Development of an intelligent electrolytic in process dressing (ELID) grinding system 1

Introduction Chapter Introduction Grinding is a machining process which uses hard abrasive particles for cutting, where the surface speed of the abrasive tool (namely grinding wheel) is relatively faster compared to other machining processes like turning and milling. Technically, grinding is a subset of cutting where each grain of abrasive functions as a microscopic single-point cutting edge with high negative rake angle. It is a very old process and has been in existence since the Stone Age when man rubbed stones against each other to produce sharp weapons. This process is conventionally regarded as finishing process to obtain a high degree of dimensional accuracy and surface finish on a part. Grinding is used in a large and diverse area of manufacturing and toolmaking. It can produce very fine finishes and very accurate dimensions; yet stock removal rate can be quite high. It is usually better suited to the machining of very hard materials. 1.1 Background Rapid progress in electronic and optical devices (such as IC chips, MEMS, smart sensors, lenses) with the increasing requirement of their performance, demand of ultra precision surface finish of the specialized glass, silicon wafer in nano-scale is growing. Introduction Conventional grinding cannot produce such surfaces with precise form accuracy; therefore, conventional grinding is still considered to be a pre-finishing process. Typical ways for achieving final finishing of different materials are lapping and polishing. Though these methods have disadvantages like poor grindability, waste water problem, mechanical damages etc. [1]. Chemical-mechanical polishing (also known as CMP) is another final finishing process for silicon wafer preparation in the IC industries. Some of the disadvantages associated with the CMP process [2] are: 1. Low efficiency due to low removal rates, 2. Non-uniform wafer surface due to the variation in relative cutting speed across the wafer surface, and 3. Relatively high cost involved in this process. In order to overcome all these drawbacks associated with the conventional finishing processes, demand for mirror surface finish by grinding (using super abrasives) to replace the polishing, lapping, etc. have been growing stronger and stronger in the manufacture of optical and electrical parts. With the aim of meeting such demand, efforts are actively being made to develop high precision grinding machines, new grinding wheels and to review the applications of these new techniques in the field of manufacturing. Though grinding with super abrasive wheel is an efficient method of achieving nano surface finish on hard and brittle materials, the method has some difficulties associated with it. One of the major problems is the preparation of the bonding matrix for the super Introduction abrasives. The abrasives also known as grits need to be held firmly by the bond material. Therefore the bond material has to be hard in nature which reduces grinding wheels’ self sharpening and self truing ability. An additional dressing mechanism is needed to ensure good protrusion of the sharp cutting grits. When we talk about attaining precision surface finish and form accuracy using grinding, it is an obvious requirement to have the grinding wheel dressed in-process. There are several methods of in-process dressing of the grinding wheel, though all of them can be subdivided into four main groups as listed below. 1. Mechanical contact method. 2. Electro thermal method. 3. Laser technology. 4. Electrochemical method. In-process dressing with mechanical contact uses separate dressing tool that comes in contact physically, with the grinding wheel. The dressing tool may be mounted on a rotating spindle or it can also be held stationary. Dressing of grinding wheel by electro-thermal method employs electro-discharge machining technique. There are basically two arrangements available for this method of wheel dressing. One is block electro-discharge method where a stationary solid block is used as the dressing electrode and another one is wire electro-discharge method where Introduction moving wire instead of a block is used for dressing the wheel. Electro thermal method of dressing is mainly used for fine grit wheels. Laser machining method is another technique to achieve the in-process dressing of the wheel. It is mainly used for vitrified bond grinding wheel. As laser beam is applied bond material becomes soft and is removed during grinding and ensures protrusion of new sharp grits. Laser can also be used as a boosting method for mechanical contact dressing. In this case laser softens the bond material and the following mechanical dresser removes it with ease. Grinding wheel dressing by electrochemical method actually employs the electrochemical machining principle to dress the wheel where the wheel is set as an anode and another metal electrode is set as a cathode. Electrolytic In-Process Dressing (ELID) grinding is one of the latest and most appropriate techniques to dress the wheel in process by electrochemical method. Electrolytic dressing of the wheel applies the basic mechanism of electrolysis. This process converts electrical energy to chemical energy in an electrolytic solution in the presence of two electrodes (anode and cathode) as shown in figure 1.1. An electrical power supply is used to