INVESTIGATIONS ON NEAR-DRY EDM AND ANALYSIS OF PROCESS PERFORMANCE

Abstract

The electric discharge machining (EDM) is an unconventional machining process. It is extensively used to generate complex profiles on electrically conductive materials having high temperature resistance and high strength. Due to its wide applicability in manufacturing industries, EDM has become the most popular machining process after conventional machining processes like turning (lathe), milling and drilling. Despite several advantages, EDM process has some limitations such as low material removal rate (MRR), high tool wear rate (TWR) and poor surface finish. In past, several attempts have been made to overcome these limitations by augmenting EDM with techniques such as electrode rotation, ultrasonic vibrations and suspensions of powders into the dielectric fluid. Although these augmentations are excellent from research perspective, but in practice, are applicable only for fewer applications. Another limitation in EDM process, is its possible environmental pollution causing characteristic. During EDM process, material removal is a consequent of thermal energy produced by series of discrete electrical sparks occurring between tool and work electrodes. These electrodes are submerged in a dielectric medium. The EDM process utilizes hydrocarbon oils as dielectric medium. These oils produce serious toxic fumes causing health hazards to the machine operator. To overcome these limitations of EDM process, dry and near-dry variants of EDM have been experimentally examined in the present research endeavor. Dry EDM process utilizes a pressurized gaseous medium as a dielectric. Whereas, near-dry EDM process utilizes a mixture of liquid and gas (two phase) as a dielectric medium. v In the present research work, the experimentation was performed in four phases to meet the research objectives. In phase I, experiments were performed on near-dry, dry and conventional EDM to compare the performance characteristics amongst these process variants. Experiments were conducted with variation in four selected EDM process parameters. The responses measured were MRR and TWR. This investigation revealed that near-dry EDM was better than other EDM process variants. Thus, near-dry EDM was selected for further experimentation. In phase II, pilot experimentation was conducted on near-dry EDM process using One-Factor-At-a-Time (OFAT) approach. This experimental investigation assisted in selection of the range of process parameters for subsequent design of experimentation. In this phase, two combinations (water-air and glycerin-air) of dielectric mediums were selected for experimentation. The phase III of experimentation was performed with three different combinations of dielectric mediums (water-air, EDM oil-air and glycerin-air) using response surface methodology (RSM). Four process parameters (current, duty factor, flushing pressure and lift) were selected for experimentation. The results of experimentation on MRR, TWR, surface roughness (SR) and recast layer were analyzed to determine the significant process parameters. The results were investigated for analysis of variance (ANOVA) for each term on the response characteristics. The best dielectric combination obtained from phase III of experimentation was further used for fabrication of straight channels in phase IV of experimentation. Taguchi’s methodology was used to optimize the response characteristics of near-dry EDM. Further, complex profiles were fabricated with optimum parameter settings. The process parameters vi viz. current, pulse on time, gap voltage, liquid flow rate and workpiece feed were selected to measure response characteristics (MRR, TWR and SR). An effort was made to realize process mechanism of near-dry EDM with the help of high speed imaging and subsequent image processing. This imaging was conducted for all phases of experiments. Scientific theory was developed with explanation on the mechanism of the near-dry EDM process by conducting detailed study. In the present investigation, an axisymmetric thermal model was also developed to predict the material removal rate in near dry EDM. Further, this model was validated with experimental results. In this experimental investigation, glycerin-air dielectric medium emerged as the best dielectric combination for near-dry EDM process. It resulted in nearly three times higher MRR compared to other combinations of dielectric mediums (water-air, EDM oil-air) investigated. Further, this combination of dielectric medium, resulted in the elimination of recast layer from the machined surfaces.