of the major real time process limitations of the EDM process.

Contradiction and Inventive Principles to Improve Material Removal Rate (MRR) of Electrical Discharge Machining (EDM)

The following TRIZ philosophy inspires an individual to look at the contradiction involved in the EDM process to increase the machining efficiency of EDM in terms of improving the MRR in the existing EDM design.

1. Somebody, someplace has already solved the problem or one similar. Creativity is finding that solution and modifying it to fit the circumstances.
   
2. Don’t accept compromises. Remove the contradictions (source of the problem).

• Improving Parameter: Amount of substance (MRR)
  
• Worsening Parameter (Undesired):Loss of time (Increase in machining time)

The full description of each of these parameters includes:⁸

• 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.

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 way to obtain better MRR without more machining time. The following inventive principles are suggested by the contradiction matrix for the parameters: amount of substance and loss of time.

• Principle 18, Mechanical Vibration
  
• Principle38, Strong Oxidants
  
• Principle 35, Parameter Change
  
• Principle 16, Partial or Excessive Action

Application of Suggested Inventive Principles

Derived by 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 principle definitions with some examples are given for better reference and to understand and validate the research findings unveiled already using trial and error approach in EDM research and to propose a new design using TRIZ, a novel tool for systematic innovation.

Principle 18, Mechanical Vibration

A.Cause an object to oscillate or vibrate. Electric carving knife with vibrating blades.
B.Increase its frequency (even up to the ultrasonic). Distribute powder with vibration.
C.Use an object’s resonant frequency. Destroy gallstones or kidney stones using ultrasonic resonance.
D.Use piezoelectric vibrators instead of mechanical ones. Quartz crystal oscillations drive high accuracy clocks.
E.Use combined ultrasonic and electromagnetic field oscillations. Mixing alloys in an induction furnace.

Principle 38, Strong Oxidants

A.Replace common air with oxygen-enriched air. Scuba diving with Nitrox or other non-air mixtures for extended endurance.
B.Replace enriched air with pure oxygen. Cut at a higher temperature using an oxy-acetylene torch. Treat wounds in a high pressure oxygen environment to kill anaerobic bacteria and aid healing.
C.Expose air or oxygen to ionizing radiation.
D.Use ionized oxygen. Ionize air to trap pollutants in an air cleaner.
E.Replace ozonized (or ionized) oxygen with ozone. Speed up chemical reactions by ionizing the gas before use.

Principle 35, Parameter Changes

A.Change an object’s physical state (to a gas, liquid or solid). Freeze the liquid centers of filled candies then dip in melted chocolate, instead of handling the messy, gooey, hot liquid. Transport oxygen or nitrogen or petroleum gas as a liquid, instead of a gas, to reduce volume.
B.Change the concentration or consistency. Liquid hand soap is concentrated and more viscous than bar soap at the point of use, making it easier to dispense in the correct amount and more sanitary when shared by several people.
C.Change the degree of flexibility. Use adjustable dampers to reduce the noise of parts falling into a container by restricting the motion of the walls of the container. Vulcanize rubber to change its flexibility and durability.
D.Change the temperature. Raise the temperature above the Curie point to change a ferromagnetic substance to a paramagnetic substance. Raise the temperature of food to cook it (changes taste, aroma, texture, chemical properties). Lower the temperature of medical specimens to preserve them for later analysis.

Principle 16, Partial or Excessive Actions

If 100 percent of an object is hard to achieve using a given solution method then by using slightly less or slightly more of the same method, the problem may be considerably easier to solve. Over spray when painting, then remove excess (or use a stencil – this is an application of principle 3, local quality and principle 9, preliminary anti-action). Fill, then “top off” when filling the gas tank of a car.

At this point in the problem solving process, appropriate problem and technological domain knowledge is important. The final solution derived from these principles was used to validate the previous research findings and to justify how well the novel approach of TRIZ can be used in emerging engineering and technological research domains to reduce research time, cost and to give the right direction and solution to research by proposing a new model of an ultrasonic assisted EDM design.