energize anode positively and cathode negatively. As the electric energy is applied ionic dissolutions occurs in the electrolytic solution and the negative and positive ions in the solution move to the positive and negative electrodes respectively. This will cause anodic metal dissolution and formation of anodic oxide. Introduction Fig 1.1: Basic principle of electrolysis The mathematical form of metallic dissolution rate is governed by Faraday’s law of electrolysis, which concludes that dissolution from anode material is directly proportional to the integration of current supplied to the electrolytic cell. The mathematical form of anodic dissolution can be written as follows, T m= ∫ idt F ⋅ M z where, m is the mass of the substance altered from anode i is the electric current passed through the cell F is Faraday constant M is the molar mass of the substance z is the valence number of ions of the substance (electrons transferred per ion) Introduction Basic elements of ELID grinding are shown in figure 1.2. ELID cell comprises of a metal bonded grinding wheel, a cathode electrode, a DC power supply and electrolyte. The wheel is connected to the positive terminal of the DC power supply thru Fig 1.2: Basic elements of ELID grinding a carbon brush whereas the electrode is connected to the negative pole of the power supply. Usually alkaline liquids are used as both electrolytes and coolant for grinding. A nozzle is used to inject the electrolyte into the gap between wheel and electrode. The gap is usually maintained to be approximately 0.1mm to 0.3 mm. An anodic oxide layer is formed on the circumference of the grinding wheel as a result of electrochemical reaction. The formation of this oxide layer is very crusial for the success of ELID grinding. It is soft and brittle in nature as compared to the original metal bond and gets easily worn off because of the excessive grinding force. The basic mechanism of ELID grinding has been explained in the figure 1.3[30]. After truing [Figure 1.3(a)], the grains and bonding material (metal) of the wheel surface are flattened. It is necessary for the trued wheel to be electrically pre-dressed to protrude the grains on the wheel surface. When pre-dressing starts [Figure 1.3(b)], the bonding Introduction Fig 1.3: Principle of ELID grinding [30] material flows out from the grinding wheel and an insulating layer composed of the oxidized bonding material is formed on the wheel surface [Figure 1.3(c)]. This insulating layer reduces the electrical conductivity of the wheel surface and prevents excessive flow-out of the bonding material from the wheel. As grinding begins [Figure 1.3(d)], diamond grains wear out and the layer also becomes worn out [Figure 1.3(e)]. As a result, the electrical conductivity of the wheel surface increases and the electrolytic dressing restarts with the flow-out of bonding material from grinding wheel. The protrusion of diamond grains from the grinding wheel therefore remains constant. This cycle is repeated during the grinding process to achieve stable grinding. The above mentioned technologies for in-process dressing of grinding wheel have their own advantages and disadvantages. The comparative analysis of all these processes can be understood from the table 1.1 given below, Introduction Table 1.1: System Comparative analysis of different in-process dressing method Mechanical Electro Laser Electrochemical contact dressing thermal machining dressing dressing dressing Bulky system complexity Die sinking Bulky method and Fairly simple design is complex simpler than system WEDM method. System noise Noisy Quiet Quiet Quiet level Any damage May to the wheel cause May cause mechanical damage May cause No thermal or thermal thermal mechanical damage from damage damage to the wheel over feeding etc Dressing tool Tool wear exists Tool wear exists wear No tool No tool wear. wear. Even though the ELID is a suitable process to obtain mirror surface finish and it has proven its advantages over other in-process dressing methods, the technology of electrolytic in-process dressing is yet to be fully optimized. ELID grinding is always a complex process in which the tool shape is changing while machining. Therefore it is necessary to monitor the change in the tool shape caused by the tool wear with some Introduction intelligent sensory system and generate the tool path by compensating the tool wear, in order to maintain high dimensional accuracy. In ELID grinding, electrolytic dressing condition is another significant factor to get successful results. In most of the earlier studies, the dressing conditions were mostly set manually and the process greatly relied on the experience of the operators. Open loop control of the ELID power supply, which is currently used, could not give the best results always. So, an intelligent electrolysis control technique using in-process sensor feedback is needed. 1.2 Main objectives This research mainly focuses to develop a smart technology that will help to improve ELID grinding by addressing the above mentioned difficulties currently faced in this area of ultra precision grinding. In order to accomplish this main objective several sub objectives have been set to be achieved. 1. The first goal is to build an intelligent machine tool for ELID grinding. In order to attain this; an existing wire cut EDM machine structure shall be reinforced with new control and sensory system to enhance the performance of ELID grinding. 