INVESTIGATIONS ON ROTARY TOOL NEAR-DRY ELECTRICAL DISCHARGE MACHINING PROCESS

Abstract

Electrical Discharge Machining (EDM) is an advanced machining process. It is primarily used to machine electrically conductive materials irrespective of their hardness, into complex shapes with high precision. In the EDM process, the material removal takes place due to erosion caused by a series of discrete electrical discharges between the tool electrode and the work material. Both the electrodes are submerged in the dielectric medium such as hydrocarbon oil or deionized water. Commercially available EDM is also known as conventional EDM uses hydrocarbon oil as a dielectric medium. The burning of hydrocarbon oil produces harmful fumes, that pollute the environment and also harmful to the operator. Low material removal rate, high tool wear rate and issues related to environmental pollution are the main disadvantages of conventional EDM. In order to overcome these problems, researchers have developed several process variants such as dry EDM, powder mixed EDM, and near dry EDM, etc. In dry EDM, a gaseous dielectric medium is used instead of hydrocarbon oil. The dry-EDM process improved the debris flushing, material removal rate (MRR), and lower the tool wear rate. However, low MRR with non-oxygen gas, short of stability, deposition of debris on the electrodes, and odor of burning is the disadvantage of dry-EDM. Near-dry EDM, another process variant of EDM gained success in dealing with the abovementioned problem. Near dry EDM uses a two-phase flow of liquid and gas as a dielectric medium. The dielectric in this process is supplied through the hollow tool. Due to the presence of liquid phase in dielectric, debris reattachment to electrodes is minimal in neardry EDM. The presence of liquid medium in the dielectric helps to solidify the debris particle and thus flush away from the IEG. In the present research work, the rotary tool near-dry EDM (RT-ND-EDM) was investigated to understand the effect of process parameters. The experiments were divided into five phases to meet the research objectives. In phase I, the comparative assessment of RT-ND-EDM process with ND-EDM process was performed using water-air as well as glycerin-air dielectric mediums. The comparative assessment was based on the MRR and machined surface characteristics such as debris deposition, micro-cracks, etc. The experiments were performed using one factor at a time (OFAT) approach. In phase II, the effect of oxygen gas in RT-ND-EDM process was explored comprehensively. The experiments were performed by modifying the RT-ND-EDM facility iv used in phase I. Oxygen content, peak current, tool rotation speed, pulse on time, gas pressure, and liquid flow rate were selected as input process parameters. The experiments were conducted using one factor at a time (OFAT) approach. In addition, sustainability measures of RT-ND-EDM process with oxygen mixed dielectric medium were discussed. Investigation in phase III was aimed to explore the capabilities of rotary tool near-dry electrical discharge drilling using the glycerin-air dielectric medium. Experiments were designed and conducted using response surface methodology (RSM). RSM was used to develop a quadratic model between input parameters (tool rotation speed, current, pulse on time, LFR, and air pressure) and response parameters (MRR, surface roughness, and hole overcut). In phase IV, experiments were performed with different tool geometry to enhance the RTND- EDM process performance. To understand the effects of tool geometry in RT-ND-EDM process on the response characteristics such as MRR and HOC. Also, the effects of tool rotation speed, current, pulse on time, gas pressure, and liquid flow rate on the performance of RT-ND-EDM process were evaluated. Investigation in phase V was aimed to explore the capabilities of RT-ND-EDM process for the synthesis of micro- and nano- iron alloy particles. The experiments were performed on the developed RT-ND-EDM facility. Iron alloy particles were synthesized on HSS work material using water-air, and glycerin-air as the dielectric mediums. The characterization of particles produced in the process was performed. In addition, the benefits and potential of RT-ND-EDM process, as an environmental-friendly process to generate nanoparticles with higher production rate, is discussed. The developed RT-ND-EDM facility was demonstrated using water-air, oxygen mixed water-air, and glycerin-air dielectric mediums. The developed process is capable of using tailored dielectric medium by controlling the amount and type of both liquid and gaseous medium. Tool rotation along with the high-pressure dielectric mediums in the developed process improved the process performance in terms of material removal and surface quality. In addition, RT-ND-EDM process is capable to drill deep holes in difficult to machine material. Extensive experimentation was performed to identify the key machining parameters and their effects. The effect of current, pulse on time, pulse off time, gas pressure, liquid flow rate, lift settings, oxygen content were identified as key parameters. The parameters were identified for both high material removal and better surface quality. Oxygen mixed RT-ND-EDM process resulted in higher MRR due to oxidation of the work material. However, there are materials which do not promote oxidation. For such materials, the v glycerin-air dielectric medium is the best solution. Glycerin-air dielectric medium resulted in higher MRR as compared to other dielectric mediums due to availability of more thermal energy in the IEG. RT-ND-EDM process is a convenient approach to generate micro- and nano-particles of conductive materials irrespective of their strength, hardness, and melting point temperature with the advantage of simplicity, high production rate, low cost. It can be successfully utilized for the synthesis of micro- and nano-particles of difficult to machine material of various sizes, composition, and desired properties by choosing the appropriate dielectric medium, tool material, and other input process parameters.