What is Electrochemical Grinding Equipment? Scientific and metallurgical developments have placed unusual demands on the metalworking industry. The challenges faced by industry are not only modern materials with high strength-to-weight ratios, but also with new fabrication requirements demanding greater precision and surface integrity. Electrochemical Grinding Equipment (ECG) is an ideal machining process that provides a better, faster, and more cost effective metal cutting and grinding solution for today's toughest materials. Unlike conventional grinding techniques, Electrochemical Grinding offers the ability to machine difficult materials independent of their hardness or strength. This is because Electrochemical Grinding is an entirely different machining process in which electrical energy combines with chemical energy for metal removal. Since Electrochemical Grinding Equipment does not rely solely on an abrasive process, the results are precise cuts free of heat, stress, burrs and mechanical distortions. Electrochemical Reaction Electrochemical oxidation and reduction occurs on the surface of electrodes when an electric current is passed between the electrodes through an electrolyte fluid. An electrochemical potential between the electrodes causes current to flow from the anode to the cathode in the DC circuit. In Electrochemical Grinding, the anode is the work piece, and the cathode is the conductive grinding wheel. A continuous stream of electrolyte flows at the interface of the grinding wheel and work piece and conducts the current in the circuit. The electrolyte fluid is a conductive aqueous solution consisting of a mixture of chemical salts and other additives. At the positive electrode, or anode, oxidation of the work piece dissolves the surface of the metal and forms a metal oxide film. The film is electrically insulating, and acts as a barrier against the electrochemical cutting action of the process. The abrasives in the rotating grinding wheel continually remove this film and expose a fresh surface for oxidation. Metal deposition on the grinding wheel (cathode) is avoided by proper choice of electrolyte. Dissolution of the metal, combined with the mechanical removal of the oxides, results in an efficient low-stress cut. Electrical Energy According to Faraday's Laws the quantity of chemical change occurring at an electrode is directly proportional to the amount of current passing between the electrodes. Low voltage, high current electrical energy supplied by a properly designed DC power supply is central to the Electrochemical Grinding process. Since the voltages are low, spark discharge and the associated heat are avoided. In addition, the low voltages used prevent any electrical shock hazard to the operator. Productivity Factors With Electrochemical Grinding, the rate of metal removal is directly proportional to the current flowing across the contact surface between the wheel (cathode) and work piece (anode). The higher the amperage, the faster the rate of chemical change and stock removal. "Machinability" of a metal depends more upon its conductivity and electrochemical reactivity than its hardness or strength. Electrochemical Grinding Equipment can be used successfully on all electrochemically reactive and conductive materials.



Cross Process Innovations Section 4. Cross Abrasive Processes--WJM/USM/ECM/EDM/MAP Abrasives are commonly used in both conventional and non-traditional machining processes. Abrasives are hard grits which acts as micro cutters during mahcining. Mechincal energy can be good solutions to some difficulties in non-traditional processes when it is used properly. Abrasive WJM Water jet machining(WJM) is mainly used to cut and slit porous nonmetals such as wood, paper, leather, and foam. However it is not efficient for hard material machining. When abrasives are mixed in the water jet, Abrasive Waterjet Machining, a new and more powerful process is realized. Both WJM and AWJM use the principle of pressurizing water to extremely high pressures, and allowing the water to escape through a very small opening (orifice). Water jets machining use the beam of water exiting the orifice (or jewel) to cut soft stuff like diapers and candy bars, but are not effective for cutting harder materials. The inlet water is typically pressurized between 20,000 and 55,000 Pounds Per Square Inch (PSI). This is forced through a tiny hole in the "Jewel", which is typically 0.010" to 0.015" in diameter. This creates a very high velocity beam of water! Abrasive water jet machining uses that same beam of water to accelerate the abrasive particles to speeds fast enough to cut through much harder materials(Fig. 13). With the help of abrasives, materials of any hardness can be cut without delamination, without thermal damag, in the same time, withvery high cutting rate and