New Experimental Design of Ultrasonic-aided Electrical Discharge Machining (UEDM) using TRIZ for Better Material Removal Rate (MRR)

As per suggested principle 18, mechanical vibration and the examples given can cause an object to oscillate or vibrate, increase its frequency even up to the ultrasonic, distribute powder with vibration, use piezoelectric vibrators instead of mechanical ones, the new design of UEDM has been modeled for better MRR.⁸ Figure 2 shows that the vibration has been given to the tool electrode by adopting principle 18, mechanical vibration and has been experimentally validated by various researchers through trial and error methods.

Figure 2: Schematic of a New Experimental Design of UEDM Using TRIZ

Validation of a New Experimental Design of Ultrasonic-aided Electrical Discharge Machining (UEDM) Using TRIZ Principles

Principle 18, Mechanical Vibration

Kamlakar Rajurkar, a professor of engineering, has indicated some future trend activities in EDM on advanced materials, mirror surface finish using powder additives and introduction of ultrasonic vibration to the electrode as one of the methods used to expand the application of EDM and to improve the machining performance on difficult to machine materials.⁹

Author, Z.N. Guo, has proposed that the higher efficiency gained by the employment of ultrasonic vibration is mainly attributed to the improvement in dielectric circulation. This facilitates the debris removal and creation of large pressure changes between the electrode and the work piece, as an enhancement of molten metal ejection from the surface of the work piece.¹⁰

Engineer, Hitoshi Ogawa, proved that the depth of micro-holes by EDM with ultrasonic vibration increases about two times without the ultrasonic vibration and machine rate increasing.¹¹

Author, Minoru Kunieda, 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.¹²

Principle 16, Partial or Excessive Action

This principle is eye opening when used with principle 3, local quality. One of the examples given is to over spray when painting then remove excess or use a stencil-this. Both applications of principle 3, local quality and principle 9, preliminary anti-action.

As per principle 3, it suggests to change an object’s structure from uniform to non-uniform, change an external environment (or external influence) from uniform to non-uniform, make each part of an object function in conditions most suitable for its operation, make each part of an object fulfill a different and useful function.

Author, Q. H. Zhang, has introduced ultrasonic vibration EDM in gas with thin walled pipe electrode (tube electrode). It validates the application of principle 3, local quality through the introduction of changing an object’s structure.¹³

Principle 35, Parameter Change

Q.H. Zhang also studied ultrasonic EDM in gas. The gas is applied through the internal hole of a thin walled pipe electrode.¹⁴

Added Advantages of New Design Ultrasonic-aided Electrical Discharge Machining (UEDM) Include:

1. Machining rate can be increased twice with ultrasonic vibration-aided machining operation when compared with conventional EDM.
   
2. Mirror surface finish can be achieved by an ultrasonic vibration assisted tool electrode with powder additives.
   
3. Tool wear rate may be considerably reduced by introducing a vibrated tube electrode.
   
4. Ultrasonic vibration gives pressure variation all along the gap and results in better flushing, debris removal, circulation of dielectric and increases the process stability.
   
5. An enhancement of molten metal ejection from the surface of the work piece.

Conclusion

This research reveals how The Theory of Inventive Problem Solving (TRIZ) has been used in electrical discharge machining (EDM) novel research to overcome the limitations of existing EDM processes in terms of increase in material removal rate (MRR) and to lower the machining time. The schematic of a new experimental design of Ultrasonic-aided Advanced Electrical Discharge Machining (UEDM) has been developed using the inventive principles of TRIZ (principle 18, mechanical vibration).

The Theory of Inventive Problem Solving is a vibrant tool in all means with its powerful search features and offers maximum results with minimal effort as compared to conventional trial and error approaches involved in EDM research since 1996 where research continues on improving the machining performance by using the trial and error approach instead of a systematic innovative approach.

It is believed that this work, using TRIZ, as a novelty in EDM research could be achieved with maximum results with minimal effort through a new design of UEDM. The Theory of Inventive Problem Solving can be used to extend EDM novel research further to model new experimental designs that improve the machining performance. The same can be tested through real-time experimentations to prove viability of the suggested TRIZ principles on the contradictions to improve MRR.

References

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