2. Open loop control of ELID power may not be the best solution as described in the previous section hence implementation of dressing on demand concept for the ELID power control and comparison of this new idea with the conventional ELID grinding is another objective for this study. Introduction 3. Grinding wheel truing is the prerequisite to maintain uniform cutting. One of the prime objectives in this research is to develop an in-process truing method by introducing segmented dressing of the grinding wheel. The performance of this proposed wheel truing concept shall also be studied. 4. In ELID grinding wheel wear is very significant which introduce form inaccuracy for curved surface machining. Therefore a wheel wear compensation method shall be implemented for spherical surface grinding. 1.3 Organization of this thesis This thesis comprises of six chapters. Chapter gives a brief overview of the new concept of ELID grinding and its history. The advantages of ELID grinding over other inprocess dressing methods of grinding wheel are described in this chapter. Finally the objectives of this study are summarized. This chapter also outlines the organization of this dissertation. A comprehensive literature review is given in the Chapter which includes three parts. First section describes different other in-process dressing and truing techniques for grinding. In the second part a detailed discussion on the current research trend in ELID grinding is given. The review of different approaches to monitor grinding wheel wear is next presented. Finally some concluding remarks on the literature review are made. 10 Introduction Chapter describes the development of a prototype machine-tool for ELID grinding. Detailed discussions on the experiments and results for evaluating the performance of the machine are also included in this chapter. This chapter also explains in details the concept of tool wear compensation in ELID grinding for spherical surface machining. The implementation of this important idea on the developed ELID grinding machine is discussed in this chapter as well. Experiments to evaluate the performance of the tool wear compensation method and their results are explained here. Chapter describes implementation and performance evaluation of dressing on demand concept for ELID grinding. This technique aims to optimize the dressing power by monitoring the grinding condition and thus to save the grinding wheel from excessive dressing. The theoretical background as well as the experimental study on the comparison between this novel concept and conventional ELID grinding is discussed elaborately in this chapter. Chapter presents the theoretical framework of closed loop controlled in-process truing for ELID grinding. In-process truing of the grinding wheel can be achieved by controlling pulse duty ratio of the ELID power supply. The algorithm for practical implementation of this concept is also included in the chapter. This chapter also describes the experimental approach to assess the performance of the proposed system. Finally Chapter concludes this dissertation with the summary of contributions. The suggestions for the future research are also recommended in this chapter. 11 [...]... ELID grinding machine is discussed in this chapter as well Experiments to evaluate the performance of the tool wear compensation method and their results are explained here Chapter 4 describes implementation and performance evaluation of dressing on demand concept for ELID grinding This technique aims to optimize the dressing power by monitoring the grinding condition and thus to save the grinding wheel...Introduction Chapter 3 describes the development of a prototype machine-tool for ELID grinding Detailed discussions on the experiments and results for evaluating the performance of the machine are also included in this chapter This chapter also explains in details the concept of tool wear compensation in ELID grinding for spherical surface machining The implementation of this important idea... excessive dressing The theoretical background as well as the experimental study on the comparison between this novel concept and conventional ELID grinding is discussed elaborately in this chapter Chapter 5 presents the theoretical framework of closed loop controlled in- process truing for ELID grinding In- process truing of the grinding wheel can be achieved by controlling pulse duty ratio of the ELID... supply The algorithm for practical implementation of this concept is also included in the chapter This chapter also describes the experimental approach to assess the performance of the proposed system Finally Chapter 6 concludes this dissertation with the summary of contributions The suggestions for the future research are also recommended in this chapter 11 . accuracy using grinding, it is an obvious requirement to have the grinding wheel dressed in- process. There are several methods of in- process dressing of the grinding wheel, though all of them can be. different in- process dressing method Mechanical contact dressing Electro thermal dressing Laser machining dressing Electrochemical dressing System complexity Bulky system Die sinking. success of ELID grinding. It is soft and brittle in nature as compared to the original metal bond and gets easily worn off because of the excessive grinding force. The basic mechanism of ELID grinding