the capability of cutting very large thickness. Fig.14 shows a 2" thick piece of 304 stainless steel cut by AWJM[1]. Electrochemical Assistance of Ultrasonic Machining Processes Ultrasonic machining is a mechanical material removal process used to erode holes and cavities in hard or brittle workpieces by using shaped tools, high frequency mechanical motion, and an abrasive slurry. Ultrasonic machining is able to effectively machine all materials harder than HRc 40, whether or not the material is an electrical conductor or an insulator[2]. Due to hardness limits, ultrasonic machining of some metallic materials is very slow and tool wear becomes a serious problem[3]. Electrochemical interactions can be integrated into the ultrasonic machining process to solve this difficulty (Figure 15). In electrochemical machining, anode dissolution is accompanied by the formation of a brittle oxide layer. Therefore, the abrasive grains in EC assisted USM act mainly on the brittle oxide layer rather than on the underneath material of the workpiece. Because the oxide film is removed by abrasion, the electrochemical dissolution process is accelerated due to depassivation of the material surface. The efficiency of both processes are improved. Machining rate is greater and the tool wear is less. An increase in the current density enhances the machining productivity and reduces the tool electrode wear, but decreases the accuracy. Both ultrasonic interaction and anodic dissolution can result in good surface finish. At the final step of operation, the machining can use small current density to improve surface finish. Abrasive ECM/EDM/ECDM The drawback and advntage of mechanical machining is that it directly contacts the workpiece and it is usually not sensitive to electrical or chemical fields. On the other hand, ECM and EDM process may be influenced by the oxidation or passivation layer. The addition of abrasives can help to breakdown the passivation layers and improve the machining rate. Also the abrasive grains that are in contact with the workpiece surface acts to remove the soft nonreactive oxide layer in ECM, thus exposing fresh metal to further electrolytic action. Conventional abrasive machining suffers from too fast wheel wear out, while in abrasive non-traditional machining, the majority of machining is done by machining other than mechanical machining. Thus the life of the wheel is greatly elongated. The benifits of both conventional and no-traditional processes are both enhanced. Besides abrasive waterjet machining, which directly uses the intensified mechanical machining effects of abrasives, other non-traditional abrasive machining processes are: Abrasive Electrical Discharge Machining (AEDM), Abrasive Electrochemical Machining (AECM), and Abrasive Electro-Chemical-Discharge Machining (AECDM). The tools are conducting electrodes containing abrasive grains, such as grinding wheels or abrasive sticks using conductive bonding agent or simply free abrasive grit in electrolyte. The types of tools and relative movements that are used in various processes are shown in Figure 16. Abrasive Electrochemical Grinding (AECG), Abrasive Discharge Grinding (AEDG), and Electrochemical Honing (ECH) use abrasive tool with conductive bond. Abrasive Electrochemical Finishing (AECF), Abrasive Electrical Discharge Finishing (AEDF), and Ultrasonic Electrochemical Machining (USECM) use free abrasive grains. Abrasive Electrochemical Machining Processes Electrochemical grinding using conductive bonded abrasive tool (AECG) is particularly effective for the machining of difficult-to-cut materials, such as sintered carbides, creep resisting alloys (eg. Inconel, Nimonic), titanium alloys, metallic composites (eg. PCD-Co, Al-SiC, Al-Al2O3). The increase in performance results from microcutting, electrochemical dissolution, breakdown of surface oxidation layer, and enhanced flow of the fresh electolyte in the gap between the grinding wheel and the electrode. Material is removed through a combination of electrochemical action and conventional mechanical grainging. Approximately 90% of the material is removed through electrochemical action and only 10% by mechanical griding. Because only a small amount of material is removed by grinding action, the wheel life is typically 10 times longer than the life of a conventional grinding wheel. An additional factor contribution to the lone life of electrochemical grinding wheels is the reduced contact arc--only a fraction of the grinding wheel directly contacts the workpiece sompared with conventional grinding wheel due to the gap produced by chemical machining. Figure 17 shows the scheme of ECG. The electrochemical interaction changes the microcutting conditions. Due to chemical effects, microhardness of the dissolved surface decreases, which reduces the tangential cutting forces Ft and normal cutting forces Fn. The electro-chemical heterogeneity (electric potentials and electro-chemical coefficients) at individual grains and grain boundariescan induce nonuniform phase dissolution rate. Mechanical grinding helps reduce these nonuniformity. Considerable reduction in cutting force significantly reduces abrasive material wear (q), and provides notable savings in machining costs when using expensive diamond grinding wheels. Figure 18 shows the effect of working voltage on power of the grinding wheel drive, diamond abrasive wear, q, and relative cost of material volume unit removing, K [4]. Abrasive electrochemical grinding can be used in the shaping of complex profiles on NC grinder. For example, Figure 19 illustrates applications for these special tool-electrodes with PCD abrasives [5]. Fig. 19 Examples of AECG Contouring[5] Figure 20 illustrates the influence of combined processes on machined surface finish. The results of workpiece surface smoothing, that have been obtained in face grinding using free abrasives (curve 2), electro-chemical grinding without free abrasives (curve 1), and abrasive – electrochemical grinding (curve 3 for grain size 2 mm. and curve 4 for grain size 0.5 mm.) are shown [16]. It can be seen that in the case of electro-chemical smoothing, maximum height of surface irregularities is Rmax = 0.05 mm., i.e. same as the initial value, and in the case of mechanical grinding this height decreases to Rmax = 0.008 mm., while with the combination of both processes it is possible to achieve Rmax = 0.006 mm. After changing grain size from 2 mm to 0.5 mm., in only 160 seconds, mirror surface (Rmax = 0.002 mm.) has been obtained [6]. This example reveals the effects of electro-abrasive machining processes and their potential in microfinishing and nanotechnology. Abrasive Electrical Discharge Grinding(AEDG) Introducing mechanical effects into EDG process leads to increase in metal removal rate. For example, in the case of Al-SiC composite, removal rate of the AEDG process (te = 10 ms, and n = 500 rpm) is about 5 times greater than that of EDM process, and about twice of EDG process. The increase in productivity of AEDG process is attributed to improvements in hydrodynamic conditions of dielectric flow. Abrasive EDM has beed discussed in Section 2 and will not be repeated here. Some Ultra High Precision Surface Finishing Processes Ultra high precision polishing technologies are used mainly in semiconductor industry. In such applications, less than 0.1 mm accuracy is required, the surface roughness should be less than 0.01mm. In order to obtain such smooth surfaces, the material surface should be machined with a controllable removal depth less than 0.001mm. Chemical interactions play more important roles than mechanical or physical actions at such dimension scales as shown in Table 1 [7]. Table 1 When the minimal removal depth at the contact between the work surface and the tool tip is large (approximately 1mm or larger), removal of the material is caused by the mechanical deformation mechanism through either plastic deformation or brittle fracture. On the other hand, when the material surface is machined with a minimal removal depth less than 0.1 mm, the chemical effects is the more important material removal mechanism because the increase in surface area due to particle size reduction. Machining from chemical action is greater than that from mechanical action. This means that even in conventional mechanical machining processes, chemical interactions may be the dominant mechanism of material removal in ultrapresion machining processes. One typical instance is the Elastic Emission Machining in which ultrafine abrasive powder mechanically bombards the material surface and elastically sputters the atoms, molecules or clusters of atoms/molecules from the surface. It is known that even in this mechanical mode of machining, removal rates considerably depend on the chemical affinity of the workpiece material and the abrasive powder[7]. In Mechanical-Chemical Polishing (MCP), chemically reactive abrasive powders softer than the workpiece are used. Neither indentations nor scratches of the abrasives would occur. MCP process using soft powder abrasive is especially suitable for very hard materials which are difficult-to-machine by conventional polishing processes. New polishing processes have been developed with the assistance of various energy fields, such as magnetic, electrical and ultrasonic field. Figure 21 shows the Magnetic Abrasive Polishing (MAP) process. Storng magnetic field drives the magnetic abrasives towards the specimen surface. This process is applicable not only to flat surfaces but also to inner and outer sculptured surfaces. For example, inner surface of a stainless steel pipe can be polished to a roughness of 0.3mm in a few minutes.